RADIOACTIVITY IN THE ENVIRONMENT A companion series to the Journal of Environmental Radioactivity Series Editor M.S.Baxter Ampfield House ClachanSeil Argyll, Scotland, UK Volume 1: Plutonium intheEnvironment (A. Kudo,Editor) Volume 2: Interactionsof MicroorganismswithRadionuclides (F.R. Livens andM.Keith-Roach, Editors) Volume 3: Radioactive Falloutafter NuclearExplosions andAccidents (Yu.A.Izrael, Author) Volume 4: ModellingRadioactivityin theEnvironment (E.M. Scott,Editor) Volume 5: Sedimentary Processes: QuantificationUsingRadionuclides (J.CarrollandI.Lerche, Authors) Volume 6: Marine Radioactivity (H.D. Livingston, Editor) Volume 7: TheNaturalRadiation EnvironmentVII (J.P.McLaughlin, S.E. Simopoulos andF.Steinha¨usler, Editors) Volume 8: Radionuclides inthe Environment (P.P. Povinec andJ.A.Sanchez-Cabeza, Editors) Volume 9: DeepGeologicalDisposal ofRadioactive Waste (R.Alexander andL.McKinley, Editors) Volume 10: Radioactivityin theTerrestrialEnvironment (G. Shaw, Editor) Volume 11: Analysis of EnvironmentRadionuclides (P.P. Povinec, Editor) Volume 12: Radioactive Aerosols (C.Papastefanou, Author) Volume 13: U–Th Series NuclidesinAquatic Systems (S. KrishnaswamiandJ. Kirk Cochran, Editors) VOLUME THIRTEEN R E ADIOACTIVITY IN THE NVIRONMENT U–T SERIES NUCLIDES h IN AQUATIC SYSTEMS Editors S. KRISHNASWAMI Physical Research Laboratory, Navrangpura, Ahmedabad, India J. KIRK COCHRAN Marine Sciences Research Center, School of Marine and Atmospheric Sciences, Stony Brook University, Stony Brook, New York, USA Amsterdam (cid:2)Boston (cid:2)Heidelberg(cid:2)London (cid:2)NewYork (cid:2)Oxford Paris (cid:2)San Diego (cid:2)San Francisco(cid:2)Singapore(cid:2)Sydney(cid:2)Tokyo Elsevier LinacreHouse,JordanHill,OxfordOX28DP,UK Radarweg29,POBox211,1000AEAmsterdam,TheNetherlands Firstedition2008 Copyrightr2008ElsevierLtd.Allrightsreserved Nopartofthispublicationmaybereproduced,storedinaretrievalsystem or transmittedinanyformorbyanymeanselectronic,mechanical,photocopying, recordingorotherwisewithoutthepriorwrittenpermissionofthepublisher PermissionsmaybesoughtdirectlyfromElsevier’sScience&TechnologyRights DepartmentinOxford,UK:phone(+44)(0)1865843830;fax(+44)(0)1865853333; email:[email protected] requestonlineby visitingtheElsevierwebsiteathttp://www.elsevier.com/locate/permissions,andselecting ObtainingpermissiontouseElsevier material Notice Noresponsibilityisassumedbythepublisherforanyinjuryand/ordamagetopersons or propertyasamatterofproductsliability,negligenceorotherwise,orfromanyuse oroperationof anymethods,products,instructionsor ideascontainedinthematerial herein.Becauseofrapidadvancesinthemedicalsciences,inparticular,independent verificationofdiagnosesanddrugdosagesshouldbemade LibraryofCongressCataloging-in-PublicationData AcatalogrecordforthisbookisavailablefromtheLibraryofCongress BritishLibraryCataloguinginPublicationData AcataloguerecordforthisbookisavailablefromtheBritishLibrary ISBN:978-0-08-045012-4 ISSN:1569-4860 For informationonallElsevier publications visitourwebsiteatbooks.elsevier.com PrintedandboundinHungary 08 09 10 11 12 10 9 8 7 6 5 4 3 2 1 D K K. T EDICATION TO ARL UREKIAN This volume is dedicated to our guru, colleague and friend Karl K. Turekian. Karl, through his numerous students and post-docs, many drawn from various international institutes, has been involved in virtually every aspect of the applications of U- and Th-series nuclides to aquatic systems described in this volume. His interest in the U- and Th-series began in the 1960s with the use of 230Th and 231Pa to date ocean deposits. The advent of the GEOSECS program during the late 1960s and early 1970s led Karl’s group to become involved in determiningtheoceanicdistributionsof210Poand210Pbandthefactorscontrolling them. The importance of atmospheric supply of 210Pb to the surface ocean prompted Karl’s group to actively pursue the atmospheric chemistry and transport ofRnanditsdaughtersthroughmeasurementsof210Pbinair,soilsandsaltmarshes and to participate in programs such as the SEAREX. These studies not only generated volumes of data on the emanation rate of Rn from soils and the deposition flux of 210Pb from the atmosphere but also provided a better v vi DedicationtoKarlK.Turekian understanding