Chapter 1
“Environmental Isotope Geochemistry”: Past, Present and Future Mark Baskaran
1.1 Introduction and Early History A large number of radioactive and stable isotopes of the first 95 elements in the periodic table that occur in the environment have provided a tremendous wealth of information towards unraveling many secrets of our Earth and its environmental health. These isotopes, because of their suitable geochemical and nuclear properties, serve as tracers and chronometers to investigate a variety of topics that include chronology of rocks and minerals, reconstruction of sea-level changes, paleoclimates, and paleoenvironments, erosion and weathering rates of rocks and minerals, rockwater interactions, material transport within and between various reservoirs of earth, and magmatic processes. Isotopic data have also provided information on time scales of mixing processes in the oceans and atmosphere, as well as residence times of oceanic constituents and gases in the atmosphere. Arguably the most important milestone on the application of isotopes to earth science is the determination of the age of the Earth and our solar system. Isotope-based dating methods serve as the gold-standard and are routinely used to validate other non-isotope-based dating methods. Dating of hominid fossils provides a handle to understand the evolution and migration pattern of humans and stable isotope analyses of organic matter, as well as phosphate in bones and teeth in recovered fossils provide evidence for food sources consumed by humans and other animals (e.g., Abelson 1988; Schwarcz and Schoeninger 2011). To put it succinctly,
M. Baskaran (*) Department of Geology, Wayne State University, Detroit, MI 48202, USA e-mail:
[email protected]
our current understanding of the chronological evolution of the earth, its exterior and interior processes occurring on time scales of minutes to billions of years and the reconstruction of the evolution of human civilization has been developed in great part by the measurement of isotopic ratios. The field of isotope geochemistry started taking its roots shortly after the discovery of “radioactivity” (term coined by Marie Curie) in 1896 by Henri Becquerel (Becquerel 1896; Curie 1898). Within a few years of this remarkable discovery, Rutherford reported an exponential decrease of activity of a radioactive substance with time and introduced the concept of half-lives opening the door for age determination of natural substances containing radioactive elements (Rutherford 1900). The term isotope was introduced by Soddy in 1913 (Soddy 1913). The rules governing the transmutation of elements during radioactive decay was simultaneously established by Soddy (1913) and Fajans (1913). Secular equilibrium between radioactive parent and daughter was first described by Rutherford and Soddy (1902). The first radiometric age determination of a geologic sample was made on a sample of pitchblende in 1905 by Rutherford and ages of a variety of other minerals were subsequently made by Strutt (1905) and Boltwood (1907) using the U-He and U-Pb systems. The complete 238U and 232Th chain was established by 1913 and is similar to the one that is in use today (e.g., Ivanovich and Harmon 1992; Henderson 2003). The disequilibria between the members of the U-Th series resulting from differences in geochemical properties of different elements within the chain opened a new field of research to investigate aqueous geochemical processes, rock-water interaction, dating of inorganic precipitates, detrital and biogenic sediments and archaeological objects (e.g., Ivanovich and Harmon
M. Baskaran (ed.), Handbook of Environmental Isotope Geochemistry, Advances in Isotope Geochemistry, DOI 10.1007/978-3-642-10637-8_1, # Springer-Verlag Berlin Heidelberg 2011
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1992; Bourdon et al. 2003; Krishnaswami and Cochran 2008). One of the key discoveries in this field was made when large fractionations of 238U and 234U were observed in rocks, leachates of rocks and natural waters (Cherdyntsev 1955; Thurber 1962). This disequilibrium is caused by a nuclear effect resulting from the displacement of 234U (in some cases release into the surrounding aqueous phase from mineral grain surfaces) by recoil during alpha decay (238U decays to 234Th which is displaced from the original position of 238U due to recoil), in contrast to the fractionation caused by differences in the geochemical properties between different members of the decay chain. Soon after the discovery of radioactivity, Victor Hess (1912) measured the radiation levels in the atmosphere at various altitudes using a Geiger counter (developed in 1908) and reported that the radiation levels increased with altitude. He attributed this to radiation coming from outer space called cosmic radiation and now commonly called cosmic rays. Cosmic rays