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The Geological Society of America 18888 2 20130 Special Paper 500 2013 CELEBRATING ADVANCES IN GEOSCIENCE

Smaller, better, more: Five decades of advances in geochemistry

Clark M. Johnson* Department of Geoscience, University of Wisconsin, Madison, Wisconsin 53706, USA, and National Aeronautics and Space Administration (NASA) Astrobiology Institute, Wisconsin Team

Scott M. McLennan Department of Geoscience, State University of New York at Stony Brook, Stony Brook, New York 11794, USA

Harry Y. McSween Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, Tennessee 37996, USA

Roger E. Summons Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA, and National Aeronautics and Space Administration (NASA) Astrobiology Institute, MIT Team

ABSTRACT

Many of the discoveries made in geochemistry over the last 50 yr have been driven by technological advances that have allowed analysis of smaller samples, attainment of better instrumental precision and accuracy or computational capability, and automation that has provided many more data. These advances occurred during development of revolutionary concepts, such as plate tectonics, which has provided an overarching framework for interpreting many geochemical studies. Also, spacecraft exploration of other planetary bodies, including analyses of returned lunar samples and remote sensing of Mars, has added an additional dimension to geochemistry. Determinations of elemental compositions of minerals and rocks, either through in situ analysis by various techniques (e.g., electron microprobe, secondary ion mass spectrometry [SIMS], synchrotron X-ray fl uorescence [XRF], laser ablation) or bulk analysis (e.g., XRF, inductively coupled plasma–atomic emission spectrometry [ICP-AES], inductively coupled plasma–mass spectrometry [ICP-MS]), have become essential approaches to many geochemical studies at levels of sensitivity and spatial resolution undreamed of fi ve decades ago. Although major-element distributions in igneous rocks have been understood at a basic level for some time, advances using major-element abundances to understand sedimentary provenance and processes have been especially noteworthy during the past half-century. The great diversity of trace elements in terms of geochemical behavior (e.g., lithophile, siderophile, etc.) has made them invaluable to many studies, providing unique constraints on redox conditions, mineral-melt and mineral-fl uid reactions, and planetary differentiation.

*[email protected]

Johnson, C.M., McLennan, S.M., McSween, H.Y., and Summons, R.E., 2013, Smaller, better, more: Five decades of advances in geochemistry, in Bickford, M.E., ed., The Web of Geological Sciences: Advances, Impacts, and Interactions: Geological Society of America Special Paper 500, p. 1–44, doi:10.1130/2013.2500(08). For permission to copy, contact [email protected]. © 2013 The Geological Society of America. All rights reserved.

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Signifi cant advances in microanalytical techniques have markedly improved experimental determinations of trace-element partitioning among phases and in characterizing elemental distributions in rocks and minerals using two- dimensional (2-D) and three-dimensional (3-D) mapping. Rare earth elements, in particular, have proved to be invaluable tracers of magmatic, sedimentary, aqueous, redox, and cos- mochemical processes, and siderophile trace elements form a basis for modeling many aspects of planetary accretion and early evolution. An anomalous amount of iridium at Mesozoic-Cenozoic boundary has revolutionized our view of one of Earth’s most important biologic extinctions. Isotopic variations, whether produced by stable or radiogenic isotopes, provide a third dimension to the Periodic Table of Elements, and tremendous advances in instru- mentation since the early 1960s have greatly broadened this fi eld of geochemistry. Early work outlined the stable H and O isotope fi ngerprints of natural waters and water-rock interactions, and stable C and S isotope studies defi ned the biological frac- tionations that occur by photosynthesis and microbial sulfate reduction, respectively, topics that have since been applied to problems relating to the evolution of life and Earth’s atmosphere. Recent work on stable O isotopes has documented the likelihood that liquid water existed >4 b.y. ago on Earth, which profoundly affects our view of Earth’s evolution. New work on “nontraditional” stable isotopes has investigated redox cycling over Earth’s history, as has study of non-mass-dependent stable iso- tope variations. New approaches using stable isotopes as paleothermometers include exploiting the unique energetics of bonds between rare stable isotopes. Early work on the radiogenic Rb-Sr and U-Th-Pb isotope systems documented the key distinc- tions between continental crust and mantle, setting the stage for later tracing of mass fl uxes via plate tectonics, as well as documenting the great antiquity of continental crust formation and mantle differentiation on Earth. The Sm-Nd and Lu-Hf isotope systems provided a temporal context for earlier studies of rare earth element varia- tions in nature, including new constraints on crustal growth rates and mechanisms extending back earlier than 4 Ga. The siderophile Re-Os isotope system has been used to study the accretion of planetary bodies, core-mantle interaction, and the nature of the ancient lithospheric mantle. The branch of geochemistry that deals with fossilized organic molecules had its origins in elucidating the processes and pathways that led to petroleum formation. As awareness of the richness and diversity of organic compounds that can be preserved in sedimentary rocks grew, this gave way to the broader endeavor of molecular paleo- biology. Despite great challenges in tying specifi c biomolecules to groups of organisms, or to metabolic processes, as well as issues of preservation mechanisms, molecular paleobiology remains a prime approach for studying the history of microorganisms, which have been the dominant life form for most of Earth’s history and yet are rarely preserved in the fossil record. Work on molecular biomarkers has produced numer- ous paleoenvironmental proxies for the chemistry and redox state (euxinia, anoxic, oxic) of the ancient oceans, as well as new paleoclimate records. The biochemical diversity of relatively simple life forms, including bacteria and archaea, has provided a wealth of lipid biomarkers that inform us about the evolution of metabolisms over Earth history, including oxygenic and anoxygenic photosynthesis, methanogenesis, and methanotrophy, and these records have been tied into stable isotope variations of many individual chemical elements (C, H, N, O, S, Fe, Mo, etc.), which provide a broad view of the biogeochemical evolution and biologically catalyzed redox cycling of Earth, and, potentially, other planetary bodies. Although many geochemists focus exclusively on terrestrial problems, research over the past fi ve decades has been intimately linked to the chemistry of other solar system bodies and the universe beyond. We routinely rely on meteorite falls, inter- planetary dust particles, and Moon rocks for a baseline for comparison to Earth, which has been extensively differentiated and repeatedly resurfaced. Sophisticated spe500-08 1st pgs page 3

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remote-sensing capabilities based on past and current spacecraft missions are enabling active study of other planetary bodies such as the Moon, Mercury, and Mars. Ideas about nucleosynthesis within stars are tested by reference to the measured isotopic compositions of tiny presolar grains extracted from chondrites. Short-lived radio- nuclides in meteorites provide a detailed record of the condensation, mixing, and differentiation history of the earliest solar system. Mass-independent oxygen isotope fractionation in extraterrestrial samples may identify photochemical processes in the early solar nebula. More broadly, the temperature stabilities of elements and minerals constrain the sequence of nebular condensation, which provides a fi rst-order explana- tion for the bulk composition of the terrestrial planets relative to the planets of the outer solar system. Organic compounds from space inform us on the delivery of com- plex organic molecules to the early Earth, which likely infl uenced the earliest organic chemistry reactions, which in turn must have affected the origin and evolution of life. Chemical characterizations of samples of the Moon from the Apollo missions have provided the key data to recognize the Moon’s formation by impact of a Mars-size object with Earth and the likelihood that both bodies solidifi ed from magma oceans. The individual subfi elds in geochemistry are becoming increasingly integrated, where systems are now viewed in a more holistic fashion, such as multi-element or multi-isotopic studies of biogeochemical cycles. Such approaches seem likely to con- tinue in the future, and they offer a comprehensive way to test multiple hypotheses and address geologic questions that continue to be important as we use geochemistry to better understand the geologic history of Earth and the solar system.

INTRODUCTION revealed a disturbing lack of agreement among the major geo- chemical laboratories of the world (e.g., Fairbairn et al., 1951), Fifty years ago, in the early 1960s, modern geochemistry and analytical geochemists continued to struggle with data qual- had begun to take shape, and from efforts during the fi rst half ity until the demanding standards of lunar sample analysis seeped of the twentieth century, many of its fundamental questions had through the broader geochemical community. Eventually, major come into focus. Even biogeochemistry, arguably the most mod- strides were made as geochemical analyses moved away from ern branch of geochemistry, had been pioneered years earlier techniques based on wet chemistry to those based on X-ray or by Vernadsky and Baas Becking. The fi rst journal specifi cally mass spectra, allowing a marked increase in sample throughput. devoted to the fi eld, Geochimica et Cosmochimica Acta, began The use of chemical modeling in geochemistry has taken place publication in 1950. Two years later, the fi rst English-language almost entirely within the past 50 yr (Bethke, 2008). Garrels and geochemistry text was published by Mason (1952), building on Thompson (1962) fi rst modeled the speciation of seawater, but the pioneering efforts of his mentor Goldschmidt (whose classic the use of computers in geochemical modeling was not intro- tome was published posthumously two years later; Goldschmidt, duced until the work of Helgeson (1968). 1954). The year 1952 also witnessed Nobel Prize–winning chem- Accordingly, our judgment is that developments in analyti- ist Harold Urey publish his seminal book, The Planets, widely cal, experimental, and modeling methods have largely facilitated considered a pivotal event in what would become the fi eld of the major advances of modern geochemistry during the last fi ve cosmochemistry. In 1961, the American president John F. Ken- decades. These developments have followed three simple but fun- nedy laid down the challenge of sending humans to the Moon, damental themes: smaller, better, and more. By smaller, we mean and within a decade, intense competition for the opportunity to the capability to analyze increasingly smaller samples and thus study returned lunar samples would result in major advances in to evaluate geochemical problems at increasingly fi ner scales. geochemical techniques and thought. For example, analytical capabilities now often achieve subfemto- On the other hand, the technical aspects of modern geochem- gram (10-15 g) sensitivities, permitting study of vanishingly small istry were very much in their infancy 50 yr ago. Three examples samples, such as individual interstellar dust particles, or isotopic illustrate this in the period leading up to our 50 yr review time analysis of micron-sized regions of individual mineral grains. By frame. High-pressure, high-temperature experimental methods, better, we mean that modern geochemical methods permit ever- pioneered at the Geophysical Laboratory by Norman Bowen dur- increasing precision, accuracy, and resolution, greater control on ing the fi rst half of the twentieth century, were still only capable experimental conditions, and more stringent constraints on geo- of exploring conditions relevant to the upper few kilometers of chemical modeling. Thus, it is possible to measure many isotopic Earth by the early 1960s. In the early 1950s, distribution of the ratios with precisions approaching parts per million, and Moore’s fi rst international geochemical rock standards, G-1 and W-1, law has witnessed >106 improvement in computing capabilities spe500-08 1st pgs page 4

