International Conference on Isotopes and Environmental Studies, Volume 8: Aquatic Forum 2004, 25-29 October, Monaco (Radioactivity in the Environment)

RADIONUCLIDES IN THE ENVIRONMENT International Conference on Isotopes in Environmental Studies: Aquatic Forum 2004, 25–...

Author: P. Povinec | J.A. Sanchez-Cabeza

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RADIONUCLIDES IN THE ENVIRONMENT International Conference on Isotopes in Environmental Studies: Aquatic Forum 2004, 25–29 October, Monaco

RADIOACTIVITY IN THE ENVIRONMENT A companion series to the Journal of Environmental Radioactivity Series Editor M.S. Baxter Ampfield House Clachan Seil Argyll, Scotland, UK Volume 1: Plutonium in the Environment (A. Kudo, Editor) Volume 2: Interactions of Microorganisms with Radionuclides (F.R. Livens and M. Keith-Roach, Editors) Volume 3: Radioactive Fallout after Nuclear Explosions and Accidents (Yu.A. Izrael, Author) Volume 4: Modelling Radioactivity in the Environment (E.M. Scott, Editor) Volume 5: Sedimentary Processes: Quantification Using Radionuclides (J. Carroll and I. Lerche, Authors) Volume 6: Marine Radioactivity (H.D. Livingston, Editor) Volume 7: The Natural Radiation Environment VII (J.P. McLaughlin, S.E. Simopoulos and F. Steinhäusler, Editors) Volume 8: Radionuclides in the Environment (P.P. Povinec and J.A. Sanchez-Cabeza, Editors)

RADIONUCLIDES IN THE ENVIRONMENT International Conference on Isotopes in Environmental Studies: Aquatic Forum 2004, 25–29 October, Monaco

Editors P.P. Povinec Comenius University, Bratislava, Slovakia and IAEA-MEL, Monaco

J.A. Sanchez-Cabeza IAEA-MEL, Monaco

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© 2006 Elsevier Ltd. All rights reserved. This work is protected under copyright by Elsevier, and the following terms and conditions apply to its use: Photocopying Single photocopies of single chapters may be made for personal use as allowed by national copyright laws. Permission of the Publisher and payment of a fee is required for all other photocopying, including multiple or systematic copying, copying for advertising or promotional purposes, resale, and all forms of document delivery. Special rates are available for educational institutions that wish to make photocopies for non-profit educational classroom use. Permissions may be sought directly from Elsevier’s Rights Department in Oxford, UK: phone: (+44) 1865 843830; fax: (+44) 1865 853333; e-mail: [email protected]. Requests may also be completed on-line via the Elsevier homepage (http://www.elsevier.com/locate/permissions). In the USA, users may clear permissions and make payments through the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923, USA; phone: (+1) (978) 7508400; fax: (+1) (978) 7504744; and in the UK through the Copyright Licensing Agency Rapid Clearance Service (CLARCS), 90 Tottenham Court Road, London WlP 0LP, UK; phone: (+44) 20 7631 5555; fax: (+44) 20 7631 5500. Other countries may have a local reprographic rights agency for payments. Derivative Works Tables of contents may be reproduced for internal circulation, but permission of Elsevier Science is required for external resale or distribution of such material. Permission of the Publisher is required for all other derivative works, including compilations and translations. Electronic Storage or Usage Permission of the Publisher is required to store or use electronically any material contained in this work, including any chapter or part of a chapter. Except as outlined above, no part of this work may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without prior written permission of the Publisher. Address permissions requests to: Elsevier’s Rights Department, at the phone, fax and e-mail addresses noted above. Notice No responsibility is assumed by the Publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the material herein. Because of rapid advances in the medical sciences, in particular, independent verification of diagnoses and drug dosages should be made.

First edition 2006 Library of Congress Cataloging in Publication Data A catalog record from the Library of Congress. British Library Cataloguing in Publication Data A catalogue record from the British Library. ISBN-13: 978-0-08-044909-8 ISBN-10: 0-08-044909-3 ISSN: 1569-4860 The paper used in this publication meets the requirements of ANSI/NISO Z39.48-1992 (Permanence of Paper). Printed in The Netherlands.

v

Preface

Radioactive and stable isotopes have been applied as tracers for better understanding of environmental processes for about fifty years, contributing to diverse areas such as atmospheric transport, ocean circulation, groundwater hydrology, sedimentation processes and climate change. However, research has always been limited by the techniques available for sampling and analysis. Recently we have seen important achievements, such as the use of robotic systems based on remotely operating vehicles (ROV) and autonomous underwater vehicles (AUV) to sample the ocean environment and the use of satellite information for the optimisation of sampling programmes. In the field of analytical technologies we have moved from simple radiochemical methods and gas counters to robotic radiochemical technologies and sophisticated detectors working on line with powerful computers, often situated deep underground to protect them against the cosmic radiation background. The philosophy of analysis of long-lived radionuclides has changed from the concept of counting decays to the counting of atoms using highly sensitive mass spectrometers working either with low energy ions, such as inductively coupled mass spectrometry (ICPMS), thermal ionisation mass spectrometry (TIMS) and resonance ionisation mass spectrometry (RIMS), or with ions accelerated to hundreds of MeV in accelerator mass spectrometers (AMS). These new developments in sampling and analytical techniques have been accompanied by changes in the philosophy and organisation of research, as institutional and national investigations have been replaced by global international projects such as World Ocean Circulation Experiment (WOCE), Joint Global Ocean Flux Study (JGOFS), Climate Variability and Predictability Study (CLIVAR), Past Global Changes (PAGES), Worldwide Marine Radioactivity Studies (WOMARS), Global Marine Biochemistry of Trace Elements and Isotopes (GEOTRACES), Southern Hemisphere Ocean Tracer Study (SHOTS), etc. These and other topics for the better understanding of key processes in the aquatic environment, responsible for its future development and its protection, were at the forefront of the IAEA’s International Conference on Isotopes in Environmental Studies – AQUATIC FORUM 2004 convened in Monaco from 25 to 29 October 2004, which was the most important gathering of the year of isotope environmental scientists. The conference was organised by the IAEA’s Marine Environment Laboratory in cooperation with the Intergovernmental Oceanographic Commission of UNESCO, the International Hydrological Programme of UNESCO, the Commission Internationale pour l’Exploration Scientifique de la Mer Mediterranée, and the Abdus Salam International Centre for Theoretical Physics. The conference was hosted by

vi

Preface

the Principality of Monaco. Over 320 experts from 60 IAEA Member States and 6 international organisations delivered 185 oral presentations in 6 plenary and 31 parallel sessions and made 130 poster presentations. The conference reviewed the present state of the art isotopic methods for investigation of the aquatic environment. Four workshops were held simultaneously: • ATOMS-Med Workshop – development of a project proposal for oceanographic investigations in the Eastern Mediterranean. • El Niño – Research Coordination Meeting of the new IAEA Coordinated Research Project investigating climate change using isotopic records in the marine environment. • CELLAR Workshop – Collaboration of European Low-Level Underground Laboratories. • GSI Workshop on Groundwater–Seawater Interactions in coastal zones, organised in cooperation with the IAPSO Commission on Groundwater–Seawater Interactions. The main conference highlights, which included the latest developments in the field, were: (i) new information on behaviour, transport and distribution of isotopes in the aquatic environment; (ii) recent climate change records using isotopic tracers in the environment (tree rings, corals, sediments); (iii) global oceanic studies by WOCE, WOMARS, SHOTS and GEOTRACERS projects; (iv) impacts of groundwater–seawater interactions on coastal zones; (v) groundwater dynamics and modelling, important for management of freshwater sources; (vi) new trends in radioecological investigations, concentrating on the protection of marine biota against radioactive contamination; (vii) transfers in analytical technologies from bulk analyses to particle and compound specific analyses of environmental samples; (viii) development of new isotopic techniques, such as AMS, RIMS and ICPMS, and their applications in environmental studies; (ix) new trends in radiometrics underground techniques; (x) new in situ radiometrics technologies, and many other exciting topics which were presented and discussed during the Conference. The contents of the book are assembled from selected papers presented during the Conference in plenary and parallel oral sessions. The Proceedings published by the IAEA contains the other papers presented in both oral and poster sessions which could not be included here for space reasons. In our opinion, the proceedings constitute an important contribution to environmental isotopic research. Finally, the Scientific Secretary of the Conference (P.P. Povinec), would like to thank colleagues on the Scientific Committee, those of sponsoring organisations, session chairmen, speakers in oral and poster session, colleagues at IAEA for their help in preparation and organisation of the Conference, and in general all the participants for their contributions to the success of the Conference. Special thanks are due to the scientific reviewers who donated their time and expertise to assure the high scientific quality of the papers presented in this book, namely to T. Altzitzoglou, R. Aravena, D. Arnold, G. Barci-Funel, G. Barrocu, D.L. Biddulph, E. Boaretto, A. Bode, R. Bojanowski, D. Boust, W.C. Burnett, J. Carroll, J. Dean, S. de Mora, J. de Oliveira, R. Delfanti, E. Duran, V. Egorov, V.I. Ferronsky, H. Florou, S.W. Fowler, T. Gäfvert, R. García-Tenorio, J.R. Gat, J.M. Godoy, S. Grabowska, E. Güngör, T. Hamilton, I. Harms, S. Hauser, G. Heusser, K. Hirose, A. Hogg, E. Holm, T. Honda, G.H. Hong, M. Horvat, N. Horvatinˇci´c, X. Hou, M. Hult, Y. Igarashi, Y. Ikeuchi, M. Iosjpe, T. Ito, C. Jeandel, G. Jia, D. Jones, G. Kanish, R.M. Key, W.E. Kieser, M. Köhler, K. Komura, E. Kontar, M. Korun, A. Kryshev, R. Ladygiene, M. Laubenstein, G. Lazorenko, M.-H. Lee,

Preface

vii

S.-H. Lee, J. Mattila, R.L. Michel, J.W. Mietelski, J. Miralles, M. Nakano, S. Nielsen, B. Obeli´c, C. Papucci, H. Pettersson, J. Paatero, T.-S. Park, A.P. McNichol, G. Polykarpov, W. Plastino, A. Priller, C.R. Quètel, G. Raisbeck, P. Rajec, J.-L. Rays, N. Reguigui, P. Roos, K. Rozanski, B. Salbu, M. Schubert, M. Schwaiger, F.W. Schwartz, S. Shima, K. Sihra, T. Stieglitz, A. Stolarz, R. Szymczak, Y. Tateda, H. Thebault, P. Theodorsson, I. Tolosa, O. Togawa, C. Tsabaris, D. Tsumune, S. Valulovsky, J. Vives i Battle, O. Voitsekhovitch, L. Vöröss, M. Warnau and E. Wyse. In publishing this book and the IAEA Proceedings we aim to make the use of isotopes more widespread in the environmental disciplines and to further stimulate work in this exciting field. P.P. Povinec, J.A. Sanchez-Cabeza, Editors IAEA-MEL Monaco

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Contents

Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

v

1. Environmental isotope tracers . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1

1. Application of accelerator mass spectrometry to environmental and paleoclimate studies at the University of Arizona by A.J. Timothy Jull, George S. Burr, J. Warren Beck, Gregory W.L. Hodgins, Dana L. Biddulph, John Gann, Arthur L. Hatheway, Todd E. Lange and Nathaniel A. Lifton . . . . . . . .

3

2. Discriminating biogenic and anthropogenic chlorinated organic compounds using multi-isotope analyses of individual compounds by Kazushi Aranami, Steven J. Rowland and James W. Readman . . . . . . . . . . . . . . . . . .

24

3. Shift in stable water isotopes in precipitation in the Andean Amazon: Implications of deforestation or greenhouse impacts? by A. HendersonSellers and K. McGuffie . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

39

2. Oceanic radionuclide tracers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

51

4. Southern Hemisphere Ocean Tracer Study (SHOTS): An overview and preliminary results by M. Aoyama, M. Fukasawa, K. Hirose, R.F.C. Mantoura, P.P. Povinec, C.S. Kim and K. Komura . . . . . . . . . . . . . . . . . . . .

53

5. Plutonium isotopes in seawater of the North Pacific: Effects of close-in fallout by K. Hirose, M. Aoyama, C.S. Kim, C.K. Kim and P.P. Povinec . . .

67

6. Distribution of anthropogenic radionuclides in the water column off Rokkasho, Japan by Shigeki Shima, Shin-ichi Gasa, Ken-ichi Iseda, Tomoharu Nakayama and Hisao Kawamura . . . . . . . . . . . . . . . . . . . . . . . .

83

ix

x

Contents

7.

Artificial radionuclides in the Yellow Sea: Inputs and redistribution by G.H. Hong, C.S. Chung, S.-H. Lee, S.H. Kim, M. Baskaran, H.M. Lee, Y.I. Kim, D.B. Yang and C.K. Kim . . . . . . . . . . . . . . . . . . . . . . .

3. Radionuclides in the European seas . . . . . . . . . . . . . . . . . . . . . . . . . . 8.

9.

10.

96 135

Distribution of anthropogenic radionuclides in the water column of the south-western Mediterranean Sea by S.-H. Lee, F.R. Mantoura, P.P. Povinec, J.A. Sanchez-Cabeza, J.-L. Pontis, A. Mahjoub, A. Noureddine, M. Boulahdid, L. Chouba, M. Samaali and N. Reguigui . . . . . . . . . . .

137

Distribution of anthropogenic radionuclides in Moroccan coastal waters and sediments by M. Benmansour, A. Laissaoui, S. Benbrahim, M. Ibn Majah, A. Chafik and P.P. Povinec . . . . . . . . . . . . . . . . . . . . . . . . . . .

148

137 Cs

in seawater and sediment along the Algerian coast by A. Noureddine, M. Menacer, R. Boudjenoun, M. Benkrid, M. Boulahdid, M. Kadi-hanifi, S.-H. Lee and P.P. Povinec . . . . . . . . . . . . . . . . . . . . . . . . . . .

156

11. Physical and chemical characteristics of 137 Cs in the Baltic Sea by Galina Lujanien˙e, K˛estutis Jokšas, Beata Šilobritien˙e and Rasa Mork¯unien˙e . . .

165

4. Radioecological studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

181

12. Comparison of the MARINA II dispersion model with CSERAM for estimating concentrations of radionuclides in UK waters by Kamaljit Sihra, Antony Bexon and John Aldridge . . . . . . . . . . . . . . . . . . . . . . .

183

13. Assessment of the discharge of NORM to the North Sea from produced water by the Norwegian oil and gas industry by T. Gäfvert, I. Færevik and A.L. Rudjord . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

193

14. Uranium mining and ore processing in Ukraine – radioecological effects on the Dnipro River water ecosystem and human health by O. Voitsekhovitch, Y. Soroka and T. Lavrova . . . . . . . . . . . . . . . . . . . . . . . . . . . .

206

and 241 Am distributions in an alpine wetland, Boréon (France) by Maïa Schertz, Hervé Michel, Geneviève Barci-Funel and Vittorio Barci . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

215

16. Concentrations and characteristics of uranium isotopes in drinking waters collected in Italy and the Balkan regions and their radiological impact on the public by Guogang Jia, Giancarlo Torri, Umberto Sansone, Piera Innocenzi, Silvia Rosamilia, Antonio Di Lullo and Stefania Gaudino . . . . . . . . . .

223

15.

90 Sr, 137 Cs, 238 Pu, 239/240 Pu

Contents

17. The radiological evaluation of uranium, radium and radon in metallic and thermo-metallic springs in Ikaria Island, the eastern Aegean Sea, Greece by H. Florou, K. Kehagia, A. Savidou and G. Trabidou . . . . . . . . . . . . . 5. Isotope biomonitors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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235 243

18. Bioaccumulation of radiocaesium in Arctic seals from Northeast Greenland by JoLynn Carroll, Kristina Rissanen and Tore Haug . . . . . . . . . . . . .

245

19. Anthropogenic radionuclides in biota samples from the Caspian Sea by J. Gastaud, B. Oregioni, S.V. Pagava, M.K. Pham and P.P. Povinec . . . . .

255

20.

21.

210 Po

in fish, algae, mussel and beach sediment samples collected along the Turkish coast of the Black Sea by Nurdan Güngör, Emin Güngör, B. Gül Göktepe and Güler Köksal . . . . . . . . . . . . . . . . . . . . . . . . . . .

265

210 Po in mussels (Mytilus galloprovincialis) and sediments along the Turkish

coast of the Aegean Sea by Aysun U˘gur, Güngör Yener, Sayhan Topcuo˘glu, U˘gur Sunlu, Serpil Aközcan and Banu Özden . . . . . . . . . . . . . . . . .

272

22. Stable nitrogen isotopes reveal weak dependence of trophic position of planktivorous fish on individual size: A consequence of omnivorism and mobility by Antonio Bode, Pablo Carrera and Carmela Porteiro . . . . . . . .

281

6. Isotope hydrology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

295

23. Radiocarbon loss from DIC in vadose water flow above the Judea Aquifer, Israel by Israel Carmi, Mariana Stiller, Joel Kronfeld, Yoseph Yechieli, Miriam Bar-Matthews, Avner Ayalon and Elisabetta Boaretto . . . . . . . . . . . .

297

24. Stable water isotopes as tools for basin-scale water cycle: Diagnosis of the Murray–Darling by A. Henderson-Sellers, P. Airey, K. McGuffie and D.J.M. Stone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

307

25. Isotopic characteristics of the Sava River basin in Slovenia by Nives Ogrinc, Tjaša Kanduˇc and Janja Vaupotiˇc . . . . . . . . . . . . . . . . . . . . . . . .

317

26.

222 Rn

as a tracer for the estimation of infiltration of surface waters into aquifers by M. Schubert, K. Knoeller, H.-C. Treutler, H. Weiss and J. Dehnert . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

326

27. Monitoring of geochemical and geophysical parameters in the Gran Sasso aquifer by Wolfango Plastino . . . . . . . . . . . . . . . . . . . . . . . . . .

335

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Contents

7. Groundwater–seawater interactions . . . . . . . . . . . . . . . . . . . . . . . . . .

343

28. Coastal water exchange rate studies at the southeastern Brazilian margin using Ra isotopes as tracers by Joselene de Oliveira, Mathew Charette, Mathew Allen, Elisabete de Santis Braga and Valdenir Veronese Furtado . . . . . .

345

29. Submarine groundwater discharge in the southeastern Mediterranean (Israel) by Y. Weinstein, G. Less, U. Kafri and B. Herut . . . . . . . . . . . . . . .

360

30. Submarine groundwater discharge investigations in Sicilian and Brazilian coastal waters using an underwater gamma-ray spectrometer by Pavel P. Povinec, Isabelle Levy-Palomo, Jean-Francois Comanducci, Joselene de Oliveira, Benjamino Oregioni and Agata M.G. Privitera . . . .

373

31. Isotope hydrochemical investigation of saline intrusion in the coastal aquifer of Karachi, Pakistan by A. Mashiatullah, R.M. Qureshi, M.A. Tasneem, T. Javed, C.B. Gaye, E. Ahmad and N. Ahmad . . . . . . . . . . . . . . . .

382

8. Coastal radionuclide studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

395

32. Temporal variations and behaviour of 90 Sr and 137 Cs in precipitation, river water and seawater in Japan by Yoshihiro Ikeuchi . . . . . . . . . . . . . .

397

33. Isotope fractionations and radiocarbon ages of beach rock samples collected from the Nansei Islands, southwest of Japan by Kunio Omoto . . . . . . .

406

34. A model of recent sedimentation in the Cananeia–Iguape estuary, Brazil by R.T. Saito, R.C.L. Figueira, M.G. Tessler and I.I.L. Cunha . . . . . . . . .

419

9. Modelling of environmental processes . . . . . . . . . . . . . . . . . . . . . . . .

431

35. Simulation of the advection–diffusion–scavenging processes for 137 Cs and 239,240 Pu in the Japan Sea by Masanao Nakano . . . . . . . . . . . . . . . .

433

36. A biokinetic model for the uptake and depuration of radioiodine by the edible periwinkle Littorina littorea by J. Vives i Batlle, R.C. Wilson, P. McDonald and T.G. Parker . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

449

37. Environmental modelling: Modified approach for compartmental models by M. Iosjpe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

463

38. Assessment of 137 Cs outspread in the Lithuanian part of the Baltic Sea by L. Davuliene, N. Tarasiuk, N. Spirkauskaite, G. Trinkunas and L. Valkunas

477

Contents

10. Radiometrics techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

xiii

493

39. Low-level germanium gamma-ray spectrometry at the µBq/kg level and future developments towards higher sensitivity by G. Heusser, M. Laubenstein and H. Neder . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

495

40. Depth profiles of environmental neutron fluxes in water and lead by Y. Hamajima and K. Komura . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

511

41. Radiocarbon measurement by liquid scintillation spectrometry at the Gran Sasso National Laboratory by Wolfango Plastino and Lauri Kaihola

520

42. Monte Carlo simulation of the muon-induced background of an antiCompton gamma-ray spectrometer placed in a surface and underground laboratory by Pavol Vojtyla and Pavel P. Povinec . . . . . . . . . . . . . . . . .

529

43. IAEA-MEL’s underground counting laboratory – The design and main characteristics by P.P. Povinec, J.-F. Comanducci, I. Levy-Palomo and F. Avaullee . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

538

44. Levels of airborne radionuclides at Hegura Island, Japan by K. Komura, N. Muguntha Manikandan, Y. Yamaguchi, M. Inoue, T. Abe and Y. Murata

554

45. The use of sodium iodide detectors to locate buried radioactive particles in the seabed off Dounreay nuclear facility, Scotland by J. Toole, S.C. Innes, M. Liddiard, J. Cassidy and S. Russ . . . . . . . . . . . . . . . . . . . . . .

562

11. Mass spectrometry techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . .

579

46. Isotope selective ultratrace analysis of plutonium by resonance ionisation mass spectrometry by Stefan Bürger, Razvan Aurel Buda, Horst Geckeis, Gerhard Huber, Jens Volker Kratz, Peter Kunz, Christoph Lierse von Gostomski, Gerd Passler, Ariane Remmert and Norbert Trautmann . . . . . . .

581

47. Two 60-year records of 129 I from coral skeletons in the South Pacific Ocean by D.L. Biddulph, J.W. Beck, G.S. Burr and D.J. Donahue . . . . . . . . .

592

48. Factors influencing the determination of ultra low levels of Pu-isotopes by sector field ICP-MS by Per Roos . . . . . . . . . . . . . . . . . . . . . . . .

599

12. Management of data quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49. Recent IAEA reference materials and intercomparison exercises for radionuclides in the marine environment by M.K. Pham, J. Gastaud, J. La Rosa, S.-H. Lee, I. Levy-Palomo, B. Oregioni and P.P. Povinec . . . . . . . . . .

615

617

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Contents

50. Towards quality excellence in radioanalytical laboratories at STUK, Finland by Tarja K. Ikäheimonen, Seppo Klemola and Pia Vesterbacka . . . . . . .

629

Author Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

641

Keyword Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

643

1. Environmental isotope tracers


3

Application of accelerator mass spectrometry to environmental and paleoclimate studies at the University of Arizona A.J. Timothy Jull* , George S. Burr, J. Warren Beck, Gregory W.L. Hodgins, Dana L. Biddulph, John Gann, Arthur L. Hatheway, Todd E. Lange, Nathaniel A. Lifton University of Arizona, Tucson, AZ 85721, USA Abstract A wide range of climatic, geologic and archaeological records can be characterized by measuring their 14 C and 10 Be concentrations, using accelerator mass spectrometry (AMS). These records are found not only in the traditional sampling sites such as lake sediments and ice cores, but also in diverse natural records. The purpose of this paper is to highlight some selected applications of AMS at the University of Arizona, including sample preparation, applications of AMS radiocarbon dating to learning about climatic changes in the past, modern 14 C studies, and 10 Be and 129 I measurements. Keywords: Paleoclimate studies, Accelerator mass spectrometry, Radiocarbon dating, 14 C, 10 Be, 129 I

1. Introduction A firm chronology is an important key to the understanding of past climatic changes and their relationship to other events. To be able to correlate distinct climatic features, we must be able to correlate independently-dated events. Hence, the improvement in the radiocarbon calibration curve over the last 26,400 yr has allowed us to cross-correlate fluctuations in the 14 C curve with climatic fluctuations in such things as ice-core records. This capability has improved attempts to cross-correlate different climatic events observed in one record with other proxy records. This extension of the calibration curve used tree rings to about 11,500 calibrated years and beyond that used corals and varved marine sediments. Other newer but perhaps less-reliable records can take us back to the limits of radiocarbon dating, using lake sediments and speleothem records. * Corresponding author. Address: NSF Arizona AMS Laboratory, University of Arizona, Physics Building, 1118 East Fourth St, Tucson, AZ 85721, USA; phone: (+1) 520 621 6816; fax: (+1) 520 621 9619; e-mail: [email protected]

RADIOACTIVITY IN THE ENVIRONMENT VOLUME 8 ISSN 1569-4860/DOI 10.1016/S1569-4860(05)08001-0

© 2006 Elsevier Ltd. All rights reserved.

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A.J.T. Jull et al.

We should also note that the same event might have a different manifestation or have a phase lag in different regions. Hence, it is also important that the underlying geochronology is sound. This is particularly true during the Glacial/Interglacial transition, which is of great interest due to the scale of climatic change at that time. During the Holocene, we also observe appreciable climatic fluctuations. These are less well understood, but may be associated with solar forcing (Damon and Sonnett, 1991). Other periodicities in the Holocene climatic record can often be related to solar fluctuations, the most obvious being in the medieval warm period and the Maunder minimum, periods associated with colder weather in Europe. There is a variety of literature on this subject. In recent years, millennial-scale periodicities (e.g. Bond et al., 1997; Alley et al., 2001) are recognized in a number of records, including varved lake sediments, loess deposits, marine records and forest-fire records. In this paper, we will highlight some climatic signals, which can be well dated using the small-sample capabilities of accelerator mass spectrometry (AMS). These signals can be seen not only in the climate record but affect the extinction of mega fauna as well as archeological events. We will also discuss the use of AMS for 10 Be and 129 I studies.

2. Improvements in the radiocarbon calibration The radiocarbon “calibration curve” was originally established by studying the changes in 14 C content of known-age tree rings. The first trees studied were Bristlecone Pines from the White Mountains of California. Tree ring 14 C measurements from living and dead wood from these long-lived trees were cross-correlated to establish a 14 C chronology longer than the life of a single tree (see Fig. 1). Dendrochronology, the science of correlating and cross-referencing variations in the widths of tree rings has been used to produce even longer chronologies using German and Irish Oak tree records, incorporating subfossil wood recovered from buried logs in river sediments and peat bogs. Currently, the continuous tree-ring sequence extends back 11,500 years. We know that the production rate of radiocarbon in the atmosphere changes with time and there are also changes in the amount of CO2 exchange with the oceans. Both these effects lead to changes in the “apparent” age, which we term the radiocarbon age of the sample. The radiocarbon age is defined simply from the amount of 14 C relative to “modern carbon”, defined as 1950 AD wood, where the industrial effect of reduced 14 C has been removed, effectively this is 1850 AD wood age-corrected back to 1950 AD. Using these assumptions, the radiocarbon age is easily computed as 14 C sample

Radiocarbon age = −8033 ln 14

Cmodern

.

There have been continuing improvements in the length of the radiocarbon time-scale. The improvement in the radiocarbon calibration curve over the last 26,000 yr has allowed us to cross-correlate fluctuations in the 14 C curve directly with those in the ice-core record. This capability has improved attempts to cross-correlate different climatic events observed in one record with other proxy records. This extension of the calibration curve used tree rings to about 11,500 calibrated years and beyond that used corals and varved marine sediments. There

Application of accelerator mass spectrometry

5

Fig. 1. Ancient Bristlecone pines from the White Mountains, California.

are also newer but as yet less well-established records which should take us back to the limits of radiocarbon dating, using lake sediments and speleothem records. An example of a section of the calibration curve is given in Fig. 2. The fluctuations in this curve demonstrate changes in the 14 C production rate and/or changes in the terrestrial carbon cycle have occurred. Intriguingly, it has often been noted that reversals (negative excursions to younger apparent age) are associated with cold climatic events and lowered carbon content of the atmosphere. This suggests a possibility of linkages to climate driven by carbon-cycle changes, such as might be produced by ocean circulation changes. They could also, of course, be due to a higher production rate. Excursions to higher 14 C age are likely due to reductions in production rate or increases in the ventilation of 14 C-depleted carbon from the deep ocean. Conversely, excursions to lower 14 C age are likely due to reductions in production rate or attenuated ocean ventilation rates. Periods of constant 14 C over an extended period of time can also be explained by reductions in production rate. These differences can be understood as the number of 14 C atoms in a given column of atmosphere, which can be approximated for times short compared to the half-life as N14 = P14 t,

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Fig. 2. Section of the tree-ring calibration curve. The radiocarbon age is given on the vertical axis and the calendar age on the horizontal axis.

where P is a complex function of production rate over the period the 14 C remains in the atmosphere (1–2 yr) and t is the (short) residence time of 14 C in the atmosphere. This approximation is valid since the residence time is very short compared to the mean life of 8,267 yr. Our modelling results suggest that the major features of this record cannot be produced with solar variability or terrestrial magnetic field modulation alone, but also require significant changes in the carbon cycle. This is also true of the substantial decline observed between 26–11 ka BP (from ∼700h to ∼100h), which is considered too large to be solely a result of reduced production from increased shielding by the Earth’s magnetic field. Superimposed on these large-scale trends are millennial and sub-millennial variations that apparently coincide with abrupt shifts in climate as recorded in the Greenland ice cores. Coral records offer the highest possible resolution for radiocarbon calibration beyond the limit of the tree-ring chronologies, because of their relatively fast growth rate. However, the existing radiocarbon calibration contains few continuously banded examples from corals. Two exceptions are a Diploastrea coral head from Vanuatu which lived during the Younger Dryas (Burr et al., 1998) and a Goniastrea coral from Papua New Guinea which lived approximately 13,000 years ago, according to U–Th dating. This subannual record (Fig. 3) is the highest resolution radiocarbon calibration record from that time period. Speleothems offer similar advantages to coral, with a time resolution of decades or better. One such record – obtained from a stalagmite recovered from a cave in the Bahamas – provides a nearly continuous record of atmospheric 14 C from 45 to 11 ka (Beck et al., 2001;


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Fig. 3. Radiocarbon age versus coral band number for a Goniastrea coral collected at 47 m depth, Huon Peninsula, Papua New Guinea. One band equals 6 months.

Richards and Beck, 2001). This record (Fig. 4), derived using TIMS U, Th and Pa measurements and AMS 14 C ages reveals highly elevated and extremely variable 14 C between 45 and 33 ka BP which appear to be correlative with peaks in cosmogenic 36 Cl and 10 Be isotopes (Baumgartner et al., 1998) observed in polar ice cores. 3. Accelerator Mass Spectrometry 3.1. AMS radionuclide methods Radionuclide measurement using Accelerator Mass Spectrometry (AMS) differs from the decay counting methods in that the amount of the radionuclide in the sample is measured directly, rather than waiting for individual radioactive decay events. This makes the technique 1,000 to 10,000 times more sensitive than decay counting for 14 C, for example. In the case of radionuclides such as 10 Be, where the low-energy β-rays were always difficult to count, AMS has improved the situation much further. This sensitivity is achieved by accelerating sample atoms as ions to high energies using a particle accelerator, and using nuclear particle detection techniques. A diagram of the 3 MV AMS machine at the University of Arizona is shown in Fig. 5. Today, an external precision of about ±0.35% in 14 C content, or ±30 years in uncalibrated radiocarbon age is possible on a single 0.5-milligram-sized sample target in 20 minutes of measurement time. Samples as small as 100 micrograms or less have been successfully dated to about ±80 years BP and even smaller samples (∼10–20 ng) have been measured for special experiments. With longer counting times or when multiple targets are measured, we can reduce the single target error to about 0.2%, or better than ±20 years in radiocarbon age. In the case of longer-lived radionuclides such as 26 Al, 10 Be, 36 Cl, 41 Ca and 129 I, which were very difficult to measure using radiometric techniques, AMS has made measurements

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(A)

(B) Fig. 4. (A) Radiocarbon calibration curve and variations in atmospheric 14 C based on stalagmite GB-89-24-1 from 45 to 11 ka, and comparison with several other 14 C records. (B) 14 C record for GB-89-24-1 from 26 to 10 ka.


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Fig. 5. Photograph of the high-energy section of the 3 MV National Electrostatics AMS machine at the University of Arizona.

of small amount of these radionuclides routine (Tuniz et al., 1998; Fifield, 1999; Jull et al., 2005). 3.1.1. AMS at Arizona The AMS systems at Arizona consist of the following basic components and sequence of events. Figure 5 shows a photograph of the high-energy end of our 3 MV NEC system. There are many good descriptions of the methodology of accelerator mass spectrometry (Tuniz et al., 1998; Fifield, 1999; Jull et al., 2003; Kutschera, 2005), so we refer the reader to these publications for a detailed discussion of the operation of an AMS system. 3.1.2. Conventions and definitions The radiocarbon age is determined from the ratio of 14 C/13 C or 14 C/12 C compared to “modern”, which is defined as the expected level in 1950 AD. 14 C ages are normalized to a constant value of δ 13 C, so any isotopic fractionation effects are removed from the age calculation. Due to industrial and nuclear effects, the 1950 AD value is computed from the corrected value of 1850 AD wood. 14 C age = −8033 ln(F ), as above, where F =

14 C/13 C sample 14 C/13 C modern

is the ratios of 14 C/13 C in the sample and modern carbon, respectively. In discussion of the calibration of radiocarbon age and other estimates of the “true age”, we often use 14 C (see Stuiver and Polach, 1977), which can be defined as the divergence of the

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production rate of 14 C at the time of production, compared to present-day production. Hence, 14 C = (Fm − 1)e−(cal BP/8267) × 1000h, where Fm is the 14 C activity, expressed as fraction of modern carbon (Donahue et al., 1990b), and cal BP is the “expected true age” of the sample. If the “expected true age” is defined by another dating method, whether it be U–Th measurements, varve counting, dendrochonology, or correlation of a sediment record with ice-core records, it might be subject to other systematic errors. Hence, records based on different estimates of age or with potential reservoir effects need to be taken with caution. Beyond this time we still observe large fluctuations in the 14 C record. We do not know the cause of these fluctuations, however large changes in cosmic-ray flux, supernova events and large-scale changes in ocean circulation have been hypothesized (cf. Beck et al., 2001). Intriguingly, some of these 14 C “spikes” are also observed in the records of cosmogenic 10 Be in ice and marine sediments (Raisbeck et al., 1990; McHargue et al., 1995, 2000), and 129 I in a speleothem (see Biddulph et al., 2006, p. 612, this volume). 3.2. Chemical preparation of samples and stable-isotope analysis A very important factor in AMS measurements is proper sample preparation. It is important that the sample is clean and all contaminants have been removed. Some basic procedures for AMS radiocarbon samples at Arizona were summarized by Jull et al. (2004a): 1. Acid-base-acid method for charcoal, wood, cellulose, plant material, animal tissue: After physical inspection, samples are cleaned with 1 N HCl acid, 0.1% NaOH and 1 N HCl (acid-base-acid (ABA) pretreatment), washed with distilled water, dried, and combusted at 900◦ C with CuO. Hatté et al. (2001) discussed some modifications and potential problems with the acid-base-acid method. 2. Carbonates: In general, samples are etched with 100% H3 PO4 to remove 50–85% of the carbonate, dried and hydrolyzed with H3 PO4 as discussed by Burr et al. (1998). 3. Selective combustion for sediments. After cleaning in 1 N HCl and drying, the sample is combusted at 400◦ C in ∼0.3 atm. oxygen gas (McGeehin et al., 2001). 4. Oxidative acid cleaning for old charcoal: We have constructed a new line to clean charcoal samples using the oxidative acid method of Bird et al. (1999). This is of particular interest for charcoal samples >20 kyr. We have also applied this method to sediments from archaeological settings. 5. Textiles, parchment, canvas, art works and artifacts: The samples are given the ABA pretreatment and after washing and drying, they are Soxhlet extracted with hexane, then ethanol and finally methanol. After washing in distilled water, and drying, they are combusted at 900◦ C with CuO. Bruhn et al. (2001) proposed a more complex, 6-stage cleaning procedure for art works. Of course, for some specialized samples, other methods need to be developed for these applications. After cleaning and combustion, samples are converted to graphite using a modification of the method of Slota et al. (1987), using an Fe catalyst and Zn as the reducing agent for the reaction 2CO → CO2 + C.


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Finally, the graphite powder is pressed into a target holder of Arizona design, which is now widely used in all NEC ion sources. The measurement of 14 C follows the procedures and calculations described in detail by Donahue et al. (1990a, 1990b). Further discussion on the question of interlab differences in calculations were highlighted by McGeehin et al. (2001).

4. Sample processing improvements 4.1. Automated sample pretreatment We have introduced a number of automated or semi-automated techniques in our laboratory. We have constructed a continuous flow apparatus for the routine chemical treatment (acid– base–acid) of samples. This device allows us to process samples with successive chemical reagents using computer control. We have set up a Carlo Erba CHN analyzer and we have interfaced the CHN analyzer with a stable isotope ratio mass spectrometer. Among other things, this system now allows us to measure elemental CN ratios and δ 15 N of bones, as indicators of bone preservation and dietary input. These instruments will be linked to an automated CO2 collection system. The objective is to provide the capability for automated sample combustion, elemental analysis, stable isotope analysis, carbon dioxide purification and sample collection. 4.2. Specialized sediment sample pretreatment We have developed a new method for sediment dating which combines physical and chemical pretreatments, followed by stepped combustion at different temperatures (McGeehin et al., 2001, 2004). Inherited clay- or silt-bound carbon can be an important contaminant for conventional bulk sediment analysis. In some sediments, the humin fraction (material resistant to the acid and base pretreatment) combusting at 400◦ C (“low temperature fraction”) appears promising for dating sediment. In some sediments, the humin fraction could contain pollen, macrofossils, wood or charcoal. We are applying this method to a wide range of sedimentary environments. For low organic concentration sediments from the last glacial maximum age, the low temperature fraction can be 2–4 ka younger than the bulk fraction (Fig. 6). Results from this work assist in understanding the relationship between archaeological records and paleoclimatic records for particular sites. We plan to refine the stepped combustion method by improving our understanding of temperature oxidation thresholds and by characterizing the organic materials in each temperature fraction. Another sediment dating technique which we are studying is designed to remove contaminants left after a conventional acid–base–acid pretreatment. Preliminary results using a dichromate-wet oxidation (1 M dichromate plus 1 M sulfuric acid) method (ABOX), initially developed as a way to purify old charcoals (Bird et al., 1999) show significant reductions in the amount of total organic carbon in sediments. The remaining “oxidation-resistant” components are likely to be composed of charcoal or soot, as opposed to other forms of organic material in soils (Bird et al., 1999).

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Fig. 6. Deviation of the radiocarbon ages measured in various humin and humic acid fractions, compared to the age of plant macrofossil remains in the same sediment stratum (from McGeehin et al., 2001).

4.3. Compound-specific studies and bone chemistry We have also begun several new techniques and strategies in our laboratory for extracting carbon from highly contaminated materials that are difficult or impossible to obtain accurate radiocarbon dates using standard methods. Among these is a method for releasing carbon from bone-specific proteins using ninhydrin (Nelson, 1991; Tisnérat-Laborde et al., 2003; Hodgins and Jull, 2004). This approach of selective collagen dissolution using ninhydrin is being applied to bones found in tar pits (see Jull et al., 2002; Hodgins and Jull, 2004). These environments are highly preservative, and thus rich repositories of animal and plant remains. However, the tar hydrocarbons that thoroughly contaminate the samples present large challenges for dating. 4.4. Surface cleaning and sample extraction by oxygen plasma We have begun a research program investigating the plasma oxidation methods used by Rowe (e.g. Rowe, 2001) to extract organic carbon from organic residues on geological carbonates. This method has to date been used for 14 C dating of rock art pictographs. We anticipate that the low temperature oxidation reactions accomplished by the plasma will have applications on other types of samples.