comprise of charged particles (including highenergy charged particles) such as protons, alpha particles, electrons, helium, nuclei of other elements and subatomic particles. The high-energy charged particles entering the atmosphere interact with atmospheric constituents (N, O, Ar, etc) and produce a suite of cosmogenic radioactive isotopes, whose half-lives range from less than an hour to millions of years (Lal and Peters 1967; Krishnaswami and Lal 2008; Lal and Baskaran 2011). Some of the cosmogenic isotopes (14C, 10Be, 7Be) have found extensive applications in quantifying processes in earth surface reservoirs such as air-sea exchange, atmospheric mixing, ocean circulation and mixing, scavenging, sediment accumulation and mixing rates in aqueous systems, erosion rates, exposure ages, changes in cosmic ray production rates, and history of human civilization. Among these, 14C has been used universally as the most-robust dating tool and has contributed tremendously to our understanding of human civilization. The field of stable isotope geochemistry started taking roots with the first set of stable isotope measurements of terrestrial samples made by Murphy and Urey (1932), Nier and Gulbransen (1939), Dole and Slobod (1940) and Urey (1948), much before the discovery of cosmogenic isotopes such as 7Be, 10 Be, and 14C. The theoretical foundation of isotopic
M. Baskaran
fractionation was later provided by Urey (1947) and Bigeleisen and Mayer (1947) using the methods of statistical quantum mechanics and statistical thermodynamics. Of the 54 elements (first 82 elements are stable of which promethium and technetium are radioactive; 26 are monoisotopic elements) that have two or more stable isotopes, only six of them (H, C, N, O, S and Si) have been extensively studied, with >10,000 published papers, abstracts and theses published on C and O isotope variations since late 1930s for investigating various near earth and earth-surface processes. Fractionations caused by mass-dependent processes such as isotope-exchange reactions, physical and biological reactions result in variations in the isotopic ratios of these elements. All of these elements form chemical bonds that have a high degree of covalent character and some are found in multiple oxidation states in the environment. In contrast, variations in radiogenic stable isotope ratios, such as Pb (206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb), Sr (87Sr/86Sr), Ce (138Ce/142Ce), Nd (143Nd/144Nd), Os (187Os/186Os) and Hf (176Hf/177Hf) in natural materials depend on the differences of their initial ratios, the parent concentrations, decay constants, and time elapsed since the solid material was formed. A natural progression of the light-element stable isotope research is to look for stable isotope fractionation of transition and post-transition elements. Sporadic attempts were made to look for them during 1970’s and 1980’s, and the initial results appeared to be encouraging. Nonetheless, because of the existing technology at that time and the preconceived notion that mass-dependent fractionations among heavy elements are expected to be negligible, there was no real progress in this area of research (O’Neil 1986). Indeed, mass-dependent isotopic fractionation in transition and post-transition elements are small compared to those in light-elements. However, as will be discussed later, these inferences and conclusions have been challenged and the occurrence of isotope fractionation in transition elements is more of a rule than exception. The radiogenic isotopes (Pb (206Pb/204Pb, 207 Pb/204Pb and 208Pb/204Pb), Sr (87Sr/86Sr), Ce 138 ( Ce/142Ce), Nd (143Nd/144Nd), Hf (176Hf/177Hf) and Os (187Os/186Os) have been widely used as tracers and stratigraphic chronometers in the environment. Pb isotopes have been utilized to trace the sources of transboundary atmospheric pollution (e.g., Bollho¨fer
1 “Environmental Isotope Geochemistry”: Past, Present and Future
and Rosman 2001; Koma´rek et al. 2008), the sources of local and global Pb pollution in a variety of natural reservoirs that include lake and coastal sediments, snow and ice samples, peat deposits, tree rings, lichens and grasses (e.g., Koma´rek et al. 2008) and to trace the pathways of lead from the environment in to human bodies (e.g., Gulson 2011). The dawn of the Atomic Age started with the detonation of the first nuclear weapon in 1945. Subsequent nuclear weapon tests during 1950’s (started in 1952) and early 1960’s (implementation of Nuclear Test Ban Treaty in 1963) released a large amount of radioactive isotopes, mainly 137Cs, 90Sr, Pu and 14C to the environment. These isotopes have been extensively utilized to investigate environmental processes that have occurred since the 1950’s, a period of time that witnessed considerable environmental changes due to anthropogenic activities. Although over 70% of the fission