U-Th Series Nuclides in Aquatic Systems, Volume 13

459 Pages·2008·7.44 MB·English

by  S. Krishnaswami

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
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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.:[phone];Fax:[phone]
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).