4 Johnson et al. over the past 50 yr. By more, we mean that with modern methods, 1997; Sutton et al., 2006). A variety of microbeam methods fur- it is possible to study increasing numbers of samples and ever- ther allows for determination of major- and trace-element (and widening geological conditions. For example, high-throughput isotope) compositions of very small volumes of minerals, and to mass spectrometers allow for construction of high-resolution spatially accumulate such data into two-dimensional (2-D) maps paleoclimate proxy records; improvements in high-pressure and three-dimensional (3-D) tomographic images of the compo- experimental equipment (e.g., diamond anvils) allow for experi- sitions of rocks and minerals at micron to tens of micron scales ments to be carried out at conditions equivalent to the centers of resolution. of terrestrial planets; improvements in chemical models permit evaluation of increasingly high ionic strength aqueous condi- Major-Element Geochemistry tions. Importantly, these advances in instrumentation occurred during, and after, the emergence of plate tectonics, and in terms Although early chemical analyses were time consuming, by of providing a framework for terrestrial geochemical studies, the mid-twentieth century enough major-element data had been plate tectonics permeated work in the fi eld. Another conceptual accumulated from igneous rocks to provide a basic appreciation advance in terrestrial geochemistry has been examination of of the ways in which fundamental magmatic processes (e.g., Earth within a planetary perspective, with much insight coming partial melting, equilibrium, and fractional crystallization) con- from studies of meteorites and rocks from the Moon and Mars. trolled observed variations (e.g., Harker and alkali-Fe-Mg [or Exploration of the solar system by spacecraft also occurred dur- AFM] diagrams). Because igneous and metamorphic processes ing this time, prodding the expansion of geochemical methods are largely controlled by equilibrium, as shown, for example, by into remote sensing. Norman Bowen’s studies of igneous systems, they are amenable Our charge has been to summarize the remarkable progress to high-temperature, high-pressure experimental investigations, that has been made in geochemistry during the last 50 yr in a and experimental petrology has provided the fundamental back- journal-length chapter. We therefore begin with an apology to ground for interpreting chemical compositions. This work con- our colleagues for the large body of important work that must be tinues with the ability to carry out experiments at increasingly omitted by the constraints imposed. Here, we are able to highlight extreme conditions, for example, using diamond anvils that can only a small sample of the remarkable advances in geochemistry attain pressures approaching the center of Earth (>300 GPa; research in the last fi ve decades. This contribution is intended to Mao et al., 1990). be useful for geochemists who desire a glimpse of progress in One area where fundamental advances in major-element areas of research beyond their own, as well as generalists who geochemistry have been made over the past 50 yr is in micro- are curious about what has been going on in those laboratories analysis. The electron microprobe was developed during the fi rst down the hall. half of the twentieth century, but commercial probes only became available in the 1960s (Long, 1995). In order for these instru- GEOCHEMISTRY OF THE ELEMENTS ments to provide quantitative analyses of complex rock-forming minerals, improved understanding of the infl uences of atomic Major- and trace-element compositions of rocks, minerals, number, X-ray absorption and fl uorescence (so-called ZAF cor- and natural fl uids have long been understood to be among the rections), and various matrix corrections (e.g., Bence and Albee, most fundamental data of geochemistry. Systematic evaluations 1968; Reed, 1995) was required. Applications of rapid quanti- of the abundances of the individual elements, and the basic laws tative analysis of minerals are legion. Among the pioneering governing their distributions in rocks and minerals were largely studies were evaluation of phase relations in high-pressure, high- established by the 1960s, pioneered by the likes of Frank Clarke, temperature experiments, studies of diffusion from elemental Victor Goldschmidt, Louis Ahrens, and Ted Ringwood. Early profi les through minerals, quantitative study of coexisting min- analytical methods relied mainly on classical gravimetry and eral equilibria to constrain geothermometry, geobarometry, and X-ray spectrographs to determine element compositions (Mason, oxygen fugacity (e.g., Andersen et al., 1993), and determination 1992). During the 1950s, these laborious techniques began to give of mineral-melt partition coeffi cients in rocks and experiments way to rapid and sensitive spectrophotometric methods, with the allowing for quantitative geochemical modeling (see following). development and availability of instruments such as arc-source In addition to the electron microprobe, other microbeam optical emission spectrographs and fl ame photometry, and chem- techniques have improved spatial resolution and sensitivity ical complexing agents (e.g., EDTA, ethylenediaminetetraace- (i.e., detection limits) and allowed for elemental mapping (Jan- tic acid) that could be used for colorimetry. Advances in bulk sen and Slaughter, 1982), including 3-D tomographic imaging chemical analyses over the past 50 yr have witnessed astonishing (Jerram and Higgins, 2007). Examples include analytical elec- developments of high-sensitivity instrumentation and methods tron microscopy, secondary ionization mass spectrometry, laser- (e.g., X-ray fl uorescence, plasma-emission spectroscopy, neu- ablation sources for emission and mass spectrometers, and syn- tron activation, chromatography, thermal ionization and plasma- chrotron X-ray fl uorescence microprobes. Figure 1 provides one source mass spectrometry), allowing for increasingly rapid and example of modern geochemical mapping using a synchrotron precise measurements of increasingly smaller samples (e.g., Gill, X-ray fl uorescence microprobe. spe500-08 1st pgs page 5

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Sedimentary Geochemistry and the Chemical Index of Developed fi rst to evaluate weathering in soil and weathering Alteration (CIA) Concept profi les (Nesbitt and Young, 1984), this approach also allowed for quantitative, petrologically based understanding of the major- Unlike igneous systems, where major-element distributions element composition of siliciclastic sediments. Over the past sev- have been studied in great detail for decades, interpretations of eral decades, few papers reporting the chemical composition of the elemental geochemistry of sedimentary systems until recently siliciclastics have omitted geochemical concepts in interpreting lagged far behind. The main reason for this is that siliciclastic the data. sediments (sandstones, shales) are mostly physical mixtures and One especially infl uential diagram is the “feldspar ternary”

have been affected by multiple episodes of kinetically dominated diagram (in mole fraction), Al2O3–(CaO* + Na2O)–K2O, or water-rock interaction prior to, during, and after sedimentation. A-CN-K (Fig. 2), where CaO* is CaO in silicate minerals only Accordingly, there was no simple measure, analogous to partition (i.e., corrected for carbonates and phosphates). The A-CN-K ter- coeffi cients or Harker diagrams, that quantitatively linked bulk nary diagram captures most of the major-element (and mineral- chemistry to process. Well into the 1970s, textbooks routinely ogical) changes observed in weathering of igneous rocks (i.e., tabulated chemical analyses of sedimentary rocks (e.g., Pettijohn, alteration of feldspar and glass to clay minerals). The CIA scale

1975) but provided little in the way of quantitative interpretation. (CIA = 100 × Al2O3/[Al2O3 + CaO* + Na2O + K2O]) is shown Weathering processes were understood but focused primarily on the left side of Figure 2. This diagram also proves useful for on gains/losses in soil profi les to evaluate weathering intensity interpreting the geochemistry of sedimentary rocks for several as an index of paleoclimates. Experiments and fi eld investiga- reasons: Minerals most relevant to sedimentary rocks plot at tions made signifi cant progress in evaluating the kinetics and well-separated locations on the apices or along joins, and two- time scales of weathering (Brantley and Lebedeva, 2011; White component mixing/unmixing relationships plot as straight lines and Brantley, 1995). Garrels and Mackenzie (1971) fi rst began and follow the lever rule. Accordingly, these relations have to describe the relations between low-temperature aqueous geo- been used to quantify or constrain (1) the degree of weather- chemistry (e.g., mineral aqueous stability diagrams) and the bulk ing affecting sedimentary source regions, which typically has composition of sedimentary rocks. paleoclimatological implications; (2) mineral sorting and simple Accordingly, a major advance in sedimentary geochemistry two-component mixing; (3) average provenance and mixing of was development of the so-called chemical index of alteration provenance/mineral components; and (4) diagenesis. Variants (CIA) concepts during the 1980s and 1990s, in a series of papers on this diagram are the A-CNK-FM and A-CNKM-F diagrams by Wayne Nesbitt and coworkers (summarized in Nesbitt, 2003). (F-FeOT; M-MgO), which have less thermodynamic/kinetic foundation (Nesbitt and Young, 1984) but are especially useful in basaltic systems (Nesbitt and Wilson, 1992), and thus have been used in planetary applications (Hurowitz and McLennan, 2007; McSween and Keil, 2000).

Trace-Element Geochemistry

During the past fi ve decades, some of the most signifi cant advances in elemental chemistry have had to do with quantitative interpretations of trace-element distributions in rocks and min- erals (as pioneered by Louis Ahrens, Paul Gast, Larry Haskin, Denis Shaw, and Ross Taylor), and these advances have naturally mirrored developments in analytical geochemistry.