products 137Cs and 90Sr derived from global nuclear weapons tests have already decayed away, several of the long-lived isotopes (14C and transuranics) continue to serve as effective tracers and chronometers in environmental studies. In this present Anthropocene Era, elements of economic value are mobilized from their respective sources into various Earth’s subsystems of the lithosphere, hydrosphere, atmosphere and biosphere. With the increases in population and the spectacular sustained growth of emerging economies over the past 2–3 decades, the demand for Earth’s resources have increased exponentially. While several hundreds of millions of people are taken out of poverty as a result of global economic growth, rapid industrialization has resulted in sustained environmental degradation in many developed and emerging economies. For example, in Detroit, Michigan, USA, the average Pb concentration in soil is more than an order of magnitude higher than the average upper crustal value. The Environmental Protection Agency in the USA and many regulatory agencies in the United States and elsewhere have listed the following ten elements as priority pollutants: Cd, Cr, Cu, Pb, Hg, Ni, Se, Ag, Tl and Zn. Except for Pb, high precision measurements of the isotopes of these elements given above for environmental studies are relatively new (20% for Re; Creaser et al. 1991). A higher precision of 2‰ (2s) on the isotopic composition of Os with 4 ng Os have been reported. The fourth major break-through in the mass spectrometry, Inductively Coupled Plasma Mass Spectrometer (ICPMS) came in 1980’s, most of them in the initial generation were conventional ICPMS, comprised of quadrupole ICPMS, high-resolution sector field ICPMS (HR-ICPMS), and time of flight ICPMS (TOF-ICPMS). Very high ionization (>90%) is obtained with ICPMS for all elements at high temperature (~6,000 K), including those that have high first ionization potentials (such as the PGE elements listed above). The sample throughput is faster and sample preparation time is significantly less in ICPMS compared to TIMS. It was not until the use of multiplecollector ICPMS (MC-ICPMS) in mid 1990s that combined sector-field ICPMS with a multiple collector detector system, major strides have been made in
1 “Environmental Isotope Geochemistry”: Past, Present and Future
obtaining high precision measurements of U-Th series radionuclides as well as other transitional and posttransitional elements. This technique emerged as an alternative or in some cases superior to TIMS method (a comparison of the sample size, precision, and sensitivity of TIMS, SIMS and ICPMS (multiple-collector, laser ablation and laser ablation-multiple collector) are given in Goldstein and Stirling (2003)). Although the first U measurements with ICPMS was made nearly 20 years ago (Walder and Freedman 1992; Taylor et al. 1995), only recently high precision measurements have been achieved. High precision measurements of Cu and Zn isotopes were made for the first time in late 1990s using ICPMS equipped with multiple collectors and magnetic sector, although earlier attempt to measure differences in Zn isotopes in environmental samples were not successful due to lack of sensitivity and precision (Rosman 1972; Mare´chal et al. 1999). Highprecision Tl isotopic measurement was made in 1999 for the first time with MC- ICPMS, with a precision of 0.1–0.2‰ (Rehk€amper and Halliday 1999), although earlier attempts with TIMS resulted in relatively large errors (>2‰). It has been assumed all along that the 238 235 U/ U atomic ratio to be constant, 137.88, except for uranium in the Oklo natural nuclear fission reactor discovered in 1972 in Gabon, Africa. Measurements of unprecedented high precision were made recently of 238 235 U/ U ratios on up to 40 ppm (0.040‰) precision in seawater and other aqueous systems (Stirling et al. 2007). With the advent of MC-ICPMS and improvements in the preparation of gases for introduction into gas-source mass spectrometers, the precision for a large number of isotopes is getting better than 0.050‰. In radioactive counting, some of the short-lived radionuclides can be measured at very low levels. With a delayed-coincidence counting system, ~3,000 atoms of 224Ra corresponding to 1.1 1018 g of 224 Ra (half-life ¼ 3.66 days) or 5.0 1021 mole of 224 Ra, can be measured with a precision of ~10% (1s) (Moore 2008). This technique can be employed to measure some other short-lived radionuclides such as 223 Ra (half-life ¼ 11.435 days) and 228Th (half-life ¼ 1.913 year). The precision and sensitivity for some of the short-lived U-Th series (210Po, 234Th, 228Th and 227 Th) with radioactive counting methods (in particular with alpha and beta counting instruments) are excellent (Baskaran 2011; Baskaran et al. 2009) and most likely counting instruments will be the method of choice in the foreseeable future.