of atmospheric scavenging and transport processes. A natural extension of these studies was the use of 210Pb as a surrogate to characterize and quantify the behavior of other similar continentally derived atmospheric components. In addition, measurements of 210Pb in the deep sea by his group, as a part of the GEOSECS program, confirmed that it is scavenged on time-scales of decades. The rapid removal of 210Pb from deep waters to sediments led Karl’s group to come up with the novel approach of using 210Pb to derive quantitative estimates of particle-mixing rates in deep-sea sediments. A persistent focus for Karl’s group was the ‘model’ ocean, Long Island Sound. Althoughitisanestuary,theSoundhasmanycharacteristicsthatsimulateanocean basin. This coastal region was a proving ground for pioneering applications of U-andTh-seriesnuclidestoscavenging(234Th,210Pb),sedimentaccumulationand bioturbation (234Th, 210Pb), and water transport rates (Ra isotopes). All these contributed significantly to the understanding of the geochemistry of U- and Th-seriesnuclidesinestuarine-coastalsystemsandthusplacetheirapplicationsona firm foundation. Karl’s interests also included using U- and Th-series nuclides to determine growth rates and ages of mollusks (from wide ranging locations, including hydrothermal vents, deep-sea abyssal plains and the continental shelf), fish and corals. Karl’s group provided fundamental measurements on U- and Th-series nuclides in hydrothermal and groundwaters that helped define the extent and rates of water–rock interactions and their impact on nuclide distribution and transport. Karl has always been fascinated by the proven and potential applications of U- and Th-series nuclides in studies of Earth surface processes. Many of the ideas pursued by Karl’s group were generated during the now famous ‘coffee hour’ at Yale, where anything and everything, from religion to science and politics to baseball were passionately discussed and debated. Karl always steered the conversation to focus on the recent results brought in by students, post-docs or visitors. The discussions were lively and pushed everyone to extract the maximum from the data and develop new approaches. As well, through his editorship of the journals Journal of Geophysical Research, Earth and Planetary Science Letters, Global Biogeochemical Cycles and Geochimica et Cosmochimica Acta, Karl urged his colleagues to submit stimulating papers that, in many cases, provided important new applications of U- and Th-series nuclides. Thus, through all these activi- ties, Karl Turekian encouraged and nurtured many young scientists at Yale and elsewhere. The editors, who have worked with Karl for many years, are pleased to dedicatethisvolume‘‘U-andTh-SeriesRadionuclidesinAquaticSystems’’tohim. C ONTRIBUTORS B. Bourdon Department of Earth Sciences W.C. Burnett Florida State University F. Chabaux CNRS et Universite´ Louis Pasteur M.A. Charette Woods Hole Oceanographic Institution Zanna Chase Oregon State University T.M. Church University of Delaware J. Kirk Cochran Stony Brook University Nicholas S. Fisher Stony Brook University Scott W. Fowler Stony Brook University W. Geibert University of Edinburgh and Dunstaffnage Marine Laboratory David C. Kadko University of Miami S. Krishnaswami Physical Research Laboratory Teh-Lung Ku University of Southern California Shangde Luo National Cheng-Kung University Brent A. McKee The University of North Carolina at Chapel Hill W.S. Moore University of South Carolina xi xii Contributors D. Porcelli University of Oxford J. Riotte Universite´ de Toulouse M.M. Rutgers van der Loeff Alfred Wegener Institute for Polar and Marine Research M.M. Sarin Physical Research Laboratory Gillian M. Stewart Queens College, CUNY CHAPTER 1 Introduction S. Krishnaswami1,(cid:2) and J. Kirk Cochran2 Contents 1. Overview 1 2. Organization oftheVolume 4 Acknowledgements 7 References 8 1. Overview The discovery of radioactivity during the last decade of the 19th century (Becquerel, 1896; Curie, 1898) laid the foundation for major applications of this phenomenontoearthsciences,beginning intheearly20thcenturyandcontinuing to the present. The discovery provided earth scientists with a powerful tool to ‘‘date’’ events in Earth’s history, including the time of its formation, and to determine time-scales of contemporary processes occurring on the Earth. The introduction of a precise means of relating time to earth surface processes has led to a better understanding of how the earth system works and how the various processeshavecontributedtotheoverallevolutionoftheEarthsinceitsformation. Indeed, it was studies of U and Th minerals that led to the discovery of radioactivity, the determination of properties of emanations from radioactive substancesandtheradioactivedecaylaws(AppendicesAandB).Withinacoupleof decades of the discoveryof radioactivity, manyof the members of the U- and Th- decay series had been isolated from minerals and their decay properties established. This led to the finding that the U and Th series were made of a number of radioactive isotopes of many elements with widely different chemical properties and half-lives. The appendix presents details of the three decay series: 238U, 235Uand232Th(AppendixA,FiguresA1–A3).Eightelements:U,Pa,Th,Ra,Rn, (cid:2) Correspondingauthor.Tel.:+91-79-26314305;Fax:+91-79-26314000 E-mailaddress:[email protected] 1PhysicalResearchLaboratory,Navrangpura,Ahmedabad-380009,India 2MarineSciencesResearchCenter,SchoolofMarineandAtmosphericSciences,StonyBrookUniversity,StonyBrook, NewYork11794–5000,USA RadioactivityintheEnvironment,Volume13 r2008ElsevierLtd. ISSN1569-4860, DOI10.1016/S1569-4860(07)00001-0 Allrightsreserved. 1 2 S.KrishnaswamiandJ.KirkCochran Po, Bi and Pb make up the core of these chains. The half-lives of nuclideswithin a decay chain alsovary considerably; for example in the 232Th chain, the variation is over twenty-four orders of magnitude (Figure A3). Members of these decay chains, being different elements with quite distinct chemical properties, participate to varying degrees in physical, chemical and biogeochemical processes on the Earth. This leads to fractionation among the members within the same decay chain, resulting in radioactive disequilibrium among them. This disequilibrium is the basis for the applications of U- and Th-seriesnuclidesinearthsciences.Theextentofdisequilibriumbetweendifferent parent–daughter pairs and the time it takes to return to equilibrium provide valuable information about the processes and their time-scales (Appendix B). The chemical properties of the parent–daughter pairs and the half-lives of the daughters determine their suitability to investigate any particular process. In principle, processes occurring over very wide time-scales, from minutes to millions of years, can be studied using U- and Th-series disequilibrium systematics. Indeed, the U-andTh-seriesnuclidesaretheonlynaturaltracersavailablefor thestudyofrates of many earth processes. Such applications, some of which are described in this volume,havecontributedinrecentyearstoconsiderableadvancesinunderstanding these processes and the geochemistry of U- and Th-series nuclides. In addition to differences in chemical properties, the very phenomenon of radioactive decay can also contribute to radioactive disequilibrium among U- and Th-series nuclides. Daughter nuclides produced by alpha decay (e.g., 234Th from 238U and 222Rn from 226Ra) are subject to recoil. This can eject the daughter nuclide out of the mineral grains or displace it from the site of production, concomitantly causing damage to the crystal lattice around the site of decay (Cherdyntsev, 1971; Kigoshi, 1971). The extent to which these processes occur depends on the grain size, density and parent nuclide distribution in the mineral. Fractionation resulting from recoil effects is not element specific. Recoil as a contributor of disequilibrium is particularly important for geochemical processes involving solid–liquid (e.g., ground water and sediment pore water) and solid–gas (e.g., soils) systems and is discussed further in several chapters of this volume (Chabaux et al., Charette et al., Cochran and Kadko, and Porcelli). The application of U- and Th-series nuclides to investigate aqueous processes began nearly a century ago, soon after the discovery of radioactivity. The measurementsofRainseawater,sedimentsandmanganesenodulesbyJoly(1908a, 1908b) set the stage for the