5. Can we observe climatic signals in the radiocarbon record? It has long been recognized that the fluctuations in the 14 C record were often coincident with major climatic events, particularly cold ones. A period of “apparent” radiocarbon age which is flat is observed at various major cold phenomena such as the Oldest Dryas (15.1–14.5 cal BP), Younger Dryas (12.9–11.6 cal BP), the 8.2 ka “cold event”, 2.5 ka and the Maunder Minimum (17–19th centuries). This can be observed in a plot of 14 C age (yr BP) vs calendar years for the glacial–interglacial transition, for example. Several of these events are observed in Fig. 4. Many authors have commented on these effects. It is well beyond the scope of this paper to present all the possible examples of these types of studies, so we will summarize a few areas of research which highlight the use of radiocarbon in the climatic record and related areas.


13

5.1. Use of 14 C in modern corals and as a tracer in the ocean The in-growth of bomb 14 C into the oceans has been studied since the early GEOSEC cruises of the 1970s. 14 C variations in dissolved inorganic carbon in seawater were used to follow the different rates of carbon uptake from different parts of the world. These experiments were repeated in the 1980s and 1990s as part of the WOCE program. Both of these studies represent the distribution of radiocarbon at the time the samples were collected. Corals, which grow in the surface waters of the ocean, have since been the object of extensive study, because they preserve a record of the radiocarbon content of the surface ocean which allows temporal variations at a particular site to be studied. In addition to 14 C variations, modern corals also preserve a record of other geochemical proxies such as Sr–Ca and δ 18 O. This signal can also be correlated with the intensity of the El-Niño Southern Oscillation. Scientists at Arizona have continued their high frequency bomb pulse radiocarbon measurements of modern corals as part of their effort to generate time-slice maps of Pacific Ocean radiocarbon variations over the last 60 years. We have completed measurements on a drill core from Guadalcanal (Schmidt et al., 2004), Easter Island and the Marquesas, begun work on a record from Wallis Island, and have collected cores from Tokelau and Kiribati in collaboration with colleagues Thierry Correge of IRD (Noumea), and Julia Cole (Arizona). They have begun modelling efforts to combine these and other published and unpublished bomb pulse records (Fig. 7) with the WOCE and GEOSECS 14 C ocean surveys using a horizontal advective flux model based on multi-decadal wind stress records. Beck has also been involved with colleagues at ANU and Stanford University, to generate a record of bomb pulse 14 C variations from the Eastern Indian Ocean. This record was used in concert with another record from the Western Indian Ocean (Grumet et al., 2002) to generate an index of E–W 14 C for this period (Grumet et al., 2004). There have also been extensive studies of the 14 C record in the oceans, as part of the World Ocean Circulation Experiment (WOCE), Joint Global Ocean Flux Study (JGOFS) and other large international programs. 5.2. Forest fires There is a considerable record of forest-fire history from different regions of the world (see, e.g., Meyer et al., 1995, 2001; Turcq et al., 1998; Hallett and Walker, 2000; Pierce et al., 2004). A characteristic of many of these studies is evidence for marked periodicities, especially on century and millennial time scales of fire frequency. Typically, these studies involve either direct dating of charcoal found in alluvial and colluvial deposits (e.g. Meyer et al., 1995, 2001; Pierce et al., 2004) or studies of the charcoal record found in lake sediments (e.g. Long et al., 1998; Cumming et al., 2002). Meyer et al. (1995) demonstrated a spectacular record of forest fires from the Yellowstone National Park, which is now cited in all subsequent papers. In this record, Meyer et al. (1995) noted millennial-scale forcing and proposed that “Bond” cycles might be a forcing mechanism. Later studies showed similar periodicities, but a different phase relation at a location in southern Idaho (Pierce et al., 2004). Recently, Jull and Geertsema reported on radiocarbon dating of charcoal from paleosols and buried charcoal horizons in a unique sequence which potentially records the last 36,000

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Fig. 7. Location map of coral 14 C bomb pulse records to be used for genesis of 4-D Pacific Ocean radiocarbon variations. 14 C

years from a fan at Bear Flat, British Columbia (Fig. 8). This site included evidence for forest fire charcoal found over the last ca. 13,500 radiocarbon yr before present (yr BP) or 16,250 ± 700 calibrated yr BP. The latest evidence of fire is during the Medieval Warm Period. The charcoal ages show a periodicity in large fires on a millennial scale through the Holocene – an average of 4 fires per thousand years. Fire frequency is higher about 2,500–3,000 cal yr BP (10–11 fires/ka), ∼5,500 cal yr BP (∼5 fires/ka), ∼8,000 cal yr BP (∼8 fires/ka), and 9,000–10,000 cal yr (∼6 fires/ka). These authors concluded that fire frequency was driven by regional or global climate, as well as local phenomena. Over 50 discrete fire-related horizons have been observed. These charcoal ages show a periodicity in large fires on a millennial scale through the Holocene – an average of 4 fires per thousand years. Fire frequency is higher about 2,000–2,500 14 C yr BP (10–11 fires/ka), ∼5,000 14 C yr BP (∼5 fires/ka), ∼7,000 14 C yr BP (∼8 fires/ka), and 8,000 to 9,000 14 C yr (∼6 fires/ka). These intervals are also times of aboveaverage aggradation of the fan. Fire frequency appears to be related to climate. Recently, Alley noted that forest-fire frequency increases during North Atlantic cold events. In particular, times of higher fire frequency may follow North Atlantic ice-rafting events. Other studies in central BC have been undertaken with colleagues from the University of Northern British Columbia from sites in a wetter region of central British Columbia. A parallel study has been conducted in collaboration with colleagues at the University of New Mexico on the forest-fire history in Idaho, this record showing similar periodicities, but


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Fig. 8. Field photograph of some charcoal-rich horizons related to forest-fire debris flows at Bear Flat, British Columbia. The cyclicity of the forest fires is evident in the photograph.

with a different phase relation to the fires in British Columbia and Yellowstone Park (Pierce et al., 2004).

6.

10 Be

studies

We now routinely make measurements of the cosmogenic-radionuclide 10 Be with the new 3MV NEC AMS. 10 Be is produced in the atmosphere by spallation of oxygen and nitrogen by cosmic rays. The intensity of the cosmic-ray flux depends on galactic and solar sources, and modulation by the heliomagnetic and geomagnetic fields. After formation, 10 Be is quickly removed from the atmosphere by precipitation and deposited onto the surface of the Earth, where it is transported throughout the ocean and is eventually sequestered within marine sediments. A record of the cosmic-ray flux, modified by marine processes, may be interpreted from marine sediment cores and provides a valuable record of past geomagnetic and cosmicray phenomena. Our previous work documented the 10 Be record in the authigenic fraction of marine sediments from the Gulf of California, Leg 64, site 480 (McHargue et al., 1995), and the Blake Ridge, CH88-10P (McHargue et al., 2000). During the period of EAR01-15488, we completed work on another core from the Blake Outer Ridge, Leg 172, site 1061. The 10 Be data, normalized to the mass of the authigenic sediment fraction, is shown in Fig. 9. Normalization of 10 Be to the mass of the authigenic fraction provides a better proxy of paleomagnetic intensity in this core than normalization to the bulk sample mass or to 9 Be. In addition, normalizing 10 Be to the authigenic fraction provides good correlation with 10 Be from cores CH88-10P and DSDP 480 normalized in the same manner (not shown). This normalization makes the 10 Be increases at approximately 60,000 years (observed in polar ice), and at the Laschamp geomagnetic excursion and possible Mono Lake excursion more apparent. Future proposed

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Fig. 9. 10 Be results for the Blake Outer Ridge, DSDP Leg 172, site 1061 core. At the bottom of the figure 10 Be is normalized to the original sample mass. In the middle figure, it is normalized to 9 Be. In the top figure, 10 Be is normalized to the mass of the original sample removed by leaching, that is, the authigenic fraction of the sediment. Adapted from McHargue et al. (2000) and unpublished data.

work will revisit a core from the Gulf of California, in which the sediments are varved and the sedimentation rate is three to four times higher than that on the Blake Ridge. Of particular interest is a detailed analysis of an extreme short-term (50 years or less) 10 Be anomaly associated with the Mono Lake excursion in the Gulf of California (Leg 64, site 480), which is too short to be observable in Blake Ridge sediments. 7.

129 I

studies

Since the installation of the new NEC 3MV Pelletron accelerator we have measured more than 500 129 I samples. Sufficient data are now available to show a machine random error of


17

0.4% for iodine samples. Repeated analysis of the low-level Woodward iodine standard has yielded results in the low 10−14 range for 129 I/127 I ratios. Chemical extraction techniques for a variety of environmental samples including seawater, corals, stalagmites and seaweed have been developed. With these techniques we can measure 129 I/127 I ratios in samples with as little as 5 µg total iodine content. We have recently produced an 84-year record showing anthropogenic iodine invasion into corals in the South Pacific, and a 20,000-year record of cosmogenically produced iodine in a stalagmite from the Bahamas islands (Biddulph, 2004; Biddulph et al., 2006, p. 612, this volume). We are collaborating with IAEA and other laboratories to produce iodine standard reference materials. Recent measurements of 129 I concentrations in fish from the North and Irish Seas are in good agreement with other laboratories. We are currently in the process of measuring other IAEA samples, including IAEA produced silver iodide and Mediterranean seawater. We have plans to continue radioiodine research in stalagmites and corals. There is some preliminary data suggesting that 129 I may track changes in the geomagnetic field and the cosmicray flux. Data from a stalagmite (Biddulph et al., 2006, this volume) seems to reproduce magnetic excursions that have been seen in other radioisotope records such as 10 Be and 36 Cl. The relatively long half-life of 129 I (15.7 million years) would enable us to analyze records that go back nearly 100 million years.

8. Megafaunal extinctions Radiocarbon dates were obtained by accelerator mass spectrometry on bones of extinct large mammals recovered from tar pits. Results on some samples of Glyptodon and Holmesina (extinct large mammals similar to armadillos) yielded results of >25 ka and >21 ka, respectively. We also studied the radiocarbon ages of 3 different samples of bones from the extinct Cuban ground sloth, Parocnus brownii, which yielded dates ranging from 4,960 ± 280 to 11,880 ± 420 yr BP. In order to remove the tar component the samples were cleaned by Soxhlet extraction in benzene. A report of our studies was presented at the 9th International AMS Conference (Jull et al., 2004b). 14 C dating of a tooth from a Madagascar Hippopotamus confirms that this member of the extinct “prehistoric” megafauna actually survived until well after European colonization. Gelatinized collagen subsamples independently dated by two AMS facilities yielded ages of 99±36 and 214±40 yr BP. Calibrated at 2σ , these dates give a range of 1639–1950 cal yr BP, placing a member of this extinct megafauna securely in colonial times. Dates on other extinct taxa show that many other prehistoric megafauna broadly overlapped the human presence on the island and may have survived until colonial times as well. A summary of all the available dates has been discussed by Burney et al. (2004). We continue to locate and try to date fossils from extinct megafauna, particularly from isolated or island refugia. In collaboration with Paul Martin (Arizona) and David Burney (Fordham University), we have located samples of extinct faunal remains from Madagascar, Cuba and other island refugia, as well as North American sites (Fig. 10). Megafaunal extinctions based on the effects of human expansion have been discussed for many other parts of the world, such as Australia, New Zealand, Madagascar, and the Americas.

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Fig. 10. Major Pleistocene extinction events on the continents and larger islands, excluding Afro-Asia. Patterns indicate timing of extinctions for each region (adapted from Martin and Klein, 1984).

Apart from some isolated evidence for earlier settlement with good associations, the predominant view is that early man arrived in the western hemisphere close to the end of the last Glacial (see Nuñez et al., 1994; Martin and Klein, 1984; Meltzer et al., 1997). The conventional model assumes that early humans arrived in the new world via a Bering land bridge. We note that the Bering Strait is only 30 m deep at its shallowest point and we can assume that the last sea-level rise would have closed off this route between Asia and the Americas. The Bering land bridge should have remained intact until ∼10,000 radiocarbon years BP (11,000 calendar years). We can understand this in terms of the sea-level rise history, which occurred in two stages of ∼60 m and ∼50 m, as shown by Bard et al. (1996) and Edwards et al. (1993). The rapid expansion of early man into central North America does not appear to have occurred until about 12,000 radiocarbon years BP, although the dating of the Monte Verde site in Chile (∼12,500 14 C yr BP, Meltzer et al., 1997) suggests a slightly older time. This period appears to be during the “Older Dryas”, which is observed in many records ∼12,200 14 C yr BP (e.g. Goslar et al., 2000). Dyke et al. (2001) have summarized the available radiocarbon records for the margins of the Laurentide ice sheet, which indicate that regions east of the Rocky Mountains were ice-free at that time. In addition, the results of Jull and Geertsema and others suggest that much of this region may have been ice-free even earlier. The extinction of many megafauna at the end of the late Pleistocene is well known (Martin and Klein, 1984). Hence, the exact time of these extinctions, and whether they are caused by climate alone, or by a combination of factors, such as the expansion of humans into previously unoccupied areas is a matter of great interest. Indeed, neither of these factors alone seems to explain all the observations. Taylor has summarized the earliest radiocarbon dates on human bones from the new world. In all cases, these dates are close to the dates for the last evidence for mammoths and other large megafauna (e.g. Martin and Klein, 1984). About one half of all species of large land


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mammals of North America disappeared at the close of the Pleistocene (Martin and Klein, 1984). At some locations, we can observe the interplay of climate, early man and megafauna. The Murray Springs site in Arizona, as discussed above, is only one of many which shows the interplay of climate, the expansion of humans and the disappearance of large mammals such as mammoths. Radiocarbon ages on algal mats, which overly the mammoth remains, also fall into the period of the Younger Dryas cold event at 10,300–10,600 yr BP. This evidence suggests that the Clovis expansion occurred during the European Allerød warm period, and that we can perhaps associate the Clovis “drought” of Haynes (1991) with the Intra-Allerød cold period (IACP) which occurred about 11,400 to 11,100 14 C yr BP and was followed by a short warmer epoch before the rapid onset of the Younger Dryas at 10,900 14 C yr BP. In addition, we can also compare these radiocarbon ages to the estimates of sea-level rise determined by Bard et al. (1996) in Barbados coral and Edwards et al. (1993) in the South Pacific. In either case, the algal mat deposits date to 10.2–10.6 ka, within the Younger Dryas epoch. This cold event post-dates Clovis, and all Clovis radiocarbon measurements (see Haynes, 1984, 1991, 1992) fall between these two sea-level rise events.

9. In situ terrestrial cosmogenic 14 C We have developed an extraction method for in situ 14 C from quartz, which overcomes numerous problems encountered using previous methods (Lifton et al., 2001). Degassed lithium metaborate (LiBO2 ) is used as a flux to dissolve the quartz sample at 1100–1200◦ C inside a mullite furnace tube. These temperatures allow the use of significantly cheaper tube furnaces than previous techniques. Furthermore, blank levels of in situ 14 C are low and quite stable ((1.3 ± 0.2) × 105 14 C atoms, 2σ ). Using this technique, quartz separates from eight samples of wave-cut quartzite benches from the well-dated Bonneville (17.4 ± 0.3 cal kyr) and Provo (16.8 ± 0.3 cal kyr) shorelines of Pleistocene Lake Bonneville, Utah, and from underlying deeply shielded locations were analyzed. Replicate analyses of five aliquots of a single sample demonstrate analytical precision better than 4% (2σ ). The precision we have attained allows measurement of in situ 14 C in a 5 g quartz sample after only ∼500 years of exposure at sea level and high latitude. Current work builds on these advances by addressing basic questions regarding in situ 14 C production and attenuation characteristics, and by applying in situ 14 C to fundamental problems in cosmogenic nuclide research. The Lake Bonneville shorelines will be the focus of the international CRONUS-Earth program and the first “intercomparison” site. We have completed field work in which we collected samples from surfaces at secular equilibrium along mid- and low-latitude altitude transects to assess the altitudinal and latitudinal dependence of integrated late Quaternary in situ 14 C production rates. We also have received samples from other investigators from Namibia, Antarctica, Australia and New Zealand to better constrain in situ 14 C production rate scaling globally. Results to date from these transects (ranging from sea level to nearly 4 km altitude) confirm the viability of using in situ 14 C in saturated surfaces to constrain in situ cosmogenic nuclide production rate scaling models (Lifton et al., 2002).

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10. Conclusion We have presented some applications of accelerator mass spectrometry which have applications to paleoclimate and environmental studies using the Arizona AMS laboratory. There are many varied uses of AMS and this paper only highlights a few chosen examples. We hope that this overview is useful to the non-AMS specialist and will allow the reader some insight into the breadth of applications of AMS.

Acknowledgements The authors are grateful for the technical support of the staff of the NSF Arizona AMS Laboratory and for support from NSF Grant EAR01-15488.

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Donahue, D.J., Linick, T.W., Jull, A.J.T. (1990b). Isotope-ratio and background corrections for accelerator mass spectrometry radiocarbon measurements. Radiocarbon 32, 135–142. Dyke, A.S., Andrews, J.T., Clark, P.U., England, J., Miller, G.H., Shaw, J., Veillette, J.J. (2001). Radiocarbon dates pertinent to defining the last glacial maximum for the Laurentide and Innuitian ice sheets, Geological Survey of Canada, Open File 4120, 54 pp. Edwards, R.L., Beck, J.W., Burr, G.S., Donahue, D.J., Chappell, J.M.A., Bloom, A.L., Druffel, E.R.M., Taylor, F.W. (1993). A large drop in atmospheric C-14/C-12 and reduced melting in the Younger Dryas, documented with Th-230 ages of corals. Science 260, 962–968. Fifield, L.K. (1999). Accelerator mass spectrometry and its applications. Reports of Progress in Physics 62, 1223– 1274. Goslar, T., Arnold, M., Tisnérnat-Laborde, N., Hatté, C., Paterne, M., Ralska-Jasiewiczowa, M. (2000). Radiocarbon calibration by means of varves versus 14 C ages of terrestrial macrofossils from Lake Go´sciaz and Lake Perespilno, Poland. Radiocarbon 42, 403–414. Grumet, N.S., Guilderson, T.P., Dunbar, R.B. (2002). Meridional transport in the Indian Ocean traced by coral radiocarbon. Journal of Marine Research 60, 725–742. Grumet, N.S., Abram, N.J., Beck, J.W., Dunbar, R.B., Gagan, M.K., Guilderson, T.P., Hantoro, W.S., Suwargadi, B.W. (2004). Coral radiocarbon records of Indian Ocean water mass mixing and wind-induced upwelling along the coast of Sumatra, Indonesia. Journal of Geophysical Research – Oceans 109, C05003. Hallett, D.J., Walker, R.C. (2000). Paleoecology and its application to fire and vegetation management in Kootenay National Park, British Columbia. Journal of Paleolimnology 24, 401–414. Hatté, C., Morvan, J., Noury, C., Paterne, M. (2001). Is classical acid-alkali-acid treatment responsible for contamination? An alternative proposition. Radiocarbon 43, 177–182. Haynes, C.V. (1984). Stratigraphy and Late Pleistocene extinctions in the United States. In: Martin, P.S., Klein, R.G. (Eds.), Quaternary Extinctions. University of Arizona Press, Tucson, AZ. Haynes, C.V. (1991). Geoarchaeological and paleohydrological evidence for a Clovis-age drought in North America and its bearing on extinction. Quaternary Research 35, 438–450. Haynes, C.V. (1992). Contributions of radiocarbon dating to the geochronology of the peopling of the New World. In: Taylor, R.E., Long, A., Kra, R.S. (Eds.), Radiocarbon After Four Decades. Springer-Verlag, New York. Hodgins, G.W.L., Jull, A.J.T. (2004). Radiocarbon dating of petroleum-impregnated bone from Tar Pits using the ninhydrin reaction. Abstract, 34th International Symposium on Archaeometry, 3–7 May, 2004, Zaragosa, Spain.. Jull, A.J.T., Burr, G.S. (2005). Accelerator Mass Spectrometry: Is the future bigger or smaller? Earth and Planetary Science Letters, submitted for publication. Jull, A.J.T., Donahue, D.J., Burr, G.S., Beck, J.W., McHargue, L.R., Hatheway, A.L., Lange, T.E., O’Malley, J.M., Biddulph, D. (2002). In: Aggarwal, S.K., Alamelu, D. (Eds.), Tenth ISMAS Workshop on Mass Spectrometry. Indian Society for Mass Spectrometry, Mumbai, India, pp. 25–34. Jull, A.J.T., Burr, G.S., Beck, J.W., Donahue, D.J., Biddulph, D., Hatheway, A.L., Lange, T.E., McHargue, L.R. (2003). Accelerator mass spectrometry at Arizona: Geochronology of the climate record and connections with the ocean. Journal of Environmental Radioactivity 69, 3–19. Jull, A.J.T., Burr, G.S., McHargue, L.R., Lange, T.E., Lifton, N.A., Beck, J.W., Donahue, D.J. (2004a). New frontiers in dating of geological, paleoclimatic and anthropological applications using accelerator mass spectrometric measurements of 14 C and 10 Be in diverse samples. Global and Planetary Change 41, 309–323. Jull, A.J.T., Itturalde-Vinent, M., O’Malley, J.M., McPhee, R.D.E., McDonald, H.G., Martin, P.S., Moody, J., Rincón, A. (2004b). Nuclear Instrument and Methods in Physics Research B 223, 668–671. Kutschera, W. (2005). Progress in isotope analysis at ultra-trace level by AMS. International Journal of Mass Spectrometry 242, 145–160. Lifton, N.A., Jull, A.J.T., Quade, J. (2001). A new extraction technique and production rate estimate for in situ cosmogenic 14 C in quartz. Geochimica et Cosmochimica Acta 65, 1953–1969. Lifton, N., Pigati, J., Jull, A.J.T., Quade, J. (2002). Altitudinal variation of in situ cosmogenic C-14 production rates: Preliminary results from the Southwestern US. Geochimica et Cosmochimica Acta 66 (15A), A457. Long, C.J., Whitlock, C., Bartlein, P.J., Millspaugh, S.H. (1998). A 9000-year fire history from the Oregon Coast Range, based on a high-resolution charcoal study. Canadian Journal of Forest Research 28, 774–787. Martin, P.S., Klein, R.G. (1984). Quaternary Extinctions: A Prehistoric Revolution. University of Arizona Press, Tucson, AZ.

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McGeehin, J., Burr, G.S., Jull, A.J.T., Reines, D., Gosse, J., Davis, P.T., Muhs, D., Southon, J. (2001). Steppedcombustion 14 C dating of sediment. Radiocarbon 43 (2A), 255–262. McGeehin, J., Burr, G.S., Hodgins, G., Bennett, S.J., Robbins, J.A., Morehead, N., Markewich, H. (2004). Steppedcombustion 14 C dating of bomb carbon in lake sediment. Radiocarbon 46, 893–900. McHargue, L.R., Damon, P.E., Donahue, D.J. (1995). Enhanced cosmic-ray production of 10 Be coincident with the Mono Lake and Laschamp geomagnetic excursions. Geophysical Research Letters 22, 659–662. McHargue, L.R., Donahue, D.J., Damon, P.E., Sonett, C.P., Biddulph, D., Burr, G.S. (2000). Geomagnetic modulation of the late Pleistocene cosmic-ray flux as determined by 10 Be from Blake Outer Ridge sediments. Nuclear Instruments and Methods in Physic Research B 172, 555–561. Meltzer, D.J., Grayson, D.K., Ardila, G., Barker, A.W., Dincauze, D.F., Haynes, D.V., Mena, F., Nuñez, L.A., Stanford, D.J. (1997). On the Pleistocene antiquity of Monte Verde, southern Chile. American Antiquity 62, 659–663. Meyer, G.A., Wells, S.G., Jull, A.J.T. (1995). Fire and alluvial chronology in Yellowstone National Park: Climatic and intrinsic controls on Holocene geomorphic processes. Geological Society of America Bulletin 107, 1211–1230. Meyer, G.A., Pierce, J.L., Wood, S.H., Jull, A.J.T. (2001). Fire, storms and erosional events in the Idaho batholith. Hydrological Processes 15, 3025–3038. Nelson, D.E. (1991). A new method for carbon isotopic analysis of protein. Science 251, 552–554. Nuñez, L.A., Varela, J., Casamiquela, R., Villagrán, C. (1994). Reconstrucción multidisciplinaria de la occupación prehistórica de Quereo, Centro de Chile. Latin American Antiquity 5, 99–118. Pierce, J.L., Meyer, G.A., Jull, A.J.T. (2004). Fire-induced erosion and millennialscale climate change in northern ponderosa pine forests. Nature 432, 87–90. Raisbeck, G.M., Yiou, F., Jouzel, J., Petit, J.R. (1990). 10 Be and 2 H in polar ice cores as a probe of the solar variability influence on climate. Philoophical Transactions of the Royal Society (London) A 330, 463–470. Richards, D.A., Beck, J.W. (2001). Dramatic shifts in radiocarbon dating the last glacial period. Antiquity 75, 482– 485. Rowe, M. (2001). Dating by AMS radiocarbon analysis. In: Whitley, D.S. (Ed.), Handbook of Rock Art Research. Alta Mira Press, pp. 139–166. Schmidt, A., Burr, G.S., Taylor, F.W., O’Malley, J., Beck, J.W. (2004). A semiannual radiocarbon record of a modern coral from the Solomon Islands. Nuclear Instruments and Methods in Physics Research B 223, 420–427. Slota, P.J., Jull, A.J.T., Linick, T.W., Toolin, L.J. (1987). Preparation of small samples for 14 C accelerator targets by catalytic reduction of CO. Radiocarbon 29 (3), 303–306. Stuiver, M., Polach, H. (1977). Reporting of 14 C data: Discussion. Radiocarbon 19, 355–363. Tisnérat-Laborde, N., Valadas, H., Kaltnecker, E., Arnold, M. (2003). AMS radiocarbon dating of bones at LSCE. Radiocarbon 45, 409–419. Tuniz, C., Bird, J.R., Fink, D., Herzog, G.F. (1998). Accelerator Mass Spectrometry: Ultrasensitive analysis for global science. CRC Press, Boca Raton, FL, 371 pp. Turcq, B., Siffedine, A., Martin, L., Absy, M.L., Soubies, F., Suguio, K., Volkmer-Ribeiro, C. (1998). Amazonia rainforest fires: A lacustrine record of 7000 years. Ambio 27, 139–142.

Further reading Alley, R.A., Clark, P.U., Keigwin, L.D., Webb, R.S. (1999). Making sense of millennial-scale climate change. In: Clark, P.U. et al. (Eds.), Mechanisms of Global Climate Change at Millennial Time Scales. Geophysics Monograph, vol. 112. American Geophysical Union, Washington, DC, pp. 385–394. Bird, M.I., Fifield, L.K., Santos, G.M., Beaumont, P.B., Zhou, Y., di Tada, M.I., Hausladen, P.A. (2003). Radiocarbon dating from 40 to 60 ka BP at Border Cave, South Africa. Quarternary Science Reviews 22, 943–947. Burr, G.S., Galang, C., Taylor, F.W., Gallup, C., Edwards, R.L., Cutler, K., Quirk, B. (2005). Radiocarbon results from a 13 ka BP coral from the Huon Peninsula, Papua New Guinea. Radiocarbon 46, 1211–1224. Cockburn, H.A.P., Summerfield, M.A. (2004). Geomorphological applications of cosmogenic isotope analysis. Progress in Physical Geography 28, 1–42. Druffel, E.R.M., Griffin, S., Hwang, J., Komada, T., Beupre, S.R., Druffel-Rodriguez, K.C., Santos, G.M., Southon, J. (2004). Variability of monthly radiocarbon during the 1760s in corals from the Galapagos Islands. Radiocarbon 46, 627–631.


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Gagan, M.K., Ayliffe, L.K., Beck, J.W., Cole, J.L., Druffel, E.R.M., Dunbar, R., Schrag, D.P. (2000). New views of tropical paleoenvironments from corals. Quaternary Science Reviews 19, 45–64. Gosse, J., Phillips, F.M. (2001). Terrestrial in-situ cosmogenic nuclides: Theory and application. Quarternary Science Reviews 20, 1475–1560. Grottoli, A.G., Gilli, S.T., Druffel, E.R.M., Dunbar, R.B. (2003). Decadal timescale shift in the 14 C record of a central equatorial Pacific coral. Radiocarbon 45, 91–99. Hallett, D.J., Lepofksy, D.S., Mathewes, R.W., Lertzman, K.P. (2003). 11,000 years of fire history and climate in the mountain hemlock rain forests of southwestern British Columbia based on sedimentary charcoal. Canadian Journal of Forest Research 33, 292–312. Hu, F.S., Kaufmann, D., Yoneji, S., Nelson, D., Shemesh, A., Huang, Y., Tian, J., Bond, G., Clegg, B., Brown, T. (2003). Cyclic variation and solar forcing of Holocene climate in the Alaskan subarctic. Science 301, 1890–1893. Hughen, K.A., Baillie, M.G.L., Bard, E., Beck, J.W., Bertrand, C.J., Blackwell, P.G., Buck, C.E., Burr, G.S., Cutler, K., Damon, P.E., Edwards, R.L., Fairbanks, R., Friedrich, M., Guilderson, T.P., Kromer, B., McCormac, G., Manning, S., Bronk Ramsey, C., Reimer, P.J., Reimer, R.W., Remmele, S., Southon, J.R., Stuiver, M., Tamalo, S., Taylor, F.W., van der Plicht, J., Wehenmeyer, C.E. (2005). MARINE04 Radiocarbon age calibration 0–26 ka cal BP. Radiocarbon 46, 1059–1086. Jull, A.J.T., Haynes, C.V. Jr, Donahue, D.J., Burr, G.S., Beck, J.W. (1999). Radiocarbon ages of early man in the New World and the influence of climate change. In: Evin, J. et al. (Eds.), Proc. 3rd International Conference “Archaeologie et 14 C”. Lyon, France, 6–10 April, 1998, pp. 239–343. Revu d’Archaeometrie, Suppl. 1999 et Soc. Préhist. Fr. Mémoire no. 26. Lertzman, K., Gavin, D., Hallett, D., Brubaker, L., Lepofsky, D., Mathewes, R. (2002). Long-term fire regime estimated from soil charcoal in coastal temperate rain forests. Conservation Ecology 6 (2), paper 5 [on-line journal]. Lifton, N.A., Pigati, J., Jull, A.J.T., Quade, J., Bierman, P., Kober, F. (2003). Testing cosmogenic nuclide production rate scaling models using in-situ 14 C from surfaces at secular equilibrium (abstract). Geochimica et Cosmochimica Acta 67, A253. Markgraf, V. (2001). Interhemispheric Climate Linkages. Academic Press, New York. Maslin, M., Seidov, D., Lowe, J. (2001). In: Seidov, D., Haupt, B.J., Maslin, M. (Eds.), The Oceans and Rapid Climate Change: Past, Present and Future. American Geophysical Union, Washington, DC, pp. 9–52. Masters, A.M. (1990). Changes in forest fire frequency in Kootenay National Park, Canadian Rockies. Canadian Journal of Botany 68, 1763–1767. McNichol, A.P., Jull, A.J.T., Burr, G.S. (2001). Converting AMS data to radiocarbon values: Considerations and conventions. Radiocarbon 43, 313–320. Meyer, G.A., Wells, S.G., Balling, R.C., Jull, A.J.T. (1992). Response of alluvial systems to fire and climate change in Yellowstone National Park. Nature 357, 147–150. Nuñez, L., Grosjean, M., Cartajena, I. (2001). In: Markgraf, V. (Ed.), Interhemispheric Climate Linkages. Academic Press, New York, pp. 105–117. Reimer, P.J., Baille, M.G., Bard, E., Bayless, A., Beck, J.W., Blackwell, P.G., Buck, C.E., Burr, G.S., Cutler, K., Damon, P.E., Edwards, R.L., Fairbanks, R., Friedrich, M., Guilderson, T.P., Herring, C., Hughen, K.A., Kromer, B., McCormac, G., Manning, S., Ramsey, C.B., Reimer, R.W., Remmele, S., Southon, J.R., Stuiver, M., Tamalo, S., Taylor, F.W., van der Plicht, J., Wehenmeyer, C.E. (2005). IntCal04 Terrestrial radiocarbon calibration 0–26 ka cal BP. Radiocarbon 46, 1029–1058.

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Discriminating biogenic and anthropogenic chlorinated organic compounds using multi-isotope analyses of individual compounds Kazushi Aranamia,b,* , Steven J. Rowlandc , James W. Readmanb a Environmental Chemodynamics Section, Environmental Chemistry Division, National Institute for Environmental

Studies (NIES), Tsukuba, Ibaraki, 305-8506, Japan b Plymouth Marine Laboratory, Prospect Place, The Hoe, Plymouth, Devon PL1 3DK, United Kingdom c Petroleum and Environmental Geochemistry Group, School of Earth, Ocean and Environmental Sciences,

University of Plymouth, Drake Circus, Plymouth, Devon PL4 8AA, United Kingdom Abstract The number of known naturally occurring chlorinated organic compounds (COCs) is rapidly increasing and now totals approximately 2200. In order to investigate anthropogenic threats relating to COCs, the influence of natural COCs also needs to be understood. Compound-specific isotopic analyses (CSIA) of both stable (13 C) and radio(14 C) carbon on environmental samples has been used successfully to evaluate the origin and fate of some organic compounds (e.g. n-alkanes and polycyclic aromatic hydrocarbons (PAHs)). Recently CSIA of 13 C, 14 C and chlorine (37 Cl) isotopes have also been applied to COCs. In this paper, we review recent data for isotopic signatures and fractionations of COCs. The potential for discriminating the origins of COCs using CSIA is discussed. CSIA of 13 C is primarily applied to identify the origin of biosynthesized volatile COCs. CSIA of 37 Cl also has potential to discriminate biogenic COCs. In addition, stable isotopic fractionations of COCs associated with chloro-respiration offer potential for monitoring the environmental behavior of COCs. Keywords: Chlorinated organic compounds (COCs), Compound-specific isotopic analysis (CSIA), Stable carbon isotope (13 C), Stable chlorine isotope (37 Cl), Compound-specific radiocarbon analysis (CSRA)

1. Introduction Humans have synthesized many chlorinated organic compounds (COCs) which have been discharged into the environment. COCs made for industrial or agricultural practices were initially hailed as affording substantial improvements in the quality of life. However, their persistence, toxicity and bioaccumulative potential gave rise to concern (Carson, 1962; Colborn et al., 1996). COCs such as polychlorinated biphenyls (PCBs) or dichlorodiphenyl* Corresponding author. Address: National Institute for Environmental Studies, 16-2, Onogawa, Tsukuba, Ibaraki, 305-8506, Japan; phone and fax: (+81) 29 850 2902; e-mail: [email protected]



Discriminating chlorinated organic compounds

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trichloroethane (DDT) have, more recently, been shown to have an endocrine disrupting influence on health (IPCS Assessment Report, 1995). Volatile COCs such as chlorofluorocarbons (CFCs) or hydrochlorofluorocarbons (HCFCs) have a catalytic influence depleting ozone in the stratosphere (IPCC Second Assessment Report, 1995). Thus, many people now consider COCs as an undesirable man-made legacy and many COCs have been discriminated from future use. Approximately 2200 natural COCs have been identified (Gribble, 2003), whilst thirty years ago, this number was only 150 (Siuda and DeBernardis, 1973). In many cases, these compounds have been investigated for pharmacological interest due to their biological activities, which include antifungal, antibacterial, antineoplastic, antiviral (e.g. anti-HIV), antiinflammatory, and other activities (Butler and Walker, 1993). They can be found in marine and terrestrial plants, marine animals, bacteria, fungi, some higher animals, and a few mammals, including humans. The oceans are the single largest source of biogenic COCs (Gribble, 1994, 2003; Winterton, 2000). The compounds have presumably evolved for chemical defense purposes (Paul et al., 1987; Pawlik, 1993) and are biosynthesized through chloroperoxidases (CPOs) (Butler and Walker, 1993; Urhahn and Ballschmiter, 1998; Ballschmiter, 2003). CPOs are enzymes secreted by bacteria or micro-algae associated with the host that catalyze chlorination as follows: Org-H + Cl− + H2 O2 + H+ → Org-Cl + 2H2 O. In contrast to chlorination, some bacterial de-chlorination reactions can occur under oxidative or reductive conditions through enzyme-catalyzed or metabolic processes (Fetzner, 1998). These have potential for use in bioremediation of materials contaminated with COCs. In the natural environment, biosynthesized COCs cycle through chlorination and dechlorination reactions (Fig. 1) which are highly stereo-selective (Fu et al., 1992) and result in isotopic fractionation (Reddy et al., 2002a). This suggests that stable isotopic signatures (37 Cl/35 Cl or 13 C/12 C) of individual COCs in environmental samples might allow discrimination between biogenic and anthropogenic origins. In addition, radiocarbon dating of individual COCs might also be used to distinguish between biogenic (contemporary 14 C) and

Fig. 1. Schematic for the biogeochemical cycle of COCs.

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anthropogenic (14 C-free) compounds. Here, we review recent data on isotopic signatures and fractionations of COCs and discuss the potential for discriminating COC origins using compound-specific isotopic analyses (CSIA).

2. Biogenic chlorinated compounds Methyl chloride (CH3 Cl) and chloroform (CHCl3 ) are well known as major biogenic COCs. Terrestrial CH3 Cl sources include: biomass burning (Crutzen et al., 1979; Lobert et al., 1999), fungi (Harper, 1985) and higher plants (Yokouchi et al., 2002). Contributions from these are thought to exceed oceanic sources (Lovelock, 1975; Gschwend et al., 1985; Moore et al., 1996; Keene et al., 1999). For CHCl3 , terrestrial (soil) sources (Hoekstra et al., 1998; Haselmann et al., 2000; Laturnus et al., 2000) are thought to be comparable to oceanic sources (Nightingale et al., 1995; Laturnus et al., 2002; McCulloch, 2003). Trichloroacetate (TCA), which is used as a herbicide and is formed via the atmospheric breakdown of trichloroethene (TCE) and tetrachloroethene (PCE), is also produced naturally in soils even without CPOs in the presence of Cl− and H2 O2 (McCulloch, 2002; Schöler et al., 2003; Hoekstra, 2003; Fahimi et al., 2003; Ahlers et al., 2003). Persistent and bioaccumulative biogenic COCs, such as chlorinated anisoles (Haglund et al., 1997; Führer and Ballschmiter, 1998), chlorinated bipyrroles (Tittlemier et al., 1999, 2002; Vetter et al., 2000, 2003; Wu et al., 2002) and dioxins (Hashimoto et al., 1995; Silk et al., 1997; Hoekstra et al., 1999; Ferrario et al., 2000; Gaus et al., 2001; Green et al., 2001) have been reported in both terrestrial and marine environments (Fig. 2).

3. Stable carbon isotopes CSIA of stable carbon (13 C) in the complex mixtures found in environmental samples is usually measured using on-line gas chromatography/combustion/isotope ratio monitoring mass spectrometry (GC/C/IRMS or irm-GC/MS). Stable carbon isotope ratios are expressed in the delta notation δ 13 C (h) = (Rsample /Rstandard − 1) × 1000, where Rsample and Rstandard are the 13 C/12 C ratios of a sample and standard, respectively. The international standard for carbon is the Vienna Pee Dee Belemnite (VPDB) from the National Institute of Standards and Technology (NIST). A sample size of 10–100 ng C is typically required for continuous flow GC/C/IRMS and delivers a reproducibility of 0.1–1.0h (Eakin et al., 1992; Merritt et al., 1995; Rudolph et al., 1997). CSIA of oceanic sediments or atmospheric aerosols have been successfully applied to the investigation of the origin and fate of hydrocarbons (e.g. n-alkanes and polycyclic aromatic hydrocarbons (PAHs)). In early studies, the stable carbon isotopic signatures of individual n-alkanes seemed to reflect the origins of oceanic phytoplankton, terrestrial C3 and C4 plants, and petroleum (Freeman et al., 1990, 1994; Hayes et al., 1990; Rieley et al., 1991; Schoell et al., 1992, 1994a, 1994b; Hayes, 1993; Eglinton, 1994; Andrews et al., 1995;


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Fig. 2. Examples of persistent biogenic COCs. 1 Haglund et al. (1997); 2 Führer and Ballschmiter (1998); 3 Vetter et al. (2000, 2003); 4 Wu et al. (2002); 5 Tittlemier et al. (1999, 2002); 6 Hashimoto et al. (1995); 7 Silk et al. (1997); 8 Ferrario et al. (2000); 9 Gaus et al. (2001); 10 Green et al. (2001); 11 Hoekstra et al. (1999).