Partition Coeffi cients Figure 1. Synchrotron X-ray fl uorescence continu- One such advance was the determination of partition coef- ous scan mapping of a Jurassic crinoid stem taken at fi cients (solid-liquid, solid-gas; expressed as Kd) from experi- the Brookhaven National Synchrotron Light Source ments and natural rock systems over a broad range of pressure- Beamline X-26A. Image is 6.1 mm by 5.3 mm, col- lected with a pixel size of 10 mm at a 100 ms/pixel temperature-composition, beginning in earnest during the 1970s collection rate. Three maps of Ca (green), Fe (blue), and 1980s. These data provided the foundation to the fi eld of and Sr (red) are superimposed, with green being trace-element modeling of igneous processes (see reviews in suppressed in this image. Areas of high strontium Allègre and Minster [1978] and Green [1994]). Early studies content (red) are magnesian calcite, whereas the were limited by the detection limits of the electron microprobe blue areas are ferroan calcite infi ll. Image is courte- sy of Steve Sutton and Tony Lanzirotti (University used to measure trace-element abundances in minerals and melts. of Chicago/Brookhaven) and Troy Rasbury (State Experiments were highly constrained by the necessary balancing University of New York at Stony Brook). act between having trace-element abundances high enough to be spe500-08 1st pgs page 6

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Figure 2. Ternary diagrams plotting molar proportions of Al2O3–(CaO* + Na2O)–K2O, or A-CN-K. CaO* refers to cal- cium in the silicate components only (i.e., corrected for carbonates and phosphates). The chemical index of alteration (CIA) scale is shown on the left side of the diagram. Also plotted are selected igneous and sedimentary minerals, average compositions of major igneous rock types, average shale, and the range for most river waters. The horizontal line con- necting plagioclase and K-feldspar separates the lower part of the diagram dominated by primary igneous minerals (i.e., unweathered) from the upper part dominated by clay minerals (i.e., weathered). (A) Typical weathering trends where the arrows schematically represent the general pathways observed for increasing degrees of weathering for various rock types, based on fi eld studies of weathering profi les and on thermodynamic-kinetic modeling. (B) Plot of suspended sedi- ment from many of the world’s major rivers, illustrating how the overall effects of weathering are preserved within the composition of sediments.

detected by the microprobe but not exceeding the Henry’s law Sm-Nd, Lu-Hf), discussed later herein. REEs have been used to limit of remaining a trace element within the system (i.e., ai = address problems ranging from constraining the earliest history of ⇒ kxi as xi 0 in some phase where i = element of interest, a = the solar nebula based on variations in condensation temperatures, activity, x = molar concentration, and k = Henry’s law constant). coupled with measurements of their abundances in Ca-Al–rich The development of highly sensitive modern microbeam tech- inclusions and minerals in meteorites (Mason and Taylor, 1982), to niques (e.g., ion microprobe, synchrotron X-ray fl uorescence, tracing the movement of water masses through the oceans based on laser-ablation mass spectrometry) now allows for precise deter- their short residence times (Piepgras and Wasserburg, 1980). The mination of a wide range of partition coeffi cients, including those past fi ve decades have also witnessed developments in the use of mineral/melt -3 for highly incompatible elements (i.e., Kd < 10 ), under REEs as components for a variety of high-technology applications, conditions that represent nature (e.g., Frei et al., 2009a). such that these elements are now considered to be strategic met- als, thus providing further impetus for future geochemical research Rare Earth Elements (Haxel et al., 2005). Modern research on the REEs dates from the The rare earth elements (REEs; La–Lu, Y1) are the most stud- development of effi cient separation methods and high-precision ied and infl uential group of trace elements, and they have provided instrumental analytical techniques in the early 1960s that resolved, crucial evidence for a wide variety of geochemical and cosmo- in the affi rmative, the long-standing question of whether or not chemical processes. In addition to elemental abundances, the REEs these elements could be fractionated during formation of Earth’s are used for several important radiogenic isotope systems (e.g., crust (Haskin and Gehl, 1962). By geochemical standards, REEs are an extremely coherent group in terms of size (ionic radius), charge, mineral cation site 1Scandium may also be considered a REE but typically is not included with the others in geochemical discussions due to its smaller size and differing partition- coordination, lithophile behavior, and aqueous complexing and ing behavior. speciation characteristics. From a planetary perspective, REEs spe500-08 1st pgs page 7

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do not fractionate signifi cantly during planetary formation from remobilization beyond the mineralogical scale during weather- the solar nebula, and, accordingly, average chondritic meteorite ing, diagenesis, and metamorphism, except under conditions of abundances serve as a useful reference for examining planetary very high fl uid/rock ratios. REEs are particle-reactive and tend and geological processes. Under geological conditions, REEs are to be effi ciently scavenged from seawater by sediment particles, trivalent, except for the distinctive redox chemistries of europium resulting in very low residence times, ranging from ~50 yr (Ce) (Eu3+ or Eu2+) and cerium (Ce3+ or Ce4+), which result in unique to ~2900 yr (Lu). In seawater, Ce is readily oxidized on Mn-oxide insights into magmatic and aqueous processes, respectively. particle surfaces, and insoluble ceric oxides and hydroxides are Reduction of Eu (17% increase in ionic radius) occurs under then preferentially removed from seawater compared to the other highly reducing conditions, which, with rare exception, exist only trivalent REEs, resulting in extreme negative Ce anomalies. The within magmatic environments. On the other hand, oxidation stability constants for most REE complexes tend to increase with of Ce (15% decrease in ionic radius) is common under surfi cial increasing atomic number, resulting in the light REEs (La–Sm) aqueous conditions, such as those encountered during weathering. being preferentially scavenged over Gd–Lu. The overall effect of The REEs have proven particularly valuable for constraining these characteristics is that REEs in natural fl uids vary by many magmatic processes because their mineral/melt partition coeffi - orders of magnitude in absolute abundances, with highly variable cients, which (apart from Eu2+) vary smoothly as a function of chondrite-normalized patterns, providing a very sensitive tracer. atomic number, vary over several orders of magnitude among the common rock-forming minerals (Fig. 3). For example, the ionic radius of Eu2+ is virtually identical to that of Sr2+ and readily substitutes for Ca2+ in plagioclase. Accordingly, the presence of Eu anomalies in magmatic rocks commonly results from frac- tionation of plagioclase during partial melting or crystallization. Since plagioclase is stable only up to ~1 GPa pressure (~40 km depth on Earth), the presence of Eu enrichments or depletions in magmatic rocks generally indicates relatively shallow condi- tions. The ubiquitous presence of chondrite-normalized negative Eu anomalies in sedimentary rocks is thus interpreted to indicate that intracrustal igneous differentiation processes dominated for- mation of the upper continental crust (the source of sedimentary REEs). In another example, the presence of very steep REE pat- terns that are depleted in heavy REEs (high Gd/Yb) in magmatic rocks is taken to indicate fractionation of garnet, a mineral only stable at pressures higher than ~1 GPa in ultramafi c systems and thus indicative of mantle origins. Accordingly, the origin of steep REE patterns in the ubiquitous Archean tonalite-trondhjemite- granodiorite suites has been central to development of models for the origin of Archean continental crust, and determination of whether or not the modern style of plate tectonics was operating at that time (Taylor and McLennan, 2009). Although all REEs are cosmochemically refractory elements

(50% Tcondensation > 1300 K at 10 Pa), slight differences in their condensation temperatures lead to remarkably complex REE patterns in 4.567 Ga refractory Ca-Al–rich inclusions, the oldest material preserved in certain chondritic meteorites. In addition to unusual and highly variable overall shapes, the REE patterns of these objects also have both positive and negative anomalies involving the least refractory REEs (Ce, Eu, and Yb). These pat- terns thus provide persuasive evidence for very complex but rela- tively localized evaporation-condensation processes in the early solar nebula (Mason and Taylor, 1982). The REEs have proven to be extremely useful tracers for understanding a wide range of aqueous processes (Byrne and Figure 3. Plot of mineral-melt partition coeffi cients (Kd) Sholkovitz, 1996), primarily because they tend not to partition for the rare earth elements (REEs) in major minerals in into the aqueous fl uid (Dfl uid/solid <<1) during fl uid-rock interac- equilibrium with basaltic melts. Data are from a compila- tions. Accordingly, REEs have been shown to be resistant to tion in White (2012). spe500-08 1st pgs page 8