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1.4 Future Forecast for 25 Years from Now We have come a long way over the past ~100 years in improving the precision of isotopic analyses. In the first stable isotope paper (Murphy and Urey 1932), the relative abundances of the nitrogen and oxygen isotopes on natural samples had a precision of about 10% and now we are at the threshold of reaching a precision of ~10 ppm (10,000 times better precision). Technological advances will continue to drive the new and innovative application of the tracer techniques. For example, 1% uncertainties in 14C measurements in the late 1970’s with AMS was considered to be major advance (compared to the beta counting), but over the last 10 years, 3‰ has been the state-of-theart. Attempts have been made to achieve a precision below 2‰, while reaching 1‰ still remains a challenge (Synal and Wacker 2010). This is probably applicable to other key cosmogenic radionuclides including 10Be, 26Al, 36Cl, and 129I. The conventional paradigm that no significant mass-dependent isotopic fractionations is expected in alkali and alkaline earth elements that commonly bond ionically or elements that are heavy where the mass difference between the heavier and lighter isotope is small is undergoing a major shift. Now the accepted view is that chemical, physical and biological processes that take place at normal environmental conditions result in measurable variations in the isotopic ratios of heavy elements. It behooves us to ask the question: Will the instrumental developments be in the driver-seat for the scientific breakthroughs in the applications of isotopes (both radioactive and stable) for earth and environmental studies? Although the foundations for mass-dependent isotope fractionation resulting from kinetic and equilibrium processes were established in 1940’s, the refinements in those theoretical foundations over the past 6 decades are minor. It is likely that when the precision improves by a factor of 5–10 (to 10 ppm level), better understanding of the fractionation mechanisms could result in multiple fractionation laws and could result in the reevaluation of the reference mass fractionation line for lighter stable isotope ratios (e.g., C, O, N). In the case of environmental forensics for organic pollutants, the future research in the source identification and fate and
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transport of pesticides, herbicides and other POPs, VOCs, and other organic pollutants appears to depend on the analysis of molecular compounds at a higher precision. Compound-specific stable carbon isotope measurements of dissolved chlorinated ethane (PCE and TCE) in groundwater provide evidence for reductive dechlorination of chlorinated hydrocarbons. For example, the d13C values were found to be more positive (18.0‰) in down-gradient wells compared to those in the source region (25.0 to 26.0‰), paving the way for the quantification of extent of biodegradation between the zones of the contaminant plume (Sherwood Lollar et al. 2001). Multiple compound-specific isotopic analyses (d37Cl, d81Br, d13C, dD) on chlorinated compounds with gas chromatography interfaced to ICPMS with much improved precision and sensitivity could provide a powerful tool in source(s) identification, fate and transport of organic pollutants including emerging contaminants, in aqueous systems. When the biological fractionation is well understood and the precision is significantly improved from the present limit, then, isotopes of Zn and Cd and certain redox-sensitive elements such as Cr, Cu, Fe, Se, Hg, Tl, and U could provide insight on the biogeochemical processes in marine and lacustrine environments. One of the major concerns in the surface waters is the ever-increasing temporal and spatial extent of harmful algal blooms (HAB). The isotopes of macro- and micro-nutrient elements [macro- (N and P) and micro-(Fe, Cr, Mn Se, Zn, Mo, I); for P, oxygen isotope ratios in phosphate can be used] could serve as effective tracers to investigate the factors and processes that lead to the formation and sustenance of HAB. Fractionation of these transition and posttransition elements caused during smelting operation could result in isotopically light elements in the vapor phase and when the condensation of the vapor phase takes place in the environment, gradient in the isotopic ratios of elements from the source of release to farther distances is expected and the isotopic ratios could provide a tool for tracing industrial sources of these elements (Weiss et al. 2008). In conclusion, if we were to plot the sheer number of publications or the total funding made available for conducting isotope-related research in earth and environmental science versus time since the 1930’s, the slope would suggest that we have every reason to be optimistic. We eagerly a wait the ground-breaking
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research that will be made in this field in the next 25 years! Acknowledgments I thank S. Krishnaswami, Jim O’Neil and Peter Swarzenski for their in-depth reviews which resulted in considerable improvement of this chapter. Some of their suggestions on the past and present status of the work are also included in this revised version.
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