Rogers and Savard, 1999). However, the larger isotopic fractionations associated with bacterial metabolism have more recently been shown to be of particular significance (Botz et al., 1996; Summons et al., 1998; Teece et al., 1999). Other investigations have demonstrated that 13 C measurements of individual PAHs provide a useful tool for discriminating between pyrolytic and petrogenic origins (O’Malley et al., 1994, 1996, 1997; Ballentine et al., 1996; Smirnov et al., 1998; Norman et al., 1999; McRae et al., 1999; Okuda et al., 2002a, 2002b, 2002c). Recent papers addressing PAHs have also shown that certain compounds, particularly perylene, in tropical sediments have depleted 13 C (McRae et al., 2000; Wilcke et al., 2002; Fahimi et al., 2003) and contemporary 14 C (Reddy et al., 2002b). This indicates the production of some natural PAHs, probably under anaerobic conditions.

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CSIA of COCs has also been used to monitor isotopic fractionations associated with biodegradation of anthropogenic chlorinated chemicals, for example TCE and PCE (Hunkeler et al., 1999, 2003; Dayan et al., 1999; Lobert et al., 1999; Bloom et al., 2000; Slater et al., 2001, 2002; Bill et al., 2001; Song et al., 2002; Barth et al., 2002; Kirtland et al., 2003; Chu et al., 2004) and PCBs (Drenzek et al., 2001). CSIA of biogenic COCs has shown that atmospheric CH3 Cl has more depleted δ 13 C compared to non-methane hydrocarbons (NMHCs) or CFCs (Rudolph et al., 1997; Tsunogai et al., 1999; Thompson et al., 2002). Also, CH3 Cl emitted from terrestrial higher plants (Harper et al., 2001, 2003) and coastal salt marshes (Bill et al., 2002; Rhew et al., 2002) has extremely depleted e 13 C signatures. Identification of 13 C end-members of CH3 Cl by CSIA could prove useful for estimating contributions from the various sources to atmospheric CH3 Cl. Stable carbon isotopic ratios of volatile and persistent halogenated organic compounds are summarized in Fig. 3. Volatile compounds normally exist as gas phase and are reactive in the atmosphere. Persistent compounds are relatively stable in the environment and accumulate in the biosphere, but harmful to organisms and humans. The values of the volatile biogenic COCs such as CH3 Cl and CHCl3 tend toward more depleted δ 13 C, although the range of values is large. Unfortunately there is currently no data for persistent biogenic COCs. If, however, we look at the brominated persistent biogenic compound 2-(3 ,5 -dibromo-2 -methoxyphenoxy)3,5-dibromoanisole (Reddy et al., 2002c), this has a value (n = 1) relatively close to those of CH3 Cl. However, in order to apply CSIA to distinguish between anthropogenic and biogenic

Fig. 3. Stable carbon isotopic signatures of selected COCs (error bars represent the ranges of reported isotopic values). (a) 2-(3 ,5 -dibromo-2 -methoxyphenoxy)-3,5-dibromoanisole; (b) average value of background tropospheric CH3 Cl (Thompson et al., 2002); 1 Reddy et al. (2002c); 2 Drenzek et al. (2002); 3 van Warmerdam et al. (1995), Beneteau et al. (1999); 4 Holt et al. (1997); 5 Jendrzejewski et al. (1997); 6 Jendrzejewski et al. (2001); 7 Rudolph et al. (1997); 8 Tsunogai et al. (1999); 9 Harper et al. (2001, 2003).


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persistent COCs, more 13 C data on persistent biogenic COCs are required. In addition, supplementary chemical information such as structural/molecular (enantiomer; isomer/congener) or isotopic (37 Cl or 14 C) signatures, which result from highly stereo-selective and isotopic fractionation or utilization of contemporary carbon of biosynthesized COCs, might be needed. 4. Stable chlorine isotopes Stable chlorine (37 Cl) isotope ratios are currently measured using dual-inlet stable isotope ratio mass spectrometry (SIRMS) (Long et al., 1993; Eggenkamp et al., 1995; Holt et al., 1997; Jendrzejewski et al., 1997; Rosenbaum et al., 2000) and thermal ionization mass spectrometry (TIMS) (Xiao and Zhang, 1992; Magenheim et al., 1994; Xiao et al., 1995, 2002; Rosenbaum et al., 2000; Numata et al., 2001; Holmstrand et al., 2004). Again, the information is expressed in the delta notation: δ 37 Cl (h) = (Rsample /Rstandard − 1) × 1000, where Rsample and Rstandard are the 37 Cl/35 Cl ratios of a sample and standard, respectively. There is no international standard for chlorine isotope ratios. The isotopic variations throughout the oceans are small and are typically less than analytical precision. This results from the relatively short circulation time of the ocean (1.5 × 103 yr) compared with the long oceanic residence time for chlorine (87 × 106 yr). Hence, Standard Mean Ocean Chloride (SMOC) has been proposed as the reference standard (Kaufmann et al., 1984; Holmstrand et al., 2004). A sample size of 3 µg Cl is required for TIMS and 300 µg Cl for SIRMS, both affording a reproducibility of 200h) because the pesticide was derived from a non-petrochemical source (synthesized during the 1950s–1970s


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Fig. 5. Radiocarbon dating of selected COCs (error bars represent the ranges of reported isotopic values). (a) 2-(3 ,5 -dibromo-2 -methoxyphenoxy)-3,5-dibromoanisole (b) 1,1 -dimethyl-3,3 ,4,4 -tetrabromo-5,5 dichloro-2,2 -bipyrrole. 1 Reddy et al. (2002c); 2 Reddy et al. (2004).

by the photo-chlorination of camphene, an isomerization product of α-pinene extracted from pine tree stumps) (Saleh, 1991). 14 C values of 1,1 -dimethyl-3,3 ,4,4 -tetrabromo-5,5 -dichloro-2,2 -bipyrrole (DBPBr4 Cl2 ) in marine animals are approximately −460h corresponding to conventional 14 C ages of ∼5000 years before present (BP) (Reddy et al., 2004). The authors suggest three possibilities to explain the depleted 14 C values: (1) approximately equal inputs from both natural and industrial sources, (2) naturally biosynthesized compounds derived from pre-aged carbon such as oceanic dissolved organic carbon (DOC) and (3) extremely persistent biosynthesized compounds with a relatively long residence time.

6. Conclusions Stable isotopic compositions of COCs in the environment depend on (1) isotopic components of the end-members of the source utilized, (2) isotopic fractionation associated with CPO-catalyzed chlorination during production of the COCs, (3) isotopic fractionation associated with metabolic de-chlorination, (4) isotopic residence times in the samples. Radiocarbon signatures, however, depend only upon the time passed (not entirely) since the atmospheric 14 C was assimilated by plants during photosynthesis. The isotopic data for COCs are summarized in Table 1. The larger is difference between anthropogenic and biogenic isotopic end members, the better is a tool for source identification. Therefore, CSIA of 13 C could be applied to identify the origin of biosynthesized volatile

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Table 1 Summary of isotopic data of COCs Isotope Chemical property

Source

Isotopic end-member of source (h)

Enrichment factor with CPOchlorination (h)

Enrichment factor with chlororespiration (h)

Analytical method

Sample size

13 C

Anthropogenic Biogenic Anthropogenic Biogenic

−30 to −20 (ca. −35) −35 to −25 −75 to −40

? (>0) (>0) (20–40)

? ? 1–27 ?

GC/C/IRMS

10–100 ng


−5 to 1 (−14 to −10) −3 to 4 −6 to 4

(∼3.5) ∼11 ? ?

(0–1.5) ? 0–13 ?

SIRMS

∼300 µg

TIMS

∼3 µg


ca. −1000 −500 to 100 – –

– – – –

– – – –

Persistent Volatile

37 Cl

Persistent Volatile

14 C

Persistent Volatile

PCGC–AMS ∼25 µg

Notes: Values in parentheses are not directly measured. Bold values are thought to be used for discriminating biogenic COCs. Enrichment factors with chloro-respiration are usually expressed as negative values.

COCs, while CSIA of 37 Cl and CSRA would have a potential for discriminating persistent biogenic COCs. In particular, CSRA appears to be the most powerful tool for source identification, because an isotopic difference between dead and alive carbon is very large. However, the relatively large sample size required for AMS may limit the application of CSRA to discriminating biogenic and anthropogenic COCs. Moreover, stable isotopic fractionations of COCs associated with chloro-respiration could be used for monitoring the environmental behavior of COCs. Although a compound-specific multi-isotope approach has the potential to evaluate the origin and fate of COCs in the environment, isotopic data for individual COCs is very scarce to date. Advances in the compound-specific multi-isotope approach are needed to provide the essential temporal and process-related information necessary to generate predictive biogeochemical models to track anthropogenic disturbances.

Acknowledgements The authors are grateful to Dr. Kimiyoshi Kitamura of NIES (National Institute for Environmental Studies, Japan) and staff of National Marine Biological Library, Plymouth for their help with the literature search. This work was supported by Grant-in-Aid for JSPS (the Japan Society for the Promotion of Science) Fellows.


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Further reading Fabbri, D., Vassura, I., Sun, C.G., Snape, C.E., McRae, C., Fallick, A.E. (2003). Source apportionment of polycyclic aromatic hydrocarbons in a coastal lagoon by molecular and isotopic characterization. Marine Chemistry 84, 123–135. Lollar, B.S., Slater, G.F., Ahad, J., Sleep, B., Spivack, J., Brennan, M., MacKenzie, P. (1999). Contrasting carbon isotope fractionation during biodegradation of trichloroethylene and toluene: Implications for intrinsic bioremediation. Organic Geochemistry 30, 813–820. Slater, J.F., Currie, L.A., Dibb, J.E., Benner, B.A. (2002). Distinguishing the relative contribution of fossil fuel and biomass combustion aerosols deposited at Summit, Greenland through isotopic and molecular characterization of insoluble carbon. Atmospheric Environment 36, 4463–4477.

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Shift in stable water isotopes in precipitation in the Andean Amazon: Implications of deforestation or greenhouse impacts? A. Henderson-Sellersa , K. McGuffieb,* a Australian Science and Technology Organisation, Lucas Heights, Australia b University of Technology, Sydney, Australia

Abstract Changes in the O and H isotopes in precipitation have been linked to greenhouse warming, but no signal attributable to Amazonian deforestation has been reported. Recent data from the Andes exhibit a seasonally contrasting signal which is consistent with large-area removal of forest. Specifically, at Izobamba, in the far west of the basin, the seasonality in isotopic depletions has become enhanced between 1972 and 2000. The observed more negative isotopic ratios in the wet season are consistent with increases in runoff fraction and/or reductions in recycling through nonfractionating processes. The dry season result (statistically significant less negative isotopic ratios) is harder to explain and could be due to a decrease in fractionating recycling (i.e. partial evaporation from water bodies). Application of a simple isotopic catchment model suggests that these isotopic changes in precipitation may be the result of large-scale deforestation in the Amazon Basin. Isotopically-enabled numerical models are needed to establish regional validity. Keywords: Amazon, Stable water isotopes, Deforestation, Transpiration, Re-evaporation, Runoff, Greenhouse

1. Climate and land-use change in the Amazon Deforestation throughout the world’s humid tropics is acknowledged to be a serious issue but adequate estimates of rates, extent and possible recovery are hard to ascertain (Achard et al., 2002). The Intergovernmental Panel on Climate Change (IPCC) has pointed out that “for tropical countries, deforestation estimates are very uncertain and could be in error by as much as ±50%” (Watson et al., 2000) while Glantz et al. (1997) detail the problems of attempting to assess rates and processes of deforestation in a careful study focused on the Amazon. Despite the acknowledged uncertainties, all data point to continuing forest removal (Fig. 1(a)). Even recent and careful studies that point to lower rates still identify the Brazilian Amazon as * Corresponding author. Address: Department of Applied Physics, University of Technology, Sydney, P.O. Box 123, Broadway, NSW 2007, Australia; phone: (+61) 2 9514 2072; fax: (+61) 2 9514 2219; e-mail: [email protected]



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A. Henderson-Sellers and K. McGuffie

(a)

(b)

Fig. 1. (a) Rates of deforestation in thousands of square kilometers per year and total area affected by deforestation as a percentage of original forested area. Rates for 1975–1983 are deduced from data presented in Fearnside (1987). Later data on area deforested annually (squares) from INPE (1998) and other sources. Rates for years without data have been estimated (thin line). (b) South America showing the Amazon Basin area within which forest is removed in GCM simulations and the location of Izobamba (filled circle) and other GNIP stations from which data are analyzed.

having among the highest deforestation rates in sampled ‘hot spots’: 4.4% and 2.4% in Acre and Rondonia, cf. 3.2–5.9% in Central Sumatra (Achard et al., 2002). Removing dense tropical moist forest and replacing it with agricultural pasture or scrub and grasses that regrow naturally causes significant shifts in hydrology and ecology. These have been measured for many years and models have been employed to try to predict the impacts of continued deforestation in the Amazon Basin (Fig. 1(b)). Henderson-Sellers and Gornitz (1984) conducted the first Amazonian deforestation simulation with a Global Climate Model (GCM) and since then many modelling groups have contributed additional estimates (see McGuffie et al., 1998). The joint impact of large-scale deforestation and greenhouse gas-induced change has only recently begun to be assessed. Zhang et al. (2001) and Costa and Foley (2000) both employed GCMs, the latter’s including a sophisticated land-surface scheme allowing stomatal closure feedback to be included. Table 1 lists the combined effects of greenhouse warming and deforestation predicted by these models. In all cases, temperatures increase and evaporation and precipitation both decrease. Adding greenhouse gases produces larger temperature increases but diminishes the impact on the hydrological cycle. Without stomatal closure in the CO2 enriched atmosphere, Zhang et al. (2001) find no change in the evaporation to precipitation ratio (E/P ) but including this feedback allows Costa and Foley (2000) to predict increased evaporative demand despite overall reductions in the hydrological cycling. At present, there are no data that can be used to assess the verity of predictions of the impact of future deforestation and greenhouse gas enrichment. The search for data with which to evaluate and, hopefully, improve GCM simulations is the basis of this paper.

Shift in stable water isotopes in precipitation in the Andean Amazon

41

Table 1 Results from two recent GCM simulations of the impacts of deforesting the Amazon (Defor.) and this combined (Both) with doubling the global CO2 content of the atmosphere (2 × CO2 ). The Costa and Foley (2000) model also incorporates stomatal response to CO2 changes, i.e. closure in enriched CO2 atmosphere Recent GCM simulations Precipitation (mm)

Evaporation (mm)

E/P (%)

Zhang et al. (2001) (no stomatal feedback) Deforestation +0.3 Both 2 × CO2 and defor. +2.9

−402 −317

−222 −179

55 56

Costa and Foley (2000) (with plant physiology) Deforestation +1.4 Both 2 × CO2 and defor. +3.5

−266 −153

−223 −146

83 95

Temperature (K)

2. Basin-scale change in the Amazon 2.1. Stable water isotopes The stable isotopes of hydrogen, oxygen and carbon are employed widely in Earth system science (e.g. Gat, 1996; Petit et al., 1999; Townsend et al., 2002) including in GCM simulations (e.g. Joussaume et al., 1984; Hoffman et al., 2000). Ratios of 18 O/16 O and 2 H/1 H have been used to quantify aspects of the hydro-climate of the Amazon Basin for over 30 years (Salati and Vose, 1984). Coupled with measurement of isotopes in water sources such as precipitation and vapor, stable isotope characteristics in rivers have been able to provide insight into basin-integrated hydro-climates (Gibson and Edwards, 2002). In the Amazon, interception of rainfall by the plant canopy is the source of re-evaporated isotopically ‘enriched’ water (e.g. Salati et al., 1979; Victoria et al., 1991). This process causes lower continental depletion in heavy isotopes here than in other major continents (Gat and Matsui, 1991). Improvement of isotope monitoring in large river basins (e.g. Gibson et al., 2002) may support a wide range of programmes exploring water budget management and modelling (Rozanski et al., 1993; Gat, 2000). In the Canadian Arctic, Gibson (2002) showed that stable water isotopes offer a means of monitoring the partitioning of evaporation and transpiration in basin-integrated discharge signals while, in semi-arid Australia, the downstream enrichment of heavier isotopes can be used to gauge progressive evaporative loss (Stone et al., 2003). The total latent fluxes in humid basins such as the Amazon can be much larger than the isotopically non-fractionating plant-mediated transpiration, resulting in evaporative enrichment of atmospheric moisture (Victoria et al., 1991; Henderson-Sellers et al., 2002). This paper extends the results of Henderson-Sellers et al. (2002) using stable water isotope data from Izobamba from 1972 up to 2001, to evaluate possible changes in the hydrological cycle over this time. 2.2. Application of stable isotopes to detecting Amazonian climatic variations The Amazon Basin recycles about half its rainfall with a water recycling time of about 5.5 days (Salati et al., 1979). Although field measurements of isotopes within the canopy (Leopoldo,

42


1981) are somewhat contradictory, the most likely source of observed differences is believed to be different moisture sources (e.g. Matsui et al., 1983; Vuille et al., 2003a) and the fate of water intercepted by the canopy. In a complete simulation of the Amazon’s forest hydrology, the land surface must correctly partition the moisture fluxes between water evaporation (fractionating), transpiration (non-fractionating), re-evaporated canopy intercepted rainfall (nonfractionating if complete) and runoff fraction (Martinelli et al., 1996). Taking advantage of the relatively straightforward nature of the atmospheric circulation over the Amazon Basin (e.g. Jones and Carvalho, 2002; Foley et al., 2002), Gat and Matsui (1991) employed a simple steady state model of the Amazon to demonstrate that some of the water recycling is from isotopically fractionating sources. Comparing their model with pre-1981 data from the International Atomic Energy Agency/World Meteorological Organization (IAEA/WMO) Global Network for Isotopes in Precipitation (GNIP, 1999), they suggested that 20–40% of the recycled moisture within the basin is derived from fractionating evaporation from sources such as lakes, the river or standing water. Victoria et al. (1991) combined isotopic observations from Belem and Manaus (1972–1986) and the model of Dall’Olio (1976) and Salati et al. (1979) to confirm that such fractionating evaporation contributes up to 40% of the total evaporation during the dry season (see also Gat, 2000). Henderson-Sellers et al. (2002) detected statistically significant temporal changes (1965– 1990) in water isotopic signatures in the central Amazon. Differences determined in deuterium excess were found to be consistent with recent GCM simulations only if there had been a relative increase in evaporation from non-fractionating water sources over the investigated period. They found no significant change in dry season isotopic characteristics despite earlier predictions of such land-use change impacts and concluded that the pre-1990 Amazonian stable isotope record is more consistent with the predicted effects of greenhouse warming (possibly combined with forest removal) than with the model-predicted effects of deforestation alone (Henderson-Sellers et al., 2002). Vuille et al. (2003a, 2003b) emphasize that δ 18 O is influenced by several factors but suggest that the dominant mode of ENSO (El Niño – Southern Oscillation) is likely to be important. However, Botta et al. (2002) used principal component analysis of Amazonian climate data to show that ENSO is not the major, or even the most important, mode of climate variability in the Amazon Basin. They found that the dominant mode of climate variability in the Amazon, which explains about 35% of the inter-annual variance of precipitation and 56% of the temperature variance, has a period of 24–28 years. By comparison, ENSO explains only 21% of the total variance in annual mean precipitation and temperature.

3. Precipitation isotopes in the Andean Amazon Stable isotopic data in rainfall continue to be collected by the IAEA/WMO in the Amazon Basin as part of a global monitoring program at around 550 stations worldwide (e.g. Rozanski et al., 1993; Gibson et al., 2002). Of the various stations that have operated within the area of the Amazon Basin, the most recent and extensive data sets are available from Izobamba (0.37◦ S, 78.55◦ W and 3058 m ASL) on the eastern flank of the Andes, situated in the upper part of the Amazon Basin. IAEA data now available for Izobamba bring the station record


43

Fig. 2. Monthly mean δ 18 O values (h) from Izobamba for all months (January–December) from 1972 to 2001 with least square linear trend lines shown in each monthly panel.

up to date and provide a record of 30 years (1972–2001), covering the periods of intense deforestation since the early 1970s (Fig. 1(a)) (McGuffie and Henderson-Sellers, 2004). Analysis of data for Izobamba shows upper Amazon Basin isotopic ratios shifting over 25 years (Fig. 2). The recent data show that the wet season (November–May) isotopic ratios have tended to become more negative while, in the dry season (June–October), ratios have tended to become more positive (Fig. 2). Although these changes are not all statistically significant (only May, June, July and November are statistically significant: shown by the black bars in Fig. 3), analysis of trend information for each month shows coherence in month-tomonth changes (Fig. 2). Seasonality in isotopic depletions in the upper part of the Amazon Basin, as represented by Izobamba, has increased over the last 25 years. Table 2 shows that at Izobamba the dry season depletions have decreased and the wet season depletions have increased. The dry season changes in both isotopic ratios are statistically significant. The wet season months where statistically significant changes are seen are those flanking the dry season (i.e. May and November) – see Fig. 3. These seasonal changes in 18 O and D signatures between 1972–1986 and 1987–2001 may indicate the impact of deforestation in the wet season: vegetation removal prompting less recycling and less re-insertion of heavy isotopes into the basin hydrological system. Figs. 2 and 3 suggest a shift away from non-fractionating sources (to fractionating sources) in the wet season and less effective recycling (i.e. a smaller fraction of precipitation is re-evaporated) of water (from fractionating sources) in the dry season. The hypothesis that these seasonally opposing changes could be the result of forest removal is examined in the next section using a simple model that allows for both fractionating evaporation (e.g. from lakes) and nonfractionating (transpiration).

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Fig. 3. δ 18 O in precipitation differences (1987–2001 minus 1972–1986) at Izobamba together with monthly mean wind roses (from NCEP reanalysis, Kanamitsu et al., 2001) and mean total precipitation (mm). Statistical significance (95% level) in the δ 18 O differences is shown by black bars (significant difference) cf. gray (not significantly different). The wind roses show that the dominant air flow is from the forested basin (see Fig. 1(b)). Table 2 Wet (November–May) and dry (June–October) season average depletions of δ 18 O and δD (h) from precipitation at Izobamba between two periods (1972–1986 and 1987–2001) together with the full time extent standard deviation. Dry season differences are statistically significant (shown in bold) at the 95% level, with differences exceeding ±1 SD Season/Period

δ 18 O

Diff.

SD

δD

Diff.

SD

Wet (November–May) 1972–1986 1987–2001

−11.0 −12.0

−1.0

0.5 0.6

−77.8 −83.4

−5.6

4.8 4.8

Dry (June–October) 1972–1986 1987–2001

−10.6 −9.2

+1.4

0.5 0.4

−75.0 −61.3

+13.7

3.6 3.2

4. Modelling water isotopes at the basin scale No GCM stable water isotopic models have yet been applied to tropical deforestation simulations that include changes in lakes and open water surfaces (Henderson-Sellers et al., 2002; Vuille et al., 2003a, 2003b). However, Gat and Matsui (1991) developed a simple basin-scale representation of the relative impact of transpiration (and canopy re-evaporation) as compared to open water evaporation: the latter being fractionating while the former are not. The model gives the isotopic ratio in the precipitation, δp as δp = δp,o + γ log f,

(1)

where γ =

(l + (1 − h)(l + r))ε + l(1 − h)Ck , l + r(1 − h)

(2)


45

l is the fraction of the precipitation which is re-evaporated from lakes and other open water surfaces, r is the total runoff fraction of the basin, ε is the equilibrium isotopic fractionation factor, Ck is the kinetic fractionation factor and h is the relative humidity. The fraction transpired by the forest is therefore t = 1 − r − l. The quantity f represents the amount of ‘pristine’ oceanic moisture that remains after the advection over the forest canopy and δp,o the depletion of precipitation not influenced by the forest recycling of moisture. This model is applied to a location representative of the Andean Amazon which receives moisture that has passed over the Amazon Basin (e.g. Izobamba). It is assumed that only ∼20% of the moisture is unchanged from its oceanic composition and the relative humidity is ∼70% (i.e. f = 0.2 and h = 0.7). Using Equations (1) and (2), the relative impacts of greater lake evaporation, a larger transpiration fraction and greater runoff fraction in Fig. 4(a) can be compared with the shift in the Izobamba GNIP data in Fig. 4(b), suggesting explanations for the observed changes in isotopic depletions. The relationship of 18 O and D in precipitation has changed between the 1970s and the first few years of this century at Izobamba in three distinct ways: (i) values have become more depleted; (ii) the distribution slope has become less steep; and (iii) seasonal separation has become greater as the dry season precipitation becomes less depleted while the wet season precipitation has become more depleted. These alterations can be seen in Fig. 4(b) and interpreted by reference to Fig. 4(a). The lengthening of the data distribution in Fig. 4(b) especially towards more depleted values in the 1990s and 2000s is consistent with less non-fractionating recycling (i.e. less transpiration and/or canopy re-evaporation): a likely result of deforestation. The local D/18 O slope for monthly means shifts from 8.1 to 7.8. This change, although not statistically significant, is

Fig. 4. (a) Model results showing schematically the relative impacts of changes in transpiration (t), lake evaporation (l) and runoff (r) on the δ 18 O and δD relationship. Open block arrows show the effect of modifying each of these parameters. (b) Scatter plot of all available GNIP data for Izobamba. Temporal changes in the data distribution include a greater range in isotopic depletions, especially a tendency towards greater depletions recently (more points to the lower left in the 1990s and 2000s) and seasonal shifts. The latter are shown as pairs (solid filled circles) of four-monthly mean values for decadal wet and dry seasons (1970s and 2000s) displayed by offsetting the δD values by ±30h for the dry and wet season respectively. These seasonal movements are consistent with the model results in Fig. 4(a) of less non-fractionating evaporation (transpiration and/or canopy re-evaporation) in the wet season and more non-fractionating (transpiration) and/or less fractionating (open water and lake evaporation) in the dry season.

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consistent with more evaporation from lakes or a greater runoff fraction (Fig. 4(a)), i.e. a result expected due to less vegetation impact on isotopic depletion. The seasonal ‘separation’ can be seen most readily by considering the (18 O, D) changes in, for example, the averages of the four wettest and four driest months in the decades of the 1970s and 2000s. In the dry season, these alter respectively from (−11.2, −79.8) to (−10.4, −74.1) and in the wet season from (−12.9, −92.5) to (−15.1, −110.8). These changes are illustrated in Fig. 4(b) by the pairs of points vertically displaced from the data distribution by ±30h of D for clarity. These movements underline the seasonal trends shown in Table 2: statistically significant in the dry season and clear (but not statistically significant) in the wet season. The results in Figs. 2 and 3 show: (i) during the dry season, isotopic ratios become less negative (statistically significant) at Izobamba (in air affected by the Amazon, e.g. Fig. 3); and (ii) opposite changes in the wet season. Comparing the catchment model with Izobamba data suggests that the recently observed increased data spread could be explained by less nonfractionating evaporation (transpiration and canopy re-evaporation) by the forest canopy over which the moisture has traveled and that a tendency for a decrease in the slope of the local meteoric line might imply either more lake evaporation or a greater runoff fraction (Fig. 4). These seasonal shifts suggest less non-fractionating evaporation (transpiration and/or canopy re-evaporation) in the wet season and more non-fractionating (transpiration) and/or less fractionating (from open water and lake evaporation) in the dry season.

5. Recent impacts of Amazonian deforestation Henderson-Sellers et al. (2002) proposed that their observed isotopic changes might be due to greenhouse intensification of the hydrological cycle masking any land-use change impact. Alternative explanations for their results include: isotope data to 1990 only were available; the statistically significant wet season changes reported might be related to ENSO events or other climatic variations that modify the regional circulation and hence affect the moisture climatology of the Amazon (cf. Vuille et al., 2003b; Botta et al., 2002); that no information on fluxes from simulated open water as a surface type in the Amazon GCM experiments was considered; and that the selected model sets failed to correctly simulate the relative components of transpiration and re-evaporated canopy interception at least in the Amazon dry season and perhaps throughout the year (Henderson-Sellers et al., 2004). Two recent assessments of the area of surface water suggest that its extent is significant. Richey et al. (2002) found that in May around 20% of the main Amazon River area is flooded. Specifically, they determined that, typically, the region is most flooded in May (350,000 km2 ), with an annual mean flooded area of 250,000 km2 (Richey et al., 2002). Foley et al. (2002) used the HYDRA land-surface scheme to simulate the extent of flooding in neutral (i.e. neither El Niño nor La Niña) years and found the areal range to be from 20,378 to 170,079 km2 . They also determined that, while both El Niños and La Niñas enlarge the minimum, La Niñas also increase the maximum (by 13,979 km2 ) while in El Niños the maximum area flooded is reduced. Overall, the estimates of open water range from about 350,000 km2 to about half this: an area that must be included in isotopic and other hydrological modelling. During the wet season, the isotopic ratios of 18 O and D in precipitation at Izobamba have become more negative over the period from the mid-1970s to 2000. These changes could


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be caused by (i) an increase in the fraction of precipitation that appears as runoff fraction, (ii) a reduction in the fraction of recycling that occurs through non-fractionating processes, i.e. transpiration, and/or (iii) an increase in recycling that occurs through fractionating processes, i.e. evaporation from open bodies of water. Causes (i), (ii) and possibly (iii) could reasonably be associated with a loss of forest cover and thus be an isotopic signal of deforestation. During the dry season, isotopic ratios of 18 O and D in precipitation at Izobamba have changed in the opposite direction over the same period. These changes could be caused by a decrease in the fraction of water recycled through fractionating processes, i.e. evaporation from open bodies of water. These results are either complementary or contradictory to those of Henderson-Sellers et al. (2002) who showed a likely impact in the central Amazon due to an apparent intensification of hydrologic cycling, itself, perhaps, the result of greenhouse warming. To determine which of these apparent causes of observed isotopic precipitation disturbances is correct or whether both need to be invoked will require the use of isotopically-enabled Global Climate Models including appropriately complex and isotopically-enabled land-surface schemes. Such models could establish if the observed trends are more than station-specific and also examine alternative explanations including large-scale circulation changes.

References Achard, F., Eva, H.D., Stibig, H.-J., Mayaux, Ph., Gallego, J., Richards, T., Malingreau, J.-P. (2002). Determination of deforestation rates of the world’s humid tropical forests. Science 297, 999–1002. Botta, A., Ramankutty, N., Foley, J.A. (2002). Long-term variations of climate and carbon fluxes over the Amazon basin. Geophysical Research Letters 29, 1319, doi:10.1029/2001GL013607. Costa, M.H., Foley, J.A. (2000). Combined effects of deforestation and doubled atmospheric CO2 concentrations on the climate of Amazonia. Journal of Climate 13, 18–34. Dall’Olio, A. (1976). A composicao isotopica das precipitacoes de Brasil: modelos isotermicos e a influencia da evapotranspiracao na Bracia Amazonica. MSc. thesis. Piracicaba, Universidada de São Paulo, 180 pp. (In Portuguese.) Fearnside, P.M. (1987). Causes of deforestation in the Brazilian Amazon. In: Dickinson, R.E. (Ed.), The Geophysiology of Amazonia Vegetation and Climate Interactions. Wiley, New York, pp. 37–61. Foley, J.A., Botta, A., Coe, M.T., Costa, M.H. (2002). El Niño – Southern Oscillation and the climate, ecosystems and rivers of Amazonia. Global Biogeochemical Cycles 16 (4), 1132, doi:10.1029/2002GB001872. Gat, J.R. (1996). Oxygen and hydrogen isotopes in the hydrological cycle. Annual Review of Earth and Planetary Sciences 24, 225–262. Gat, J.R. (2000). Atmospheric water balance – The isotopic perspective. Hydrological Processes 14, 1357–1369. Gat, J.R., Matsui, E. (1991). Atmospheric water balance in the Amazon Basin: An isotopic evapotranspiration model. Journal of Geophysical Research 96 (D7), 13,179–13,188. Gibson, J.J. (2002). Short-term evaporation and water budget comparisons in shallow Arctic lakes using non-steady isotope mass balance. Journal of Hydrology 264, 242–261. Gibson, J.J., Edwards, T.W.D. (2002). Regional water balance trends and evaporation–transpiration partitioning from a stable isotope survey of lakes in northern Canada. Global Biogeochemical Cycles 16 (2), 1026, doi:10.1029/2001GB001839. Gibson, J.J., Aggrawal, P., Hogan, J., Kendall, C., Martinelli, L.A., Stichler, W., Rank, D., Goni, I., Choudury, M., Gat, J., Bhattacharya, S., Sugimoto, A., Fekete, B., Pietroniro, A., Maurer, T., Panarello, H., Stone, D., Seyler, P., Maurice-Bourgoin, L., Herzceg, A. (2002). Isotope studies in large river basins: A new global research focus. EOS 83 (52), 613–617. Glantz, M.H., Brook, A.T., Parisi, P. (1997). Rates and processes of Amazon deforestation. Available at http://www.isse.ucar.edu/rates/index.html.

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GNIP, Global Network for Isotopes in Precipitation (1999). The IAEA/WMO GNIP Database, Release 3, October 1999. Available at http://isohis.iaea.org/search.asp. Henderson-Sellers, A., Gornitz, V. (1984). Possible climatic impacts of land cover transformations, with particular emphasis on tropical deforestation. Climatic Change 6, 231–258. Henderson-Sellers, A., McGuffie, K., Zhang, H. (2002). Stable isotopes as validation tools for global climate model predictions of the impact of Amazonian deforestation. Journal of Climate 15, 2664–2677. Henderson-Sellers, A., McGuffie, K., Noone, D., Irannejad, P. (2004). Using stable water isotopes to evaluate basinscale simulations of surface water budgets. Journal of Hydrometeorology 5 (4), 805–822. Hoffman, G., Jouzel, J., Masson, V. (2000). Stable isotopes in atmospheric general circulation models. Hydrological Processes 14, 1385–1406. INPE (1998). Average annual deforestation rate in the legal Amazon. Available at http://www.inpe.br/amz-04.htm. Jones, C., Carvalho, L.M.V. (2002). Active break phases in the South American monsoon system. Journal of Climate 15, 905–914. Joussaume, S., Sadourny, R., Jouzel, J. (1984). A general circulation model of water isotopes cycles in the atmosphere. Nature 311, 24–29. Kanamitsu, M., Kousky, V., van den Dool, H., Jenne, R., Fiorino, M. (2001). The NCEP–NCAR 50-year re-analysis: Monthly means CD-ROM and documentation. Bulletin of the American Meteorological Society 82, 247–268. Leopoldo, R.R. (1981). Aspetos hidrologicos at florista amazonica denga na regiao de Manaus. PhD thesis. University National Estado São Paulo, Botucato SP, Brasil, 180 pp. McGuffie, K., Henderson-Sellers, A. (2004). Stable water isotope characterization of human and natural impacts on land-atmosphere exchanges in the Amazon basin. Journal of Geophysical Research – Atmospheres 109, D17104, doi:10.1029/2003JD004388. McGuffie, K., Henderson-Sellers, A., Zhang, H. (1998). Modelling climatic impacts of future rainforest destruction. In: Maloney, B.K. (Ed.), Human Activities and the Tropical Rainforest. Kluwer, Dordrecht, The Netherlands, pp. 169–193. Martinelli, L.A., Victoria, R.L., Sternberg, L.S.L., Ribeirio, A., Moreira, M.Z. (1996). Using stable isotopes to determine sources of evaporated water to the atmosphere in the Amazon basin. Journal of Hydrology 183, 191–204. Matsui, E., Salati, E., Ribeiro, M.N.G., Reis, C.M., Tancredi, A.C.S.N.F., Gat, J.R. (1983). Precipitation in the Central Amazon Basin: The isotopic composition of rain and atmospheric moisture at Belem and Manaus. Acta Amazonica 13, 307–369. Petit, J.R., Jouzel, J., Raynaud, D., Barkov, N.I., Barnola, J.-M., Basile, I., Bender, M., Chappellaz, J., Davisk, M., Delaygue, G., Delmotte, M., Kotlyakov, V.M., Legrand, M., Lipenkov, V.Y., Lorius, C., Pépin, L., Ritz, C., Saltzmank, E., Stievenard, M. (1999). Climate and atmospheric history of the past 420000 years from the Vostok ice core, Antarctica. Nature 399, 429–436. Richey, J.E., Melack, J.M., Aufdenkampe, A.K., Ballester, V.M., Hess, L.L. (2002). Outgassing from Amazonian rivers and wetlands as a large tropical source of atmospheric CO2 . Nature 416, 617–620. Rozanski, K., Araguas-Araguas, L., Gonfiantini, R. (1993). Isotopic patterns in modern global precipitation. Climate Change in Continental Isotopic Records. Geophysical Monographs 78, 1–36. Salati, E., Vose, P.B. (1984). Amazon Basin: A system in equilibrium. Science 225, 129–137. Salati, E., Olio, A.D., Matsui, E., Gat, J.R. (1979). Recycling of water in the Amazon Basin: An isotopic study. Water Resources Research 15 (5), 1250–1258. Stone, D.W., Henderson-Sellers, A., Airey, P., McGuffie, K. (2003). Murray Darling basin isotope observations: An essential component of the Australian CEOP. EOS Transactions AGU 84 (46). AGU Fall Meeting Suppl., H22I-08, December 8–12, 2003. Townsend, A.R., Asner, G.P., White, J.W.C. (2002). Land use effects on atmospheric 13 C imply a sizable terrestrial CO2 sink in tropical latitudes. Geophysical Research Letters 29 (10), 1426, doi:10.1029/2001GL013454. Victoria, R.L., Martinelli, L.A., Mortatti, J., Richey, J. (1991). Mechanisms of water recycling in the Amazon Basin: Isotopic insights. Ambio 20 (8), 384–387. Vuille, M., Bradley, R.S., Werner, M., Healy, R., Keimig, F. (2003a). Modelling δ 18 O in precipitation over the tropical Americas: 1. Interannual variability and climatic controls. Journal of Geophysical Research 108 (D6), 4174, doi:10.1029/2001JD002038. Vuille, M., Bradley, R.S., Healy, R., Werner, M., Hardy, D.R., Thompson, L.G., Keimig, F. (2003b). Modelling δ 18 O in precipitation over the tropical Americas: 2. Simulation of the stable isotope signal in Andean ice cores. Journal of Geophysical Research 108 (D6), 4175, doi:10.1029/2001JD002039.


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Watson, R.T., Noble, I.R., Bolin, B., Ravindranath, N.H., Verardo, D.J., Dokken, D.J. (2000). Land Use, Land Use Changes and Forestry: A Special Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, 388 pp. Zhang, H., Henderson-Sellers, A., McGuffie, K. (2001). The compounding effects of tropical deforestation and greenhouse warming on climate. Climatic Change 49, 309–338.


2. Oceanic radionuclide tracers


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Southern Hemisphere Ocean Tracer Study (SHOTS): An overview and preliminary results M. Aoyamaa,* , M. Fukasawab , K. Hirosea , R.F.C. Mantourac , P.P. Povinecc , C.S. Kimd , K. Komurae a Geochemical Research Department, Meteorological Research Institute (MRI), Tsukuba 305-0052, Japan b Ocean Research Department, Japan Agency for Marine-Earth Science and Technology (JAMSTEC),

Yokosuka, Kanagawa 237-0061, Japan c Marine Environment Laboratory, International Atomic Energy Agency (IAEA-MEL), MC-98000, Monaco d Environmental Radioactivity Assessment Department, Korea Institute of Nuclear Safety (KINS),

Seoul, Republic of Korea e Low-Level Radioactivity Laboratory (LLRL), Institute of Nature and Environmental Technology,

Wake, Tatsunokuchi, Ishikawa 923-1224, Japan Abstract Approximately 900 seawater samples were collected in the subtropical gyres in the South Pacific, the South Atlantic and the South Indian Ocean in 2003–2004 during the BEAGLE2003 cruise. Preliminary results show that 137 Cs and 239,240 Pu concentrations in surface waters were comparable with those in the subtropical gyres in the North Pacific. The 137 Cs profile shows a smaller effect of subduction on water mass transport. The estimated 137 Cs inventory of 800 Bq m−2 is about a half of the inventory in the North Pacific at the same latitude. The 239,240 Pu concentration at sub-surface maximum is one order of magnitude lower than that observed in the North Pacific, which is a remarkable difference when compared with 137 Cs profiles. Measured 240 Pu/239 Pu atom ratios are 0.20–0.24, statistically not different from the global fallout ratio. The SHOTS collaboration will produce a comprehensive dataset on anthropogenic radionuclides in the Southern Ocean. The obtained results will contribute to better understanding of processes in the water column using radionuclides as tracers, and they will improve our knowledge of circulation processes in the ocean, important for better understanding of climate change. Keywords: Anthropogenic radionuclides, 137 Cs, Plutonium isotopes, SHOTS, Seawater, Water column, Pacific Ocean, Atlantic Ocean, Indian Ocean, Southern Ocean

1. Introduction The main source of anthropogenic radionuclides in the world ocean is global fallout from atmospheric nuclear tests carried out between 1945 and 1980. Several global studies have been carried out in the world ocean (e.g. GEOSECS (Bowen et al., 1980), WOCE (Schlosser et al., * Corresponding author. Address: MRI, 1-1 Nagamine, Tsukuba 305-0052, Japan; phone: (+81) 29 853 8719; fax: (+81) 29 853 8728; e-mail: [email protected]



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Fig. 1. BEAGLE2003 cruise track.