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Siderophile and Platinum Group Elements an important means with which to trace mass fl uxes among A second group of trace elements that has proven especially Earth’s reservoirs that are independent of concentration effects useful for evaluating geochemical phenomena is the siderophile in the sense of activity coeffi cients and the thermodynamics of trace elements in general, including the refractory and highly mixing. Historically, stable and radiogenic isotopes were studied siderophile platinum group elements (PGEs = Ru, Rh, Pd, Os, by different laboratories, refl ecting the distinct instrumentation Ir, Pt). The major value of highly siderophile elements is their required by various techniques, but such boundaries have now strong partitioning into metal phases during metal-silicate equi- blurred. Our theme of “smaller, better, more” is embodied in the librium (Dmetal liquid/silicate liquid >> 103; Righter, 2003). Accordingly, history of development of mass spectrometers. Modern mass their distributions provide insight into the nature of core-forming spectrometers capable of high-precision isotopic measurements processes in planets and in the parent bodies of the various mete- are based on the Nier (1940) geometry, and by the 1960s, the orite classes, and into the evolution of planetary mantles after isotope ratio mass spectrometers (IRMS) available to the stable extraction of cores. isotope community had a well-established dual-inlet system for One key discovery, based on PGE abundances, was that comparing sample and standard gases, as well as simultane- the mass extinction at the ca. 65 Ma Mesozoic-Cenozoic ous collection of two isotopes. Later, continuous-fl ow systems (Cretaceous-Paleogene; K-Pg) boundary was related to the would come online (Hayes et al., 1990), and after that, online impact of an asteroid, thus opening the door to an entirely new laser fl uorination systems (Sharp, 1990). The thermal ionization way to consider the relations between biologic evolution and geo- mass spectrometers (TIMS) available to the radiogenic isotope logic history. Alvarez et al. (1980) determined that clay-rich sedi- community 50 yr ago, however, could not match the precision ment deposited exactly at the K-Pg boundary was highly elevated of dual-inlet, double-collector IRMS instruments. As electron- in iridium, typically by ~20–200 times the levels in the enclosing ics continued to improve, as well as the addition of multicol- sediments (Fig. 4). Since PGEs (including Ir) occur at very low lection, precisions attainable using TIMS instruments increased levels in Earth’s mantle and crust due to sequestration into the markedly in the 1980s (Thirlwall, 1991). Negative ion-capable metallic core during planetary differentiation, they argued that the TIMS later provided a breakthrough for the Re-Os isotope sys- likely source of globally distributed Ir was from impact of a large tem (Creaser et al., 1991). A major innovation for both stable and meteorite. Mass-balance calculations indicated the impactor, radiogenic isotope geochemistry was development of a multicol- assuming a CI chondrite composition, would have been ~10 km lector, magnetic-sector–based inductively coupled plasma–mass in diameter. Much subsequent work, including the presence spectrometer (MC-ICP-MS) in the 1990s (Halliday et al., 1998). within the K-Pg sediment of additional PGE enrichments, as well Finally, efforts have focused on in situ isotopic analysis at the as Re-Os isotope data, in addition to the presence of soot, shocked micron scale from the 1990s to present. This includes second- quartz, stishovite, and impact glass spherules, reinforced this ary ion mass spectrometry (SIMS, or ion microprobe), where model, and discovery of the buried ~200-km-diameter Chicxulub the current state-of-the-art, large-radius, multicollector instru- crater in Mexico with a 65.0 Ma age (e.g., Swisher et al., 1992), ments can determine d18O precisions, for example, of ±0.3‰ on has confi rmed that a major impact took place at that time. ~10 µm spots (Valley and Kita, 2009). Laser-ablation (LA) Siderophile element distributions are also important in cos- coupled to MC-ICP-MS has also emerged as an important tech- mochemistry (McSween and Huss, 2010). In addition to elemental nique for in situ isotopic analysis, a technique widely applied, for abundances, siderophile elements possess important radiogenic example, to Hf isotope analysis (Griffi n et al., 2000), and new isotope systems (e.g., Re-Os, Pt-Os, Hf-W) that provide details research is currently investigating ultrafast (femtosecond; 10-15 s) about planetary differentiation time scales (Kleine et al., 2009; lasers (Poitrasson et al., 2003). Shirey and Walker, 1998). The family of siderophile elements Next, we touch on some of the major stable and radiogenic encompasses a broad range of siderophile tendencies (as measured isotope systems used in geochemistry in the last fi ve decades. by metal-silicate partition coeffi cients under different pressure [P], Space limitations prevent us from covering some important isoto- temperature [T], and oxygen fugacity [fO2] conditions), volatilities pic systems, such as rare gas isotopes, which have seen important (condensation temperatures), and incompatibilities (mineral/sili- applications, for example, to mantle evolution using 3He/4He ratios cate melt partition coeffi cients). Accordingly, they provide insights (Kurz et al., 1982). Other important isotopic systems we have into core-mantle differentiation and silicate mantle melting in the omitted include cosmogenic radionuclides, which, in solid Earth terrestrial planets, Moon, and meteorite parent bodies (Righter, geochemistry, have provided unique constraints on sediment sub- 2003; Righter and Drake, 1996; Walker, 2009). duction using isotopes such as 10Be (Tera et al., 1986).

ISOTOPE GEOCHEMISTRY Stable Isotopes

The fi eld of isotope geochemistry offers the opportunity to Broadly, stable isotope variations refl ect partitioning look at the Periodic Table of Elements in a “third dimension,” between phases that results from differences in zero-point ener- where isotopic variations may result from stable isotope frac- gies for isotopically substituted species, although in detail, com- tionation or radioactive decay. Isotope geochemistry provides plexities exist that are mass-independent or refl ect nonstochastic spe500-08 1st pgs page 9

Five decades of advances in geochemistry 9

distribution of rare isotopes. The basic thermodynamic frame- et al., 1964), and especially large seasonal and temperature work for stable isotope geochemistry was laid out by Bigeleisen effects were documented in the d18O values of precipitation in and Mayer (1947) and Urey (1947). Here, we fi rst focus on the polar regions (Dansgaard et al., 1969; Epstein et al., 1963), set- main players that existed 50 yr ago, H, C, O, and S, followed ting the stage for use of oxygen isotopes from ice cores to infer by a discussion of newer stable isotope systems that have been paleoclimate. The δD-d18O relations of hydrothermal waters that developed since then. underwent fl uid-rock interaction were recognized to be different from those produced by evaporation of meteoric waters (Craig, Hydrogen and Oxygen Isotopes 1966). Initial efforts began to study the d18O values of the ancient The importance of water in the geologic cycle logically oceans when Perry (1967) suggested that the Precambrian oceans placed H and O isotopes as early targets in stable isotope geo- had much lower d18O values than the modern ocean based on chemistry (Fig. 5). Signifi cant accomplishments in the 1960s analysis of cherts. Muehlenbachs and Clayton (1976) proposed were numerous. The canonical relation between D/H and the important concept that the d18O value of seawater was buff- 18O/16O ratios of meteoric waters was determined (Craig, 1961), ered near zero by extensive water-rock interactions at mid-ocean defi ning the meteoric water line, a relation that is a key factor ridges (MORs), which would imply that the d18O value of ancient to understanding the origin of fl uids and fl uid-rock interactions. seawater would be roughly invariant as long as plate tectonics The kinetic and equilibrium isotopic fractionations associated operated. A landmark study of the Skaergaard intrusion com- with meteoric waters, as well as temperature, latitude, and oro- bined detailed fi eld studies with numerical modeling of convec- graphic effects, were established (Craig et al., 1963; Friedman tive heat transport to quantify water-rock interaction at a fossil hydrothermal system (Norton and Knight, 1977; Norton and Taylor, 1979). Studies of ophiolites (Gregory and Taylor, 1981) confi rmed that extensive hydrothermal interaction at MORs has

Figure 5. δD-d18O variations for waters and rocks. Meteoric water line (MWL) and standard mean ocean water (SMOW) are shown for refer- ence, as is box for magmatic waters. Hydrothermal fl uids are shown for Salton Sea (Craig, 1966) and Lassen (Janik et al., 1983), which Figure 4. Stratigraphic variations in iridium (Ir) concentrations are shifted from the MWL, where the arrows mark increasing rock (parts per billion by mass) in sedimentary rocks at the Cretaceous- infl uence (low water/rock ratios). Sedimentary formation waters may Paleogene (Tertiary in older nomenclature) boundary in the vicinity lie along a slope lower than that of the MWL, refl ecting kinetic effects of Gubbio, Italy. Analyses are for 2 molar nitric acid insoluble resi- of evaporation (fi eld shown for oil-fi eld brines from California; see dues of limestones and boundary clay. The highly elevated Ir abun- Sheppard, 1986). Hydrothermally altered igneous rocks have distinct dances at this stratigraphic boundary were the fi rst direct evidence δD-d18O trends compared to waters, and data shown for the Idaho that a major meteorite impact was involved with the mass extinctions batholith (Criss and Taylor, 1983); arrow marks direction of increas- at the 65 Ma Mesozoic-Cenozoic boundary. Figure is adapted from ing water infl uence (high water/rock ratios). Diagram is adapted from Alvarez et al. (1980). those in Criss (1999). spe500-08 1st pgs page 10