applications of U- and Th-series disequilibrium to oceanography. These early studies showed that manganese nodules and surface sediments from the deep sea were enriched in Ra relative to coastal sediments and sedimentaryrocks,apropertyattributedbyJoly(1908b)toprecipitationofRafrom seawater. Petterson (1937), about three decades later, based on the observation that thereisverylittleThinseawater,suggestedthatthehighRaindeep-seasedimentswas a consequence of removal of 230Th (ionium), the parent of 226Ra (Figure A1) from seawater to sediments and the subsequent in-growth of 226Ra from it. This laid the foundation for the ionium dating method for deep-sea sediments, an applica- tion which continues to be widely used today in paleoceanographic research (Appendix B; Chase, this volume). Introduction 3 Further studies of 230Th and 226Ra in sediments and Th isotopes in seawater during the ensuing two decades (Piggot and Urry, 1941; Urry, 1942; Isaac and Picciotto,1953;Kroll,1953,1954;PicciottoandWilgain,1954;Koczyetal.,1957) ledtokeyfindingsandinferenceswhichformthecornerstonesofU-andTh-series applications in oceanography. These include the confirmation of the considerable deficiencyof230Threlativeto238Uinseawaterconsistentwiththehypothesisofits rapidremoval,the‘‘excess’’of228Thover232Thinseawateranditsinterpretationin terms of production from 228Ra supplied from rivers and sediments, and the migratory behavior of 226Ra from sediments to seawater. In terms of concepts, these studies led to the development of a constant flux model for 230Th supply to deep-sea sediments, and the existence of non-equilibrium radioactive systems and models to describe the changes of daughter nuclides with depth (time) in sediment cores. Further developments in the field closely followed advances in sampling, analytical chemistry and instrumentation. The advent of alpha and gamma spectrometric techniques, low-level beta counting, Rn extraction and scintillation counting, large volume sampling for water and particulate matter all led to a surge of activities on U- and Th-series nuclides during the 1960s and 1970s. During this period, the 230Th- and 231Pa-based dating methods for deep-sea sediments became fully established (Volchok and Kulp, 1957; Goldberg and Koide, 1958, 1962; Rosholt et al., 1961; Chase, this volume; Rutgers van der Loeff and Geibert, this volume), large-scale measurements of 226Ra and 228Ra were made in the global oceans to exploit their application as circulation tracers (Broecker et al., 1967; Ku and Luo, this volume) and the use of the 222Rn–226Ra system to determine parameters of air–sea gas exchange was developed (Broecker, 1965; Church and Sarin, this volume). Considerable efforts also were directed towards studies of short-lived nuclides both in seawater and sediments. The discovery of 234Th–238U and 228Th–228Ra disequilibrium in surface waters (Moore and Sackett, 1964; Bhat et al., 1969; Broeckeretal.,1973)and210Pb–226Rainthedeepsea(Craigetal.,1973)provided a better understanding of scavenging processes in the ocean and their time-scales (Appendix B; Rutgers van der Loeff and Geibert, this volume). These findings also opened new areas of research on trace element scavenging and on the use of 234Th and 210Pb as proxies to determine fluxes of biogeochemically important elements through the water column. The rapid removal of 234Th and 210Pb to sediments led to the development of their application to determine sedimentation and particle-mixing rates in coastal and deep-sea sediments (Koide et al., 1972; Aller and Cochran, 1976; Nozaki et al., 1977). Further, characteristics of solute– particle interactions and time scales of particle dynamics in seawater were obtained from the measurements of Th isotopes in dissolved and particulate phases (Nozaki et al., 1981; Bacon and Anderson, 1982). The discovery of 234U excess over 238U in aqueous systems, and the role of alpha recoil in contributing to this disequilibrium was an important advance in the application of U- and Th-series disequilibrium to aquatic systems (Thurber, 1962; Chalov et al., 1964; Cherdyntsev, 1971). The 234U–238U disequilibrium provided another chronometer to date uranium-sequestering marine deposits (e.g., corals,