1999), WOMARS (Povinec and Togawa, 1998; Povinec, 2003)), however, anthropogenic radionuclides in the Southern Ocean have not been studied well and data on the distribution, inventory and behavior of anthropogenic radionuclides have been missing (Hamilton et al., 1996; Livingston and Povinec, 2000). Radionuclide data stored in the IAEA’s Global Marine Radioactivity Database (GLOMARD; IAEA, 2000; Povinec et al., 2005), as well as in the Meteorological Research Institute’s (MRI) HAM Pacific database (Aoyama and Hirose, 2004), include only a few hundred records for 137 Cs concentrations in seawater of the Southern Ocean, compared to several ten thousand records for the Northern Hemisphere Oceans. Radionuclide tracer studies in the Southern Hemisphere Oceans continue to be a major objective in worldwide marine radioactivity research. The ship opportunity to carry out sampling in the Southern Ocean was available during the Blue Earth Global Expedition 2003 (BEAGLE2003) conducted on the R/V Mirai during August 2003–March 2004 (Fig. 1) by the JAMSTEC. The BEAGLE2003 expedition was following the WOCE track in the Southern Ocean, revisiting WOCE stations with the aim of determining any changes in oceanographic parameters (Wijffels et al., 1998), distribution of 14 C and other tracers in the water column. In connection with the BEAGLE2003 expedition a project on the South Hemisphere Ocean Tracer Study (SHOTS) has been developed with the aim to study the distribution of radioactive and stable isotopes, and other non-radioactive tracers in the Southern Ocean, which plays a dominant role in the Earth climate. The project will focus on analysis of radionuclides in water samples for better understanding of circulation and water column processes. The main target radionuclides with the most frequent analyses will be tritium, radiocarbon, 90 Sr, 137 Cs and plutonium isotopes, however, other radionuclides will be analyzed as well, supported by oceanographic and non-radioactive tracer analyses such as nutrients. The co-analysis of nutrients and O2 with radionuclides offers a unique opportunity to determine the in situ phys-

Southern Hemisphere Ocean Tracer Study (SHOTS): An overview and preliminary results

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ical and biogeochemical dynamics of anthropogenic radionuclides in the South Hemisphere oceans. The SHOTS international collaboration will produce a comprehensive dataset on anthropogenic radionuclides in the Southern Ocean. A complete hydrographic dataset from the BEAGLE2003 cruise will be produced by JAMSTEC. The obtained results will contribute to better understanding of processes in the water column using radionuclides as tracers, as well as for studying circulation processes in the ocean towards a better understanding of climate change. In this paper, we present and discuss preliminary results obtained from the South Pacific Ocean.

2. Sampling and analyses 2.1. Sampling onboard Sampling work in the subtropical gyres in the Southern Hemisphere Oceans was carried out at 93 stations which included both surface and water column stations (Fig. 1). The average distance for surface water stations was about 270 km. A CTD (SBE 9 plus) with a Rosette Multi-bottle Sampler (RMS) was used to measure oceanographic parameters at 491 stations throughout the cruise. The cruise details for hydrographic observations are available at the JAMSTEC web site (http://www.jamstec.go.jp/mirai/2003/data_2003.html). CTD/RMS water profile sampling for radionuclide measurements was done in 57 of 491 stations. The sampling depths at each station were 1, 100, 200, 400, 600, 800 and 1000 dbar, and approximately every 500 dbar below 1000 dbar (e.g. 1400, 2000, 2400, 3000, 3500, 4000, etc., down to the sea bottom). The location of stations and sampling depths for samples collected in the South Pacific Ocean are shown in Fig. 2. The sample volumes for surface water were around 85 L, while water column samples varied from 5 to 20 L depending on availability. The total number of samples is close to 900 (Table 1) with a total weight of about 22,000 kg. Seawater samples for radionuclide analyses were filtered onboard through a membrane filter with a pore size of 0.45 µm (Millipore HA), and acidified using nitric acid (Aoyama et al., 2000), while samples for tritium measurements were stored in 1 liter bottles without acidification. 2.2. Radionuclide analyses 239,240 Pu, 137 Cs and 90 Sr analyses of collected samples have been carried out in MRI (Aoyama

et al., 2000; Hirose et al., 2001), IAEA-MEL (La Rosa et al., 2001; Lee et al., 2001; Povinec et al., 2001), KINS (Kim et al., 2000, 2002) and LLRL (Komura, 1997; Hirose et al., 2005) following their respective radiochemical procedures. At LLRL, in order to achieve very low background in gamma spectrometry, high efficiency well type HPGe detectors, shielded with very low activity lead, were placed in the Ogoya underground facility, specially designed for low level counting. The detection limit for 137 Cs is 0.18 mBq for a counting time of 1 week. Since the volume of water column samples is relatively small (5–20) liters, the deep water samples are measured at Ogoya underground facility to obtain reliable results for 137 Cs.

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Fig. 2. SHOTS sampling locations and depths for the Pacific Ocean. Table 1 SHOTS samples

Surface Water column

Total

Volume (L)

Pacific Ocean

85 20 10–18 1000 m depth) did not change. These findings suggest that the 239,240 Pu concentrations in mid depths (0–500 m depth) in the mid-latitude region of the North Pacific have been continuously decreasing during the past three decades. This change cannot be explained by a simple biogeochemical process including only vertical transport such as particle scavenging because there is no marked increase of the 239,240 Pu concentrations in deeper water layers (>1000 m). Therefore, the decrease of the 239,240 Pu concentrations in the mid-depth in the mid-latitude region of the North Pacific is lead by exchange with water masses having lower 239,240 Pu concentrations due to horizontal advection. Since there was no GEOSECS site adjacent to our station in the Equatorial North Pacific, we compared our data with a more eastern station (5◦ 53 N, 172◦ 01 W) in the Equatorial North Pacific. The vertical 239,240 Pu profile in the Equatorial North Pacific in the depth layers from the surface to 1000 m did not show significant change during the past three decades. A similar

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(a)

(b)

Fig. 3. The vertical profiles of 239,240 Pu concentrations in the water column of the North Pacific. Open and closed circles show present observations and GEOSECS data (1973), respectively, (a) the eastern North Pacific; (b) the Equatorial North Pacific.

tendency has been observed in the subtropical western North Pacific (Livingston et al., 2001; Povinec et al., 2003). On the other hand, there is a significant difference in 239,240 Pu concentrations in deep waters between equatorial GEOSECS and present sites; deep 239,240 Pu concentrations in the present station near the Pacific Proving Ground nuclear test sites were higher than those in the eastern station (GEOSECS site). This finding suggests that 239,240 Pu from the close-in fallout did not spread in the central Equatorial Pacific, which is consistent with the previous observations that radionuclides from Bikini explosions were initially injected into sea areas northwards of Bikini (Miyake et al., 1955). 3.3. Deep 239,240 Pu The North Pacific deep waters contain significant amounts of 239,240 Pu (Bowen et al., 1980; Povinec et al., 2003). It is interesting to know the timescale of 239,240 Pu introduction into deep waters and processes which were responsible for its vertical transport into deep layers. Livingston et al. (2001) suggested that 239,240 Pu concentrations in deep waters (below 2000 m) have almost been maintained at constant level during the past two decades. In order to elucidate the present features in the distribution of deep 239,240 Pu, we have depicted latitudinal

Plutonium isotopes in seawater of the North Pacific

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distributions of deep 239,240 Pu in two different layers (the upper layer: 2000–3500 m, and the lower layer: 4000–6000 m) along the 165◦ E and 180◦ E longitude, in which data (Aoyama and Hirose, 2004), collected from 1978 to 2002, and from 1973 to 1988, respectively, have been included. The results along the 165◦ E longitude are shown in Figs. 4(a),(b). The latitudinal distribution of the 239,240 Pu concentrations in the upper deep layer is showing two peaks; one peak occurs in the mid-latitude region (30–40◦ N), corresponding to the latitudinal pattern of global fallout. The second peak appears in the south subtropical region (10–15◦ N), corresponding to the close-in fallout from the Pacific Ground nuclear test sites. Lower 239,240 Pu concentrations in the upper deep layer occurred in the north subtropical (20–25◦ N) and equatorial regions (0–7/8◦ N). The 239,240 Pu concentrations in the lower deep layer were generally lower than that in the upper deep layer. The latitudinal distribution of the 239,240 Pu concentrations in the

Fig. 4. The latitudinal distribution of 239,240 Pu in deep waters. Open and closed circles show historical data and present data including the IAEA’97 Pacific cruise (Povinec et al., 2003), respectively. (a) The upper deep layer (2000–3500 m) along 165◦ E; (b) the lower deep layer (4000–6000 m) along 165◦ E; (c) the upper deep layer (2000–3500 m) along 180◦ ; (d) the lower deep layer (4000–6000 m) along 180◦ .

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Fig. 4. (Continued.)

lower deep layer (Fig. 4(b)) is also showing peaks between 20–30◦ N and 10–15◦ N, however, the picture is not so for the upper deep layer. The latitudinal distributions of the 239,240 Pu concentrations in deep waters along the 180◦ E longitude (Figs. 4(c),(d)) show similar patterns as for the 165◦ E longitude. The 239,240 Pu concentrations in the upper deep layer along 180◦ E are slightly lower than those in the corresponding depth layer along 165◦ E. The 239,240 Pu concentrations in the lower deep layer showed, however, a peak in the mid-latitude region (30–40◦ N) with higher 239,240 Pu concentrations than in the upper deep layer. Buesseler (1997) observed an enhanced 240 Pu/239 Pu ratio, a typical signature of close-in fallout, in a sediment sample collected in the central North Pacific (38◦ 00 N, 179◦ 45 W). The close-in fallout plutonium may be preferentially transported into deeper layers including sediments because the chemical and physical properties of particles carrying close-in fallout plutonium are different from those of global fallout (Adams et al., 1960). Therefore, close-in fallout from Bikini or Enewetak Atolls may contribute to 239,240 Pu concentrations in deeper layer waters of the central North Pacific as well.


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These findings suggest that a geographical structure of deep 239,240 Pu concentrations exists in the western and central North Pacific. The subarctic Pacific is recognized as a highly productive area in the North Pacific. Therefore, it is expected that larger vertical 239,240 Pu transport and remineralization in deep waters occur as a result of higher particle export fluxes in the ocean interior. However, the geographical distributions of deep 239,240 Pu suggest that there is little contribution from biogeochemical cycling of 239,240 Pu to the North Pacific deep waters (>2000 m). Model studies conducted by Tsumune et al. (2003) suggest that there is less contribution of biogeochemical vertical transport to deep 239,240 Pu (>2000 m depth) although about four decades have passed since the surface injection of 239,240 Pu. These findings suggest that a fraction of fallout 239,240 Pu-bearing particles was rapidly transported to deep layers and dissolved in deep waters. Therefore, the geographical distributions of 239,240 Pu in deep waters reflect spatial distributions of global fallout and close-in fallout together. 3.4. Plutonium isotope ratios Recent developments in high resolution ICP-MS techniques allow us to determine plutonium isotopic ratios with high precision (Kim et al., 2000, 2002; Taylor et al., 2001). Measured 240 Pu/239 Pu atom ratios in water samples are summarized in Table 1. The 240 Pu/239 Pu ratios in the North Pacific waters ranged from 0.16 to 0.28, which are in the range of the 240 Pu/239 Pu ratio of global fallout (0.18) to that in close-in fallout (0.30). The 240 Pu/239 Pu ratios in shallower water layers tend to be lower than that in deep waters, although the depth trend of 240 Pu/239 Pu ratios is not statistically significant (the correlation coefficient R 2 = 0.1) because of larger analytical uncertainties obtained for the deep water samples. The 240 Pu/239 Pu atom ratios in deep waters (>2000 m) were around 0.25, which coincides with the results obtained by Buesseler (1997). The observed elevated 240 Pu/239 Pu ratios thus document a presence of close-in fallout plutonium in North Pacific waters. Significantly lower 240 Pu/239 Pu atom ratios were found in shallower water layers of the Equatorial North Pacific (7◦ 59 N, 164◦ 54 E), which may be due to intrusion of the subsurface waters carrying a global fallout 239,240 Pu signal as a result of southwestward motion. 3.5. Inventory of

239,240 Pu

in the water column

The 239,240 Pu inventory in the water column of the western North Pacific was calculated by interpolating the 239,240 Pu concentrations measured at each water depth. The results are summarized in Table 2. The 239,240 Pu inventory in the mid-latitude region of the western North Pacific decreased when comparing with GEOSECS data (Livingston et al., 2001; Povinec et al., 2003). The decrease in the 239,240 Pu inventory in the mid-latitude regions is attributable to a plutonium deficiency in the upper layer (0–1000 m depth). Similar trends in the North Pacific mid-latitude region have been observed also for 137 Cs (Aoyama and Hirose, 2003). On the other hand, there is no temporal decrease of the 239,240 Pu inventories in the subtropical and the Equatorial North Pacific water columns (Livingston et al., 2001), although the present 239,240 Pu inventory in the Equatorial North Pacific is larger than that of the corresponding GEOSECS station. The 239,240 Pu inventories in the western North Pacific shows a flat latitudinal distribution (100–130 Bq m−2 ), except for the Equatorial Pacific. The largest 239,240 Pu inventory (130 Bq m−2 ) was observed in the North Equatorial Current region (15◦ 30 N, 159◦ 31 W), the site being located northwards downstream of Bikini. It must

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Table 2

239,240 Pu inventories in the western North Pacific (Bq m−2 )

Location 34◦ 60 N, 146◦ 00 E 30◦ 34 N, 170◦ 37 E 11◦ 26 N, 164◦ 52 E 11◦ 30 N, 161◦ 45 E 15◦ 30 N, 159◦ 31 E 7◦ 59 N, 164◦ 54 E

Depth (m)

239,240 Pu

5927 5472 4537 3690 5557 5138

101 122 115 110 129 67∗∗

239,240 Pu (close-in)

239,240 Pu (global)

239,240 Pu

GEOSECS 37∗ 51∗ 62∗ 64∗ 79∗ 26∗∗∗ (22–32)

64∗ 71∗ 53∗ 46∗ 50∗ 41∗∗∗ (45–35)

140 150 110

40

∗ The global and close-in 239,240 Pu inventories were calculated from 238 Pu inventories using Equation (1). The data of 239,240 Pu and 238 Pu concentrations in the water column obtained in the IAEA’97 cruise (Povinec et al., 2003) were used. ∗∗ This work. ∗∗∗ The inventories were calculated from 240 Pu/239 Pu atom ratios at each depth using Equation (2). Values in paren-

thesis were calculated from different 240 Pu/239 Pu atom ratios (0.30 and 0.36) of close-in fallout.

be noted that the present 239,240 Pu water column inventories in the western North Pacific are larger than that of total 239,240 Pu deposition in corresponding latitudes originating from global fallout (5 Bq m−2 (0–10◦ N) to 67 Bq m−2 (30–40◦ N)) (Hardy et al., 1973). Therefore for better understanding of plutonium behavior in the western North Pacific, it is important to know the contribution of close-in fallout plutonium from the Pacific Proving Ground testing. The plutonium isotope signature is a clue for estimation of individual contributions from global and close-in fallout 239,240 Pu, because the isotope ratios of global fallout plutonium (238 Pu/239,240 Pu and 240 Pu/239 Pu) differ from the close-in fallout ratios. The fallout plutonium originating from the Pacific Proving Ground testing (Bravo test, conducted on February 28, 1954) is characterized by lower 238 Pu/239,240 Pu activity ratio (0.001) and higher 240 Pu/239 Pu atom ratio (0.33), which were determined for archived samples directly contaminated by the Bravo test (Komura et al., 1984). The low 238 Pu/239,240 Pu activity ratios from the Bravo test were recorded in the Ross ice sheet, Antarctica, as well (Koide et al., 1979). The plutonium isotope ratios depend on individual nuclear explosions; for example, a higher 240 Pu/239 Pu atom ratio (0.36) was reported for the Ivy Mike explosion on November 1, 1952 (Diamond et al., 1960). If plutonium in the water column of the western North Pacific consists of only two components, i.e., global fallout and close-in fallout (the Bravo test as a representative), then 239,240 Pu inventories derived from individual fallout can be estimated from plutonium isotope data based on a simple mixing model. The 239,240 Pu water column inventory originating from global fallout based on 238 Pu/239,240 Pu activity ratios is estimated from the following equation: IWT,2 − IWT,1 RB IWG,1 = (1) , RG − RB where IWT,1 and IWG,1 are the total inventories of 239,240 Pu in the water column and from global fallout, respectively, and IWT,2 is the total inventory of 238 Pu in the water column.


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RG and RB denote the 238 Pu/239,240 Pu activity ratios in global fallout (0.03), and in close-in fallout (0.001), respectively. We calculated global fallout and close-in fallout 239,240 Pu inventories for the IAEA’97 cruise data (Povinec et al., 2003), which contains a complete data set including 238 Pu concentrations in the water column. The results are summarized in Table 2. On the other hand, the 239,240 Pu concentrations derived from the Bikini nuclear explosions can be estimated from the 240 Pu/239 Pu atom ratios by the following equation: −1 and AB = AO (1 + λR RB ) 1 + λR RB + (1 + λR RG )Q (2) −1 Q = (RO − RB )(RG − RO ) , where AB and AO are the Bikini-derived and observed 239,240 Pu concentrations, respectively, λR is the ratio of the radioactive decay constants of 239 Pu and 240 Pu, and RB , RG and RO are atom ratios in close-in fallout from Bikini, in global fallout and in analyzed water samples, respectively. The Bikini-derived 239,240 Pu inventory is calculated from the estimated Bikiniderived 239,240 Pu concentrations in each depth. The results are summarized in Table 2. It must be noted that the estimated inventories from global fallout and close-in fallout have significant uncertainties (less than 50%) because 238 Pu determinations have large uncertainties due to its low concentrations, and the variation range of the 240 Pu/239 Pu atom ratios between global fallout and close-in fallout is relatively narrow. The results suggest that a significant amount of 239,240 Pu derived from close-in fallout has spread over the subtropical region of the western North Pacific. The close-in fallout 239,240 Pu inventories peaked in the North Equatorial Current region (15◦ 30 N), and gradually decreased northward. A significant amount of the current 239,240 Pu inventory in the subtropical western North Pacific is still represented by the close-in fallout 239,240 Pu. The gap in the 239,240 Pu inventories derived from the close-in fallout, between the subtropical and Equatorial North Pacific, may be due to the fact that the close-in fallout occurred north of Bikini and Enewetak islands (Miyake et al., 1955). The 239,240 Pu inventory originating from global fallout is from 41 to 71 Bq m−2 , showing a latitudinal distribution with a maximum in the mid-latitude region, and a minimum in the Equatorial Pacific. Comparing with the latitudinal distribution of global fallout 239,240 Pu (Hardy et al., 1973) and GEOSECS data (Bowen et al., 1980), the 239,240 Pu inventories have gradually decreased in the mid-latitude region and increased in the Equatorial Pacific. Therefore, a latitudinal gradient of the 239,240 Pu water column inventory from global fallout in the western North Pacific is smaller than the 239,240 Pu deposition due to global fallout (Hardy et al., 1973). This finding suggests that a significant amount of global fallout 239,240 Pu, mostly deposited in the mid-latitude region, has been transported southward in the North Pacific during the last four decades, which is consistent with the 137 Cs inventory (Aoyama and Hirose, 2003).

4. Conclusions We examined the temporal and spatial distributions of the 239,240 Pu concentrations in seawater using present and historical databases (Aoyama and Hirose, 2004) with the aim to better understand the behavior of 239,240 Pu in the North Pacific Ocean. The observed 239,240 Pu concentrations in surface waters showed temporal variations; it decreased exponentially in the

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mid-latitude and the North Equatorial Current regions, whereas in the Equatorial Pacific they have been stable since 1985. The vertical profiles of 239,240 Pu in the water column of the North Pacific are still showing typical patterns with a surface minimum, a mid-depth maximum and, thereafter, a gradual decrease with increasing water depth. The temporal variations in the 239,240 Pu vertical profiles depend on the sea areas; in the mid-latitude region, the 239,240 Pu maximum layer deepened with time and the corresponding 239,240 Pu concentrations decreased, whereas there does not seem to be any change of the 239,240 Pu vertical profiles in the subtropical and Equatorial North Pacific. The 239,240 Pu concentrations in deep waters (>2000 m depth) of the North Pacific have a spatial structure, which seems to reflect input patterns of global fallout and close-in fallout. This finding suggests that there is little temporal variability of deep 239,240 Pu over a time scale of several decades. Plutonium isotope signatures suggest that large amounts of 239,240 Pu were injected into the subtropical western North Pacific as close-in fallout from the Pacific Proving Ground nuclear testing in the early 1950s and spread due to advection and diffusion in the North Pacific during the past five decades. 239,240 Pu inventories from global and close-in fallout were estimated using plutonium isotope ratios. The close-in fallout 239,240 Pu still occupies a significant part of the 239,240 Pu water column inventory in the subtropical North Pacific. The inventory of the global fallout 239,240 Pu has showed a latitudinal distribution with high values in the mid-latitude regions, which gradually decrease to the south. Plutonium in seawater of the North Pacific showed different behavior between the shallower layer (2000 m depth); in the shallower layer, 239,240 Pu shows larger temporal variability as a result of physical processes such as advection and mixing as well as biogeochemical processes, whereas deep 239,240 Pu seems to behave conservatively. Acknowledgements The authors thank the captain and the crew of the R/V Ryofu-maru for on-board water sampling, M. Fukasawa of JAMSTEC for providing water samples collected in the eastern North Pacific on the RV Mirai cruise, and I. Koshino for radiochemical analysis of plutonium. The authors also thank three anonymous reviewers for constructive comments and suggestions. This study has been supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan. The Agency is grateful for the support provided to its Marine Environment Laboratory by the Government of the Principality of Monaco. References Adams, C.E., Farlow, N.H., Schell, W.R. (1960). The compositions, structures and origins of radioactive fall-out particles. Geochimica et Cosmochimica Acta 18, 42–56. Aoyama, M., Hirose, K. (1995). The temporal and spatial variation of 137 Cs concentrations in the western North Pacific and marginal seas during the period from 1979 to 1988. Journal of Environmental Radioactivity 29, 57– 74.


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Aoyama, M., Hirose, K. (2003). Temporal variation of 137 Cs inventory in the Pacific Ocean. Journal of Environmental Radioactivity 69, 107–117. Aoyama, M., Hirose, K. (2004). Artificial radionuclides in the Pacific Ocean – HAM database. The Scientific World Journal 4, 200–215. Bowen, V.T., Noshkin, V.E., Livingston, H.D., Volchok, H.L. (1980). Fallout radionuclides in the Pacific Ocean: Vertical and horizontal distributions, largely from GEOSECS stations. Earth and Planetary Science Letters 49, 411–434. Buesseler, K.O. (1997). The isotopic signature of fallout plutonium in the North Pacific. Journal of Environmental Radioactivity 36, 69–83. Buesseler, K.O., Scholkovitz, E.R. (1987). The geochemistry of fallout plutonium in the North Atlantic: II. 240 Pu/239 Pu ratios and their significance. Geochimica et Cosmochimica Acta 51, 2623–2637. Diamond, H., Fields, P.R., Stevens, C.S., Studier, M.H., Fried, S.M., Inghram, M.G., Hess, D.C., Pyle, G.L., Mech, J.F., Manning, W.M. (1960). Heavy isotope abundances in Mike thermonuclear device. Physical Review 119, 2000–2004. Fowler, S.W., Ballestra, S., LaRosa, J., Fukai, R. (1983). Vertical transport of particulate-associated plutonium and americium in the upper water column of the Northeast Pacific. Deep-Sea Research 30, 1221–1233. Hardy, E.P., Krey, P.W., Volchok, H.L. (1973). Global inventory and distribution of fallout plutonium. Nature 241, 444–445. Harley, J.H. (1980). Plutonium in the environment – A review. Journal of Radiation Research 21, 83–104. HASL (1973). Global atmospheric plutonium-239 and plutonium isotopic ratios for 1959–1970. In: Fallout Program Quarterly Summary Report, HASL-237. US Department of Energy, pp. III-2–III-28. Hicks, H.G., Barr, D.W. (1984). Nevada test site fallout atom ratios: 240 Pu/239 Pu and 241 Pu/239 Pu. Lawrence Livermore National Laboratory, UCRL-53499/1, p. 4. Hirose, K. (1997). Complexation scavenging of plutonium in the ocean. In: Germain, P., Guary, J.C., Guéguéniat, P., Métivier, H. (Eds.), Radionuclides in the Oceans: Input and Inventories. Les Editions de Physique, Les Ulis, pp. 96–97. Hirose, K., Aoyama, M. (2003a). Analysis of 137 Cs and 239,240 Pu concentrations in surface waters of the Pacific Ocean. Deep-Sea Research II 50, 2675–2700. Hirose, K., Aoyama, M. (2003b). Present background levels of 137 Cs and 239,240 Pu concentrations in the Pacific. Journal of Environmental Radioactivity 69, 53–60. Hirose, K., Sugimura, Y. (1985). A new method of plutonium speciation in seawater. Journal of Radioanalytical and Nuclear Chemistry, Articles 92, 363–369. Hirose, K., Igarashi, Y., Aoyama, M., Miyao, T. (2001a). Long-term trends of plutonium fallout observed in Japan. In: Kudo, A. (Ed.), Plutonium in the Environment. Elsevier Science, Amsterdam, pp. 251–266. Hirose, K., Aoyama, M., Miyao, T., Igarashi, Y. (2001b). Plutonium in seawaters of the western North Pacific. Journal of Radioanalytical and Nuclear Chemistry, Articles 248, 771–776. Hirose, K., Miyao, T., Aoyama, M., Igarashi, Y. (2002). Plutonium isotopes in the Sea of Japan. Journal of Radioanalytical and Nuclear Chemistry, Articles 252, 293–299. Hisamatsu, S., Sakanoue, M. (1978). Determination of transuranium elements in a so-called “Bikini Ash” sample and in marine sediment samples collected near Bikini atoll. Health Physics 35, 301–307. Kim, C.S., Kim, C.K., Lee, J.I., Lee, K.J. (2000). Rapid determination of Pu isotopes and atom ratios in small amounts of environmental samples by an on-line sample pre-treatment system and isotope dilution high resolution inductively coupled plasma mass spectrometry. Journal of Analytical Atomic Spectrometry 15, 247–255. Kim, C.S., Kim, C.K., Lee, K.J. (2002). Determination of Pu isotopes in seawater by an on-line sequential injection technique with sector field inductively coupled plasma mass spectrometry. Analytical Chemistry 74, 3824–3832. Koide, M., Michel, R., Goldberg, E.D., Herron, M.M., Langway Jr., C.C. (1979). Depositional history of artificial radonuclides in the Ross ice sheet, Antarctica. Earth and Planetary Science Letters 44, 205–233. Komura, K., Sakanoue, M., Yamamoto, M. (1984). Determination of 240 Pu/239 Pu ratio in environmental samples based on the measurement of LX/X-ray activity ratio. Health Physics 46, 1213–1219. Krey, P.W., Hardy, E.P., Paxhucki, C., Rourke, F., Coluzza, J., Benson, W.K. (1976). Mass isotopic composition of global fall-out plutonium in soil. In: Transuranium Nuclides in the Environment, IAEA-SM-199/39. IAEA, Vienna, pp. 671–678. Livingston, H.D., Anderson, R.F. (1983). Large particle transport of plutonium and other fallout radionuclides to the deep ocean. Nature 303, 228–230.

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Distribution of anthropogenic radionuclides in the water column off Rokkasho, Japan Shigeki Shima* , Shin-ichi Gasa, Ken-ichi Iseda, Tomoharu Nakayama, Hisao Kawamura Japan Marine Science Foundation, Mutsu, Aomori 035-0064, Japan Abstract Anthropogenic iodine-129, carbon-14, plutonium-239,240, strontium-90, caesium-137 and tritium were determined together with oceanographic parameters in the water column off Rokkasho where liquid wastes from a reprocessing plant will be discharged. This sea area is located at the mixing zone of the subtropical and the subarctic waters. The differences in radionuclide concentrations between the subtropical and the subarctic waters are presented and discussed. Except for 129 I, the observed concentrations of anthropogenic radionuclides in the water column offshore of Rokkasho can be explained by global fallout and processes in the water column. However, the atom ratio of iodine to caesium was ten times as high as that of global fallout, therefore contributions from reprocessing plants should be responsible for the observed elevated 129 I levels in the Northwest Pacific. Keywords: Anthropogenic radionuclides, 3 H, 14 C, 129 I, 239,240 Pu, Reprocessing plant, Seawater, Coastal water, Water column, Oceanic observation, Water masses, Rokkasho, Northwest Pacific

1. Introduction The first commercial facility for reprocessing nuclear spent fuel in Japan is to be opened in summer 2006 in Rokkasho, in the north-eastern part of Japan. The facility will routinely release gaseous and liquid wastes containing radionuclides into the surrounding environment. The liquid wastes will be discharged into coastal waters at a distance of 3 km from the coastal line. The Rokkasho area is situated offshore from Sanriku, in the boundary where the subarctic (Oyashio current – OC) and subtropical (Kuroshio current – KC) currents meet (Tomczak and Godfery, 2001). The Tsugaru Warm Current (TWC) flows into this region through the Tsugaru Strait from the Sea of Japan. These three water masses with different origins coexist in the surface layer of this domain. It has been known that there are differences in radionuclide concentrations between the subtropical and the subarctic gyre in the Pacific Ocean (Aoyama * Corresponding author. Address: Japan Marine Science Foundation, 4-24 Minato-machi, Mutsu, Aomori 035-0064, Japan; phone: (+81) 175 22 9111; fax: (+81) 175 22 9112; e-mail: [email protected]

RADIOACTIVITY IN THE ENVIRONMENT VOLUME 8 ISSN 1569-4860/DOI 10.1016/S1569-4860(05)08006-X


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et al., 2001; Hirose and Aoyama, 2003). Therefore, it is important to understand the distribution of anthropogenic radionuclides in the region in advance of the operation of the nuclear reprocessing plant.

2. Oceanographic scope The area investigated in this paper is defined as the area from 39◦ N to 42.5◦ N and from 141◦ E to 143.5◦ E. It includes the Kuroshio/Oyashio mixed water region, where subtropical and subarctic waters meet and interact with each other. Therefore this region is also called the perturbed region (Kawai, 1972). The area is influenced by the TWC, the OC and the KC with several small or meso-scale eddies, including the Kuroshio warm-core ring. The Kuroshio begins west of the Philippines where the North Equatorial Current advects westward, continues northward east of Taiwan, flows along the eastern coast of the Japan Islands, and deviates from Honshu Island around 141◦ E, 35◦ N. The OC is formed by the Okhotsk Sea waters and the Kamchatka Current west of the Kamchatka Peninsula at about 55◦ N. It flows southward just south of Hokkaido Island and splits into two paths called the First and Second Oyashio Intrusion. A branch of the KC flows into the Japan Sea via the Korea Strait and flows out to the Pacific Ocean through the Tsugaru Strait (sill depth about 200 m) as the TWC. The volume transport from the strait varies seasonally, with volume transport of about 2.8 Sv in September and of only 1.2 Sv in March (Toba et al., 1982). This water advects partly southward along Honshu Island (the main island), while another part moves eastward against the advance of the OC. Since the path of the TWC varies seasonally and the nutrient-rich OC meets in this region, the area off Sanriku is known to be complex in oceanographic conditions and currents system, as well as in high fishing activities (Tomczak and Godfery, 2001; Conlon, 1982).

3. Materials and methods 3.1. Sampling and in situ measurements A CTD (SBE 20)/MBS with multi-bottle sampler SBE 32, SeaBirds Inc.) and LVS (largevolume sampler, 120 L Van Dom bottle) were used on October 2001 and June 2002 for water sampling at the stations shown in Fig. 1. In 2001 the stations were visited along the meridional section at 142.5◦ E, and in 2002 along the zonal section at 40.5◦ N, because of the behaviour of the TWC, i.e. within the TWC gyre in autumn, and across the coastal streamline of the TWC in early summer. For each station, surface and bottom water and three intermediate water samples (at 200, 300 and 500 m depths) were collected in order to obtain information on the distribution of anthropogenic radionuclides in this area. For 90 Sr, 137 Cs, and 239,240 Pu analyses, 120 L water samples were acidified with hydrochloric acid (pH = 1). One-litre samples for 14 C analyses were stored in special glass bottles with air-tight cups in order to avoid contamination from the surrounding air. Additionally, during each cruise at about hundred sites, including the water sampling stations, CTD observations were also conducted using the SBE 20 in order to get information on oceanographic conditions in this area.

Distribution of anthropogenic radionuclides in the water column off Rokkasho

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Fig. 1. Location of sampling stations with bathymetry. The maximum depth of all stations is below 1,800 m.

3.2. Analytical methods Tritium in seawater was enriched using a solid polymer electrolyte (SPE) method, and its activity was determined in a low background liquid scintillation counter. Caesium-137 in seawater was pre-concentrated by adsorption on ammonium molybdophosphate (AMP), which was collected by filtration and counted by an HPGe detector with lead shielding. Carbon-14 (as dissolved inorganic carbon, DIC) from seawater was extracted as CO2 by acidification with phosphoric acid. After a purification of CO2 in a vacuum line, a graphite target was prepared, and the carbon isotope ratio was measured in the JAERI accelerator mass spectrometer (AMS) in Mutsu. δ 13 C measurements were carried out using an isotope mass spectrometer. The 14 C activity in seawater samples is expressed by 14 C (h), defined as 14 C = (Fm − 1) × 103 , where Fm (a fraction of modern carbon) is the measured AMS ratio of 14 C to 13 C, normalised to δ 13 C of −25h (Donahue et al., 1990). After recovery of iodine from seawater and its purification by the n-hexane extraction method, silver iodide targets for AMS were prepared. Iodine-129 was measured by AMS at the Isotrace Laboratory, University of Toronto. The 129 I concentrations are expressed in 106 atoms/L. Strontium in seawater was separated and purified by an ion exchange method and the activity of 90 Sr was determined by counting the β-rays emitted by its 90 Y daughter in radioactive equilibrium using a low background gas flow counter. Plutonium isotopes were separated by co-precipitation from seawater, purified by use of an anion exchange column, and electroplated onto a stainless steel disc. 238 Pu and 239,240 Pu activities were measured by α-spectrometry. Measured activity concentrations of radionuclides (except 14 C and 129 I) are expressed in mBq/L.

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4. Results and discussion 4.1. Oceanographic observations Vertical profiles of temperature and salinity above a water depth of 700 m varied seasonally and spatially (Fig. 2), however, below the 700 m water depth they were almost the same during sampling periods. Intrusion of water masses with lower temperatures and salinities appeared in the upper layer, as shown in Fig. 2. Salinity in the upper water layer of station 01W-1 was the lowest amongst all the stations. The highest salinity above the depth of 80 m was found at station 02W-1. The density (sigma-t) profiles also largely fluctuated in the upper layer, since they depend on in situ temperature and salinity. Moreover, the complexity of oceanographic conditions was inferred from a large difference (∼200 m) in water depths that have the same density in the upper layer. In addition to large vertical variations, the horizontal distributions of temperature and salinity were also complicated. For example, the distribution of temperature at 200 m depth is shown in Fig. 3. In June, the warmer TWC distributes along the coastal side of the Japanese main island and the colder Oyashio occupied the greater part of this area. The warmer water mass around latitude 41◦ N and longitude 143◦ E seems to derive from the warm core separated from the Kuroshio in the Kuroshio Extension region (Japan Coastal Guard, 2002). Although the TWC widely spread in this region in October, the expanse of the TWC is smaller than usual (Shima et al., 2000). The intrusion of the OC was recognised in the middle zone of this area. The distributions of temperature and salinity were also reflected in the

Fig. 2. Vertical profiles of temperature, salinity and density (sigma-t).


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Fig. 3. Horizontal distribution of temperature at the depth of 200 m.

Table 1 Definition of water masses off Sanriku Water mass

Definition

TWC Kuroshio Oyashio Deep water Coastal Oyashio Surface water

33.7 S < 34.2, T 5 and σT 24 S > 34.2 and σT < 26.7 S > 33, T < 7 and σT < 26.7, except for T > 5 and S 33.7 σT > 26.7 > T , except for T > 5 and 33.7 S < 34.2 S < 33 and T < 2 Other than the above

water density. The difference in water depths having the same density reached hundreds of meters. Compared with the horizontal distributions of temperature, the isobathymetric line of the same density is deeper in the warm region than that in the colder one. It is well known that the TWC flows along the Japanese coastal line in winter to spring (called coastal mode) and forms a clockwise gyre in summer to autumn (called gyre mode) (Conlon, 1982; Hishida, 1987). The observed horizontal distributions of temperature appeared to reflect the characteristic flow pattern of the currents. The classification of water masses in the area was done following the objectives of research (Hanawa and Mitsudera, 1986; Yasuda et al., 1988; Inagake and Saitoh, 1998). In this paper, Hanawa’s criterion was adopted for classifying the water system distribution in the upper layer offshore of Sanriku (Table 1). According to this criterion, thirty large volume water samples were collected in these systems: TWC (6 samples), the Oyashio (4), the deep water (16), the

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surface water (3), and the Kuroshio (1). The classification of all sampling stations is shown in Figs. 4a, 4b, 4c. It can be seen that the TWC, the surface water, and the Kuroshio dominated over the Oyashio in this region. 4.2. Distribution of radionuclides 4.2.1. Carbon-14 As shown in Fig. 4a, 14 C values in surface waters were in excess to the modern carbon ratio, from 45 to 75h, except for station 01W-1 (−14h). The average value of 14 C in surface waters in the North Pacific Basin obtained from the WOCE project was 75h. The values in the western subarctic zone were lower than those in the subtropical one (Key et al., 2002). As the 14 C values in the subarctic gyre (45–50◦ N, 165◦ E in P13N line of the WOCE) are negative, the water with negative 14 C value observed in the subsurface of station 01W-1 must be of Oyashio origin. Vertical profiles of 14 C decreased with depth rapidly and monotonically, except for near the sea surface. In stations 01W-2, 01W-3 and 02W-3 where the mixing layer

Fig. 4a. Vertical profile of 14 C in the water column. The box plot on the right shows the water masses as classified in this paper.

Fig. 4b. Vertical profile of 129 I in the water column. The box plot on the right shows the water masses as classified in this paper.


89

Fig. 4c. Vertical profile of 239,240 Pu in the water column. The box plot on the right shows the water masses as classified in this paper.

Fig. 5. Plot of 14 C vs. density.

reached down to 200 m in depth, the 14 C values in surface waters were almost the same, or less than those at a depth of 200 m. Although the vertical profiles of 14 C in each cruise are divided into two groups, relationships between 14 C and σT (sigma-t) show no difference between the 14 C profiles. As shown in Fig. 5, the 14 C values are within uncertainties the same for densities of (24.5–26.5)σT , except for the value in surface water of station 01W-1. Most of the bomb-produced 14 C remains in this area in shallow waters with density < 26.5σT . Recently, instead of the SiO2 –14 C relationship (Broecker et al., 1995), a new method has been proposed to infer natural 14 C in the ocean (Rubin and Key, 2002). However, as alkalinity data are required, we could not use this method, as we do not have such data available for each cruise. Using Broecker’s relationship between 14 C and dissolved SiO2 , the bomb-produced 14 C has penetrated to an isopycnal of about 27.0.