10 Johnson et al. been responsible for fi xing the d18O value of seawater. If the d18O C reservoirs (Schidlowski et al., 1975). An important exception value of seawater was relatively constant, the low-d18O values was Paleoproterozoic carbonates, which had unusually positive of Precambrian cherts could be interpreted to refl ect very high d13C values, fi rst documented in the Lomagundi Group, Rhodesia ocean temperatures (Knauth and Epstein, 1976), although the (Schidlowski et al., 1975), and later shown to be global and cor- possibility exists that very old samples have later exchanged with relative with a major rise in atmospheric oxygen, likely, at least fl uids that lowered their d18O values after deposition. in part, refl ecting organic C burial (Karhu and Holland, 1996). In Oxygen isotope studies of igneous and metamorphic rocks contrast to the generally zero d13C values for Ca-Mg carbonates, in the 1970s and 1980s focused on (1) use of d18O values as a Fe-rich carbonates from Precambrian banded iron formations tracer of the sources of magmas, and (2) application of O isotope (BIFs) have signifi cantly negative d13C values. Early workers thermometry to metamorphism (Valley, 1986) and hydrother- suggested microbial oxidation of organic carbon as an explana- mal alteration (Gregory and Criss, 1986). The large decreases in tion (Becker and Clayton, 1972), but later work favored verti- d18O values that occur from meteoric hydrothermal alteration at cal zonation in d13C values for dissolved inorganic carbon (DIC) elevated temperature (e.g., Criss and Taylor, 1983), or the large in the oceans (Beukes et al., 1990; Winter and Knauth, 1992). increases in d18O that occur during weathering (e.g., Savin and More recently, geochemical modeling and studies of shelf-to- Epstein, 1970) produce a wide range in d18O values for upper- basin transects suggest that vertical zonation in d13C values for crustal rocks that may be incorporated into magmas. Elevated DIC is unlikely, swinging the interpretation for negative d13C val- d18O values are characteristic of “S-type” granites, a term used ues back to microbial respiration (Beukes and Gutzmer, 2008; for granites that contain a sedimentary component, and con- Fischer et al., 2009). fi rmed by O isotopes (O’Neil et al., 1977). In contrast, Friedman By the 1970s, there was already a substantial database for et al. (1974) discovered that magmas could attain low-d18O val- C isotope compositions of kerogen from Precambrian shales as ues through interaction with meteoric waters directly, or through old as 3.4 Ga, documenting that d13C values for organic carbon assimilation of hydrothermally altered country rocks. Collec- generally lay between -25‰ and -35‰ (Oehler et al., 1972). An tively, this discussion points to one of the most important contri- important exception was discovery of highly negative d13C val- butions of stable O isotope geochemistry: A rock that has a d18O ues for organic carbon in sedimentary rocks between ca. 2.8 and value that is distinct from that of the mantle probably contains O 2.6 Ga in age, down to −60‰. Such low values are accepted to that was cycled near the surface of Earth in the presence of water refl ect a role for involvement of methane, and Hayes (1983) spec- (Taylor and Sheppard, 1986). ulated that aerobic methanotrophy might have been responsible. One of the most profound demonstrations of the use of O Follow-up work has confi rmed these unusually low d13C values isotopes as a “water tracer” is found in the >4 Ga zircons from and documented correlations with sedimentary facies that sug- the Jack Hills, Western Australia, which have elevated d18O val- gest a transition from an anaerobic ecosystem to one supported ues, providing compelling evidence that liquid water existed on by oxygenic photosynthesis at the end of the Archean (Eigen- Earth over 4 b.y. ago (Cavosie et al., 2005; Mojzsis et al., 2001; brode and Freeman, 2006). Peck et al., 2001). This work, in fact, highlights the importance in Looking at the oldest known sedimentary rocks, Schid- isotope geochemistry of moving away from bulk sample analy- lowski et al. (1979) noted that graphite in metasedimentary rocks sis to in situ approaches (e.g., Valley and Kita, 2009). Without from the Isua belt, SW Greenland, did not generally reach the the ability to make precise, and accurate, O isotope analyses of low-d13C values that are characteristically thought to refl ect pho- micron-sized spots, the elevated d18O values found in the Jack tosynthetically fi xed C, and this was interpreted to refl ect the Hills zircons would not have been discovered, and the concept of effects of metamorphism (Schidlowski, 1987). The importance a “cool early Earth” (Valley et al., 2002) would not have arisen. of obtaining “primary” d13C values from C from Early Archean rocks, despite their commonly high grade of metamorphism, Carbon Isotopes has generated numerous studies. Mojzsis et al. (1996) docu- By 1960, it was already well established that photosynthetic mented d13C values for graphite from Akilia Island, SW Green- 13 fi xation of CO2 into organic carbon produced a decrease in d C land, obtained via in situ SIMS analysis, that were signifi cantly values by ~25‰–30‰ (Park and Epstein, 1960). In the follow- lower than those measured in bulk samples, but this work has ing years, the fi rst surveys of d13C values for marine carbonates come under criticism on a number of fronts, including arguments and organic C confi rmed that the overall isotopic fractionation (1) that the graphite is not photosynthetic in origin, but instead between these C reservoirs can be found in natural samples, a breakdown product of Fe-bearing carbonates (van Zuilen et including those from Precambrian rocks (Hoefs and Schid- al., 2002), (2) that the sample analyzed was not a sedimentary lowski, 1967; Keith and Weber, 1964). As the database for C rock but a metasomatic dike (Fedo and Whitehouse, 2002), and isotope compositions of carbonates greatly expanded in the fol- (3) that the graphite analyzed was not enclosed in apatite and lowing decade, it became clear that the vast majority of Ca-Mg therefore isolated from the effects of metamorphism (Lepland et marine carbonates had a restricted range in d13C values near al., 2005). The question of when the fi rst C isotope compositions zero for most of Earth history, which was interpreted to refl ect that suggest photosynthesis appeared on Earth remains an impor- a relatively constant balance between the organic and inorganic tant one, although there is no consensus on the answer. spe500-08 1st pgs page 11

Five decades of advances in geochemistry 11

Recent work in C isotope geochemistry has expanded into be recognized in the same single pyrite grain (Williford et al., in situ isotopic analyses, including studies of organic carbon and 2011). Work on ca. 3.4 Ga rocks from the Pilbara craton, Aus- individual microfossils (House et al., 2000). Increasingly, it is rec- tralia, has documented signifi cant S isotope variations on the ognized that the range in d13C values for individual microfossils, micron scale, which have been generally interpreted to refl ect or kerogen on submillimeter scales, is much larger than would biological cycling of S, including S0 disproportionation and sul- be suggested by bulk C isotope analysis. Such an approach has fate reduction (Philippot et al., 2007; Wacey et al., 2011). A very affected the range in C isotope fractionations inferred between large range in d34S values, >35‰, was observed in pillow lava organic and inorganic C, which in turn may constrain atmo- textures in the Barberton greenstone belt, which were interpreted

spheric CO2 contents (Kaufman and Xiao, 2003), test the indig- to refl ect microbial microboring, and which McLoughlin et al. enous nature of molecular biomarkers (Rasmussen et al., 2008), (2012) suggested provides evidence for a Paleoarchean subsea- or distinguish between eukaryotic and cyanobacterial photosyn- fl oor biosphere. thesis (Williford et al., 2013). Mass-Independent Stable Isotopes Sulfur Isotopes Most stable isotope variations in terrestrial systems fraction- Early studies of S isotope variations in the laboratory and ate in a mass-dependent manner. Mass-independent fraction- natural environments had documented a large fractionation in ations (MIF) for O isotopes (and others) are commonly observed 34S/32S ratios during microbial sulfate reduction that was depen- in extraterrestrial samples (Birck, 2004), and will be discussed dent upon the rate of reduction and the abundance of sulfate, and later in this review. Sulfur MIF (commonly termed “S-MIF”) has similar ranges in isotopic compositions had been documented been studied extensively in ancient sedimentary rocks as a tracer

in modern marine sediments (Kaplan et al., 1963; Thode et al., of past atmospheric O2 contents. The most common mechanism 1961). In contrast to the strongly positive d34S values measured called upon to produce S-MIF is photolysis reactions involving for Phanerozoic sulfates, Perry et al. (1971) found that older SO2 and H2S in the upper atmosphere, based on ultraviolet (UV) than 3 Ga barites from South Africa had only slightly positive radiation experiments (Farquhar et al., 2001, 2000b). Because d34S values, which they interpreted to refl ect very low seawa- ozone is a strong absorbent of UV in the atmosphere, its pres-

ter sulfate contents, suggesting very low atmospheric O2 con- ence would greatly decrease S-MIF in aerosols, which sug- tents. Detailed experimental work on microbial reduction and gests that the S-MIF recorded in Archean and Paleoproterozoic

S disproportionation in the 1990s demonstrated that extremely rocks indicates very low atmospheric O2 contents (Farquhar et large 34S/32S fractionations could be produced during microbial al., 2000a). Photochemical modeling suggests that transport of S cycling that involved S species of intermediate oxidation state S-MIF to the sedimentary cycle requires ambient atmospheric such as sulfi te and thiosulfate (Canfi eld and Thamdrup, 1994; oxygen contents less than 10−5 of present day (Pavlov and Kast- Canfi eld and Teske, 1996). This in turn suggested that the excep- ing, 2002). The specifi c mechanisms and pathways responsible tionally low d34S values in sedimentary sulfi des of younger than for creating S-MIF in the ancient rock record, however, remain 1 Ga age refl ected an increase in the oxidative component of the unclear, where initial UV experiments may not have adequately S cycle, which in turn provided strong evidence for a major rise modeled the full spectrum of UV radiation, and the roles of other

in atmospheric O2 contents in the Neoproterozoic. The decrease gases, including methane and inert gases, are issues that have in d34S values of sulfi des in the Neoproterozoic, relative to ear- been raised (e.g., Domagal-Goldman et al., 2008; Lyons, 2009; lier time periods, was accompanied by an increase in the esti- Masterson et al., 2011; Zahnle et al., 2006). Further complex- mated d34S values for seawater sulfate (Canfi eld, 2001). ity arises from the fi nding that thermochemical sulfate reduction Burdett et al. (1989) proposed that the isotopic composi- by organics can produce mild S-MIF (Watanabe et al., 2009). tions of seawater sulfate may be determined through analysis Nevertheless, most workers accept S-MIF in Archean and Paleo- of carbonate-associated sulfate (CAS), an approach that greatly proterozoic sedimentary sulfi des as indicating essentially anoxic extends the lithologies that may be used to infer ancient seawater conditions in Earth’s atmosphere at this time, although research S. The CAS proxy proved particularly valuable for the ancient continues on the ancient rock record, as well as the mechanisms rock record, where the d34S values of ancient seawater sulfate to produce MIF. had been previously inferred indirectly based on the maximum values obtained in a suite of sedimentary sulfi des. Application of Nontraditional Stable Isotopes the CAS proxy to the Proterozoic has confi rmed expectations that Nearly three quarters of the elements on the Periodic Table seawater sulfate contents were very low in the Paleoproterozoic, of Elements have two or more stable isotopes, but beyond the before or immediately after the Great Oxidation Event (GOE), stable isotope systems discussed here, and a few others, most but rose substantially in the Neoproterozoic at the time of the elements have remained relatively unexplored due to analytical second increase in atmospheric oxygen (Kah et al., 2004). barriers or the opinion that the range of isotopic variations in The last decade has seen a rapid increase in in-situ S isotope nature is too small to be worth the trouble. Vanguard efforts in studies, primarily focused on pyrite. These efforts have shown the “nontraditional” stable isotopes include early studies of Li that detrital, authigenic, and hydrothermal components may (Chan, 1987), Si (Douthitt, 1982), and Ca (Russell et al., 1978) spe500-08 1st pgs page 12

12 Johnson et al. isotopes. The largest isotopic variations of the “nontraditional” of paleotemperatures, as well as the study of atmospheric gases 7 6 stable isotopes are observed for Li, where the range in Li/ Li (Eiler, 2007). For CO2 and carbonate, the measured enrichment in nature exceeds 75‰ (Tomascak, 2004). Smaller variations in 13C-18O bonds relative to that expected for a random distribu- 13 18 Δ are found for intermediate-mass elements, where, for example, tion of C- O bonds is defi ned as 47, refl ecting nominal mass 26Mg/24Mg, 44Ca/40Ca, 53Cr/52Cr, 56Fe/54Fe, 65Cu/63Cu, 80Se/76Se, 47 for 13C18O16O, and this has been shown to be an inverse func- and 98Mo/95Mo ratios vary by ~5‰–10‰ in natural samples tion of temperature (Ghosh et al., 2006). Because the enhanced (e.g., Albarède, 2004; Anbar, 2004; Beard and Johnson, 2004a; stability, or “clumping,” of 13C-18O bonds, under equilibrium con- DePaolo, 2004; Johnson and Bullen, 2004; Young and Galy, ditions, refl ects internal, homogeneous equilibrium that is inde- 13 18 Δ 2004). Results from additional stable isotope systems are being pendent of the bulk d C or d O values, 47 provides an “inter- regularly reported at meetings and in publications, extending nal” thermometer that does not require knowledge of the isotopic all the way up to mass U (e.g., Weyer et al., 2008). The fi eld is composition of the fl uid from which the carbonate precipitated. changing so rapidly that the fi rst review of the subject in 2004 is This critical aspect of “clumped-isotope” geochemistry obviates now signifi cantly out of date (Johnson et al., 2004). the need to know, or assume, the d18O or d13C value of ancient One of the most intensively studied “nontraditional” stable seawater when extracting paleotemperatures from marine car- isotope systems has been Fe. The large bonding changes that bonates. Clumped stable isotopes therefore hold great promise occur between Fe3+ and Fe2+ species in solutions and minerals for resolving debates such as the temperatures of the Archean were recognized early as the major driving force for producing oceans (Kasting et al., 2006; Knauth, 2005). An important com- Fe isotope fractionations (Beard et al., 1999; Bullen et al., 2001; ponent to using clumped isotope thermometry is the preliminary Johnson et al., 2002; Polyakov and Mineev, 2000). The vast observation that the “vital” effects that are known to fractionate majority of Fe in the crust has a δ56Fe value near zero, includ- 13C/12C and 18O/16O ratios during biologically catalyzed carbon- ing low-C, low-S sedimentary rocks (Beard et al., 2003a, 2003b) ate formation in marine environments do not apparently affect Δ and most igneous rocks, although there are small variations in 47 values (Thiagarajan et al., 2011; Tripati et al., 2010). There high-temperature rocks (Beard and Johnson, 2004b). The largest seems little doubt that as the very demanding analytical issues of range in δ56Fe values, however, is restricted to organic-rich shales clumped-isotope geochemistry are tackled by more laboratories, and BIFs, which vary from −4‰ to +2‰ (Johnson et al., 2008a; and refi ned with improvements and new directions in instrumen- Rouxel et al., 2005; Yamaguchi et al., 2005). The origin of such tation and experimental studies, this area of stable isotope geo- large variations is debated, where some workers argue that they chemistry will expand. refl ect variable extents of oxidation of aqueous Fe2+ (Anbar and Rouxel, 2007), and others interpret such variations, particularly Radiogenic Isotopes in Fe-rich rocks, to refl ect microbial Fe3+ reduction (Johnson et al., 2008b). Recently, multiple stable isotope systems have been Radiogenic isotopes may be used for geochronologic infor- used to study redox-driven geochemical cycling, greatly expand- mation, or as a genetic tracer. Here, we will generally cover the ing the utility of stable Fe isotopes, including Fe and S isotopes latter, as the subject of geochronology is covered in another (Archer and Vance, 2006), Cr isotopes and Fe redox changes chapter in this series, but it should be recognized that radiogenic (Frei et al., 2009b), Fe and C isotopes, including carbonate C isotope systems often provide both types of information. The fol- (Heimann et al., 2010) and organic C (Czaja et al., 2010), and lowing discussion is grouped by isotope system, as is tradition- Fe and Mo isotopes (Czaja et al., 2012). In summary, the fi eld ally done, but we note that modern radiogenic isotope studies of nontraditional stable isotopes is growing rapidly, although commonly combine multiple isotopic systems, blurring such progress is currently hampered by the relative paucity of stable grouping of subjects. isotope fractionation factors as compared to other stable isotope systems that have been studied for several decades. Rb-Sr 87 87 The isotope Rb decays to Sr with a half-life (t1/2) of 49 b.y. Clumped Stable Isotopes Work in the 1960s established that Precambrian continental Stable isotope compositions generally consider substitution crust had signifi cantly higher 87Sr/86Sr ratios than mantle-derived of a single minor isotope, such as 13C16O16O or 12C18O16O in car- rocks, refl ecting the higher time-integrated 87Rb/86Sr ratios of bon dioxide because multiply substituted minor isotopes, such as continental crust (Faure and Hurley, 1963). Early studies of the 13C18O16O, are very low in abundance in nature. Recently, how- mantle documented Sr isotope heterogeneity across tholeiitic and ever, the unique aspects of multiply substituted minor isotopes alkaline basalts, but they documented a generally nonradiogenic have been exploited (Wang et al., 2004). Now termed “clumped- (low 87Sr/86Sr) isotopic composition that was similar to achondrite isotope” geochemistry, to describe, for example, the enhanced meteorites (Engel et al., 1965). In contrast, the fi rst studies of gra- 13 18 87 86 stability of C- O bonds in CO2 and carbonates relative to a nitic batholiths showed intermediate Sr/ Sr ratios, suggesting random or stochastic distribution of minor isotopes, this fi eld they were composed of mixtures of mantle and crustal material of stable isotope geochemistry promises great breakthroughs (Hurley et al., 1965). A landmark paper by Kistler and Peterman in paleoclimate studies, which require accurate determination (1973) on the Sierra Nevada batholith documented across-arc spe500-08 1st pgs page 13

Five decades of advances in geochemistry 13 variations in 87Sr/86Sr ratios that correlated with outcrop patterns minerals, Sr isotope measurements of melt inclusions document of Precambrian rocks. Following contouring of the data, they very large 87Sr/86Sr variations in olivine-hosted melt inclusions proposed that a line of initial 87Sr/86Sr = 0.706 marked the bound- from oceanic basalts, which Jackson and Hart (2006) interpreted ary of the continental crust. The 1980s saw an increased focus on to refl ect mixing of primitive magmas that originated from interaction between arc magmas and continental crust, including enriched and depleted mantle reservoirs. many studies that combined O and Sr isotopes. Taylor (1980) and DePaolo (1981b) advanced the idea of coupled assimilation and U-Th-Pb fractional crystallization, drawing upon heat balance arguments The U-Th-Pb system involves decay of three parent isotopes 238 206 235 207 put forth decades earlier by Norman Bowen. Numerous stud- to distinct Pb isotopes: U- Pb (t1/2 = 4.5 b.y.), U- Pb (t1/2 = 232 208 ies on the covariation of O and Sr isotopes in granitic batholiths 0.7 b.y.), and Th- Pb (t1/2 = 14 b.y.). Early efforts on the provided constraints on the crustal lithologies that were blended U-Th-Pb isotope system focused on determining the evolution in arc magmas, as well as insights into deep crustal architecture of the continental crust and mantle, given the framework that (e.g., Fleck and Criss, 1985; Solomon and Taylor, 1989). Patterson’s (1956) Pb-Pb age of Earth provided. Broad surveys There has long been an interest in the Sr isotope composition of galena of various ages confi rmed that a single-stage growth of seawater, and Faure et al. (1965) made one of the fi rst measure- curve for the crust can explain some data (Kanasewich and Far- ments of modern seawater, found that it was isotopically homo- quhar, 1965). In contrast, Patterson and Tatsumoto (1964) stud- geneous, and proposed that the isotopic composition refl ected a ied detrital feldspars as a widespread measure of North American mixture of weathering inputs from continental and young vol- continental crust and found that the relatively high abundance of canic components. This homogeneity was confi rmed by later 207Pb, which can only have been produced early in Earth’s his- studies, leading to the conclusion that the residence time of Sr tory, required a two-stage growth curve that included an early in the oceans far exceeds the time scales of water mass mixing. U/Pb differentiation event between 3.5 and 2.5 Ga, which they The fi rst detailed survey of marine carbonates of Phanerozoic age interpreted to be the primary formation age for the crust. Arm- sketched out the broad outlines of seawater 87Sr/86Sr variations, strong (1968) recognized, as did many others, that the average identifying periods of radiogenic (high 87Sr/86Sr) compositions isotopic composition of modern Pb plotted to the right of the geo- in the Carboniferous and late Cenozoic, and relatively nonradio- chron (the Pb-Pb array of the solar system, inferred to include genic (low 87Sr/86Sr) compositions in the Cretaceous (Peterman et the terrestrial planets), that is, at high 206Pb/204Pb on a 206Pb/204Pb- al., 1970). Archean-age carbonates were found to be very nonra- 207Pb/204Pb diagram; this was commonly referred to at the time as diogenic, which was interpreted to indicate minimal input from “anomalous” or “future” Pb, and it was recognized that this likely continental crust, possibly refl ecting small continental exposure required a multistage history of U/Pb evolution. The scope of the (Veizer and Compston, 1976). Spooner (1976) made the break- Pb isotope mass-balance problem (sometimes referred to as the through that the nonradiogenic Sr fl ux was more likely to refl ect “Pb paradox”; Allègre, 1968) became clear in studies of basaltic MOR hydrothermal input rather than erosion of exposed young rocks from the ocean basins, where most oceanic basalts lay to volcanic rocks. As mass spectrometry precision improved, both the right of the geochron, requiring some sort of multistage his- broad (Burke et al., 1982; Veizer et al., 1983) and detailed (Hess tory (Gast et al., 1964; Tatsumoto, 1966). An important attempt et al., 1986) Sr isotope studies of carbonates, including shells and at solving the “Pb paradox” and the failure of single-stage growth foraminifera, provided a highly resolved Sr seawater curve. This curves came from the “Stacey-Kramers” average crustal growth curve has been used for both stratigraphic chronology and to infer curve, used to this day, which is a two-stage growth curve that changes due to sea-level variations, tectonic activity, weathering, involved a global U/Pb enrichment event at 3.7 Ga, possibly and continental-scale glaciations in the Phanerozoic (DePaolo, refl ecting the fi rst major differentiation event on Earth (Stacey 1986; Elderfi eld, 1986; Raymo et al., 1988). and Kramers, 1975). The “plumbotectonics” model put forth The Rb-Sr isotope research outlined here provided a broad in the late 1970s, and refi ned in following years, attempted to view of Sr isotope variations in the crust and mantle, and in the explain the transport of Pb between the major reservoirs of Earth, last decade, there has been an increase in work on determining and offered a possible explanation to the Pb paradox (Doe and 87Sr/86Sr variations in individual minerals, rather that bulk sam- Zartman, 1979; Zartman and Doe, 1981). Moving forward, Pb ples, using in situ approaches, which have included microdrill- isotope studies, as well as Sr isotopes, became increasingly inte- ing, SIMS, and LA-ICP-MS. For example, studies on feldspar grated with Sm-Nd isotope studies, which provided important phenocrysts in igneous rocks have been used to determine open- breakthroughs in understanding of crust-mantle evolution; these versus closed-system behavior in magmatic systems, where both are discussed in the next section. magma recharge and crustal assimilation have been documented Recent work on in situ Pb isotope analysis by SIMS and from intracrystal variations in 87Sr/86Sr ratios (Davidson et al., LA-ICP-MS has been applied to subjects ranging from sedimen- 2001; Knesel et al., 1999). In addition, Sr isotope profi les in tary provenance to melt inclusions. Pb isotope analyses of single phenocrysts potentially provide information on crystal residence detrital feldspars in both modern (Alizai et al., 2011) and ancient times when coupled with diffusion modeling across discontinui- (Tyrrell et al., 2007) sediments have been used to infer paleodrain- ties (Davidson et al., 2007). In addition to analysis of individual age confi gurations, the extent of sediment recycling, and mineral spe500-08 1st pgs page 14