90

S. Shima et al.

The average values of 14 C for each water mass are (40.7 ± 37.9)h for the TWC, (38.3 ± 36.2)h for the Oyashio and (105 ± 82.4)h for the deep water (see Table 1). Although there are large uncertainties in these values, the differences between water masses have been recognised. These differences in water masses offshore of Rokkasho may disappear due to disturbances in mixing at the outlet of the Tsugaru Strait, as well as due to high primary production. 4.2.2. Iodine-129 Concentrations of 129 I in surface waters are from (17 ± 4) × 106 to (23 ± 4) × 106 atoms/L, and it looks like there are not temporal and spatial variations similar to that observed for 14 C. Similar concentrations have been observed in surface waters of the Japan Sea ((12–31) × 106 atoms/kg) and in coastal water of Vladivostok harbour (58×106 atoms/kg; Cooper et al., 2001). Concentrations in the subtropical zone of the Northwest Pacific were similar (Povinec et al., 2000). The surface concentrations around Japan are almost two orders of magnitude lower than those in the Barents and Kara Seas in the Arctic, where European nuclear fuel reprocessing effluents dominate the 129 I signal (Smith et al., 1998). As mentioned later in discussion of activity ratios, the observed 129 I concentrations in the water column may be higher than that expected from global fallout. The vertical profiles of 129 I show rapid decrease with depth, as observed for 14 C, although the inversion of the 129 I concentration slightly appears in the upper layers (Fig. 4b). The 129 I concentrations of bottom water below 1,400 m depth were found to be under the detection limit. In the Pacific and the Sea of Japan, 129 I was found in deep waters, at several thousand meters. The observed 129 I profiles may be caused by shallower bathymetry, or differences in circulation of aged deep water (Schlosser et al., 2001). From the plot of 129 I concentrations as a function of sigma-t, the concentrations rapidly decrease below the isopycnal of about 26.7. The water depth of this isopycnal surface corresponds to 100–400 m. The plot of 129 I vs. 14 C shows that the 129 I concentration is decreasing with the water age (Fig. 6). The straight line in this figure is the 129 I concentration expected from global fallout. The difference between the observed and expected 129 I values suggests that another source of 129 I may influence its concentration offshore of Rokkasho. The average concentrations of 129 I in each water system are (15.8 ± 1.9) × 106 , (12.0 ± 2.6) × 106 and (6.8 ± 5.7) × 106 atoms/L for the TWC, Oyashio and deep waters, respectively. The 129 I concentration in the TWC is slightly higher than that in the Oyashio. If iodine in the ocean behaves similarly as radiocarbon, the 129 I concentration may be higher in the subtropical than in the subarctic zone. 4.2.3. Plutonium-239,240 The concentrations of 239,240 Pu in surface water were under the limit of detection (5108, 3100, 100 and 3.2 Bq m−2 , respectively. Our values are in the high end of the earlier reported values (Hölgye and Filgas, 1995; Lee et al., 1996). The vertical distributions of 239+240 Pu and 137 Cs in the soil column are similar, the highest massic activities are at the surface, and remain nearly constant to the depth of 12 cm (Fig. 10). Then their massic activities decrease significantly, and at around 20 cm below the surface they are below our detection limits. The depth distribution pattern for 90 Sr shows a smoother decrease with depth, probably due to its losses at the surface. 210 Pb massic activity was also measured in the soil core to elucidate the retention behavior of these fallout radionuclides in the soil, since the excess 210 Pb is also supplied from the atmosphere into the soil. The downcore distribution of excess 210 Pb is very similar to that of 137 Cs and 239+240 Pu (Fig. 10). The 210 Pb concentration decreases logarithmically with depth, and it approaches to the supported level at approximately 12 cm below the surface. Therefore, the penetration rate of these radionuclides is estimated to be larger than 0.2 cm yr−1 . The apparent down core logarithmic decrease of the excess 210 Pb suggested that an apparently undis-

Fig. 10. Soil depth profiles of 90 Sr, 137 Cs, 239+240 Pu, and 210 Pb at the mid-western part of the Korean Peninsula (Kwangrung Arboretum, 37◦ 44 N, 127◦ E).

110

G.H. Hong et al.

turbed soil core sample was retrieved from the Kwangrung National Arboretum. Atmospheric 210 Pb flux was estimated by multiplying the decay constant of 210 Pb and the excess 210 Pb inventory (Nozaki et al., 1978). The estimated atmospheric 210 Pb flux calculated from the soil profile (Fig. 1) is 20 mBq cm−2 yr−1 , which is about 2/3 of the regional atmospheric 210 Pb flux (Nozaki et al., 1973). The denudation rate in this drainage basin, estimated using the total river discharge and suspended matter concentration, yields 0.047 g cm−2 yr−1 (Hong et al., 2002). If we assume the soil is eroded from the top of the soil, then, the annual 210 Pb removal flux would be 12 mBq cm−2 yr−1 , that could account for the remaining 1/3 of the regional atmospheric 210 Pb flux, although this estimate is based on only one soil core. However, this estimate will result unusually shorter drainage residence time for 210 Pb. For the sake of further discussion, we assume that the soil inventories of artificial radionuclides may be underestimated by as much as 1/3 of the total inventory in the region. 3.3. River input A few data were collected for the dissolved concentrations of 90 Sr, 137 Cs and 239+240 Pu of the major Korean rivers flowing into the Yellow Sea in 1999 and 2001. The range of values reported for the activity concentrations of 90 Sr, 137 Cs and 239+240 Pu were 2.8–4.3, 0.25–0.68 mBq l−1 , and 2.7–3.1 µBq l−1 , respectively (Table 4). Similar values for 90 Sr and 137 Cs were also observed in Ebro River in Spain (Pujol and Sanchez-Cabeza, 2000), but lower than in other measurements in North America (Cornet et al., 1995). Although the available data set is rather limited, a preliminary estimation on the riverine input of artificial radionuclides from the Korean Peninsula was carried out using the river discharge rate and the average dissolved activity concentrations and surface soil concentrations of artificial radionuclides (Table 5). The sum of the discharge from major rivers in South Korea (Han, Keum, Youngsan, Anseoung, Sapkyo, Mankyoung, and Dongjin) into the Yellow Sea is estimated to be 32.9 × 109 m3 yr−1 of water and 20.1 × 106 t yr−1 of suspended particulate matter. Furthermore, if we include all the major rivers from the China (Huanghe, Changjiang, and Aprock), then 217.7 × 109 m3 yr−1 of water and 504.9 × 106 t yr−1 of suspended particulate matter Table 4 Activity concentrations of artificial radionuclides in rivers discharging into the Yellow Sea. All quoted uncertainties are 1 sigma standard deviations River

Sampling date

River discharge∗ (m3 s−1 )

90 Sr

137 Cs (mBq l−1 )

Han

January 1999 April 1999 June 1999 September 1999 December 1999 June 2001 June 1999 January 2001

377.4 2,405.5 1,749.2 418.4 99.6 353.4 884.9 18.3

3.39 ± 0.10 2.31 ± 0.07 3.03 ± 0.11 3.04 ± 0.09 2.85 ± 0.07 4.33 ± 0.14 3.00 ± 0.07 3.20 ± 0.09

0.46 ± 0.16 0.68 ± 0.13

Keum Youngsan

(mBq l−1 )

239+240 Pu (µBq l−1 )

2.70 ± 0.16 3.07 ± 0.18

0.25 ± 0.13 0.32 ± 0.10

∗ Han River Flood Control Service, Ministry of Construction and Transportation, Republic of Korea.

2.94 ± 0.17 3.04 ± 0.18

Artificial radionuclides in the Yellow Sea: Inputs and redistribution

111

Table 5 Riverine discharge of artificial radionuclides into the Yellow Sea from the south Korean Peninsula Riverine flux (S. Korean rivers)

90 Sr (109 Bq yr−1 )

137 Cs (109 Bq yr−1 )

239+240 Pu (109 Bq yr−1 )

Dissolved flux Particulate flux Total

103 104 208

60 1202 1262

0.10 24.1 24.2

(Hong et al., 2002) would additionally be flowing in to the Yellow Sea. The contribution from the Changjiang was taken to be 14% of the total discharge as estimated by Liu et al. (2003). The large excess of 239+240 Pu over the global fallout in the Yellow Sea was also attributed to the Changjiang River by Nagaya and Nakamura (1992). Most of the 239+240 Pu in river water is likely associated with particulate matter. For example, in the Rhone River, France, ∼80% of plutonium isotopes were found to be associated with the suspended material, and ∼20% was found to be exported as dissolved species (Eyrolle et al., 2004). 3.4. Seawater The dissolved activity concentrations of 90 Sr, 137 Cs and 239+240 Pu in the southeastern part of the Yellow Sea in the period of 1994–2000 are reported in Table 6. The average surface activity concentrations of 90 Sr, 137 Cs and 239+240 Pu in the Yellow Sea in 1999–2000 were found to be 2.1 ± 0.2, 2.7 ± 0.2 mBq kg−1 , and 4.2 ± 1.2 µBq kg−1 , respectively. The Yellow Sea is being fed from the East China Sea including a branch of Kuroshio Current originating from the western North Pacific subtropical gyre, and it is diluted with freshwater discharges from rivers in summer. Therefore, salinity in surface water of the Yellow Sea is highest in winter and lowest in autumn (Figs. 11–14). The temporal change in water characteristics was also reflected in the activity concentration of these artificial radionuclides. The average activity concentration of 90 Sr was higher in winter–spring (2.1 ± 0.4 and 2.4 ± 0.7 mBq kg−1 in February 1999 and April 2000, respectively) than summer–autumn (1.9 ± 0.3 and 2.0 ± 0.3 mBq kg−1 in August 1999 and September 2000, respectively). We observed a weak correlation of 90 Sr activity concentration and salinity (90 Sr (mBq kg−1 ) = 12.655–0.324 × Salinity, r = −0.523, P > 0.01), which may suggest that river waters are the main source of 90 Sr flowing into the Yellow Sea. The activity ratios of 137 Cs/90 Sr in the Yellow Sea varied from 1.1 to 1.5, showing lower values than that of the global fallout (1.6), which further suggest continuing terrestrial sources. However, there were no discernible relationship for 137 Cs and 239+240 Pu with respect to salinity in the Yellow Sea. The water column in the Yellow Sea is vertically well mixed during winter and stratified in summer. In general, 90 Sr activity concentrations were generally higher in surface water than in bottom water in winter (February 1999, Table 6) and 137 Cs and 239+240 Pu activity concentrations were lower in the surface water than in the bottom water (February 1999 and April 2000, Table 6), although the Yellow Sea is well mixed in winter and early spring period (Hong et al., 1999b). In summer, 239+240 Pu activity concentrations increased with depth more

112

Table 6 Activity concentrations of dissolved 90 Sr, 137 Cs, 239+240 Pu in the Yellow Sea (1994–2000). All quoted uncertainties are 1 sigma standard deviations Date

02-Jul-94 02-Jul-94 12-Dec-93 12-Dec-93 12-Dec-93 02-Jul-94 01-Jul-94 01-Jul-94 29-Jun-94

February 24– March 2, 1999

YS9406 S9406 CO9312 CO9312 CO9312 YS9406 YS9406 YS9406 YS9406

Stn.

Coordinates

Water depth (m)

Sampling depth (m)

90 Sr

Temp. (◦ C)

Sal.

0 0 0 0 0 0 0 0 0

22.28 20.47 13.11 14.33 17.71 21.28 19.88 22.70 20.84

32.70 32.42 32.87 33.11 34.27 32.57 32.32 31.87 31.11

3.29 ± 0.04 3.79 ± 0.06

(mBq kg−1 )

137 Cs (mBq kg−1 )

239+240 Pu (µBq kg−1 )

3.06 ± 0.25 2.93 ± 0.21 2.74 ± 0.20 2.74 ± 0.18 3.21 ± 0.21 3.78 ± 0.22 2.65 ± 0.22 2.72 ± 0.20 2.21 ± 0.16

2.3 ± 0.4 5.2 ± 0.6 8.1 ± 0.6 6.5 ± 0.6 12.7 ± 1.1 5.8 ± 0.6 5.3 ± 0.8 5.3 ± 0.6 4.1 ± 0.6

Latitude

Longitude

A3 A5 D2 D3 D5 B4 C1 C5 D3

33◦ 52.42 N 35◦ 07.45 N 34◦ 10.00 N 34◦ 00.00 N 33◦ 40.00 N 34◦ 30.53 N 33◦ 30.08 N 33◦ 59.94 N 33◦ 30.18 N

124◦ 00.00 E 125◦ 00.16 E 126◦ 00.00 E 126◦ 00.00 E 126◦ 00.00 E 124◦ 30.01 E 122◦ 30.18 E 124◦ 30.01 E 125◦ 59.90 E

R 3 4 5 6 8 11 12

37◦ 22 N 37◦ 19 N 37◦ 19 N 37◦ 17 N 37◦ 12 N 37◦ 09 N 37◦ 15 N 37◦ 18 N

126◦ 14 E 125◦ 55 E 125◦ 59 E 125◦ 58 E 125◦ 47 E 125◦ 57 E 126◦ 01 E 126◦ 05 E

20 34 9 10 46 35 43 33

0 0 0 0 0 0 0 0

19.6 16.9 16.96 17.1 17.7 17.2 17.2 17.5

31.55 31.68 31.74 31.75 31.92 31.77 31.75 31.85

2.61 ± 0.07 2.66 ± 0.08 2.47 ± 0.06 2.68 ± 0.15 2.76 ± 0.11 2.47 ± 0.06 2.47 ± 0.05

2.85 ± 0.11 2.65 ± 0.12 2.62 ± 0.18 2.81 ± 0.18 2.84 ± 0.10 2.71 ± 0.11 2.94 ± 0.16 2.57 ± 0.17

B1 B 1 B2

36◦ 54.9 N 36◦ 44.1 N 37◦ 07 N

126◦ 00 E 126◦ 00 E 125◦ 30 E

63 64 43

B 2 B3 B4 B5

36◦ 24.7 N 37◦ 00 N 36◦ 59.9 N 37◦ 0.12 N

125◦ 0.28 E 125◦ 00 E 124◦ 30.1 E 124◦ 0.35 E

35 54 80 80

C1 C 1

36◦ 00 N 35◦ 29.7 N

126◦ 27 E 124◦ 00 E

19 82

0 0 0 38 0 0 0 0 75 0 0

4.22 5.11 3.50 3.55 5.72 6.24 7.35 7.88 7.89 5.30 8.67

31.87 32.06 31.88 31.89 32.17 32.50 32.38 32.40 32.39 32.14 32.53

2.22 ± 0.07 2.46 ± 0.06 3.00 ± 0.08 1.79 ± 0.06 2.01 ± 0.08 2.39 ± 0.05 2.37 ± 0.03 2.60 ± 0.10 1.89 ± 0.04 2.08 ± 0.07 1.97 ± 0.12

2.29 ± 0.18 2.50 ± 0.23 2.36 ± 0.18 3.18 ± 0.29 2.36 ± 0.22 2.94 ± 0.24 2.75 ± 0.22 2.79 ± 0.18 3.12 ± 0.22 3.01 ± 0.21 2.39 ± 0.14

2.93 ± 0.04 3.37 ± 0.09 2.89 ± 0.05 2.74 ± 0.06

G.H. Hong et al.

July 4–6, 1995

Cruise

Table 6 (Continued) Date

Stn.

Coordinates Latitude

Longitude

Water depth (m)

C2 C3

36◦ 00 N 36◦ 00 N

126◦ 17.5 E 126◦ 00 E

27 43

C4 C5 C6

35◦ 59.9 N 36◦ 00 N 36◦ 00 N

125◦ 30.1 E 124◦ 59.7 E 124◦ 30.1 E

55 80 85

C7 D5 E2

36◦ N 35◦ 0.36 N 34◦ N

124◦ E 124◦ E 126◦ E

77 84 80

E3 E4 E5 E6 F1

34◦ N 33◦ 59.8 N 34◦ N 34◦ N 33◦ 35 N

125◦ 31.05 E 124◦ 59.6 E 124◦ 30 E 124◦ E 125◦ E

64 98 82 81 89

F2

33◦ 0.2 N

124◦ 15.1 E

65

A2 B1 B3

37◦ 25 N 36◦ 44.1 N 37◦ N

26◦ 10 E 126◦ 00 E 125◦ E

23 75 54

B5

37◦ N

124◦ E

80

90 Sr (mBq kg−1 )

137 Cs

239+240 Pu

(mBq kg−1 )

(µBq kg−1 )

32.19 32.37 32.37 32.45 32.52 32.48 32.51 32.51 32.55 32.69 34.05 32.64 32.67 32.81 33.04 33.92 34.00 33.67

2.19 ± 0.12 2.31 ± 0.08 1.94 ± 0.11 2.58 ± 0.18 1.50 ± 0.13 2.54 ± 0.13 2.04 ± 0.07 2.50 ± 0.07 2.40 ± 0.09 1.88 ± 0.05 1.56 ± 0.06 1.98 ± 0.06 2.01 ± 0.09 1.95 ± 0.06 1.47 ± 0.13 1.93 ± 0.09 1.88 ± 0.10 1.55 ± 0.08

2.98 ± 0.21 2.65 ± 0.19 2.62 ± 0.37 2.52 ± 0.19 2.65 ± 0.21 2.94 ± 0.23 2.47 ± 0.23 2.64 ± 0.22 3.01 ± 0.20 2.84 ± 0.18 2.50 ± 0.23 2.83 ± 0.26 2.41 ± 0.18 3.38 ± 0.22 2.11 ± 0.22 2.69 ± 0.29 2.67 ± 0.23 1.78 ± 0.24

27.13 31.16 31.33 31.34 31.82 32.14 31.93 32.45 32.40 32.40

1.98 ± 0.06 1.50 ± 0.07 2.10 ± 0.08 1.52 ± 0.06 1.72 ± 0.05 1.70 ± 0.05 2.24 ± 0.10 1.86 ± 0.06 2.22 ± 0.09 2.68 ± 0.10

2.25 ± 0.20 2.42 ± 0.21 2.88 ± 0.28 3.13 ± 0.30 2.08 ± 0.16 2.95 ± 0.27 2.14 ± 0.20 3.41 ± 0.39 3.34 ± 0.52 2.73 ± 0.16

Sampling depth (m)

Temp. (◦ C)

Sal.

0 0 39 0 0 0 80 0 0 0 75 0 0 0 0 0 84 0

5.74 6.41 6.33 6.34 7.12 7.59 7.57 8.30 8.45 8.71 12.65 8.32 9.36 10.14 10.51 12.35 12.62 11.98

0 0 0 10 30 49 0 20 40 75

22.84 20.39 24.90 22.39 16.95 11.70 26.14 17.39 9.25 9.25

2.61 ± 1.02 2.65 ± 0.79 4.18 ± 1.66 1.25 ± 0.89 5.52 ± 1.77 3.04 ± 0.63 5.76 ± 1.18 5.64 ± 0.80 3.46 ± 1.04


August 7–17, 1999

Cruise

113

114


Stn.

137 Cs (mBq kg−1 )

239+240 Pu (µBq kg−1 )

30.23 31.57 31.64 31.51 32.48 32.07 32.16 32.66 33.00 31.66 31.66 32.54 32.54 31.72 32.06 32.56 32.55 30.53

2.11 ± 0.09 1.66 ± 0.05 1.62 ± 0.05 1.83 ± 0.05 1.80 ± 0.05 1.57 ± 0.08 2.03 ± 0.07 1.75 ± 0.07 2.13 ± 0.06 1.80 ± 0.06 1.77 ± 0.06 1.72 ± 0.07 1.94 ± 0.07 1.18 ± 0.03 1.58 ± 0.06 1.67 ± 0.08 2.12 ± 0.08 2.09 ± 0.09

2.69 ± 0.16 2.84 ± 0.21 2.49 ± 0.22 2.86 ± 0.25 2.98 ± 0.17 2.48 ± 0.20 2.89 ± 0.25 2.92 ± 0.22 2.72 ± 0.19 2.06 ± 0.15 2.70 ± 0.39 2.98 ± 0.29 3.40 ± 0.42 1.90 ± 0.16 2.40 ± 0.31 2.18 ± 0.22 2.96 ± 0.19 2.26 ± 0.20

2.03 ± 0.56 3.13 ± 1.41 2.06 ± 0.89 7.40 ± 2.40 3.61 ± 0.65 2.48 ± 0.44 3.84 ± 0.56 2.36 ± 0.47 6.04 ± 1.01 3.70 ± 1.16 3.59 ± 1.25 4.33 ± 0.66 11.29 ± 3.48 3.39 ± 1.37 2.50 ± 0.91 6.90 ± 1.20 7.17 ± 1.17 3.58 ± 0.78

31.62 32.05 32.36 31.30 31.92 32.46 32.47 32.13 32.13 32.13

2.04 ± 0.35 3.59 ± 0.39 2.02 ± 0.29 3.55 ± 0.50 2.34 ± 0.31 2.63 ± 0.17 2.01 ± 0.23 2.78 ± 0.14 2.65 ± 0.16 1.84 ± 0.23

2.12 ± 0.18 2.06 ± 0.12 2.33 ± 0.16 2.44 ± 0.22 2.25 ± 0.15 1.62 ± 0.09 2.70 ± 0.23 1.90 ± 0.20 2.27 ± 0.26 2.59 ± 0.15

2.75 ± 0.43 2.33 ± 0.52 4.02 ± 042 2.67 ± 0.43 3.41 ± 0.54 5.43 ± 0.63 3.62 ± 0.35 8.45 ± 0.96 7.99 ± 0.98

Sampling depth (m)

Temp. (◦ C)

C1 C5

36◦ N 36◦ N

126◦ 27 E 124◦ 30.1 E

19 80

C7

36◦ N

124◦ E

77

D4

35◦ N

124◦ 30 E

92

E4

34◦ N

124◦ E

98

F1

33◦ 35 N

125◦ E

89

0 0 20 40 75 0 20 40 70 0 20 50 87 0 20 75 93 0

24.54 25.83 24.52 6.55 8.49 26.17 24.52 10.23 10.10 25.06 25.03 8.78 8.74 24.84 22.55 8.98 8.93 25.38

B 1

36◦ 44.1 N 37◦ N 36◦ 59.9 N 36◦ N 36◦ N 36◦ N 36◦ N 35◦ N

126◦ 00 E 125◦ E 124◦ 30.1 E 126◦ 27 E 126◦ E 124◦ 30.1 E 124◦ E 125◦ 43 E

64 54 80 19 38 85 76 21

0 0 0 0 0 0 0 0 10 20

6.22 7.12 7.89 10.38 7.59 8.45 8.45 8.40 8.37 8.16

B3 B4 C1 C3 C6 C7 D1

Coordinates

(mBq kg−1 )

Longitude

Water depth (m)

Sal.

Latitude

90 Sr

G.H. Hong et al.

April 10–22, 2000

Cruise


Cruise

Stn.

Coordinates Longitude

D2

35◦ N

125◦ 30 E

86

D3

35◦ N

125◦ E

87

D4

35◦ N

124◦ 30 E

92

D5

35◦ 0.36 N

124◦ E

84

124◦ E 124◦ 15.1 E 124◦ 15.1 E

81 65 0

E6 34◦ N F2 33◦ 0.2 N DEACHUNG-DO 33◦ 0.2 N September 14–27, 2000

B1 B3 B5

36◦ 54.9 N 37◦ N 37◦ 0.12 N

126◦ E 125◦ E 124◦ 0.35 E

63 54 80

C1 C5 C7

36◦ N 36◦ N 36◦ N

126◦ 27 E 124◦ 59.7 E 124◦ E

19 80 77

D1 D5 E3 E4

35◦ N 35◦ 0.36 N 34◦ N 33◦ 59.8 N

125◦ 43 E 124◦ E 125◦ 31.05 E 124◦ 59.6 E

74 84 64 98

E6

34◦ N

124◦ E

81

90 Sr

137 Cs

239+240 Pu

(mBq kg−1 )

(mBq kg−1 )

(µBq kg−1 )

32.15 32.24 32.34 32.60 − 32.60 32.59 32.57 32.57 32.75 32.43

2.81 ± 0.13 2.21 ± 0.22 1.95 ± 0.22 1.83 ± 0.05 1.73 ± 0.15 1.72 ± 0.04 2.54 ± 0.13 1.61 ± 0.12 1.59 ± 0.12 3.96 ± 0.35 2.52 ± 0.12 2.73 ± 0.09

1.96 ± 0.14 2.69 ± 0.18 2.34 ± 0.13 5.15 ± 0.25 2.08 ± 0.12 2.57 ± 0.15 2.28 ± 0.11 2.95 ± 0.18 2.23 ± 0.13 1.95 ± 0.12 1.95 ± 0.12 2.53 ± 0.23

5.76 ± 2.06 4.88 ± 0.70 6.20 ± 0.66 5.17 ± 0.64 4.09 ± 0.60 3.61 ± 0.68 6.02 ± 0.86 4.53 ± 1.15 8.13 ± 1.42 4.11 ± 0.68 2.61 ± 0.31

30.85 30.52 31.82 32.31 24.69 31.22 31.87 32.41 31.47 31.46 31.90 32.00 32.90 32.05

2.16 ± 0.08 2.10 ± 0.05 2.38 ± 0.07 2.09 ± 0.08 1.68 ± 0.09 1.77 ± 0.07 1.92 ± 0.09 2.13 ± 0.08 1.97 ± 0.09 2.53 ± 0.09 1.67 ± 0.09 1.90 ± 0.08 1.48 ± 0.10 2.03 ± 0.09

3.36 ± 0.16 3.03 ± 0.13 2.77 ± 0.12 3.50 ± 0.22 2.27 ± 0.14 3.08 ± 0.09 2.94 ± 0.07 4.03 ± 0.17 2.65 ± 0.15 2.43 ± 0.12 1.35 ± 0.08 2.65 ± 0.13 2.87 ± 0.09 2.70 ± 0.14

2.33 ± 0.34 2.78 ± 0.72 4.04 ± 0.59

Sampling Temp. depth (◦ C) (m)

Sal.

0 20 40 0 82 0 87 0 79 0 0 0

7.66 7.52 6.98 8.31 − 8.49 7.59 8.66 8.66 9.52 10.94

0 0 0 75 0 0 0 72 0 0 0 0 0 0

21.05 21.08 21.61 8.26 21.74 21.01 21.15 7.21 20.47 20.66 23.16 22.99 11.39 22.50

1.76 ± 0.33 3.46 ± 0.55 2.78 ± 0.43 5.23 ± 0.96 3.91 ± 0.65 2.55 ± 0.30 2.04 ± 0.32 2.21 ± 0.32


Latitude

Water depth (m)

2.06 ± 0.39 115

116

G.H. Hong et al.

Fig. 11. Distribution of temperature (T , ◦ C), salinity, 90 Sr (mBq kg−1 ) and 137 Cs (mBq kg−1 ) in the surface water of the Yellow Sea in February 1999.

than 137 Cs, while 90 Sr were stable, especially below the thermocline (Fig. 15), probably due to the particle scavenging in the surface and subsequent sinking and regeneration in the bottom layer of the sea. These nutrient-like features and similar 239+240 Pu activity concentrations at corresponding depths were found in the nearby East Sea (Sea of Japan) as well (Lee et al., 2003a). In general, spatial distribution patterns of radionuclides in the Yellow Sea were not pronounced for most of the seasons probably due to the presence of many rivers at various locations along the entire coast of the sea and active mixing of the water column due to its shallow depth (54 >0.46

5 22

433 150

27 320

1 7 7

>0.62 0.35 0.06

16 38 54

100 117 100

88 112 78


Table 8 (Continued)

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Fig. 22. 210 Pb-derived sediment accumulation rate (mg cm−2 yr−1 ) in the Yellow Sea.

ported for their inventories and penetration depths in coastal sediments (Carpenter et al., 1987; Baskaran and Naidu, 1995). The 210 Pb-derived sediment accumulation rates revealed higher sediment accumulation rates along the coastal areas and lower sedimentation rates in the middle of the Yellow Sea (Fig. 22). They varied from 20 mg cm−2 yr−1 (0.02 cm yr−1 ) to 1590 mg cm−2 yr−1 (1.0 cm yr−1 ) with a geometric mean of 106 mg cm−2 yr−1 and the geometric standard deviation of 2.51 (Table 8). These values are similar to those obtained in previous studies (Alexander et al., 1991). Although the sediment accumulation rate varies a great deal in a relatively small area, a preliminary estimate of the sediment burial fluxes of particulate 90 Sr, 137 Cs and 239+240 Pu, based on the average 210 Pb-derived sediment accumulation rate, the average surface sediment deposition rates of 90 Sr, 137 Cs and 239+240 Pu for the entire Yellow Sea were found to be about 1.7 ± 1.3, 2.5 ± 2.2 and 0.3 ± 0.2 Bq m−2 yr−1 , respectively. However, high sedimentation regions of the Chinese coastal area were not included in this exercise, therefore the current estimate should be regarded as a minimum one. 3.6. Oceanic boundary conditions In order to compare levels of 90 Sr, 137 Cs and 239,240 Pu in the Yellow Sea (Table 6), oceanic boundary conditions were also determined in the Korea (Tsushima) Strait, East China Sea (Table 9) and the tropical Northwestern Pacific Ocean (Table 10) in 1993 and 1994. The average dissolved 90 Sr activity concentrations were 3.17 ± 0.39, 2.57 ± 0.48, 2.58 ± 0.32 mBq kg−1 in the Yellow, East China Sea and Tropical Pacific, respectively. The average dissolved 137 Cs activity concentrations were 2.89 ± 0.44, 2.83 ± 0.34, 2.56 ± 0.24 mBq kg−1 in the Yellow,


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Table 9 Surface water distribution 90 Sr, 137 Cs, and 239+240 Pu activity concentrations in the Korea (Tsushima) Strait and East China Sea in December 1993 to September 1994. All quoted uncertainties are 1 sigma standard deviations Cruise

Stn.

Location

Date

Temp. Sal. (◦ C)

90 Sr

137 Cs

Latitude

Longitude

CO9312 A2 A4 A6 A8 A10

Korea Strait 34.4120◦ N 34.1862◦ N 33.9602◦ N 33.7343◦ N 33.5083◦ N

128.4590◦ E 128.6445◦ E 128.8297◦ E 129.0148◦ E 129.2000◦ E

06-Dec-93 06-Dec-93 06-Dec-93 06-Dec-93 07-Dec-93

14.61 17.67 18.70 20.31 19.05

33.42 33.87 34.11 34.58 34.30

3.34 ± 0.20 3.54 ± 0.30 3.20 ± 0.24 3.47 ± 0.15 2.26 ± 0.18

CO9312 CO9312 CO9312 CO9312 CO9312 CO9312 CO9312 CO9312 YS9406 YS9406 YS9406 CO9408 CO9408 CO9408 CO9408 CO9408

East China Sea∗∗ 30◦ 30.00 N 127◦ 0.00 E 30◦ 00.00 N 128◦ 20.00 E 33◦ 20.00 N 126◦ 00.00 E 33◦ 00.00 N 126◦ 00.00 E 31◦ 30.00 N 126◦ 00.00 E 31◦ 30.00 N 127◦ 20.00 E 31◦ 30.00 N 128◦ 20.00 E 31◦ 30.00 N 129◦ 20.00 E 31◦ 59.98 N 123◦ 00.08 E 32◦ 14.94 N 124◦ 00.07 E 32◦ 41.24 N 125◦ 45.08 E 32◦ 00.12 N 125◦ 59.83 E 29◦ 18.51 N 125◦ 15.14 E 28◦ 37.59 N 128◦ 00.04 E 29◦ 56.37 N 126◦ 44.95 E 29◦ 22.54 N 129◦ 00.11 E

10-Dec-93 10-Dec-93 12-Dec-93 12-Dec-93 08-Dec-93 08-Dec-93 08-Dec-93 08-Dec-93 30-Jun-94 30-Jun-94 30-Jun-94 05-Sept-94 03-Sept-94 02-Sept-94 01-Sept-94 01-Sept-94

20.37 22.27 18.35 18.90 19.57 21.40 21.24 20.75 24.48 19.48 21.51 28.43 28.82 28.53 28.42 28.98

34.37 34.78 34.52 34.44 34.13 34.64 34.60 34.51 30.11 31.14 31.82 31.61 34.04 34.63 33.95 34.56

3.23 ± 0.23 2.69 ± 0.22 1.84 ± 0.14 3.08 ± 0.23 3.02 ± 0.23 3.31 ± 0.19 2.88 ± 0.23 3.08 ± 0.25 2.73 ± 0.21 2.77 ± 0.22 2.79 ± 0.17 3.03 ± 0.22 2.56 ± 0.27 2.76 ± 0.16 2.93 ± 0.24 2.60 ± 0.21

H1 H6 D7 D9 B1 B4 B7 B10 E1 E3 E7 D10 L3 L11 J1 J9

239+240 Pu∗

(mBq kg−1 ) (mBq kg−1 ) (µBq kg−1 )

2.38 ± 0.04 2.56 ± 0.08 3.34 ± 0.06 2.82 ± 0.10 2.73 ± 0.07 2.84 ± 0.05 2.02 ± 0.03 1.87 ± 0.04

6.5 ± 0.6 3.9 ± 0.5 8.0 ± 0.6 7.0 ± 0.8 9.3 ± 0.8 5.2 ± 0.7 4.5 ± 0.5 8.1 ± 0.6 5.1 ± 0.6 4.9 ± 0.7 3.9 ± 0.5 7.0 ± 1.3 4.8 ± 1.7 6.3 ± 1.5 3.8 ± 0.9 5.0 ± 0.9

∗239+240 Pu data were reported earlier by Lee et al. (2003a). ∗∗ East China Sea was assigned the region south of Cheju Island (33◦ 30 N) for convenience.

East China Sea and tropical Northwestern Pacific, respectively. The highest activity concentration of dissolved 137 Cs was found in the Korea Strait (3.16 ± 0.52) where the chemical constituents are largely derived from the Yellow Sea, although the water mass in the Korea Strait originated from the Yellow Sea and the Kuroshio (Suk et al., 1996). The average 239+240 Pu activity concentrations were 6.14 ± 2.92, 5.83 ± 1.68 and 3.52 ± 1.22 mBq kg−1 in the Yellow Sea, East China Sea and tropical Northwestern Pacific, respectively. The radionuclide activity concentrations observed in the Yellow Sea were higher than in the Northwest Pacific (Fig. 23). In the early 1980s, the radionuclide concentrations were also higher in the Bohai Sea (northern part of the Yellow Sea) than in the East China Sea (Li et al., 1994). Therefore the Yellow Sea supply artificial radionuclides via water and particle transport to the North Pacific Ocean. A comprehensive study on the atom ratios of 240 Pu/239 Pu may reveal more information on the inter-basin redistribution of these radionuclides (Kim et al., 2004).

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Table 10 Surface water distribution 90 Sr, 137 Cs, and 239+240 Pu activity concentrations in the tropical Northwestern Pacific Ocean during April 1994. All quoted uncertainties are 1 sigma standard deviations Cruise

TG9404

Location

Water depth

Latitude

Longitude

25◦ N 20◦ N 15◦ N 10◦ N 5◦ N 2◦ N

132◦ E 133◦ 30 E 137◦ E 137◦ E 137◦ E 137◦ E

0 0 0 0 0 0 200 500 1000 2000

Temp. (◦ C)

Sal.

– – – 29.20 29.64 29.73 20.02 7.75 4.76 2.18

– – – 34.05 33.93 34.22 35.28 34.63 34.60 34.68

90 Sr

137 Cs

239+240 Pu

(mBq kg−1 )

(mBq kg−1 )

(µBq kg−1 )

2.71 ± 0.11 3.07 ± 0.08 2.66 ± 0.07 2.35 ± 0.07 2.54 ± 0.05 2.14 ± 0.10 2.05 ± 0.08

3.00 ± 0.13 2.58 ± 0.10 2.32 ± 0.10 2.60 ± 0.11 2.43 ± 0.10 2.43 ± 0.11

3.6 ± 0.7 3.3 ± 0.9 2.1 ± 0.8 5.1 ± 1.1 4.7 ± 0.8 2.3 ± 0.6 30.1 ± 2.7 4.3 ± 0.7 15.9 ± 2.2 8.9 ± 1.6

0.14 ± 0.02 0.15 ± 0.02

Fig. 23. Distribution of dissolved 90 Sr, 137 Cs, 239+240 Pu activity concentrations in the Northwest Pacific Ocean in the period of December 1993 to September 1994.

4. Conclusions 90 Sr, 137 Cs

and 239+240 Pu were studied in seawater and bottom sediments in the Yellow Sea during 1994–2000, along with atmospheric and riverine input of these radionuclides. From the present investigation, the following conclusions can be drawn: (i) The deposition of 239+240 Pu in the mid-eastern coast of the Yellow Sea (Ansan, Korea) was about 6.1–8.1 mBq m−2 yr−1 with highest values in the early spring. The monthly total deposition of 239+240 Pu in Ansan was very similar to that observed in Tsukuba, Japan, despite that Tsukuba is located further east more than 1000 km. Resuspension of surface soils from the arid regions of China and the subsequent deposition of fallout radionuclides from previously deposited debris on land has become an important source of radionuclides for the down wind sides including Yellow Sea


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as well as Korean Peninsula. (ii) The wet deposition of these radionuclides during individual rainout events also showed the highest values in the spring, further supporting the tropospheric transport of these radionuclides in the region. (iii) The river water activity concentrations for 90 Sr, 137 Cs and 239+240 Pu were 2.8–4.3, 0.25–0.68 mBq l−1 and 2.7–3.1 µBq l−1 , respectively. (iii) The average activity concentrations of 90 Sr, 137 Cs and 239+240 Pu in surface water of the Yellow Sea were 2.17, 2.7 mBq kg−1 and 4.2 µBq kg−1 , respectively. They appeared to be supplied mainly from the land and their concentrations appeared to be higher towards the river mouth. During the summer stratification period, vertical segregation was pronounced for Cs and Pu isotopes. (iv) Despite the wide variation in the local water characteristics from which samples were taken, 90 Sr and 137 Cs activity concentrations exponentially decreased with time for the period of 1960–1994. Using the historical data the half-residence times of 90 Sr and 90 Cs in surface waters for this period were estimated to be 7 years. The current levels in the sea appeared to be relatively stable since the early part of the 1990s. (v) The average sedimentary 90 Sr, 137 Cs and 239+240 Pu massic activities in the seafloor were 1.6, 2.4 and 0.29 Bq kg−1 , respectively. The 210 Pb-derived sediment accumulation and mixing rates have been found useful for radionuclide sedimentation dynamics studies. (vi) The Yellow Sea appears to be currently supplying 90 Sr, 137 Cs, and 239+240 Pu to the East China Sea.

Acknowledgements The authors thank Professor Pavel P. Povinec (Marine Environment Laboratory, IAEA) for his continued encouragement and frequent advice which helped shape this paper. We also thank two anonymous reviewers for their constructive comments which improved greatly this paper. K.Y. Choi, S.K. Park, S.H. Chung, and D.S. Moon had provided laboratory assistance. This study was supported by Korea Ocean Research and Development Institute (PE89400 and BSPN 99383).