14 Johnson et al. diagenesis. In situ Pb isotope analyses of feldspar phenocrysts in Covariation of Sr and Nd isotopes in orogenic arcs showed large caldera-related volcanic systems have shown correlations that many arc lavas were shifted to high 87Sr/86Sr ratios relative between the proportion of mantle and crustal components in phe- to the Sr-Nd mantle array (Hawkesworth et al., 1979). Coupled nocryst cargos and eruptive frequency and volume (Simon et al., with new Sr and Nd isotope data from ophiolites that showed 2007). Initial work on olivine-hosted melt inclusions in oceanic strong shifts in 87Sr/86Sr ratios but invariant 143Nd/144Nd during basalt demonstrated very large ranges in isotopic compositions hydrothermal alteration at MORs (McCulloch et al., 1981), that greatly exceed those measured in bulk samples, and such preferential shifts in 87Sr/86Sr in orogenic arcs were interpreted ranges have been inferred to refl ect blending of mantle melts to refl ect a subduction component derived from the altered slab. from distinct mantle reservoirs, in addition to wall-rock interac- A landmark study by Hildreth and Moorbath (1988) proposed tions in the plumbing system (Saal et al., 1998). Later work has that mixing, assimilation, storage, and homogenization of arc suggested less extreme ranges in the isotopic compositions of magmas extensively occurred in the lower crust during pond- melt inclusions, but it has also highlighted the utility of in situ ing of mantle-derived basaltic magmas near the Moho, a model isotopic analysis in distinguishing between mantle and crustal known as “MASH,” and one that still provides a framework for processes (Paul et al., 2011). arc magmatism. The relative coherence of Sm/Nd ratios dur- ing melting provided a means of “seeing through” recent mag- Sm-Nd matic events to infer the time when Nd was extracted from the Arrival of the Sm-Nd isotope system on the scene in the 1970s mantle, an approach that led to the concept of “Nd model ages” opened new research avenues due to the relatively restricted REE (DePaolo, 1981a). In western North America, Nd model age variations in a large variety of rocks, as compared to Rb/Sr and U/Pb. The most widely used decay system has been 147Sm-143Nd 146 142 (t1/2 = 106 b.y.), although the Sm- Nd (t1/2 = 0.1 b.y.) decay system has also been used to trace early solar system processes. A powerful component of the Sm-Nd isotope system was the uni- form nature of chondrite meteorites, which refl ected generally limited Sm/Nd fractionation (Jacobsen and Wasserburg, 1980), and this led to a reference reservoir, the chondritic uniform reser- voir, or “CHUR” (DePaolo and Wasserburg, 1976), which would be used extensively in studies of terrestrial rocks for inferring the age of differentiation events. Early work documented a broad anticorrelation between 87Sr/86Sr and 143Nd/144Nd ratios in ter- restrial samples (DePaolo and Wasserburg, 1976; O’Nions et al., 1977; Richard et al., 1976), which was recognized as refl ecting the distinct time-integrated Rb/Sr and Sm/Nd ratios of “depleted” (high 143Nd/144Nd) and “enriched” (low 143Nd/144Nd) components. For oceanic lavas, this relation was initially termed “the mantle array.” In the three components of Sr, Nd, and Pb isotopes, a “mantle plane” was proposed (Zindler et al., 1982), later modifi ed in multiple component space by the landmark paper by Zindler Figure 6. 143Nd/144Nd-87Sr/86Sr variations in the mantle as inferred and Hart (1986), who defi ned four principal mantle components: from oceanic mafi c lavas and mantle-derived xenoliths. Box for mid- a depleted mantle (DMM) anchored by mid-ocean-ridge basalt ocean-ridge basalts (MORB) defi nes a depleted mantle (DMM) com- (MORB), a high 238U/204Pb (μ) and high 206Pb/204Pb component ponent, and ocean-island basalts (OIB) extend from MORB to lower 143Nd/144Nd and higher 87Sr/86Sr ratios toward two enriched mantle (HIMU), defi ned by ocean-island basalts (OIBs) such as St. Hel- (EM) components: EM I (low 143Nd/144Nd, moderate 87Sr/86Sr) and ena, a moderate 87Sr/86Sr and low 143Nd/144Nd enriched component EM II (moderate 143Nd/144Nd and high 87Sr/86Sr). A high U/Pb compo- (EM I) represented by Walvis Ridge, and a high 87Sr/86Sr and low nent (HIMU) is subtly defi ned for Nd-Sr isotopes, but it is prominent 143Nd/144Nd enriched component (EM II), represented by Samoa. for Pb isotopes. These mantle components were discussed in detail It is generally accepted that the DMM component refl ects deple- in Zindler and Hart (1986) and Hart (1988). Nd-Sr isotope varia- tions for xenoliths derived from the subcontinental mantle greatly tion of the mantle through long-term production of continental extend the range observed for oceanic basalts, refl ecting isolation crust, and the three “enriched” components (HIMU, EM I, and from the asthenosphere, but the data are consistent with the DMM, EM II) are generally thought to refl ect lithospheric recycling (Fig. HIMU, EMI, and EM II components (Hawkesworth et al., 1990). 6). Mantle xenoliths from the subcontinental lithospheric mantle Subduction-related volcanic arcs may deviate from the OIB fi eld, 87 86 demonstrated that the DMM, EM I, and EM II components can sometimes toward relatively high Sr/ Sr ratios, possibly refl ecting a component from hydrothermally altered oceanic crust (McCulloch et be identifi ed in the subcontinental lithospheric mantle for some al., 1981). Subduction-related volcanic rocks may also extend to very isotopic systems such as Sr and Nd (Hawkesworth et al., 1990; low 143Nd/144Nd and high 87Sr/86Sr ratios (data shown for Martinique; Menzies, 1989). Davidson, 1983). Diagram is adapted from those in Dickin (1995). spe500-08 1st pgs page 15