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3. Radionuclides in the European seas


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Distribution of anthropogenic radionuclides in the water column of the south-western Mediterranean Sea S.-H. Leea,* , F.R. Mantouraa , P.P. Povineca , J.A. Sanchez-Cabezaa , J.-L. Pontisa , A. Mahjoubb , A. Noureddinec , M. Boulahdidd , L. Choubae , M. Samaalif , N. Reguiguif a International Atomic Energy Agency, Marine Environment Laboratory, 98000, Monaco b International Atomic Energy Agency, Department of Technical Cooperation, Vienna, Austria c Commissariat à l’Energie Atomique, Centre de Recherche Nucléaire d’Alger, 16000 Alger, Algeria d Institut des Sciences de la Mer et de l’Aménagement du Littoral, Dely Ibrahim, 16320 Alger, Algeria e Institut National des Sciences et Technologies Marines, TN-2060 La Goulette, Tunisia f Centre National des Sciences et Technologies Nucléaires, Sidi Thabet, TN-2020 Ariana, Tunisia

Abstract In the framework of the IAEA’s Technical Co-operation project RAF7/004, international research cruises were carried out in 2001 and 2004 in order to assess the distribution of anthropogenic radionuclides in the south-western (SW) Mediterranean Sea. Measured surface 239,240 Pu activity concentrations were slightly lower, while 241 Am concentrations were comparable to those observed in the north-western (NW) part of the Sea. The subsurface 239,240 Pu maximum was found in the Algerian Basin at 250 m, but in the Sardinia Channel it was much deeper (1000 m). 137 Cs activity concentration gradually decreased with depth, but higher values were observed at the bottom layers in the Sardinia and Sicily Channels. Higher 137 Cs activity concentrations observed in the Sicily Channel may be influenced by the Levantine Intermediate Water (LIW), carrying higher concentrations due to the Chernobyl accident. Higher concentrations of conservative anthropogenic radionuclides (90 Sr and 137 Cs) and lower concentrations of non-conservative radionuclides (239,240 Pu and 241 Am) were found in the bottom layer at the Sardinia Channel, possibly indicating an intrusion of surface waters. The accumulated inventories of 239,240 Pu, 241 Am, 137 Cs and 90 Sr are lower than those found in the NW Mediterranean, and also lower than the global fallout deposition in these latitudes. 238 Pu/239,240 Pu activity ratios indicate that plutonium in the SW Mediterranean is of global fallout origin. The 241 Am/239,240 Pu activity ratios are much lower than that of global fallout due to the enhanced scavenging of 241 Am from the water column. Keywords: Anthropogenic radionuclides, 90 Sr, 137 Cs, 241 Am, Pu isotopes, Water column, Mean residence time, Algerian Basin, Sardinia Channel, Sicily Channel, South Mediterranean

* Corresponding author. Address: IAEA-MEL, 4 Quai Antoine 1er, MC 98000, Monaco; phone: (+377) 97977229;

fax: (+377) 97977273; e-mail: [email protected] RADIOACTIVITY IN THE ENVIRONMENT VOLUME 8 ISSN 1569-4860/DOI 10.1016/S1569-4860(05)08008-3


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1. Introduction The Mediterranean Sea is a semi-enclosed basin connected to the North Atlantic Ocean through the narrow and shallow Strait of Gibraltar, exchanging water, salt, heat and other substances. The basin is characterized by low precipitation and high evaporation. The surface water coming from Gibraltar and the intermediate water from the eastern Mediterranean cross the central Mediterranean region. The Algerian Current transports Modified Atlantic Water (MAW) across the Algerian Basin to the Sardinia Channel. East of the Sardinia Channel, a minor part of MAW penetrates into the Tyrrhenian Sea, and the major part enters the eastern Mediterranean through the Sicily Channel (Sammari et al., 1999). Below MAW, Levantine Intermediate Water (LIW) flows westward (Stansfield et al., 2003). Most of LIW and TDW (Tyrrhenian Deep Water) flow out from the Tyrrhenian Sea through the Sardinia Channel, along the Sardinian slope (Millot, 1999). As the straits play a key role in controlling the flows and the water exchange, the area between Sardinia, Sicily and Tunisia is thought to be an important region for the comprehension of exchange of water masses between the eastern and western Mediterranean basins (Astraldi et al., 1998). Marine radiotracer studies can effectively contribute to a better understanding of water dynamics and the behavior of contaminants in the Mediterranean Sea. Nevertheless, available data on isotope tracers for the Tunisia–Sardinia–Sicily area are scarce. In the framework of the IAEA’s Technical Co-operation project “Contamination Assessment of the South Mediterranean Sea (RAF/7/004)”, carried out in collaboration with several institutes from the North African countries, the IAEA-MEL organized four expeditions to sample seawater, sediment and biota in coastal zones of Morocco, Algeria and Tunisia. The present work aims to investigate the distribution of anthropogenic radionuclides such as plutonium isotopes, 241 Am, 137 Cs, and 90 Sr in the water column of the SW Mediterranean Sea, along the Algerian and Tunisian coasts.

2. Materials and methods Oceanographic cruises in the SW Mediterranean Sea were carried out from 8 August to 23 August 2001 along the Algerian coast and from 14 June to 29 June 2004 along the Tunisian coast using a research vessel “Mohamed Seddik Benyahia” owned by “Institut des Sciences de la Mer et de l’Aménagement du Littoral”, Algeria. The study area is shown in Fig. 1. A large volume water sampler (250 L) was used for collecting water column samples. All seawater samples were pumped through a membrane filter (0.45 µm pore, 293 mm diameter). Samples (∼150 L) were taken at several depths down to a maximum depth of 1500 m. They were acidified with HCl to pH 1–2 and tracers were added (242 Pu, 134 Cs, 243 Am and 85 Sr). The radiochemical and counting techniques used have already been described in detail by La Rosa et al. (2001) and Lee et al. (2005), therefore they will be only shortly described here. Plutonium and americium were coprecipitated with MnO2 . 8 M HNO3 was added to re-dissolve the precipitate followed by plutonium purification by anion exchange. For the separation of americium, the anion exchange column eluate fraction was evaporated. Ca-oxalate precipitations and TRU extraction chromatography were used for Am purification. An alcohol ion exchange step was then performed to separate rare earth elements from Am. The

Anthropogenic radionuclides in the water column of the south-western Mediterranean Sea

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Fig. 1. Sampling stations in the SW Mediterranean Sea in 2001 (◦) and 2004 (•).

final column eluates, containing the purified Pu and Am isotopes, were evaporated to dryness, treated to remove organic traces, electrodeposited onto stainless steel discs and measured by alpha-ray spectrometry. After the MnO2 precipitation, an appropriate amount of ammonium molybdophosphate (AMP) was mixed with the re-acidified supernatant solution to adsorb Cs on the AMP. The AMP precipitate was dissolved with 10 mL of 10 M NaOH, transferred to a standardized container and diluted to an appropriate volume for gamma-ray spectrometry analysis. 90 Sr was analyzed by oxalate precipitation of the supernatants after sequential separation of 137 Cs. The oxalate fraction was ashed and strontium was precipitated as strontium nitrates in concentrated HNO3 . For further purification, the strontium nitrates were set aside for 14 days to enable full in-growth of 90 Y. Yttrium was precipitated as yttrium oxalate after the addition of stable yttrium, and analyzed in a gas proportional counter. The chemical recoveries of 85 Sr and 137 Cs were determined by gamma-ray spectrometry, and by gravimetric method for yttrium precipitates. IAEA-381 (Irish Sea water) reference material was used to assess data quality. The replicate analyses of investigated radionuclides were within the 95% confidence intervals.

3. Results and discussion 3.1. Algerian Basin 239,240 Pu activity concentrations in surface waters of the Algerian Basin ranged from 6.5 ±0.5

to 7.0 ± 0.4 µBq L−1 (Table 1 and Fig. 2). The 239,240 Pu activity concentration increased significantly with depth up to 250 m (21 ± 1 µBq L−1 ) and then remained constant up

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Table 1 Activity concentrations of 239,240 Pu and 241 Am in the water column of the SW Mediterranean. The quoted uncertainties represent 1 sigma (σ ) standard deviations Sampling date

Location

Depth (m)

239,240 Pu (µBq L−1 )

238 Pu (µBq L−1 )

241 Am (µBq L−1 )

238 Pu/239,240 Pu

241 Am/239,240 Pu

St. 1 Jijel

13/Aug/01

36◦ 54 N, 05◦ 32 E

0 250 550 950 1200

6.5 ± 0.5 20.9 ± 0.9 22.2 ± 1.1 22.4 ± 1.0 21.7 ± 1.0

1.21 ± 0.23 0.80 ± 0.14 0.62 ± 0.15 0.90 ± 0.16 1.16 ± 0.19

1.29 ± 0.13 1.07 ± 0.12 1.51 ± 0.16 1.90 ± 0.18 2.02 ± 0.27

0.19 ± 0.04 0.038 ± 0.007 0.028 ± 0.007 0.040 ± 0.007 0.054 ± 0.009

0.20 ± 0.03 0.051 ± 0.006 0.068 ± 0.008 0.085 ± 0.009 0.093 ± 0.013

24.0 ± 0.4

Total inventory (Bq m−2 )

1.0 ± 0.1

1.9 ± 0.1

St. 2 Alger

18/Aug/01

36◦ 54 N, 03◦ 20 E

0 250

7.0 ± 0.4 7.7 ± 0.3

0.28 ± 0.07 0.40 ± 0.07

0.74 ± 0.10 0.70 ± 0.10

0.041 ± 0.010 0.052 ± 0.009

0.110 ± 0.020 0.090 ± 0.010

St. 1 Sicily Channel

20/June/04

37◦ 36 N, 11◦ 28 E

0

7.9 ± 1.2

–

–

–

–

St. 3 Sardinia Channel

24/June/04

38◦ 09 N, 09◦ 07 E

0 250 500 750 1000 1250 1483

8.8 ± 0.3 14.1 ± 0.6 18.7 ± 0.7 21.8 ± 0.7 20.5 ± 0.8 21.2 ± 0.9 6.9 ± 0.3

0.36 ± 0.06 1.22 ± 0.15 0.84 ± 0.11 0.75 ± 0.09 0.85 ± 0.13 0.68 ± 0.13 0.38 ± 0.07

0.57 ± 0.08 0.62 ± 0.08 0.71 ± 0.09 0.97 ± 0.11 0.92 ± 0.22 0.96 ± 0.12 0.74 ± 0.10

0.041 ± 0.007 0.087 ± 0.011 0.045 ± 0.006 0.035 ± 0.004 0.041 ± 0.007 0.032 ± 0.006 0.055 ± 0.010

0.065 ± 0.010 0.044 ± 0.006 0.038 ± 0.005 0.045 ± 0.005 0.045 ± 0.011 0.045 ± 0.006 0.107 ± 0.016

26.0 ± 0.3

1.2 ± 0.1

1.2 ± 0.1

Total inventory (Bq m−2 )

S.-H. Lee et al.

Station


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Fig. 2. Profiles of 239,240 Pu, 241 Am and 137 Cs in the water column of the Algerian Basin in 2001 and 2004.

to 1200 m. Plutonium and americium are characterized as particle reactive elements, therefore, their vertical profiles tend to be similar to that of nutrients. The observed low concentrations of plutonium (and americium, Fig. 2) in surface water are due to scavenging by particulate matter. The vertical transport of plutonium and americium associated with sinking particles, and the subsequent regeneration of these elements as a result of the decomposition of particulate matter, causes the subsurface 239,240 Pu and 241 Am maxima observed in the water column (Bowen et al., 1980; Fowler et al., 1983; Molero et al., 1995; Povinec et al., 2003; Lee et al., 2003a, 2003b). However 241 Am, due to its higher particle affinity, is more rapidly scavenged from the surface water than plutonium. The surface 239,240 Pu activity concentrations in the Algerian Basin are slightly lower than in the NW Mediterranean Sea (Lee et al., 2003a), in the Sardinia Channel and in the Gibraltar Strait (Benmansour et al., 2006). The 239,240 Pu activity concentration observed in 2001 (21 ± 1 µBq L−1 ) at 250 m depth at Jijel (St. 1) is higher than that at St. 2 (7.7 ± 0.3 µBq L−1 ), although both stations are located in the same basin. The local variations in plutonium concentrations might be due to large mesoscale turbulences that have been observed in the Algerian Basin (Bouzinac et al., 1998). The highest 137 Cs activity concentration (3.1 ± 0.2 mBq L−1 ) is observed at the surface layer (Fig. 2, Table 2), then it decreases with depth, and a higher activity is observed again at the bottom layer (2.9 ± 0.2 µBq L−1 ). This bottom water enrichment is thought to be the due to the influence of LIW coming from the eastern Mediterranean basin, where higher concentrations of 137 Cs were observed due to the Chernobyl fallout (Papucci et al., 1996). 3.2. The Sardinia Channel The Sardinia Channel is the passage between the Algerian Basin and the Tyrrhenian Basin, limited by Sardinia Island and the northern Tunisian coast. The 239,240 Pu, 241 Am, 137 Cs and 90 Sr profiles in the water column south of Sardinia Island are shown in Fig. 3. The 239,240 Pu activity concentration has a minimum at the surface (8.8 ± 0.3 µBq L−1 ), then it is gradually increasing until reaching a maximum at 800 m (22 ± 1 µBq L−1 ), much deeper than the

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Table 2 Activity concentrations of 90 Sr and 137 Cs in the water column of the SW Mediterranean (2004). The quoted uncertainties represent 1 sigma standard deviations Station

Sampling date

Location

Depth (m)

St. 1 Sicily Channel

20/June/04

37◦ 36 N, 11◦ 28 E

0 150 350 650

38◦ 09 N, 09◦ 07 E

0 250 500 750 1000 1250 1483

37◦ 13 N, 08◦ 17 E

0 150 350 500

Total inventory (kBq m−2 ) St. 3 Sardinia Channel

24/June/04

Total inventory (kBq m−2 ) St. 4 Algerian Basin

26/June/04

Total inventory (kBq m−2 )

137 Cs (µBq L−1 )

90 Sr

137 Cs/90 Sr

1.9 ± 0.3 1.5 ± 0.2 2.1 ± 0.2 2.6 ± 0.2

1.48 ± 0.04 1.52 ± 0.04 1.60 ± 0.04 1.44 ± 0.04

1.31 ± 0.17 1.00 ± 0.12 1.31 ± 0.16 1.83 ± 0.10

(µBq L−1 )

1.3 ± 0.1

0.99 ± 0.01

1.98 ± 0.08 2.40 ± 0.17 1.73 ± 0.06 1.35 ± 0.06 1.55 ± 0.07 1.32 ± 0.07 2.18 ± 0.10

2.04 ± 0.05 2.15 ± 0.05 1.91 ± 0.05 1.83 ± 0.04 1.93 ± 0.05 1.47 ± 0.04 2.32 ± 0.06

2.58 ± 0.02

2.8 ± 0.2

3.1 ± 0.2 2.6 ± 0.2 2.5 ± 0.2 2.9 ± 0.2

– – –

1.3 ± 0.004

–

0.97 ± 0.05 1.12 ± 0.09 0.91 ± 0.04 0.74 ± 0.04 0.80 ± 0.04 0.90 ± 0.05 0.94 ± 0.05

maximum observed in the Algerian Basin. A remarkable decrease of 239,240 Pu and 241 Am activity concentrations was observed at the bottom layer. The activity concentrations of 90 Sr and 137 Cs in the water column at Sardinia Channel also varied with depth. A subsurface maximum concentration of 137 Cs was found at 250 m and then decreased with depth, although a higher concentration, comparable to the surface water concentration, was found in the bottom layer. The 90 Sr activity concentrations followed a similar but smoother pattern. Activity concentrations of 239,240 Pu, 241 Am, 90 Sr and 137 Cs observed near the bottom layer indicate an intrusion of surface waters, as documented by the temperature and salinity profiles (Fig. 3). During winter and early spring, cooling of superficial waters increases their density and induces vertical convection (Schmidt and Reyss, 1996; Bethoux et al., 2002). Also, strong mesoscale perturbations in the surface layer could affect the radiotracers distributions, as some eddies extend down to 2000 m (Bouzinac et al., 1998). Both these processes could transport high 90 Sr and 137 Cs and low 239,240 Pu and 241 Am concentrations with surface waters to the deep layer (Delfanti et al., 1994; Lee et al., 2003a). However, further studies are needed to better understand the observed radiotracers distributions in the Sardinia Channel. 3.3. The Sicily Channel The Sicily Channel provides a direct interface between the eastern and western Mediterranean. The two-layer water system in the Sicily Strait is maintained by the excess of evaporation and the specific circulation in the eastern Mediterranean (Astraldi et al., 1999). While the MAW


143

Fig. 3. Profiles of 239,240 Pu, 241 Am and 90 Sr in the water column in the Sardinia Channel in 2004.

is found in the sea surface next to the Tunisian coast, the LIW flow fills the whole bottom section of the Strait. The observed surface 239,240 Pu activity concentration (7.9 ± 1.2 µBq L−1 ) is comparable with that found in the Sardinia Channel and the Algerian Basin surface waters (Table 1). The vertical profile of 137 Cs in the Sicily Strait (Fig. 4 and Table 2) shows a subsurface minimum (at 150 m, 1.5 ± 0.2 mBq L−1 ) and a bottom maximum (at 650 m, 2.6 ± 0.2 mBq L−1 ). However, the 90 Sr vertical profile shows a constant distribution with depth (Fig. 4). A few 90 Sr results were reported by Merino et al. (1997), who found 0.57–1.77 mBq L−1 in the Catalan Sea. A recent study (Lee et al., 2003a) showed 1.3 ± 0.1 mBq L−1 in surface waters of the NW Mediterranean, in agreement with 1.48–1.60 mBq L−1 found in the Sicily Strait. The 137 Cs maximum observed in deep water might be attributed to the presence of LIW, carrying higher levels of 137 Cs from the Chernobyl accident, but not for 90 Sr. Indeed, LIW carrying higher levels of 137 Cs moves westward, passes over the Sicily Strait and enters the western Mediterranean basin (Ozsony et al., 1993; Papucci et al., 1996; Papucci and Delfanti, 1999; Stansfield et al., 2003; Lee et al., 2003a). 3.4. Radionuclide inventories in the water column Radionuclide seawater inventories were calculated and compared with global fallout inputs estimated from data on the global distribution of fallout in this latitude band (UNSCEAR, 2001). The deposited 239,240 Pu inventory in the water column is 24.0 ± 0.4 Bq m−2 for the Algerian Basin and 26.0 ± 0.3 Bq m−2 for the Sardinia Channel, respectively. These inventories are much lower than that expected from global fallout because of the small water column

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Fig. 4. Profiles of 137 Cs and 90 Sr in the water column in the Sicily Channel in 2004.

in the visited stations. The 238 Pu inventory is estimated to be 1.0 ± 0.1 Bq m−2 for the Algerian Basin and 1.2 ± 0.1 Bq m−2 for the Sardinia Channel, respectively. The 241 Am inventory in the water column is 1.9 ± 0.1 Bq m−2 for the Algerian Basin and 1.2 ± 0.1 Bq m−2 for the Sardinia Channel, respectively, which are lower than those observed in the NW Mediterranean (Lee et al., 2003a). The 137 Cs inventories in the water column in the Sicily Channel and in the Algerian Basin are 1.3 ± 0.1 and 1.3 ± 0.4 kBq m−2 , respectively. Higher 137 Cs inventory (2.58 ± 0.02 kBq m−2 ) has been observed in the Sardinia Channel. Nonetheless, those are much lower than the value (3.7 ± 0.1 kBq m−2 ) observed in the NW Mediterranean (Lee et al., 2003a) because of the smaller water column. The 90 Sr inventory in the water column at Sicily and Sardinia Channels is 0.99 ± 0.01 kBq m−2 and 2.8 ± 0.2 kBq m−2 , respectively. The 90 Sr inventory for Sardinia waters is even higher than for the NW Mediterranean (Lee et al., 2003a). The observed flat profiles of 90 Sr in the water column is possibly due to eddies. Similar flat profiles in the SW Mediterranean have also been observed by Noureddine et al. (2006) and Benmansour et al. (2006). 3.5. Radionuclide activity ratios 238 Pu/239,240 Pu

and 241 Am/239,240 Pu activity ratios in the water column are reported in Table 1. The activity ratios of 238 Pu/239,240 Pu are, within uncertainties, the same as the global fallout ratio (0.03) at these latitudes, indicating that global fallout is the main source of plutonium in the SW Mediterranean. The observed 137 Cs/90 Sr activity ratios in the water column ranged from 0.74 ± 0.04 to 1.8 ± 0.1 (Table 2), i.e. most of them are lower than the global fallout ratio of 1.6


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(UNSCEAR, 2001). This difference might be attributable to a higher scavenging rate of 137 Cs than 90 Sr in shallow coastal waters, due to its higher particle affinity, as the distribution coefficients (Kd s) for 137 Cs are higher by a factor of 500 and 10 than for 90 Sr for open ocean and coastal waters, respectively (IAEA, 2004). 4. Conclusions The distribution of anthropogenic radionuclides in the water column of the SW Mediterranean Sea gives useful indications for better understanding of oceanic processes and water transport in the area. Various oceanic processes such advection by water currents, convection by eddies and remineralization have affected the distribution of radionuclides in the water column. The main observations can be summarized as follows: (i) The surface 239,240 Pu activity concentrations in the SW Mediterranean Sea are slightly lower than the values reported for the NW region, probably because of the inflow of surface Atlantic waters; however, the surface 241 Am concentrations are comparable to those observed in NW Mediterranean waters. (ii) Higher 137 Cs activity concentrations were observed at the surface layer (MAW), and at bottom layers of the Sardinia and Sicily Channels. Higher activities observed in the deep waters of the Sardinia Channel might indicate the intrusion of surface waters. (iii) The inventories of 239,240 Pu, 241 Am and 137 Cs in the water column of the SW Mediterranean Sea are lower than those of global fallout deposition at this latitude. The inventories of 90 Sr in the water column are lower than that of global fallout deposition in the Sicily Channel, or comparable to that of global fallout in the Sardinia Channel. (iv) The 238 Pu/239,240 Pu activity ratios confirm that plutonium in the SW Mediterranean Sea is of global fallout origin. The 241 Am/239,240 Pu and 137 Cs/90 Sr activity ratios are lower than those of global fallout due to an enhanced scavenging of 241 Am and 137 Cs from the water column. Acknowledgements The authors would like to thank the Institut des Sciences de la Mer et de l’Aménagement du Littoral (Alger, Algeria), the Captain and the crew members of R/V Mohamed Seddik Benyahia, and the Algerian, Tunisian and IAEA-MEL colleagues participating in the cruises for assistance during seawater sampling. We also thank Ms. J. Gastaud and Ms. M.K. Pham for assistance during alpha- and gamma-spectrometry measurements. The IAEA is grateful for the support provided to its Marine Environment Laboratory by the Government of the Principality of Monaco. References Astraldi, M., Gasparini, G.P., Sparnocchia, S. (1998). Water masses and seasonal hydrographic conditions in the Sardinia–Sicily–Tunisia region. Rapports Commission Internationale pour l’exploration scientifique de la Mer Méditerranée 35, 122–123.

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Astraldi, M., Balopoulos, S., Candela, J., Font, J., Gacic, M., Gasparini, G.P., Manca, B., Theocharis, A., Tintore, J. (1999). The role of straits and channels in understanding the characteristics of Mediterranean circulation. Progress in Oceanography 44, 65–108. Benmansour, M., Laissaoui, A., Benbrahim, S., Ibn Majah, M., Chafik, A., Povinec, P.P. (2006). Distribution of anthropogenic radionuclides in Moroccan coastal waters and sediments. In: Povinec, P.P., Sanchez-Cabeza, J.A. (Eds.), Radionuclides in the Environment, International Conference on Isotopes in Environmental Studies: Aquatic Forum 2004. 25–29 October, Monaco. Elsevier, Amsterdam, pp. 148–155, this volume. Bethoux, J.P., Durieu de Madron, X., Nyffeler, F., Tailliez, D. (2002). Deep water in the western Mediterranean: Peculiar 1999 and 2000 characteristics, shelf formation hypothesis, variability since 1970 and geochemical inferences. Journal of Marine System 33/44, 117–131. Bouzinac, C., Font, J., Millot, C., Vazquez, J. (1998). Circulation variability the channel of Sardinia observed from in situ and altimetric data. Rapports Commission Internationale pour l’exploration scientifique de la Mer Méditerranée 35, 128–129. Bowen, V.T., Noshkin, V.E., Livingston, H.D., Volchok, H.L. (1980). Fallout radionuclides in the Pacific Ocean: Vertical and horizontal distributions, largely from GEOSECS stations. Earth Planetary and Science Letters 49, 411–434. Delfanti, R., Papucci, C., Vives i Battle, J., Downes, A.B., Mitchell, P.I. (1994). Distribution of 137 Cs and transuranics elements in seawater of the western Mediterranean Sea (Algerian Basin, Balearic Sea). In: Cigna, A., Delfanti, R., Serro, R. (Eds.), The Radiological Exposure of the Population of the European Community to Radioactivity in the Mediterranean Sea. European Commission, Luxembourg, pp. 427–439. Fowler, S.W., Ballestra, S., La Rosa, J., Fukai, R. (1983). Vertical transport of particulate-associated plutonium and americium in the upper water column of the Northeast Pacific. Deep-Sea Research 12 (A), 1221–1233. IAEA (2004). Sediment distribution coefficients and concentration factors for biota in the marine environment. IAEA Technical Reports Series No. 422. La Rosa, J.J., Burnett, W., Lee, S.H., Levy, I., Gastaud, J., Povinec, P.P. (2001). Separation of actinides, cesium and strontium from marine samples using extraction chromatography and sorbents. Journal of Radioanalytical and Nuclear Chemistry 248, 765–770. Lee, S.-H., La Rosa, J.J., Levy-Palomo, I., Oregioni, B., Pham, M.K., Povinec, P.P., Wyse, E. (2003a). Recent inputs and budgets of 90 Sr, 137 Cs, 239,240 Pu and 241 Am in the northwest Mediterranean Sea. Deep-Sea Research II 50, 2817–2834. Lee, S.-H., Gastaud, J., Povinec, P.P., Hong, G.-H., Kim, S.-H., Chung, C.-S., Lee, K.W., Pettersson, H.B.L. (2003b). Distribution of plutonium and americium in the marginal seas of the Northwest Pacific Ocean. Deep-Sea Research II 50, 2727–2750. Lee, S.-H., La Rosa, J., Gastaud, J., Povinec, P.P. (2005). The development of sequential separation methods for analysis of actinides in sediment and biological materials using anion exchange resins and extraction chromatography. Journal of Radioanalytical and Nuclear Chemistry 263 (2), 419–425. Merino, J.A., Sanchez-Cabeza, J.A., Bruach, J.M., Masque, P., Pujol, L.I. (1997). Artificial radionuclides in high resolution water column profile from the Catalan Sea (the Northwestern Mediterranean). Radioprotection – Colloques 32, C2-85–C2-90. Millot, C. (1999). Circulation in the western Mediterranean Sea. Journal of Marine Systems 20, 423–442. Molero, J., Sanchez-Cabeza, J.A., Merino, J., Pujol, Ll., Mitchell, P.I., Vidal-Quadras, A. (1995). Vertical distribution of radiocaesium, plutonium and americium in the Catalan Sea (Northwest Mediterranean). Journal of Environmental Radioactivity 26, 205–216. Noureddine, A., Menacer, M., Boudjenoun, R., Benkrid, M., Boulahdid, M., Kadi-hanifi, M., Lee, S.-H., Povinec, P.P. (2006). 137 Cs in seawater and sediment along the Algerian coast. In: Povinec, P.P., Sanchez-Cabeza, J.A. (Eds.), Radionuclides in the Environment, International Conference on Isotopes in Environmental Studies: Aquatic Forum 2004. 25–29 October, Monaco. Elsevier, Amsterdam, pp. 156–164, this volume. Ozsony, E., Hecht, A., Unluata, U., Brenner, S., Sur, H.I., Bishop, J., Latif, M.A., Rozentruub, Z., Oguz, T. (1993). A synthesis of the Levantine Basin circulation and hydrography, 1985–1990. Deep-Sea Research II 40 (6), 1075– 1119. Papucci, C., Delfanti, R. (1999). 137 Cs distribution in the eastern Mediterranean Sea: Recent change and future trends. The Science of the Total Environment 237/238, 67–75. Papucci, C., Charmasson, S., Delfanti, R., Gasco, C., Mitchell, P., Sanchez-Cabeza, J.A. (1996). Time evolution and levels of man-made radioactivity in the Mediterranean Sea. In: Guéguéniat, P., Germain, P., Métivier, H. (Eds.), Radionuclides in the Oceans: Input and Inventories. Les éditions de physique, Les Ulis, pp. 177–197.


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Povinec, P.P., Livingston, H.D., Shima, S., Aoyama, M., Gastaud, J., Goroncy, I., Hirose, K., Huynh-Ngoc, L., Ikeuchi, Y., Ito, T., La Rosa, J., Liong Wee Kwong, L., Lee, S.-H., Moriya, H., Mulsow, S., Oregioni, B., Pettersson, H., Togawa, O. (2003). IAEA’97 expedition to the NW Pacific Ocean – Results of oceanographic and radionuclide investigations of the water column. Deep-Sea Research II 50, 2607–2637. Sammari, C., Millot, C., Taupier-Letage, I., Stefani, A., Brahim, M. (1999). Hydrological characteristics in the Tunisia–Sardinia–Sicily area during spring 1995. Deep-Sea Research I 46, 1671–1703. Schmidt, S., Reyss, J.-L. (1996). Radium as internal tracer of Mediterranean outflow water. Journal of Geophysical Research 101 (C2), 3589–3596. Stansfield, K., Gasparini, G.P., Smeed, D.A. (2003). High-resolution observations of the path of the overflow from the Sicily Strait. Deep-Sea Research I 50, 1129–1149. UNSCEAR (1993). Source and effects of ionizing radiation. United Nations, New York.

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Distribution of anthropogenic radionuclides in Moroccan coastal waters and sediments M. Benmansoura,* , A. Laissaouia , S. Benbrahimb , M. Ibn Majaha , A. Chafikb , P.P. Povinecc a Centre National de l’Energie, des Sciences et des Techniques Nucléaires, Rabat, Morocco b Institut National de Recherche Halieutique, Casablanca, Morocco c International Atomic Energy Agency, Marine Environment Laboratory, Monaco

Abstract Concentrations of anthropogenic radionuclides (137 Cs, 239, 240 Pu and 241 Am) were measured in surface seawater, water column and sediment samples collected in Moroccan coastal waters. The average activity concentrations in surface water were 2.7 mBq L−1 , 8.7 µBq L−1 and 1.5 µBq L−1 for 137 Cs, 239, 240 Pu and 241 Am, respectively. The vertical distributions of 239,240 Pu in the water column of the southwest Mediterranean showed a sub-surface maximum at 500 m water depth, while 137 Cs concentrations were constant up to 900 m. On the contrary to the 239,240 Pu profile, which has been as expected, the 137 Cs profile has been affected by movement of water masses in the western Alboran Sea. The total inventories of 137 Cs and 239,240 Pu in the water column (0–900 m) were estimated to be 2170 and 23 Bq m−2 , respectively, and they were as expected for this water depth. The concentration profiles of 137 Cs and 239,240 Pu in the sediment core revealed the presence of sub-surface maximum located at the depth of 5 cm. The sedimentation rate of 0.12 cm yr−1 has been estimated using the 210 Pb profile, which indicated that the observed peaks of 137 Cs and 239,240 Pu originated in the sediment core following the maximum global fallout deposition in 1963. Keywords: Radionuclides, 137 Cs, 239,240 Pu, 241 Am, 210 Pb, Seawater, Sediment, Sedimentation rate, Strait of Gibraltar, Alboran Sea, Mediterranean Sea, Atlantic Ocean, Morocco

1. Introduction The main sources of anthropogenic radionuclides in the Mediterranean Sea are global fallout from atmospheric nuclear weapon tests carried out mainly in the fifties and the early sixties, and the Chernobyl accident which occurred in 1986 (e.g. Fukai et al., 1979; Delfanti et al., 1995; Molero et al., 1995; Papucci et al., 1996; León Vintró et al., 1999; Fowler et al., 2000; Lee et al., 2003). The global inputs of 137 Cs and 239,240 Pu to the Mediterranean Sea up to 1986 are estimated to be 12 PBq and 0.19 PBq, respectively (Holm et al., 1988; UNEP, 1992). The * Corresponding author. Address: CNESTEN, B.P. 1382, R.P. 10001, Rabat, Morocco; phone: (+212) 37819757; fax: (+212) 37803317; e-mail: [email protected]



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Chernobyl accident significantly increased the total input of 137 Cs into the environment of the Mediterranean Sea. Papucci et al. (1996) estimated that about 2.5 PBq of 137 Cs was deposited after the accident mainly in the eastern and northern parts of the Mediterranean basin. The distribution of anthropogenic radionuclides in the southwest Mediterranean has not been well established. Therefore one of the objectives of the IAEA’s regional technical cooperation project RAF/7/004 “Contamination Assessment of the South Mediterranean Sea” has been to study present radionuclide levels in the Mediterranean Sea and to use radionuclides as tracers for better understanding of processes in the water column.

2. Materials and methods 2.1. Seawater and sediment samples Samples were collected in four stations alongside the Moroccan coast which is approximately 3500 km long, which 500 km are in the southwest Mediterranean Sea. Three stations were situated in the Mediterranean Sea (the Alboran Sea and Strait of Gibraltar) and one in the Atlantic Ocean. The cruise was carried out in December 1999 using the R/V Charif Al Idrissi of the Institut National de la Recherche Halieutique (INRH). The visited sampling stations are shown in Fig. 1 and their corresponding specifications are given in Table 1. Station 2 located in the Mediterranean Sea (35◦ 47 N, 04◦ 48 W) was extensively explored by collecting water profile samples at different depths until 900 m, and a bottom sediment core collected at the water depth of 800 m. Due to low concentrations of transuranic elements, large volumes of water (∼200 L) were sampled using 30 L Niskin bottles. The water samples were promptly filtered through membrane filters of 0.45 µm pore size to remove any suspended matter.

Fig. 1. Sampling stations in the Atlantic Ocean and the Mediterranean Sea.

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Table 1 137 Cs, 239,240 Pu and 241 Am activity concentrations in surface seawater and in the water column (reported uncertainties are ±1σ ; N.D. means not determined) Station

137 Cs (mBq L−1 )

239,240 Pu (µBq L−1 )

St. 1 (Mohammadia) 33◦ 46 N, 7◦ 28 W St. 2 (Mdiq) 35◦ 47 N, 4◦ 48 W 0m 250 m 500 m 900 m

2.95 ± 0.22

8.7 ± 0.9

2.32 ± 0.18 2.46 ± 0.18 2.50 ± 0.19 2.29 ± 0.16

St. 3 (Mdiq) 35◦ 47 N, 5◦ 15 W St. 4 (Tangier) 35◦ 52 N, 5◦ 51 W

241 Am

241 Am/239,240 Pu

239,240 Pu/137 Cs

N.D.

N.D.

0.0025 ± 0.0003

8.3 ± 0.9 23 ± 2 33 ± 5 27 ± 2

1.50 ± 0.20 N.D. 7.6 ± 1.2 N.D.

0.18 ± 0.03 N.D. 0.23 ± 0.05 N.D.

0.0035 ± 0.0004 0.0089 ± 0.0010 0.0114 ± 0.0020 0.0118 ± 0.0013

2.37 ± 0.14

8.8 ± 1.4

1.56 ± 0.50

0.17 ± 0.02

0.0037 ± 0.0006

3.11 ± 0.23

8.3 ± 0.9

N.D.

N.D.

0.0027 ± 0.0003

(µBq L−1 )

Appropriate tracers of 242 Pu and 243 Am, and a stable Cs carrier were added, after acidifying the samples with concentrated HCl to pH 1–2, to serve as indicators of chemical recoveries. Sequential separations of radionuclides were carried out on board by co-precipitating Pu and Am, along with the other actinides, with manganese dioxide MnO2 . 137 Cs was precipitated by physical adsorption onto AMP. The sediment core was collected using a 40 cm × 40 cm box corer (Ocean Instruments). The core was sectioned to series of horizontal slices of 0.5–1 cm thick. The sediment samples were freeze-dried, sieved and homogenized before a non-destructive gamma-ray spectrometry analysis was carried out. 2.2. Analytical methods 137 Cs, 210 Pb, 226 Ra activities in samples were measured using an HPGe detector (n-type) with

resolution of 2 keV and the relative efficiency of 50% at 1332 keV (in comparison with the 7.6 cm height × 7.6 cm in diameter NaI(Tl) detector). The stable Cs in water samples was measured by ICPMS to estimate the chemical recovery. For alpha-emitters such as 239,240 Pu and 241 Am suitable radiochemical methods were applied for their extraction from seawater and sediment samples (Lee et al., 2001; La Rosa et al., 2001). Pu was separated from Am by anion-exchange resin AG1 × 8. Am was co-precipitated with calcium oxalate and extracted into DDCP and finally separated from rare earths by anion exchange in mineral acids–methanol media. Both Pu and Am fractions were electrodeposited on stainless steel discs and the resulting alpha-sources were analyzed by alpha-ray spectrometry using silicon solid state detectors. The analysis of 210 Pb in sediment samples was also carried using its daughter product 210 Po. After the total digestion of a sediment sample and a spontaneous deposition of Po onto silver discs, the measurements were carried out by alpha-ray spectrometry.


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3. Results and discussion 3.1. Seawater Table 1 lists 137 Cs, 239,240 Pu and 241 Am activity concentrations in surface waters for the four visited stations, as well as in water profile samples collected to a depth of 900 m at Station 2 located in the Alboran Sea. Some 241 Am activity concentrations are not given due to low recoveries of the radiochemical procedures. The 137 Cs activity concentrations in surface water of the Moroccan coast ranged from 2.32 to 3.11 mBq L−1 with an average concentration for the Mediterranean stations of 2.60 ± 0.21 mBq L−1 . These values are in the same range as those reported by other authors in previous studies regarding to the Mediterranean Sea area. Before the Chernobyl accident, which occurred in 1986, 137 Cs activity concentrations in different regions of the Mediterranean Sea ranged between 3.2–4.8 mBq L−1 and the average 137 Cs concentration was estimated to be 3.4 mBq L−1 (Fukai et al., 1980). The 137 Cs levels after 1986 increased mainly in the northern and eastern zones of the Sea. From recent Mediterranean studies (Molero et al., 1995; Merino et al., 1997; Sanchez-Cabeza and Molero, 2000; Lee et al., 2003), 137 Cs activity concentrations in surface seawater have been between 2.3 and 5.6 mBq L−1 . 137 Cs activity concentrations in the water profile of Station 2 (Fig. 2) do not show a noticeable surface maximum (Lee et al., 2003), as the concentration at 900 m water depth is comparable with the concentration at the surface. Such a homogeneous distribution of 137 Cs in the water column, which is behaving as a conservative tracer, may be due to movement of water masses in the area. Atlantic water, characterized by low salinity and density, enters the Mediterranean Sea via the Strait of Gibraltar as surface water (100–200 m water depth), while near the bottom, water masses with high salinity and density previously formed in the Mediterranean leave the Sea towards the Atlantic Ocean. The 239,240 Pu surface activity concentrations determined in this work range between 8.29 and 8.92 µBq L−1 and are comparable with the data reported by León Vintró et al. (1999) in the same zone ten years earlier, as well as with recent data published by Lee et al. (2003). The vertical distribution of 239,240 Pu in the water column of Station 2 shows a typical subsurface maximum at a depth close to 500 m (Fig. 2). This behavior particularly for plutonium was already observed in previous studies (Fukai et al., 1979; Mitchell et al., 1995; Merino et al., 1997; León Vintró et al., 1999; Fowler et al., 2000; Lee et al., 2003) and is usually

Fig. 2. Vertical distributions of 137 Cs and 239,240 Pu in the water column (Station 2).

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attributed to the removal of the plutonium from the surface in association with scavenging processes, its transport with suspended matter to a deeper water layers, and the subsequent particle dissolution/remineralization at depth, with the return of plutonium into the solution. Only a few results have been obtained for 241 Am in this work. However, on the basis of the results obtained for Stations 2 and 3 situated in the Alboran Sea, the mean 241 Am activity concentration in surface water has been 1.53 ± 0.18 µBq L−1 . This value is similar to that given by Lee et al. (2003) in a recent study in the northwest Mediterranean Sea (∼1.5 µBq L−1 ), and to that measured earlier by Molero et al. (1995) along the Spanish Mediterranean coast (∼1.0 µBq L−1 ). The 241 Am activity concentration at the depth of 500 m is 7.6 ± 1.2 µBq L−1 , much higher than at the surface. This is because of the fact that Am is even more particle reactive element than Pu, and its behavior in the water column is therefore similar to Pu. The total radionuclide inventories in the water column (Station 2, water depth 0–900 m) were estimated to be 2170 Bq m−2 for 137 Cs and 23 Bq m−2 for 239,240 Pu. They are both lower by a factor of two than the inventories reported in previous studies (Fowler et al., 2000; Lee et al., 2003), as the height of the water column at Station 2 has been smaller by about factor of two. The 239,240 Pu/137 Cs activity ratio increases with depth from 0.0035 to 0.012, as expected (Lee et al., 2003), suggesting a depletion of plutonium at the surface layer in comparison with cesium due to its preferential association with sinking particulate mater (Fowler et al., 2000; Lee et al., 2003). The 241 Am/239,240 Pu activity ratio is estimated to be 0.18 at the surface and 0.23 at the 500 m water depth, however, this change is within the reported uncertainties. 3.2. Sediment The vertical distributions of massic activities of 137 Cs and 239,240 Pu in the sediment core (Station 2) are plotted in Fig. 3. The both activity profiles revealed a presence of the subsurface maximum, located at the depth of ∼5 cm. It can be assumed that these maxima correspond to the maximum atmospheric deposition of anthropogenic radionuclides from global fallout which occurred in 1963 (Lee et al., 2003). The 137 Cs and 239,240 Pu activities ranged between 1.90–6.3 Bq kg−1 and 0.31–0.80 Bq kg−1 , respectively, and are within the range of those reported in the literature (Delfanti et al., 1995; Papucci et al., 1996; León Vintró et al., 1999). The total inventories were about 280 Bq m−2 for 137 Cs and 32 Bq m−2 for 239,240 Pu. It is worth noting that the activities and corresponding inventories of transuranics in sediment depend strongly on the sedimentation rate which is itself a function of local hydrodynamic conditions. However, as the suspended particle concentration in the Alboran Sea is very low, only a small fraction of transuranics is scavenged from the water column and deposited on the seafloor. In addition to activity profiles of the anthropogenic 137 Cs and 239,240 Pu, profiles of naturally occurring radionuclides such as 210 Pb and 226 Ra were determined in the sediment core, in order to estimate, through the excess 210 Pb, a sedimentation rate and to reconstruct the history of contaminants present in the sediment. 210 Pb found in the sediment has two components: (i) a supported 210 Pb, which comes from the 226 Ra decay that occurs in the sediment;


153

Fig. 3. Vertical distributions of 137 Cs and 239,240 Pu in the sediment core (Station 2).

Fig. 4. Excess 210 Pb profile in the sediment core (Station 2).

(ii) an unsupported or “excess” 210 Pb (210 Pbex ), which originates from 222 Rn decay in the atmosphere and the water column, and 210 Pb as its daughter product is deposited on the sediment. The second component is used in dating models of sediments. The 210 Pbex activity profile is shown in Fig. 4. The 210 Pbex activity in the top 3 cm of the sediment is almost uniform (∼600 Bq kg−1 ), which corresponds to the mixed sediment layer formed due to bioturbation processes in the sediment. Below the 3 cm layer, the activity profile is decreasing with depth following an exponential form, as a consequence of the radioactivity decay law. By applying the constant rate of supply of 210 Pbex model, a value of 0.12 cm yr−1 has been found for the average linear sedimentation rate. If the data is exploited for dating the sediment core, the peaks observed for anthropogenic 137 Cs and 239.240 Pu correspond to the period of 1962–1963, when the maximum global fallout deposition occurred.

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4. Conclusions The levels and behavior of anthropogenic radionuclides, with special emphasis on the 137 Cs, 239,240 Pu and 241 Am in Moroccan coastal waters have been presented in this paper. The most important findings can be summarized as follows: • The mean activity concentrations in surface water were around 2.7 mBq L−1 , 8.7 µBq L−1 and 1.5 µBq L−1 for 137 Cs, 239,240 Pu and 241 Am, respectively. These values are in an agreement with those reported in recent studies carried out in the western Mediterranean Sea, and are within the ranges of concentrations expected from global fallout. • The vertical distributions of 239,240 Pu in the water column of the southwest Mediterranean showed a sub-surface maximum at 500 m water depth, while 137 Cs concentrations were constant up to 900 m. On the contrary to the 239,240 Pu profile, which has been as expected, the 137 Cs profile has been affected by movement of water masses in the western Alboran Sea. • The total inventories of 137 Cs and 239,240 Pu in the water column (0–900 m) were estimated to be 2170 and 23 Bq m−2 , respectively, and they were as expected for this water depth. • The 239,240 Pu/137 Cs activity ratio was increasing with depth, suggesting a progressive separation of plutonium from cesium due to the Pu association with suspended matter. • The depth profiles of 137 Cs and 239,240 Pu in the sediment core showed a maximum peak which has been attributed to the maximum global fallout deposition which occurred in 1963. This result was confirmed by 210 Pb dating of the sediment core, estimating the sedimentation rate of 0.12 cm yr−1 .

Acknowledgements The authors thank colleagues from IAEA-MEL, CNESTEN and INRH for assistance during sampling and pre-treatment of collected samples. They also thank the Captain and the crew of the R/V Charif Al Idrissi for help during the sampling expedition. The support provided by IAEA for the RAF/7/004 project and for the sampling cruise is highly acknowledged. The IAEA is grateful for the support provided to its Marine Environment Laboratory by the Government of the Principality of Monaco.

References Delfanti, R., Desideri, D., Martinotti, W., Assunta Melti, M., Papucci, C., Queirraza, G., Testa, C., Triulzi, C. (1995). Plutonium concentration in sediment cores collected in the Mediterranean Sea. The Science of the Total Environment 173/174, 187–193. Fowler, S.W., Noshkin, V.E., La Rosa, J., Gastaud, J. (2000). Temporal variations in plutonium and americium inventories and their relation to vertical transport in the northwest Mediterranean Sea. Limnology and Oceanography 45, 446–458. Fukai, R., Holm, E., Ballestra, S. (1979). A note on vertical distribution of plutonium and americium in the Mediterranean Sea. Oceanologica Acta 2, 129–132. Fukai, R., Ballestra, S., Vas, D. (1980). Distribution of 137 Cs in the Mediterranean Sea. In: Management of Environment. Wiley Eastern Ltd, New Delhi, pp. 353–360.


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Holm, E., Fukai, R., Whitehead, N.E. (1988). Radiocesium and transuranium elements in the Mediterranean Sea: Sources, inventories and environmental levels. In: International Conference on Environmental Radioactivity in the Mediterranean Area. SNE, Barcelona, pp. 601–617. La Rosa, J., Burnett, W., Lee, S.H., Levy, I., Gastaud, J., Povinec, P.P. (2001). Separation of actinides, caesium and strontium from marine samples using extraction chromatography and adsorbents. Journal of Radioanalytical and Nuclear Chemistry 248, 765–770. Lee, S.H., Gastaud, J., La Rosa, J., Liong Wee Kwong, L., Povinec, P.P., Wyse, E., Fifield, L.K., Hausladen, P.A., Di Tada, L.M., Santos, G.M. (2001). Analysis of plutonium isotopes in marine samples by radiometrics, ICPMS and AMS techniques. Journal of Radioanalytical and Nuclear Chemistry 248, 754–764. Lee, S.-H., La Rosa, J.J., Levy-Palomo, I., Oregioni, B., Pham, M.K., Povinec, P., Wyse, E. (2003). Recent inputs and budgets of 90 Sr, 137 Cs, 239,240 Pu and 241 Am in the northwest Mediterranean Sea. Deep-Sea Research II 50, 2817–2834. León Vintró, I., Mitchell, P.I., Condren, O.M., Downes, A.B., Papucci, C., Delfanti, R. (1999). Vertical and horizontal fluxes of plutonium and americium in the western Mediterranean and the strait of Gibraltar. The Science of the Total Environment 237/238, 77–91. Merino, J., Sanchez-Cabeza, J.A., Bruach, J.M., Masqué, P., Pujol, L.I. (1997). Artificial radionuclides in high resolution water column profile from the Catalan Sea (the Northwestern Mediterranean). Radioprotection Colloques 32, C2. Mitchell, P.I., Vives, J., Batlle, I., Downes, A.B., Condren, O.M., León Vintró, L., Sanchez-Cabeza, J.A. (1995). Recent observations on the physico-chemical speciation of plutonium in the Irish Sea and the Western Mediterranean. Applied Radiation and Isotopes 46, 1190–1995. Molero, J., Sanchez-Cabeza, J.A., Merino, J., Pujol, L.I., Mitchell, P.I., Vial-Quadras, A. (1995). Vertical distribution of radiocesium, plutonium and americium in the Catalan Sea (Northwest Mediterranean). Journal of Environmental Radioactivity 26, 205–216. Papucci, C., Charmasson, S., Delfanti, R., Gasco, C., Mitchell, P., Sanchez-Cabeza, J.A. (1996). Time evolution and levels of man-made radioactivity in the Mediterranean Sea. In: Guéguéniat, P., Germain, P., Métivier, H. (Eds.), Radionuclides in the Oceans: Input and Inventories. Les Editions de Physique, Les Ulis, pp. 177–197. Sanchez-Cabeza, J.A., Molero, J. (2000). Plutonium, americium and radiocesium in the marine environment close to the Vandellos I nuclear power plant before decommissioning. Journal of Environmental Radioactivity 51, 211– 228. UNEP, United Nations Environmental Programme (1992). Assessment of the state of pollution of the Mediterranean Sea by radioactive substances. MAP Technical Reports Series No. 62. Athens 66.

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137 Cs

in seawater and sediment along the Algerian coast

A. Noureddinea,* , M. Menacera , R. Boudjenouna , M. Benkrida , M. Boulahdidb , M. Kadi-hanific , S.-H. Leed , P.P. Povinecd a Commissariat à l’Energie Atomique, Centre de Recherche Nucléaire d’Alger, Algiers, Algeria b Institut des Sciences de la Mer et de l’Aménagement du Littoral, Alger, Algeria c Universite des sciences et de la technologie Houari Boumediène, Alger, Algeria d International Atomic Energy Agency, Marine Environment Laboratory, Monaco

Abstract In order to define a baseline of natural and anthropogenic radionuclides, a sampling campaign along the Algerian coast was organised during the last quarter of 1999 to collect seawater and sediment samples. A total of 25 surface and water column samples were collected along the coast. In addition to a sediment core at Annaba station at the eastern coast, 41 surface sediment samples were collected each 25 km along the coast. 137 Cs activity concentrations in surface seawater ranged from 1.69 ± 0.11 to 3.3 ± 0.2 Bq/m3 with a mean value of 2.03 ± 0.13 Bq/m3 . A slight increase of 137 Cs activity (within a factor of two) from the western to the eastern coast of Algeria was observed. The 137 Cs water profiles had peaks at 80–100 m water depths. Massic activities of 137 Cs in surface sediment samples varied between 8.15 and 35.4 Bq/kg dry weight. The observed differences could be due to several reasons, namely the nature and composition of the sediment, and the water depth at the sampling location. Corresponding inventories of 137 Cs in the water column and sediment core were calculated, and a sedimentation rate using unsupported 210 Pb was estimated to be 0.2 cm/year. Keywords: Marine radioactivity, 137 Cs, 210 Pb, Sedimentation rate, Radionuclide inventory, Seawater, Sediment, Global fallout, Chernobyl accident, Mediterranean

1. Introduction The most common primordial radionuclides in the environment are 40 K and 238 U, 235 U and 232 Th, which are also parents of the three natural decay series, producing several radiogenic radionuclides with wide applications in environmental studies (e.g. 234 Th, 226 Ra, 222 Rn, 210 Pb, 228 Ra, etc.). In addition to that, a number of anthropogenic sources of radioactive contamination exist since the last century in the environment. These include global fallout from atmospheric * Corresponding author. Address: Centre de Recherche Nucléaire d’Alger (CRNA), 02 Bd. F. Fanon, BP 399 Algiers-Gare, 16000 Algiers, Algeria; phone: (+213) 21 43 44 44; fax: (+231) 21 43 42 80; e-mail: [email protected]



137 Cs in seawater and sediment along the Algerian coast

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nuclear weapons test carried out during the 1950s and 1960s (with a distinct peak in 1963), radioactive substances released from nuclear reprocessing facilities and other nuclear plants, dumping of radioactive wastes, and nuclear accidents, especially that of Chernobyl which occurred in 1986 (e.g. Burton, 1975; Livingston and Povinec, 2000). For the Mediterranean region the most important sources of anthropogenic radionuclides have been global fallout, the Chernobyl accident (with a release of about 10 ± 2 PBq of 137 Cs; UNEP, 1992) and nuclear facilities on the Rhône River. Anthropogenic radionuclides are introduced into the marine environment following many pathways. In order to study the behaviour of radionuclides, their transport and cumulative deposition, it is important to determine their concentrations in seawater and sediment (e.g. Calmet and Grauby, 1988; Buffoni and Cappelletti, 1997; Delfanti et al., 2001; Lee et al., 2003). The aim of this work, which was carried out in the framework of the Algerian monitoring and contamination assessment programme, and the IAEA’s Technical Cooperation project ALG/2/006, has been to define a baseline of natural and anthropogenic radionuclides in seawater and marine sediment along the Algerian coast, to estimate the corresponding inventory of 137 Cs in the water column and sediment, and to evaluate sedimentation rates using the 210 Pb method.

2. Materials and methods 2.1. Sampling The sampling campaign was organised during September–October 1999 by the Laboratory of radiological impact studies of the Centre de Recherche Nucléaire d’Alger (CRNA), on board of the research vessel M.S. Benyahia (belonging to the Institut des Sciences de la Mer et de l’Aménagement du Littoral (ISMAL)) in order to collect surface seawater, water column samples, surface sediment and a sediment core along the Algerian coast. Sampling locations are shown in Fig. 1. A total of 25 surface and water profile seawater samples (40–70 L) were collected. Surface samples were taken at a depth of approximately 5 m below the water line by means of a pumping system. Water column samples were taken using a 50 L Niskin PVC water-bottle sampler. The collected unfiltered seawater was first acidified with HCl to pH ≈ 1–2, spiked with 134 Cs for determining the chemical yield, and coprecipitated with ammonium molybdophosphate (AMP) to preconcentrate the sample to a few litres, in order to bring it to the laboratory for analysis (e.g. Roos et al., 1994). After pre-treatment and drying, the AMP precipitates were analysed in the laboratory by direct gamma-ray spectrometry. A total of 41 surface sediments were collected each 25 km along the coast from Ghazaouet station (35◦ 10 90N, 02◦ 55 857E) situated on the West, to El-Kala (near Annaba station) on the East (36◦ 59 49N, 08◦ 26 93E) (not shown in Fig. 1). A Van Veen Grab sampler was used for the sediment sampling at water depths ranging from 13 to 170 m. In addition, a sediment core was collected using a box corer at Annaba station (on the eastern coast, 36◦ 56 504N, 07◦ 47 750E), at the water depth of around 50 m. A multiparameter probe was also used at the same station to determine temperature, salinity and pH profiles of the water column. On board,

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A. Noureddine et al.

Fig. 1. Sampling stations for the water profiles and the sediment core collection.

surface layer sediments were put in plastic bags and labelled to record station co-ordinates, date and wet weight. The sediment core samples were cut with a metallic sheet each 2 cm until a depth of 22 cm. All sediment samples were then stored in a freezer to be later analysed in the laboratory by gamma-ray spectrometry. 2.2. Gamma-ray spectrometry A high purity germanium detector of 23% relative efficiency and resolution of 1.8 keV at 1332 keV (FWHM) has been used for analysis of gamma-ray emitters. The AMP seawater precipitates were placed in 100 cm3 plastic beakers, while the sediment samples after drying at 80◦ C, crushing into fine powder and homogenisation were put either into 100 cm3 plastic cylindrical containers or 500 cm3 Marinelli beakers. For the seawater samples two standards were prepared with the same geometry in which 134 Cs and 137 Cs spikes were injected (activity of 13 and 17 Bq, respectively) using two aliquots of wet AMP covering the different masses of AMP precipitates, in order to calculate the chemical yield using 134 Cs and to determine the detection efficiency using the 137 Cs spike. In the case of the sediments the detection efficiency curve was determined by measuring a standard sample of the same nature and geometry, in which a liquid radioactive solution of 152 Eu was injected. For the energies below 122 keV, a similar standard sample was spiked with a liquid radioactive source of 210 Pb. The seawater and sediment samples were counted until statistically reliable results were obtained (usually 48–72 h). A 4096 channels analyser with Genie-PC software (CANBERRA) was used to collect and analyse the spectra. The data quality was assured by analysing IAEA reference materials and by participation in intercomparison exercises: IAEA-381 – Irish Sea water for seawater samples (Povinec et al., 2002); IAEA-384 – Fangataufa lagoon sediment (Povinec and Pham, 2000); IAEA-385 – Irish Sea sediment, for sediment samples.


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3. Results and discussion 3.1. Seawater activity concentrations in surface seawater samples ranged from 1.59 ± 0.11 Bq/m3 (observed at the westernmost station at Ghazaouet) to 3.31 ± 0.21 Bq/m3 (observed at the easternmost station at Annaba), with a mean value of 2.03 ± 0.13 Bq/m3 (Table 1). This would indicate a weak west–east gradient, however, the observed levels differ only by a factor of two. As Atlantic waters with lower 137 Cs levels are entering the Mediterranean Sea as surface waters, it should be possible to observe a west–east gradient along the Algerian coast. Generally, the activity concentrations of 137 Cs in surface seawater are comparable with those measured by other authors in different regions of the Mediterranean Sea (Delfanti et al., 2001; Lee et al., 2003; Benmansour et al., 2006; Lee et al., 2006). The activity concentrations of 137 Cs in water profiles (Fig. 2) appear to increase with a water depth at the Algiers station, having a maximum of 2.41 ± 0.15 Bq/m3 at 100 m, and probably 137 Cs

Table 1 137 Cs activity concentrations in seawater along the Algerian coast

Longitude

Latitude

Sampling depth (m)

137 Cs concentration (Bq/m3 )

Skikda

36◦ 56 107N

06◦ 55 857E

surface

1.69 ± 0.11

Annaba

36◦ 56 504N

07◦ 47 750E

surface 15 25 35

3.31 ± 0.21 1.92 ± 0.12 2.67 ± 0.15 2.10 ± 0.12

Azzefoun

36◦ 55 300N

04◦ 22 000E

surface

1.81 ± 0.11

Jijel

36◦ 45 500N

05◦ 53 500E

surface

1.89 ± 0.11

Algiers

35◦ 10 90N

02◦ 05 000E

surface 50 75 100 130 160

2.09 ± 0.12 2.28 ± 0.14 2.35 ± 0.14 2.41 ± 0.15 2.31 ± 0.15 2.26 ± 0.14

Bejaia Ténès

36◦ 44 32N

05◦ 07 000E

surface 30

1.92 ± 0.12 1.72 ± 0.10

Mostaganem

36◦ 00 20N

00◦ 02 20W

surface 15 35 55

1.92 ± 0.12 1.72 ± 0.10 1.80 ± 0.12 2.29 ± 0.14

Ghazaouet

35◦ 10 90N

02◦ 05 000W

surface 50 75 100 130 160

1.59 ± 0.10 1.63 ± 0.10 1.87 ± 0.13 1.87 ± 0.12 1.72 ± 0.11 2.02 ± 0.12

Station/location

Coordinates

160


Fig. 2. 137 Cs water column profiles along the Algerian coast.

also at Ghazaouet station, having a maximum of 1.87 ± 0.13 Bq/m3 at 75–100 m. The other two stations (at Mostaganem and Annaba) were too shallow (55 and 35 m, respectively), so the subsurface 137 Cs maximum could not be observed. 137 Cs profiles in the water column of the south-western Mediterranean Sea obtained by other authors (Gheddou et al., 1999; Benmansour et al., 2006; Lee et al., 2006) seem to be similar to those obtained in this work. 3.2. Surface sediment Massic activities of 137 Cs in surface sediment ranged from 0.29 ± 0.04 Bq/kg dry weight (dw) to 13.1 ± 0.5 Bq/kg dw, with a mean value of 5.2 ± 0.2 Bq/kg dw. Surface sediments were sampled at different water depths (13–170 m) and different distances from the coast, having a variable grain size and organic carbon content (sand, mud). The variations in 137 Cs massic activities along the Algerian coast are shown in Fig. 3. The mean value of 5.2 ± 0.3 Bq/kg dw obtained for the Algerian coast is in a reasonable agreement with the mean value of 4.6 Bq/kg dw determined for the whole Mediterranean Sea (UNEP, 1992). Massic activities of 40 K in surface sediment samples, which depend on the nature of the sediments, ranged from 107 ± 4 Bq/kg dw to 630 ± 20 Bq/kg dw, with a mean value of 370 ± 20 Bq/kg dw. A weak correlation (R 2 = 0.51) is observed between 40 K and 137 Cs (Fig. 4), possibly due to their similar behaviour in the marine environment. The observed 137 Cs variations in surface sediments (Fig. 3) may be due to different water column depths and differences in physical (e.g. grain size) and chemical (e.g. organic carbon content) characteristics of the sediments.


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Fig. 3. Massic activities of 137 Cs in surface sediments collected each 25 km along the Algerian coast (from W to E).

Fig. 4. Correlation between 40 K and 137 Cs massic activities in surface sediments.

3.3. Sediment core Massic activities of 137 Cs in the sediment core collected at Annaba station (36◦ 56 504N, 07◦ 47 750E) ranged from 0.16 ± 0.02 Bq/kg dw (at the bottom of the core at 22 cm) to 12.4 ± 0.7 Bq/kg dw in the second layer (2–4 cm), which forms a peak in the distribution of 137 Cs in the core (Fig. 5). The sedimentation rate of 0.2 cm/yr estimated using the 210 Pb method suggests that the 137 Cs peak presented in Fig. 5 may be due to the Chernobyl accident.

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Fig. 5. 137 Cs in the sediment core collected at Annaba station (36◦ 56 504N, 07◦ 47 750E) compared with the profile of the eastern station (37◦ 12 07N, 08◦ 17 87E) collected in 2004 (water depth 550 m).

3.4.

137 Cs

inventory

Based on the 137 Cs activity concentrations determined in 1999, the corresponding inventory of 137 Cs in the water column along the Algerian coast was estimated to be 86 ± 9 Bq/m2 , 370 ± 20 Bq/m2 , 100 ± 10 Bq/m2 and 280 ± 20 Bq/m2 for Annaba, Algiers, Mostaganem and Ghazaouet stations, respectively. These values are relatively low, as they strongly depend on the water column depth, but are comparable with those obtained by other authors in the south-western Mediterranean when normalised for similar water depths (Lee et al., 2003, 2006). Figure 5 compares the 137 Cs sediment profile in Annaba station with one collected in 2004 at the eastern Algerian coast (37◦ 12 07N, 08◦ 17 87E; water depth 550 m). The profiles are quite similar, higher 137 Cs massic activities observed in surface layers of the eastern core may be due to different grain size composition of the sediment. The 137 Cs inventory in sediment estimated for Annaba station is 1500 ± 30 Bq/m2 , by about a factor of two lower than the total deposition due to global fallout for the latitude belt of 40–60◦ N (UNSCEAR, 2003). Due to shallow water depth almost all 137 Cs has been deposited on sediment. The total inventory (i.e. water and sediment) for Annaba station is 1600 ± 30 Bq/m2 , at least by a factor of two lower than results reported elsewhere in the Mediterranean (e.g. Lee et al., 2003), however, for water depths greater than 1000 m.


163

4. Conclusions We can conclude that anthropogenic 137 Cs found in seawater and in sediment, along the Algerian coast was affected mostly by global fallout and the Chernobyl accident. The observed 137 Cs activity concentrations in surface water and in water profiles, and the total inventories of 137 Cs in the water column are in a reasonable agreement with those given by other authors. The observed radionuclide levels in surface and core sediments have been dependant on physical characteristics (e.g. grain size) and chemical composition (e.g. organic carbon content) of sediments.

Acknowledgements The authors wish to thank ISMAL Director M. Boulahdid, the scientific team and the crew who participated in the cruise on board of R/V M.S. Benyahia for support and assistance during sampling. Thanks are also due to the IAEA for support provided through the Technical Cooperation project ALG/2/006 (1996–1999). The IAEA is grateful for the support provided to its Marine Environment Laboratory by the Government of the Principality of Monaco.

References Benmansour, M., Laissaoui, A., Benbrahim, S., Ibn Majah, M., Chafik, A., Povinec, P.P. (2006). Distribution of anthropogenic radionuclides in Moroccan coastal waters and sediments. In: Povinec, P.P., Sanchez-Cabeza, J.A. (Eds.), Radionuclides in the Environment, International Conference on Isotopes in Environmental Studies: Aquatic Forum 2004. 25–29 October, Monaco. Elsevier, Amsterdam, pp. 148–155, this volume. Buffoni, G., Cappelletti, A. (1997). On the accumulation–dispersion processes of the 137 Cs tracer in the Italian seas. Journal of Environmental Radioactivity 37, 155–173. Burton, J.D. (1975). Radioactive nuclides in the marine environment. In: Riley, J.P., Skirrow, G.S. (Eds.), second ed. Chemical Oceanography, vol. 3. Academic Press, London. Calmet, D., Grauby, A., (1988). Distribution spatio-temporelle des radioéléments anthropogéniques du bassin Méditerranéen occidental 1980–1987. In: International Conference on Environmental Radioactivity in the Mediterranean Area. Barcelone, 1988, pp. 543–568. Delfanti, R. et al. (2001). The new distribution of the tracer 137 Cs in the eastern Mediterranean relationship to the deepwater transient. Rapport du 36ème Congrès de la CIESM. CIESM, Monte Carlo (Monaco), 36 pp. Gheddou, A., Noureddine, A., Menacer, M., Boudjenoun, R., Hammadi, A. (1999). Distribution of 137 Cs in surface and deep water in the central part of Algerian littoral. Marine Pollution, IAEA-TECDOC-1098. Lee, S.-H., Larosa, J.J., Palomo, I.L., Oregioni, B., Pham, M.K., Povinec, P.P., Wyse, E. (2003). Recent inputs and budgets of 90 Sr, 137 Cs, 239,240 Pu and 241 Am in the northwest Mediterranean Sea. Deep-Sea Research II 50, 2817–2834. Lee, S.-H., Mantoura, F.R., Povinec, P.P., Sanchez-Cabeza, J.A., Pontis, J.-L., Mahjoub, A., Noureddine, A., Boulahdid, L., Samaali, M., Reguigui, N. (2006). Distribution of anthropogenic radionuclides in the water column of the south-western Mediterranean Sea. In: Povinec, P.P., Sanchez-Cabeza, J.A. (Eds.), Radionuclides in the Environment, International Conference on Isotopes in Environmental Studies: Aquatic Forum 2004. 25–29 October, Monaco. Elsevier, Amsterdam, pp. 137–147, this volume. Livingston, H.D.L., Povinec, P.P. (2000). Anthropogenic marine radioactivity. Ocean and Coastal Management 43, 689–712. Povinec, P.P., Pham, M.K. (2000). Report on the Intercomparison Run IAEA-384 Radionuclides in Fangataufa Lagoon Sediment. IAEA/MEL/68. IAEA, Vienna, 46 pp.

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Povinec, P. et al. (2002). Certified reference material for radionuclides in seawater IAEA-381 (Irish sea water). Journal of Radioanalytical and Nuclear Chemistry 251, 369–374. Roos, P., Holm, E., Persson, R.B.R. (1994). Comparison of AMP precipitate method and impregnated Cu2 [Fe(CN)6 ] filters for the determination of radio-caesium concentrations in natural waters. Nuclear Instruments and Methods in Physics Research A 39, 282–286. UNEP (1992). Assessment of the state of pollution of the Mediterranean Sea by radioactive substances. MAP Technical Reports Series 62. Athens, 60 pp. UNSCEAR (2003). Sources and effects of ionising radiation. United Nations, New York, 123 pp.

Further reading Delfanti, R., Papucci, C., Alboni, M., Lorenzelli, R., Salvi, S. (1995). 137 Cs inventories in the water column and in sediments of the west Mediterranean Sea. Rapports Commission Internationale pour l’exploration scientifique de la Mer Méditerranée 34, 226–230. Merino, J., Sanchez-Cabeza, J.A., Bruach, J.M., Masqué, P., Pujol, L. (1997). Artificial radionuclides in a high resolution water column profile from the Catalan Sea (the Northwest Mediterranean). Radioprotection – Colloques 32 (C2), 85–90. Millot, C. (1987). Circulation in Western Mediterranean Sea. Oceanologica Acta 10, 2–8. Molero, J., Sanchez-Cabeza, J.A., Merino, J., Pujol, Ll., Mitchell, P.I., Vidal-Quadras, A. (1995). Vertical distribution of radiocaesium, plutonium and americium in the Catalan Sea (Northwest Mediterranean). Journal of Environmental Radioactivity 26, 205–216. Noureddine, A., Benkrid, M., Hammadi, A., Boudjenoun, R., Menacer, M., Khaber, A., Kecir, M.S. (2003). Radioactivity distribution in surface and core sediment of the central part of the Algerian coast: An estimation of the recent sedimentation rate. Mediterranean Marine Science 4 (2), 53–58. OECD (1971). The cycling of artificial radionuclides through marine food chains. Proc. Marine Radioecology, Hamburg.

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Physical and chemical characteristics of 137Cs in the Baltic Sea Galina Lujanien˙ea,* , K˛estutis Jokšasb , Beata Šilobritien˙ea , Rasa Mork¯unien˙ec a Institute of Physics, Savanoriu ave 231, LT-02300 Vilnius, Lithuania ˛ b Institute of Geology and Geography, T. Ševˇcenkos 13, LT-2600 Vilnius, Lithuania c Vilnius Gedimino Technical University, Saul˙etekio al. 11, LT-2040 Vilnius, Lithuania

Abstract 137 Cs in seawater, suspended particles and bottom sediments was studied with the aim to better understand its behavior, redistribution and sink in the Lithuanian coastal area of the Baltic Sea. High massic activities of 137 Cs (over

1000 Bq/kg dry weight) associated with fine particles of bottom sediments can be attributed to an increase in the specific surface area and variations in mineralogical composition of sediments participating in sorption processes. A clear correlation was found both between the 137 Cs activity and the content of clay particles (r = 0.95, n = 16), as well as the amount of total organic carbon (r = 0.75, n = 16). 137 Cs activity of particulates depends strongly on the sampling depth of seawater. The study has indicated a complicated sorption–desorption behavior of 137 Cs in the Baltic Sea as a result of which it can be mobilized by suspended particles or released to seawater. The fate of 137 Cs can be considerably affected by mineralogical composition of suspended particles and bottom sediments, as well as by geochemical characteristics of seawater. Keywords: Cesium-137, Seawater, Speciation, Suspended particles, Bottom sediments, Sorption–desorption, Distribution coefficient (Kd ), Baltic Sea

1. Introduction The distribution of radionuclides between the particulate, colloidal and truly dissolved phases can strongly influence transport processes and bioavailability of contaminants in the aquatic systems. Among parameters that control the transport of radionuclides associated with particles, the size distribution of particles carrying radionuclides, their density and chemical composition are the most important. However, in the zone of interaction of fresh and saline waters under salinity and activity concentration gradients, the sorption–desorption process can lead to transformation of nuclide associations. In addition, the removal of particles of higher density and larger size occurring during settling can affect the properties of suspended matter * Corresponding author. Address: Institute of Physics, Savanoriu av. 231, LT-02300 Vilnius, Lithuania; phone: ˛ (+370) 5 264 48 56; fax: (+370) 5 260 23 17; e-mail: [email protected]



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(e.g. mineralogical composition) and can result in different binding of radionuclides and their different removal from the water column. To assess the transfer and fate of radioactive contaminants in the marine environment the information on physical and chemical parameters of radionuclide carriers is required. Radioactive contamination of the Baltic Sea was caused by three main factors such as global fallout, discharges from the reprocessing plants and the fallout after the Chernobyl accident in April 1986. At present, the average activity concentration of 137 Cs in surface water of the Baltic Sea has been estimated to be about 60 mBq/L, while the worldwide average concentration due to global fallout is about 2 mBq/L (Livingston and Povinec, 2000). The Baltic Sea is a semi-enclosed, largest brackish water and unique shallow sea in which self-cleaning processes are slow and dissolved substances remain there for a long time. Measurements of total activity concentrations of 137 Cs as well as other artificial and natural radionuclides in the Baltic Sea water have been carried out in many riparian countries (Nielsen, 1997; Nielsen et al., 1999). However, there is a lack of information on the speciation of radionuclides in the Baltic Sea, their bioavailability, migration and on self-cleaning processes. There is no doubt that consistent pattern of radionuclide migration, processes of selfcleaning and their redistribution in the environment depend on their specific chemical forms or type of binding rather than on the total element or nuclide content. However, the direct determination of speciation or binding forms of radionuclides is difficult and often hardly possible due to very low concentrations of radionuclides typically found in the environment. Therefore, the determination of physico-chemical forms or fractions in practice, using sequential extraction methods, is a reasonable compromise to evaluate the associations of radionuclides in the environmental samples (e.g. with carbonate minerals, Fe/Mn oxides and organic substances). Although the results obtained by a sequential extraction procedure are operationally defined and cannot be used as input data for thermodynamic equilibrium models, they provide useful information about the behavior of radionuclides. Knowledge about association of radionuclides with geochemical phases is important from the point of view of radiation protection, as binding of radionuclides and the stability of geochemical phases provide data on bioavailability and migration ability of radionuclides. At present, the only available tool to study radionuclide associations at low-level concentrations is sequential extraction method. These methods were criticized because of incomplete selectivity of reagents used to dissolve one particular phase without the additional attack of other geochemical phases and readsorption of analytes after release. The results obtained by Khebonian and Bauer (1987) using synthetic models and their interpretation were intensively discussed since the appearance of their publication. Re-adsorption processes of some elements were observed for iron and organic rich sediments, and non-selective extraction was found during extraction of elements from anoxic sediments. Recently obvious advantages were achieved in analyses of speciation of heavy metals (Sahuquillo et al., 2003). The most widely used sequential procedure proposed by Tessier et al. (1979) was modified in order to avoid the mentioned problems by changing the extraction time, the extractant-sample ratio, the reagent concentration ant the extraction temperature. The aim of this study was to determine speciation of 137 Cs suspended particles and bottom sediments in order to better understand its behavior, redistribution and sink in the Lithuanian coastal area of the Baltic Sea.

Physical and chemical characteristics of 137 Cs in the Baltic Sea

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2. Material and methods 2.1. Sampling Water and bottom sediment samples were collected during the expedition in the Lithuanian economical zone of the Baltic Sea, the Curonian Lagoon and near the seashore of the Baltic Sea and the Curonian Lagoon in 1999–2001. The sampling locations are shown in Fig. 1. The studied area is characterized by horizontal and vertical salinity gradient: (i) salinity ranges from 0.5h to 7h in surface waters and from 5h to 9h in near bottom waters; (ii) activity concentrations of 137 Cs vary from 1 mBq/L in fresh waters up to 100 mBq/L in seawater. Measurements performed in the Institute of Geology and Geography indicated a strong loading by particulate matter: in the Curonian Lagoon the concentration of suspended particles differed from 1 to 103 mg/L, in the Baltic near-mouth – from 2 to 29 mg/L and in the Baltic Sea open waters – from 1 to 41 mg/L (Galkus and Joksas, 1997). The seasonal variations are mostly related to plankton blooming periods. It should be noted that all studies were performed in the transitional–accumulation zone with a complex current regime. The intrusions of Baltic Sea water into the Curonian Lagoon caused by differences in the water level, different hydrometeorogical conditions and various anthropogenic activities can influence the accumulation of 137 Cs in the bottom sediments. The amount of sampled water was depended on the 137 Cs detection limit, and varied from 50 to 200 L. In addition, the suspended particle samples were collected in situ by filtering a large volume of water (∼400–1000 L) through two consecutive 5 µm and two consecutive 1 µm polypropylene Sediment Filter Cartridges (US Filter Plymouth Products). Bottom sediment samples were collected during different sampling campaigns in 1999– 2001. The bottom sediments in the Baltic Sea were collected using a Van Veen grab sampler available on the R/V “V˙ejas”. The Bottom Sampler acc. Ekman–Birge with an effective grasping area of 225 cm2 and weight of 3.5 kg was used for the bottom sediment sampling in the Curonian Lagoon. 2.2. Sample preparation The separation of the suspended matter was achieved using membrane filtration. Water (from 50 to 800 L) was filtered through Nuclepore (Dubna) membrane 0.2 µm and Filtrak 388 prefilters using the filtration equipment (Millipore) consisting of the Dispensing Pressure Vessel of 10 L and a Stainless Steel Filter Holder of 293 mm. It is very well known that the separation of suspended particles of 1–0.2 µm from large volume samples is a very time consuming and artefacts prone process, e.g. the size of separated particles can be changed significantly due to clogging (Salbu et al., 1985). To avoid the clogging effect the filtration process was carefully controlled and filters were changed on time. The activity of 137 Cs in some samples of suspended matter was close to detection limits for gamma-spectrometric detection, therefore cesium was separated radiochemically. The samples were digested using HF/HNO3 and HCl, and then cesium was precipitated as Cs3 Sb2 I9 . Chemical yield was determined gravimetrically.

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Fig. 1. Sampling locations in the Baltic Sea and the Curonian Lagoon.


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To determine aqueous speciation the filtered water passed through four subsequent cartridges filled with the impregnated Mtilon-T fiber. The Mtilon-T fiber developed in Moscow Textile Institute by modification of cellulose and polyacrylonitrile copolymer having thioamide functional groups is widely used for cesium pre-concentration (Vakulovsky et al., 1985). The Mtilon-T fibre impregnated with Ni Ferro cyanide (the efficiency – 99%) was used to separate ionic and non-ionic Cs species in seawater. The dependence of the preconcentration efficiency of 137 Cs from seawater on pH, temperature and the flow rate was studied. In order to provide a complete collection of ionic 137 Cs from seawater, four consecutive cartridges were used. The details of the separation method were described previously (Lujanien˙e et al., 1998). The ionic fraction of 137 Cs sorbed on the impregnated Mtilon-T was determined directly by gamma-spectrometry, while the fraction passing through cartridges so-called “non-ionic 137 Cs fraction” was separated from seawater radiochemically prior to measurements. For the determination of physical and chemical association of radionuclides, a separation of particles of different sizes from bottom sediment samples was performed using wet sieving and column settling techniques. The fractions >50 µm, 50–4 µm and 50 µm, 50–4 µm, 4–1 µm and 1–0.2 µm. The separation of 1–0.2 µm fraction was achieved using membrane filtration. The modified Tessier sequential extraction method was used for the investigation of radionuclide chemical forms. The following extracting agents were used: F1 – 1 M NH4 Cl, pH = 7.0 (exchangeable); F2 – 1 M NH4 C2 H3 O2 , pH = 5 CH3 COOH (carbonate bound); F3 – 0.04 M NH2 OHHCl in 25% CH3 COOH (oxide bound); F4 – 30% H2 O2 at pH = 2 (HNO3 ), then 3.2 M NH4 C2 H3 O2 in 20% HNO3 (organically bound); F5 – 40% HF, HNO3 , or measured directly by gamma-spectrometry (residual). 2.3. Measurements Total carbon (TC) and TOC were determined using a LECO CS-125 analyzer. 133 Cs concentration in seawater was determined using ICP-MS. Grain size distribution was measured by the gravimetric pipette method. Clay minerals were identified by X-ray diffraction. XRD analyses were conducted using a D8 (Bruker AXS) X-ray diffractometer. Cs from suspended matter samples separated radiochemically was measured using the proportional Emberline FHT 770 T Multi-Low-Level-Counter. 137 Cs activities were measured with HPGe detector (resolution 1.9 keV at 1.33 MeV, the relative efficiency 42%). Measuring time varied according to sample activities. An efficiency calibration of the system was performed using calibration sources (prepared from a solution supplied by Amersham, UK) of different densities and geometry that were close to measured samples. Accuracy and precision of analysis was tested in intercomparison runs, organized by the Riso National Laboratory, Denmark (mineral, sea and lake water matrices). Precision of 137 Cs measurements by gamma-spectrometry was 1 µm in size could take place. Data of different sampling campaign indicated the losses of large portion of fine particles up to 30% in the near-shore zone using the filters of 1 µm as compared with filters of the 0.2 µm pore size. In addition, this portion increased in open waters where fine particles are predominant. During transport from the Curonian Lagoon in the saline and fresh water mixing zone an intensive coagulation and deposition of particles take place (Fig. 6). However, not only a fractionation of suspended particles according to sizes (a decrease of suspended particles >1 µm and an increase in the amount of particles of 0.2–1 µm carrying 137 Cs depending on the distance from the Klaip˙eda Strait), but also changes in the chemical/mineralogical composition of suspended particles are possible. It seems that particles of various sizes and mineralogical composition can display different sorption ability. From particles of the >1 µm size 137 Cs can be released into seawater, on contrary, the fine fraction of suspended particles of 0.2–1 µm and possibly smaller, is responsible for sorption and removal of 137 Cs from the water column. Thus, different sorption–desorption behavior of cesium can be explained by different nature of particles. Possibly, the fine fraction of suspended particles collected in May 1999 consists mainly of clay minerals which can more effectively adsorb Cs even from brackish


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Fig. 5. Activity of particulate 137 Cs versus concentration of suspended particles, salinity and concentration of potassium (October 2001).

Fig. 6. Variations of activity of particulate 137 Cs (Bq/kg), concentration of suspended particles (mg/mL) depending on the distance from the shore and the Klaip˙eda Strait.

seawater containing high concentrations of potassium. The clay minerals of Muscovite group can be responsible for an increase in the activity of particulate 137 Cs during particle transport and settling in seawater. The collected suspended matter exhibited rather different sorption ability due to different cut-off filters used during the sampling performed in October 2001. However, sorption–desorption behavior of 137 Cs depends not only on the origin of suspended particles, but also on the concentration of ions, which can compete with 137 Cs for sorption places. Thus, information about the concentration of stable cesium (133 Cs) at the site is also important for quantitative assessment of accumulation of radiocesium on suspended particles, because the concentration of 133 Cs is usually higher by 6–8 orders of magnitude in comparison with 137 Cs. During the sampling campaign in October 2001 the measured concentrations of 133 Cs ranged from 0.03 to 0.05 ppb, nevertheless, any relation of massic activity of particulate 137 Cs to the concentration of 133 Cs in seawater was not observed. Perhaps, high concentrations of potassium (which varied from 50,900 to 75,700 ppb) smothered the influence of 133 Cs. Another possible explanation could be in differences in speciation and a lack of equilibrium of 133 Cs and 137 Cs isotopes in seawater. If the equilibrium between 133 Cs and

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isotopes has not been reached yet (after the Chernobyl accident), the quantitative assessment and applications of laboratory experiments for radiocesium transport are difficult because of absence of data on real Cs concentrations in seawater. The selective adsorption of Cs by clay minerals has been attributed to the large ionic radius, uncomplicated nature, and especially to its low hydration energy. Although cations with similar charge and ionic radii are expected to compete with cesium, the sequence of sorption ability of alkali elements Na+ < K+ < R+ < Cs+ is in good agreement with the sequence of effective ionic radii of alkali elements and the sequence of single ion hydration enthalpies of alkali elements (Richens, 1997). In the clay mineral muscovite, a negative fixed charge arises primarily from isomorphic substitution of Al3+ for Si4+ in the tetrahedral sheet comprising the siloxane site. In illite mineral whose composition is very close to that of muscovite, isomorphic substitution of Al3+ for Si4+ and partly of Fe2+ and Mg2+ for Al3+ enhances the stability of the Cs+ -siloxane surface complex (Jackson, 1962). Strongly sorbed Cs+ on fraered edge sites (FES), external basalt sites, or within the interlayer exists as an inner-sphere, dehydrated surface complex, which usually is much more stable than outer-sphere complexes (Kemner et al., 1997). Cesium sorbed to outer-sphere complexes can be easily desorbed and is distinguished for higher mobility in the environment, while inner-sphere sorption complexes can limit the Cs transport and bioavailability. In smectites, the isomorphous substitution in both tetrahedral and octahedral layers generates weak negative charges of sheets thus leading to the formation of a structural feature and resulting in wide ranges of cation exchange capacity, selectivity and swelling properties. Water and cations such as H+ , Na+ , Ca2+ , Mg2+ can easily penetrate into the smectite interlayer and participate in exchange processes and, for example, from a species as beidellite the sorbed cation can easily be ion-exchanged by the other cations. However, the existence of electro statically stable sorption site for a cation whose ionic size is small enough to enter the hole was estimated for montmorillonite (Onodera et al., 1998). The complicated behavior of cesium can be the result of peculiarities of its sorption to various sorption sites of different clay minerals and their mixtures. The specific surface area and mineralogical composition of particles of different sizes can differ considerably and can result in the variation of binding ability of suspended particles. 3.3. Association of 137 Cs with bottom sediments Massic activities of 137 Cs in bottom sediments of the Baltic Sea and the Curonian Lagoon ranged from 4 to 450 and from 0.4 to 596 Bq/kg dw, respectively. Studies performed on the particle size distribution and 137 Cs activities in bottom sediments indicated variations in 137 Cs activities associated with particles (62–1450 Bq/kg dw for particles 0.2–1 µm (n = 6), 62–836 Bq/kg dw for particles 1–4 µm (n = 20), 29–242 Bq/kg dw for particles 4–50 µm (n = 20) and 3–64 Bq/kg dw for particles >50 µm (n = 20). The data on the particle size distribution in six sediment samples are presented in Fig. 7. An increase in the massic activities of 137 Cs (over 1000 Bq/kg dw) observed in fine particles can be due to the increase in the specific surface area of sediments participating in sorption processes. Another possible explanation could be associated with variations in the mineralogical composition of different size particles. The presence of illite (hidromuskovite) mineral whose characteristic particles size distribution is from 0.002 to 2.9 µm, and smectite


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Fig. 7. Massic activities of 137 Cs in bottom sediments and average physico-chemical characteristics of 137 Cs in bottom sediments of the Baltic Sea (F1 – exchangeable, F2 – carbonates, F3 – oxides, F4 – organic, F5 – residue).

with typical particles size distribution from 0.01 to 1.0 µm can be expected in fine fractions (Nelson, 2003). It seems therefore that the ability of bottom sediments to bind cesium varied with the particles size due to their different mineralogical composition. Moreover, not only an increase in 137 Cs activity of sediments with a decrease in particle sizes was observed, but also wide ranges of 137 Cs activities were found for smaller particles. Possibly heterogeneous origin of sediments is responsible for wide ranges of 137 Cs activities in fine fraction. It should be noted that a comparatively smaller amount of fine particles of 0.2 µm in near bottom samples in comparison with surface ones, and the differences (or “excess”) between the surface and near bottom activities increases with sampling depth. The 137 Cs Kd values changed in the similar way (Fig. 4) and the “excess” of Kd s could be regarded as a function of residences time of particles in the water column. The residence time of clay particles of density 2.7 g/cm3 and the average diameter (which varied from 5 to 0.5 µm depending on the distance from the shore) was evaluated using differences in the sampling depth and data on the sinking velocity of particles in a non-turbulent water column at 10◦ C (Ruchin, 1957). The 134 Cs Kd and 137 Cs Kd values obtained from two kinetic and one field experiments as function of time are presented in Fig. 9. It should be noted that the residence time of particles was estimated rather approximately due to very complex natural conditions and variability of different parameters. It is obvious, that size distribution of particles should be evaluated more precisely as well. Larger particles carrying 137 Cs are predominant in the near-shore zone and their residence time decreases, but at the remote sampling stations the spectrum of particle sizes changes towards the smaller ones (Fig. 6), and therefore they are sinking with lower velocities. In addition, turbulences in natural systems will reduce sinking velocities. Moreover, a halocline can serve as a particular trap for settling particles and increasing their residence time in the near bottom layer, where higher salinity and activity concentrations of 137 Cs can affect the sorption of 137 Cs to suspended particles. It should be noted that activity concentrations of 137 Cs in surface and near-bottom water sam-

Fig. 9. Cs Kd values as a function of time (data of two laboratory experiments (I exp., II exp.) and field experiment in the Baltic Sea (BS) in May 1999).

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ples varied insignificant during the sampling campaign of May 1999. Usually the explanation of increase in activity concentrations of 137 Cs in near-bottom layer is linked to the anoxic conditions, and the diffusion of Cs ions from the bottom sediments. However, anoxic conditions are not characteristic for the study area, and an increase in concentrations of NH4 + ions (which are formed under anoxic conditions and are responsible for desorption of Cs from FES of clay minerals) in the near bottom layer was not observed (Jokšas, 2005). Thus, despite of complexity of natural conditions and variability of different parameters, this comparison indicated that the Cs Kd values obtained from various experiments differ insignificantly. The observed alterations can be mainly attributed to the differences in the size distribution of particles participating in the sorption process, to the variations in Cs concentrations in the kinetic experiments and turbulences in natural water systems. It seems that the 137 Cs K values obtained from measurements carried out in May 1999 reflect (with mentioned d uncertainties) sorption processes, however, 137 Cs Kd values calculated using the 137 Cs activity in bottom sediments can be attributed to the complicated long-term processes. Possibly, Kd data obtained from laboratory experiments can be used for short-term predictions (up to one year), but Kd values from field measurements are more suitable for appropriate long-term predictions.

4. Conclusions Our study has indicated complicated sorption–desorption behavior of 137 Cs in Baltic Sea waters as a result of which it can be mobilized by suspended particles or released to seawater. Thereby, the fate of 137 Cs can be considerably affected by mineralogical composition of suspended and bottom sediments, by characteristic geochemistry of seawater and the presence of other contaminants. Small 1–4 and 1–0.2 µm particles can play an important role in adsorption of 137 Cs from seawater. Particles distinguished for high specific adsorption ability can effectively remove Cs from water column; they can be easily re-suspended and transported over long distances.

Acknowledgements This work was performed under the auspices of IAEA under project LIT/7/002. In addition, we would like to express our gratitude to all project participants as well as to Prof. P.P. Povinec for encouragement, to Riso National Laboratory and Dr. Sven P. Nielsen for support.

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Ewais, T.A., Grant, A., Fattah, A.T.A. (2000). The role of surface coatings on sediments in sediment: Water partitioning of trace elements and radionuclides. Journal of Environmental Radioactivity 49, 55–64. Galkus, A., Joksas, K. (1997). Sedimentary Material in the Aquatic Transition Zone. Institute of Geography, Vilnius, 198 pp. (In Lithuanian.) Jackson, M.L. (1962). Interlaying of expansible layer silicates in soils by chemical weathering. In: Trans. 11th Nat. Conf. Clays Clay Min., pp. 29–46. Jokšas, K. (2005). Unpublished data. Livingston, H.D., Povinec, P.P. (2000). Anthropogenic marine radioactivity. Ocean & Coastal Management 43, 689– 712. Lujanien˙e, G., Mork¯unien˙e, R., Styra, B. (1998). Speciation of 137 Cs in the Baltic Sea water. Environmental Physics 1, 34–42. Lujanien˙e, G., Šilobritien˙e, B., Jokšas, K., Mork¯unien˙e, R. (2004). Behaviour of radiocesium in marine environment. Environmental Research, Engineering and Management 2 (28), 23–32. Lujanien˙e, G., Vilimait˙e-Šilobritien˙e, B., Jokšas, K. (2005). Accumulation of 137 Cs in bottom sediments of the Curonian Lagoon. Nukleonika 50 (1), 23–29. Nelson, S.A. (2003). Clay minerals. Geology, Mineralogy 211. Nielsen, S.P. (1997). Comparison between predicted and observed levels of 137 Cs and 90 Sr in the Baltic Sea. Radioprotection – Colloques 32, C2-387–C2-394. Nielsen, S.P., Bengtson, P., Bojanowsky, R., Hagel, P., Herrmann, J., Ilus, E., Jakobson, E., Motiejunas, S., Panteleev, Y., Skujina, A., Suplinska, M. (1999). The radiological exposure of man from radioactivity in the Baltic Sea. The Science of the Total Environment 237/238, 133–141. Kemner, K.M., Hunter, D.B., Bertsch, P.M., Kirkland, J.P., Elam, W.T. (1997). Determination of site-specific binding environments of surface sorbed cesium on clay minerals by Cs-EXAFS. Journal de Physique IV 7, 777–779. Khebonian, C., Bauer, C.F. (1987). Accuracy of selective extraction procedures for metal speciation in model aquatic sediments. Analytical Chemistry 59, 1417–1423. Onodera, Y., Iwasaki, T., Ebina, T., Hayashi, H., Torii, K., Chatterjee, A., Mimura, H. (1998). Effect of layer charge on fixation of cesium ions in smectites. Journal of Contaminant Hydrology 35, 131–140. Richens, D.T. (1997). The Chemistry of Aqua Ions, Syntheses, Structure and Reactivity. Wiley, Chichester. Appendices 2 and 3, 5. Ruchin, L.B. (1957). Methods of investigation of sedimentary material. Institute of Geology of Academy of Science USSR, vol. 1, 611 pp. (In Russian.) Sahuquillo, A., Rigol, A., Rauret, G. (2003). Overview of the use of leaching/extraction tests for risk assessment of trace metals in contaminated soils and sediments. Trends in Analytical Chemistry 22, 152–159. Salbu, B., Bjornstad, H.E., Lindstrom, N.S., Lydersen, E., Brevik, E.M., Rambaek, J.P., Paus, P.E. (1985). Size fractionation techniques in the determination of elements associated with particulate or colloidal material in natural fresh waters. Talanta 32, 907–913. Tessier, A., Campbell, P.G.C., Bisson, M. (1979). Sequential extraction procedure for the speciation of particulate trace metals. Analytical Chemistry 51, 844–851. Vakulovsky, S.M., Lishevskaja, M.O., Nikitin, A.I., Chumichiov, V.B., Shkurko, V.N. (1985). Method of preconcentration of radiocaesium from sea water using fiber sorbents. Trudy Gosudarstvennogo Okeanograficheskogo Instituta 174, 83–88.

Further reading von Gunten, H.R., Benes, P. (1995). Speciation of radionuclides in the environment. Radiochimica Acta 69, 1–29.


4. Radioecological studies


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Comparison of the MARINA II dispersion model with CSERAM for estimating concentrations of radionuclides in UK waters Kamaljit Sihraa,* , Antony Bexona , John Aldridgeb a National Radiological Protection Board, Chilton, Didcot, Oxon, OX11 0RQ, UK b The Centre for Environment, Fisheries and Aquaculture Science, Lowestoft Laboratory, Pakefield Road,

Lowestoft, Suffolk, NR33 0HT, UK Abstract A strategy was agreed in 1998 by the OSPAR Commission to achieve, by 2020, near zero concentrations of anthropogenic pollutants and near to background levels for pollutants that also occur naturally. One of the uses of the MARINA II model was to test the feasibility of this strategy, given inherited activity concentrations. The model has been validated extensively within the OSPAR region using spatial measurements of nine radionuclides, including 99 Tc, 137 Cs and 239/240 Pu, over the period 1990–2000. Typically the model is found to agree with measurements to within a factor of three, with a marginal spatial bias towards underestimating activity concentrations. In this study, future estimates of activity concentration were tested in a comparison to CSERAM, a high-resolution, physically based model of the Irish Sea, for Sellafield discharges of 137 Cs and 239/240 Pu. Both models show good agreement to the year 2000, although differences of up to an order of magnitude can be observed close to the discharging source by 2020. Typically both models agree to within a factor of five elsewhere by 2020. This study illustrates that MARINA II is a computationally inexpensive but effective tool for calculations of activity concentration in radiation protection. Keywords: MARINA II, CSERAM, Marine modelling, Model comparison, OSPAR

1. Introduction A strategy was agreed in 1998 by the OSPAR Commission (OSPAR, 1998) to achieve, by 2020, close to zero concentrations of anthropogenic radioactive substances and close to background levels for pollutants that occur naturally. To determine whether or not this is achievable requires a comprehensive and robust marine model, together with knowledge of annual radioactive discharges to the marine environment. By modelling several different future discharge strategies to 2020, including a scenario where discharges stop in the year 2000, it is possible to estimate the contribution of historic discharges to predicted activity concen* Corresponding author. Address: National Radiological Protection Board, Chilton, Didcot, Oxon, OX11 0RQ, UK; phone: (+44) 1235 831600; fax: (+44) 1235 833891; e-mail: [email protected]



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trations. Marine models, such as the MARINA II model, can be used to estimate activity concentrations for this purpose. MARINA II is a 72-compartment marine model of North European waters. It was developed from the MARINA model (CEC, 1990) in order to provide: better spatial resolution in the English Channel, Atlantic Ocean and Arctic Ocean; more realistic flows between compartments in the Irish Sea; a refined sediment model to improve remobilisation of radionuclides from the sediment layer to the water column. Activity concentrations are calculated annually; therefore seasonal effects are not resolved. However, the model is computationally efficient, a requirement necessary for radiation protection calculations where radiation doses are estimated over long periods. A brief description of the MARINA II model is given in Section 2; a more complete description of the model and its validation is given elsewhere (European Commission, 2002). Compartment models are typically tuned to observational data and therefore are most reliable over the time periods for which data are available. To test the model outside these conditions, MARINA II was compared to CSERAM, a physical, process-based, high-resolution model of the Irish Sea (Aldridge et al., 2003). This model attempts to describe the underlying physical processes directly, an approach that is clearly preferable, where data are available, to model dispersion over short timescales where physical effects, such as tides, need to be resolved. For longer term, radiation protection calculations however, the computational cost for routine multi-decadal estimates of activity concentrations are likely to be prohibitively expensive. The CSERAM model is significantly different in modelling philosophy and should therefore provide a useful insight into both the effectiveness and the limitations of the MARINA II model. The discharge scenario used in the comparison was of actual historical discharges of 239/240 Pu and 137 Cs from Sellafield from the 1950s until the end of 1999 (European Commission, 2002). Activity concentrations were calculated until 2020, as it is of interest to see the changes in concentration after the discharges cease. The discharges are presented in Section 3 and the results discussed in Section 4. 2. The dispersion models 2.1. The MARINA II model The MARINA II model is a 72-compartment model for estimating the dispersion of radionuclides in Northern European waters. It incorporates model developments carried out at the National Radiological Protection Board (NRPB), Centre d’études sur l’Evaluation de la Protection dans le domaine Nucléaire (CEPN) and the Risø National Laboratory and also recent improvements in representing marine sedimentation processes (European Commission, 1995, 2002; Lepicard and D’Ascenzo, 2000; Lepicard, 2001). The movement of water between various sea areas by processes of advection and diffusion is modelled by assuming instantaneous uniform mixing within each marine compartment with rates of annual transfer between adjacent compartments. The detail of the model compartments is greatest in northern European waters; however the model includes transfer to and recycling from the World oceans. This is important for very long-lived and mobile radionuclides, such as 14 C and 99 Tc, which adsorb weakly to sediment and potentially contribute to

Estimating concentrations of radionuclides in UK waters

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Fig. 1. Irish Sea compartments in the MARINA II model.

collective doses for long timescales. However, for comparison with the CSERAM model, only the results for the compartments in the Irish Sea are considered; details of these compartments are given in Fig. 1. The absorption of radionuclides by sediments can lead to a significant removal from the water column, due to both the partitioning between the liquid phase and suspended sediments, and the subsequent removal of the activity from the water column to bed sediments. This partitioning is described in the model using a distribution coefficient (IAEA, 1985, 2004), defined as the ratio of the concentration of a radionuclide in dry sediment (in Bq t−1 ) to its concentration in filtered water (Bq m−3 ) at equilibrium. The movement of radionuclides within the seabed after being deposited from the overlying water and the return of radionuclides to the water phase is modelled using a multilayered bed structure. This allows relevant processes, such as molecular diffusion, porewater mixing, particle mixing and sediment turnover, to be taken into account. A distinction is made between deep and coastal waters in modelling the various processes due to factors such as the extent to which exchange with sediment occurs and the abundance of biota. A complete list of all of the model parameters is given in European Commission (2002). 2.2. The CSERAM model The approach adopted in CSERAM is to resolve and model, in a realistic way, the main physical, chemical, and biological processes responsible for the transport and re-distribution of radionuclides in both dissolved and particulate form. This compares with the more traditional ‘box model’ approach of the MARINA II model in which transport processes are parameterised via advective and diffusive fluxes between compartments representing the water column and the seabed.

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Fig. 2. Compartments in the CSERAM model.

The more detailed approach relies on resolving the full tide, wind, and ‘diagnostic’ density driven flow fields derived from a numerical model (Aldridge, 1998; Aldridge et al., 2003). Where wind waves are responsible for significant sediment resuspension, a wave model (based on the JONSWOP spectrum; Hasselmann et al., 1973) is used to calculate the wave-induced stress. The third component of the modelling system (Aldridge et al., 2003) is a transport model capable of dealing with transport material in both dissolved and particulate (bound) phases. The hydrodynamic model is run on an extended grid which cover the Malin shelf and Celtic Sea regions, while the transport model is run in the Irish Sea only at a resolution of approximately 4 km (see Fig. 2). 2.3. Validation of the models Three radionuclides, 99 Tc, 137 Cs, and 239/240 Pu, were used in the validation of both models. These radionuclides were chosen either to represent a long time period of measurements (137 Cs), or to exhibit a range of behaviours in the marine environment, from a strong affinity to sediment (239/240 Pu) to high spatial dispersion (99 Tc).


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Annual radionuclide discharge data from all significant sources to the Irish Sea were considered. Discharge data were collated, for discharges from the start of operation (indicated in parentheses) until 2000, for the sites located at Heysham (1982 for Heysham 1; 1987 for Heysham 2), Sellafield (1952), Wylfa (1971). For validation, the MARINA II model considered annual discharges from all of these sites, while the CSERAM model considered discharges using both monthly and annual discharge profiles. Observational data were provided by the Centre for Environment, Fisheries and Aquaculture Science, CEFAS, based on many measurement surveys dating back as far as the mid1970s (Bexon et al., 2003a). Furthermore, only a summary of the model validations in the Irish Sea, for the radionuclides used in this model comparison, is included here; for a complete description see European Commission (2002); Bexon et al. (2003b). For 137 Cs in seawater, the MARINA II model showed good agreement with observations pre-1974 and post-1986, with differences typically of better than a factor of two and with the model systematically calculating lower activity concentrations than the observations during the latter period. The variation with time estimated by the model was also in good agreement with the observations. Between 1974 and 1986, when activity concentrations in the Irish Sea were at their highest, the MARINA II model showed differences of a factor of two to three, with no systematic model bias in activity concentration. Similar results were seen in the comparison of the total Irish Sea inventory between the CSERAM model and observations. Largest differences of a factor of 1.5 were observed between 1974 and 1986, with the model calculating lower activity concentrations post-1986 by as much as a factor of two. It was also noted that the Irish Sea flushing rate needed to be increased post-1976 in order to accurately reproduce the peak activity concentrations (Jefferies et al., 1982). For 137 Cs activity concentrations in bed sediment, the CSERAM model generally predicted lower values than the measurements, while the MARINA II model showed a spread of values both greater and less than the measurements, dependent on geographical location. Outside the Irish Sea, the MARINA II model performed well for recent years lying within a factor of two of the measured data in most cases. Calculated activity concentrations of 239/240 Pu in the Irish Sea compared better with the observed measurements than the results for 137 Cs did. Both models predicted greater activity concentrations in seawater by a factor of 1.5 compared to measurements after 1980. However, there are large variations in the individual measurements that were not modelled either by MARINA II or the more physically based CSERAM model (although both models compare better to the average of the measurements). Both models showed strong remobilisation from the bed sediment when discharges, from Sellafield in particular, were reduced. Calculated activity concentrations in the bed sediment in the Irish Sea are in reasonable agreement with the observed data (with differences of up to a factor of three). A more extensive description of the model validation for the Irish Sea can be found in (Bexon et al., 2003b). However, it should be apparent that both models give a good if not perfect representation of the marine processes that disperse the radioactivity from the discharge points into the sea. It is interesting to find that the MARINA II spatially and temporally averaged activity concentrations agree with point observations from research vessels. It is possible that these measurements are not truly representative of the surrounding area. However, agreement also with the CSERAM model, with its higher temporal resolution, suggests that the MARINA II resolution and compartment sizes are appropriate to approximate the Irish Sea

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environment. It should also be interesting to note that the physical processes modelled explicitly, and the input driving data, in CSERAM are sufficient to represent the movement of radionuclides in the Irish Sea accurately and with a reduced need for model parameterisations or empirically-based approximations. A comparison between MARINA II and CSERAM to the year 2020 should therefore provide a useful insight into the predictive ability of MARINA II in a regime outside of that which the model was tuned to.

3. The discharge scenario The discharge scenario used in the comparison was of actual historical discharges of 239/240 Pu and 137 Cs from Sellafield from the 1950s until the end of 1999 (see Fig. 3). Activity concentrations were calculated until 2020. Comparisons were produced for all Irish Sea compartments shown in Fig. 1, with the exception of the Irish Sea south compartment because it is outside the spatial extent of the CSERAM model. Model comparisons are discussed for activity concentrations in both filtered seawater and sediment.

4. Results and discussion 4.1. Caesium-137 The comparison of activity concentrations of 137 Cs in filtered seawater, between MARINA II and CSERAM, typically shows two different types of result. In the compartments close to the discharging source, there is close agreement (better than a factor of two) in calculated activity concentrations to the year 2000, while the site is discharging, and differences up to an order of magnitude by 2020, with the MARINA II model showing lower activity concentrations than CSERAM (see Fig. 4 for example). In the compartments further away from the discharging source (Irish Sea Northwest and Irish Sea West), the MARINA II model shows systematically

Fig. 3. Historical discharges from Sellafield.


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Fig. 4. Comparison of the activity concentrations of 137 Cs in filtered water, in the Cumbrian Waters compartment, from historical discharges at Sellafield.

Fig. 5. Comparison of the activity concentrations of 137 Cs in bed sediment, in the Cumbrian Waters compartment, from historical discharges at Sellafield.

greater activity concentrations to the year 2000, by as much as a factor of three, but a much better comparison to 2020. For 137 Cs in sediment, there is a larger difference in massic activities between the two models than for filtered seawater by as much as a factor of 10 (see Fig. 5 for example). The CSERAM model estimates greater activities up to the early 1980s, when source discharges are high, but also a quicker reduction with time as discharges decrease, compared to MARINA II. This trend is representative of all compartments in the Irish Sea. Given the responses of the models to calculating activity concentrations in both filtered seawater and bed sediment, it is possible to conclude that the CSERAM model reaches equilibrium between liquid and sediment phases more quickly during discharges and shows more rapid remobilisation after the source is switched off. The differences in filtered seawater activity concentrations at 2020

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for compartments close to the source maybe important for critical group doses. However, the differences are more than acceptable for collective dose calculations, to the UK population for instance. This is because collective doses are estimated by summing exposures from all compartments. 4.2. Plutonium-239/240 The comparison of 239/240 Pu activity concentrations in filtered water generally showed closer agreement between models than that for 137 Cs. Typically both models agreed to within a factor of two, as shown in Fig. 6, with the exception of the Cumbrian Waters, and Liverpool and Morecambe Bay, where differences up to a factor of 20 are observed in 2020. For the Irish Sea Northwest and West compartments, the MARINA II model showed greater activity concentrations than CSERAM, for all times to 2020, while for the other compartments, the MARINA II model showed lower activity concentrations. This spatial difference has been attributed to the CSERAM model having a time-dependent, sediment distribution coefficient until equilibrium is reached, while in MARINA II equilibrium conditions are assumed at all times. The comparison of massic activities of 239/240 Pu in sediment also showed closer agreement between models than that for 137 Cs. Both models were found to agree to within a factor of two to five for all Irish Sea compartments (see Fig. 7), with the MARINA II model calculating greater activities than CSERAM for all compartments except the Cumbrian Waters, and Liverpool and Morecambe Bay. It is possible that both the filtered water and the bed sediment differences at these locations are due to the differences in the treatment of site-specific details. The oscillations modelled in the western Irish Sea seabed activities (and water activity concentrations to some extent) by CSERAM are due to a seasonal density stratification effect, which mimics the induced summer circulation in the low tidal region to the west of the Isle of Man.

Fig. 6. Comparison of the activity concentrations of 239/240 Pu in filtered water, in the Irish Sea Northwest compartment, from historical discharges at Sellafield.


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Fig. 7. Comparison of the activity concentrations of 239/240 Pu in bed sediment, in the Irish Sea Northwest compartment, from historical discharges at Sellafield. The oscillations observed in the CSERAM calculation are due to a seasonal density stratification effect.

5. Conclusion Both the MARINA II and CSERAM models show reasonably good agreement, especially considering that the models have been developed using very different modelling approaches. In particular, CSERAM is designed to provide activity concentrations in the Irish Sea on a high-resolution grid, compared to the seven compartments representing the Irish Sea in the MARINA II model. Spatial and temporal averaging of calculated activity concentrations is therefore likely to contribute to these observed differences. The MARINA II model also considers the transport of radionuclides throughout the whole of northern European waters and is optimised to give the best overall fit to all measurements in this region. Consequently, the effect of model differences close to the source at 2020 is likely to be reduced considerably for collective dose calculations in particular. Therefore, considering the computational cost of running the model, MARINA II provides an effective tool for radiation protection predictive modelling.

Acknowledgement The work was funded by the Food Standards Agency, as part of a wider study, under contract RO1049.

References Aldridge, J.N. (1998). CSERAM: A model for prediction of marine radionuclide transport in both particulate and dissolved phases. Radiation Protection Dosimetry 75, 99–103. Aldridge, J.N., Kershaw, P., Brown, J., Young, E.F., McCubbin, D., Leonard, K. (2003). Transport of plutonium (239/240 Pu) and caesium (137 Cs) in the Irish Sea: Comparison between observations and results from sediment and contaminant transport modelling. Continental Shelf Research 23, 869–899.

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Bexon, A.P., Shaw, S., Sihra, K.S., Simmonds, J.R., Aldridge, J.N., Gurbutt, P.A., Smith, B.D. (2003a). Development of a Methodology for the Prediction of Doses from the Consumption of Marine Foodstuffs, for Past and Current Discharges. Volume 1 – Description of methodology and dataset CD-ROM. Food Standards Agency, London. Bexon, A.P., Shaw, S., Sihra, K.S., Simmonds, J.R., Aldridge, J.N., Gurbutt, P.A., Smith, B.D. (2003b). Development of a Methodology for the Prediction of Doses from the Consumption of Marine Foodstuffs, for Past and Current Discharges. Volume 2 – Technical background to the methodology. Food Standards Agency, London. CEC (1990). The radiological exposure of the population of the European Community from radioactivity in North European marine waters. Project “MARINA”, EUR 12483. EC, Luxembourg. European Commission (1995). Methodology for assessing the radiological consequences of routine releases of radionuclides to the environment. Radiation Protection 72. European Commission (2002). MARINA II. Update of the MARINA project on the radiological exposure of the European Community from radioactivity in North European marine waters. Radiation Protection 132. Hasselmann, K., Barnett, T.P., Bouws, E.C.H., Cartwright, D.E., Enke, K., Ewing, J.A., Gienapp, H., Hasselmann, D.E., Kruseman, P., Meerburg, A., Müller, P., Olbers, D.J., Richter, K., Sell, W., Walden, H. (1973). Measurements of wind-wave growth and swell decay during the Joint North Sea Wave Project (JONSWAP). Deutsche Hydrographische Zeitschrift 8 (Suppl. A), 95. IAEA (1985). Sediment kds and concentration factors for radionuclides in the marine environment. Technical Report Series No 247. IAEA, Vienna. IAEA (2004). Sediment distribution coefficients and concentration factors for biota in the marine environment. Technical Report Series No 422. IAEA, Vienna. Jefferies, D.F., Steele, A.K., Preston, A. (1982). Further studies on the distribution of 137 Cs in British coastal waters. I. Irish Sea. Deep Sea Research 29, 713–738. Lepicard, S. (2001). Review of marine models for impact assessments of radionuclide releases into the north European marine waters – contribution to MARINA II Working Group D. CEPN NTE/01/15. Lepicard, S., D’Ascenzo, L. (2000). Integration d’un decoupage pour l’Atlantique dans le logiciel POSEIDON (Version 3). Note CEPN NTE/00/09. OSPAR (1998). SINTRA Statement. Summary Record, OSPAR 98/14/1 Annex 45. OSPAR, London.

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Assessment of the discharge of NORM to the North Sea from produced water by the Norwegian oil and gas industry T. Gäfvert* , I. Færevik, A.L. Rudjord The Norwegian Radiation Protection Authority (NRPA), Norway Abstract In order to obtain a reliable estimate of the discharge of NORM (Naturally Occurring Radioactive Materials) to the North Sea from the Norwegian oil and gas industry, produced water from 41 Norwegian offshore platforms has been sampled during a five-month period from the autumn of 2003 to early 2004 and analysed for 226 Ra, 228 Ra and 210 Pb. Together with data on the volume of produced water discharged from each production platform, the total activity of radium discharged to the North Sea in 2003 has been estimated. Activity concentration of 226 Ra and 228 Ra in the samples ranged from below the detection limit (0.3–1.3 Bq l−1 ) up to 16 and 21 Bq l−1 , respectively. For 210 Pb, all results except one were below the detection limit (0.2–1.5 Bq l−1 ). For some of the platforms, short-term variations in the activity concentration were observed. The discharge of 226 Ra and 228 Ra through produced water in 2003 from the Norwegian oil and gas industry into the North Sea was estimated to be 440 GBq and 380 GBq, respectively. Dividing the total discharged activity by the total volume of produced water discharged in 2003 gives an average activity concentration of 3.3 Bq l−1 for 226 Ra and 2.8 Bq l−1 for 228 Ra in produced water from the Norwegian continental shelf. Keywords: NORM, 226 Ra, 228 Ra, 210 Pb, Oil and gas industry, Produced water, Seawater, North Sea

1. Introduction In recent years the attention to discharges of natural radioactive elements from non-nuclear industries has increased. The MARINA II study, published by the European Commission in 2003, found that discharges from phosphoric acid production and the oil and gas industry, had contributed significantly to the total input of alpha-emitting nuclides to northern European marine waters. Since the discharge of phosphogyspsum has declined during the 1990s, the discharge of produced water from the oil and gas industry, containing enhanced levels of radium, has become the major contributor of alpha-emitters according to the MARINA II study (Betti et al., 2004; MARINA II, 2003). One problem in assessing the impact of discharges from * Corresponding author. Address: NRPA, Postboks 55, N-1332 Østerås, Norway; phone: (+47) 67 162576; fax: (+47) 67 147407; e-mail: [email protected]



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non-nuclear industries is that there is generally less information available concerning the activity discharged compared with authorised discharges from the nuclear industry. A sampling programme was initiated in 2003 by the NRPA (Norwegian Radiation Protection Authority) in order to obtain a good estimate of the discharged activity of NORM, especially radium, through produced water from the Norwegian oil and gas industry. The objective was to obtain information on the activity concentrations of the alpha-emitter 226 Ra and the beta-emitters 228 Ra and 210 Pb in produced water from all 41 Norwegian platforms discharging into the North Sea, and to assess the total activity of 226 Ra and 228 Ra discharged in 2003. In order to see whether short-term variations in the activity concentration occur, samples were collected monthly from each platform during five consecutive months. The study is also considered important for work in the OSPAR radioactivity group (RSA), where relevant NORM industries will be identified, and discharge data collected during the next few years. 1.1. Produced water from the oil and gas industry Produced water constitutes the largest waste stream in terms of volume from oil and gas exploitation. In 2003, 135 × 106 m3 was discharged from the Norwegian offshore oil and gas industry into the North Sea (OLF, 2004), while 21 × 106 m3 was reinjected into the reservoirs. The produced water extracted together with the oil and gas consists of formation water naturally present within the wells, but can also be a mixture of formation water and seawater, if seawater has been injected in order to maintain pressure in the reservoir. Produced water contains a large amount of dissolved inorganic elements and organic compounds that have been leached from the surrounding geological formations (Neff, 2002). Constituents always present in varying amounts are dispersed oil, dissolved hydrocarbons, organic acids and phenols (Røe, 1998). Furthermore, produced water contains traces of production chemicals, such as scale inhibitors, that are injected into the reservoirs or the production line to facilitate production. The composition of produced water may also change over time. If seawater is injected, the composition of the produced water can change rapidly in the mixing zone if a breakthrough of seawater occurs, and in a long-term perspective the character of the water will change to that of seawater if large volumes of seawater are injected and the original formation water is extracted from the reservoir (Stephenson et al., 1994). When seawater is mixed with formation water, some elements and compounds present in the formation water will be diluted. Another effect is that elements such as barium may precipitate as sulphates, due to the low solubility product of BaSO4 . Due to the similar behaviour of barium and radium, coprecipitation of Ra with BaSO4 is also possible. An example of how Ba levels can change within a reservoir when seawater breaks through is presented in Fig. 1. This shows how Ba concentration in the produced water and seawater fraction (calculated from the concentration of sulphate ions) changed in one of the wells in the Statfjord Nord field during the period 1998–2004. Breakthrough occurred in the autumn of 2000, as shown by the increase in the fraction of seawater. Simultaneously, Ba levels declined. One year later, at the end of 2001, the seawater fraction fell sharply because production was changed to a zone in the reservoir where breakthrough had not yet occurred. The volume of produced water may also change over time. In the initial phase of production the water-to-oil ratio is generally low. If seawater or other types of water (sulphate free

Assessment of the discharge of NORM to the North Sea

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Fig. 1. Barium concentration and seawater fraction in produced water in a North Sea well in the period 1998–2004 (unpublished data from Thingvoll, Statoil, 2004).

formation water can also be injected from nearby reservoirs) is injected the water-to-oil ratio will increase, and in the final phase of production the produced water volume can be several times larger than the volume of oil produced. The average water-to-oil ratio for the Norwegian oil and gas industry in 2003 has been reported to be 0.88 (OLF, 2004). 1.2. Natural radionuclides in produced water One component that has become the subject of much attention is naturally occurring radioactive elements, foremost radium, which may be present in the produced water. Four radium isotopes exist in nature (226 Ra from the uranium decay series, 228 Ra and 224 Ra from the thorium decay series, and 223 Ra from the actinium decay series) where 226 Ra and 228 Ra are the most long-lived, with physical half-lives of 1600 and 5.75 years, respectively. Radium enters the water from the surrounding rocks by direct alpha recoil into the water or chemical leaching (the latter may be facilitated by crystalline damage caused by recoil energy from previous alpha decays in the decay series). Low sulphate concentrations in formation water, due to the reducing environment, and high salinities are two factors that enable radium to remain in solution. If sulphate ions are present radium will coprecipitate with BaSO4 , and at a high salinity other positive ions will compete for adsorption sites on, for example, clay minerals in the reservoir (Kraemer and Reid, 1984; Bloch and Key, 1981). At high Cl− ion concentrations, the formation of soluble chloride complexes with radium can also explain enhanced radium activity concentration in formation water (Zukin et al., 1987). According to a review (Jonkers et al., 1997), 226 Ra levels in produced water samples have been reported to be in the range 0.002–1200 Bq l−1 .

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At some locations, elevated activity concentrations of 210 Pb have been reported in formation water. Zukin et al. (1987) explained high levels (up to 97 Bq l−1 ), comparable to 226 Ra, in brines from a geothermal well in California, by the formation of soluble chloride complexes, high reservoir temperatures and high 222 Rn concentrations. Worden et al. (2000) reported high 210 Pb levels but low 226 Ra levels in a Triassic fluvial reservoir in the UK. The explanation of this was also the formation of soluble chloride complexes. Since the water was relatively oxidised and the concentration of sulphate was high, Ra could coprecipitate as barite (BaSO4 ). The low sulphide concentrations in the water prevented the formation of insoluble galena (PbS). 1.3. Radium in produced water from the Norwegian continental shelf In spite of the large volumes of produced water discharged to the North Sea into account, relatively few data have been published on the level of radium in produced water from the Norwegian oil and gas industry. An overview of published levels of radium in produced water sampled in the period 1995–2002 from the Norwegian continental shelf is presented in Fig 2. Strand et al. (1997) analysed produced water sampled at 11 Norwegian production platforms in 1996 with regard to 226 Ra and 228 Ra. Radium-226 ranged from below the detection limit up to 10.4 Bq l−1 , while 228 Ra was found in the range from below the detection limit up to 10.0 Bq l−1 . Of 27 samples in total, 23 samples showed activity concentrations of 226 Ra below 5 Bq l−1 . Røe (1998) published data on 226 Ra and 228 Ra levels in produced water sampled from 4 platforms (Brage, Oseberg F, Oseberg C and Troll) during the period October 1995 to August 1996, where 226 Ra and 228 Ra ranged from 6 to 9 Bq l−1 and

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