Five decades of advances in geochemistry 15 provinces correlated with Archean, Proterozoic, and Phanero- rocks from the 3.8 Ga Isua supracrustal belt, Greenland, relative zoic crustal boundaries and allowed identifi cation of older Nd to average terrestrial rocks. Consideration of coupled 142Nd/144Nd components in granitic batholiths (Bennett and DePaolo, 1987; and 143Nd/144Nd variations suggested that the 142Nd enrichment Farmer and DePaolo, 1983). Neodymium isotope data from recorded Sm/Nd fractionation in the source reservoir(s) of the Isua large caldera complexes from North America, however, identi- rocks likely occurred between 4.55 and 4.45 Ga. These results fi ed large proportions of mantle, indicating that such complexes were confi rmed by Bennett et al. (2007), who interpreted the 142Nd represented new periods of net crustal growth (Johnson, 1991), enrichment to record early mantle differentiation in the fi rst 30– suggesting that ignimbrites may contain a larger proportion 75 m.y. of Earth’s history. Boyet and Carlson (2005, 2006) com- of mantle than granitic rocks, and hence periods of net crustal pared high-precision 142Nd/144Nd data from meteorites and a wide growth (Johnson, 1993). variety of terrestrial mantle–derived rocks and documented that The Nd model age concept has been extensively applied to chondrite meteorites have 142Nd depletions on the order of 20 ppm fi ne-grained sedimentary rocks, and for samples that had deposi- relative to the average of terrestrial rocks, and they suggested that tional ages younger than 2 Ga, Nd model ages were usually older this records a major mantle differentiation event ~30 m.y. after than 2 Ga, indicating that most Phanerozoic- and Proterozoic- Earth formation. Moreover, Boyet and Carlson proposed that age sedimentary rocks have been recycled from older sources there is a missing low-142Nd/144Nd reservoir in Earth, possibly (Allègre and Rousseau, 1984; Goldstein et al., 1984; O’Nions et located at the base of the mantle, assuming that Earth formed with al., 1983). An exception is sedimentary rocks that were depos- a chondritic Sm/Nd ratio. Carlson and Boyet (2008) suggested ited at the same time as orogenic episodes, which were found to that the missing reservoir, which should have enriched incompat- contain a larger proportion of mantle Nd than nonorogenic sedi- ible trace-element contents, refl ects high-density components that ments, refl ecting net crustal growth (Michard et al., 1985). Mass- settled into the deep mantle from an early terrestrial magma ocean. age distributions for the continents calculated from Nd crustal Evidence for an incompatible element–enriched mafi c crust very residence times using sedimentary rocks indicated that ~40% of early in Earth history lies in rocks from the Nuvvuagittuq green- the present-day continental mass formed by 3.8 Ga (Jacobsen, stone belt, Canada, which has relatively low 142Nd/144Nd ratios that 1988), a conclusion that is similar to that inferred from Pb isotope overlap those of chondrites, and which has a 146Sm-142Nd “age” of compositions of detrital feldspars, as discussed earlier (Patterson ca. 4.3 Ga (O’Neil et al., 2008). A contrasting view of the “miss- and Tatsumoto, 1964). A critical component to extracting crustal ing 142Nd reservoir” is offered by Jacobsen et al. (2008), who growth curves from Nd model ages, however, is quantifying the argued that the difference in 142Nd/144Nd ratios between terrestrial extent of crustal recycling in sediments (Allègre and Rousseau, rocks and chondrite meteorites may be explained by isotopic het- 1984); as will be seen later herein, signifi cantly different crustal erogeneity during Earth’s accretion, rather than invoking a miss- growth curves have been inferred using Hf isotope data. ing reservoir in Earth’s mantle. In contrast to Sr, the residence time of Nd in seawater is very short, on the order of 102 as, and early researchers in Sm-Nd iso- Lu-Hf topes recognized that this should make the isotopic composition The isotope 176Lu decays to 176Hf with a half-life of 37 b.y. The of Nd in seawater a sensitive indictor of local input (Piepgras and large Lu/Hf fractionations that are produced by garnet, as well as Wasserburg, 1980; Piepgras et al., 1979). This early work, which the high abundance of Hf in zircon, have long spurred interest in confi rmed the isotopic provinciality of Nd in seawater, and hence this analytically diffi cult isotope system. A series of pioneering its use as a sensitive tracer of water masses, was later extended papers in the 1980s outlined the range in Hf isotope composi- back in time through analysis of Fe-Mn crusts and authigenic tions of meteorites, mantle-derived rocks, crustal zircons, and marine sediments. Examples include tracing the distinct evolu- sedimentary rocks (Patchett et al., 1981; Patchett and Tatsumoto, tion of the Pacifi c-Panthalassa and Iapetus Oceans from the Neo- 1980; Patchett et al., 1984). Hafnium and Nd isotope composi- proterozoic to present (Keto and Jacobsen, 1988), and closure tions for OIB are broadly correlated, but Hf and Nd isotopes for of the Central American isthmus that restricted water mass com- MORB are not correlated. Early work on zircons from igneous munication between the Pacifi c and North Atlantic Oceans (Bur- and metamorphic rocks as old as 3.7 Ga suggested a chondritic ton et al., 1997). In addition to Nd, Pb isotope studies of Fe-Mn Hf evolution for the crust until ca. 2.8 Ga, at which time high sediments demonstrated that Pb isotopes were also provincial, initial 176Hf/177Hf ratios appeared, indicating the presence of a commensurate with the short residence time for Pb in seawater globally differentiated mantle (Patchett et al., 1981). Later work (Abouchami and Goldstein, 1995). on detrital zircons indicated the presence of a depleted mantle by Advances in mass spectrometry in the 1990s allowed appli- 3.0 Ga (Stevenson and Patchett, 1990). Detailed studies of the Hf cation of the 146Sm-142Nd isotope system, which requires mea- isotope compositions for the oceanic mantle identifi ed HIMU, surement of 142Nd/144Nd ratios to a very high precision of several EM I, and EM II components defi ned by Sr-Nd-Pb isotope stud- parts per million. The short half-life of the 146Sm-142Nd system ies (Salters and Hart, 1991), but Hf isotopes require two DMM provides a sensitive tracer of processes in the fi rst few hundred components, one of which has high 176Hf/177Hf ratios that requires million years of solar system history. Harper and Jacobsen (1992) garnet as an important ancient component in the source regions demonstrated an average 142Nd/144Nd enrichment of 32 ppm for of MORB (Salters, 1996). spe500-08 1st pgs page 16

16 Johnson et al.

ε The late 1990s saw a marked increase in Lu-Hf research as positive Hf values up to +13 at 4.3 Ga, obtained by in situ meth- MC-ICP-MS instruments became widespread. High-precision ods using LA-MC-ICP-MS, and they argued that an active plate- Hf isotope measurements of juvenile crystalline rocks, as well as tectonic system existed at this time. Valley et al. (2006) noted ancient sediments, initially documented a limited range in initial that signifi cant errors could be introduced in calculating initial 176 177 ε Hf/ Hf ratios prior to 3.0 Ga, which stood in contrast with Hf values given the complexity of the zircons and the disparate Sm-Nd isotope data that suggested very large ranges in initial volumes involved in U-Pb (SIMS) and Lu-Hf (LA-MC-ICP-MS) 143Nd/144Nd early in Earth’s history (Vervoort and Blichert-Toft, measurements. Harrison et al. (2008) combined Pb and Lu-Hf 1999; Vervoort et al., 1999), although later work, based on detri- analysis to constrain 207Pb/206Pb ages to the same ablated volume tal zircons (see following), has since expanded the Hf isotope analyzed for Lu-Hf, and this revised approach found only nega- ε − database for Precambrian rocks by several orders of magnitude tive Hf values down to 5 at 4.2 Ga. Recently, Kemp et al. (2010) and documented a much larger range in initial 176Hf/177Hf ratios. reported LA-MC-ICP-MS Pb and Hf isotope analyses that pro- ε High-quality MC-ICP-MS data for OIBs identifi ed subtle varia- duced negative Hf values between 3.9 and 4.3 Ga in age (Fig. tions in Hf-Nd isotope arrays that provided strong support for 7), which they argued refl ects only small domains of enriched an ancient subduction of pelagic sediment that was previously (low Lu/Hf) components in primitive crust, and not widespread hypothesized but diffi cult to confi rm using TIMS data (Blichert- melting and depletion of the mantle, nor an active plate-tectonic Toft et al., 1999). system prior to 4 Ga. A major effort in combined U/Pb geochronology and Hf iso- tope studies of detrital zircons, using in situ analysis methods, Re-Pt-Os began in the early 2000s and has continued to the present. Much Radiogenic Os isotope studies have generally focused on the 187 187 190 186 of this work has been aimed at addressing the growth and evolu- Re- Os system (t1/2 = 42 b.y.), although the Pt- Os system tion of continental crust, given the wide variety of crustal growth (t1/2 = 489 b.y.) has also been explored. The Re-Pt-Os isotope sys- histories that have been proposed (e.g., Allègre and Rousseau, tem differs greatly from the radiogenic systems discussed previ- 1984; Armstrong, 1981; Condie, 1998; Hurley and Rand, 1969; ously in that these elements are siderophile/chalcophile (Shirey Taylor and McLennan, 1985). U/Pb zircon geochronology has long highlighted major peaks in crystallization ages at ca. 2.7 and ca. 1.8 Ga, as well as other peaks in more detail (e.g., Condie, 1998), leading a number of workers to conclude that crustal growth has been episodic (e.g., Rino et al., 2004). This in turn has led to models where catastrophic events in the mantle have been invoked to explain punctuated periods of very rapid crustal growth. In contrast, the distributions of Hf isotope model ages for detri- tal zircons do not show the strong peaks recorded in U/Pb ages, which suggests that the U/Pb age record may refl ect biases in preservation rather than periods of episodic crustal growth (e.g., Belousova et al., 2010; Hawkesworth et al., 2009, 2010; Voice et al., 2011). Recognizing that Hf isotope model ages in detri- tal zircons may refl ect mixtures of mantle-derived and recycled components, Kemp et al. (2006) combined U/Pb geochronology and O and Hf isotopes to separate a recycled high-d18O sedimen- tary component from juvenile, mantle-derived components that refl ect net addition to the continents. This approach has forced Figure 7. ε (t) vs. age relations for zircons from the Jack Hills, Austra- a major change in thinking about crustal growth rates based on Hf ε lia, the oldest known terrestrial samples. The Hf(t) value was calculat- detrital zircons, and it suggests that as much as 70% of current ed from initial 176Hf/177Hf ratios and 207Pb/206Pb ages (U-Pb zircon geo- continental crustal volume was produced by 3 Ga, followed by chronology). Chondrite uniform reservoir (CHUR) reference is shown, slower, but relatively uniform, rates of crustal growth since that as well as the fi eld for depleted mantle inferred from mid-ocean-ridge basalts (MORB-DM), which spans a range in present-day ε values time (Dhuime et al., 2012). Hf from CHUR to highly positive values (Salters and Hart, 1991). The Looking at the oldest detrital zircons, the Archean and limiting line for an enriched reservoir, such as continental crust, is Hadean zircons from the Jack Hills, Australia, have been a logi- shown at Lu/Hf = 0. Some previous studies reported a limited number ε cal target of Lu-Hf studies. Early work on mineral separates by of positive Hf(t) values (Blichert-Toft and Albarède, 2008; Harrison Amelin et al. (2000, 1999) highlighted the diffi culty in inter- et al., 2005), which would indicate widespread mantle depletion, but other studies found only negative ε (t) values (Amelin et al., 1999; preting both Lu-Hf and U-Pb data from complex zircons, and Hf Harrison et al., 2008; Kemp et al., 2010); such values indicate the pres- they found little evidence for enriched or depleted components ence of an enriched component, possibly tied to crust formation, but at 3.8 Ga, suggesting minimal differentiation on Earth by this this does not provide conclusive proof of widespread mantle depletion time. Later work by Harrison et al. (2005) reported remarkably and continental crust formation. spe500-08 1st pgs page 17

Five decades of advances in geochemistry 17 and Walker, 1998), and the major inventories exist in Earth’s ments in 186Os/188Os and 187Os/188Os in modern OIBs that they core, distantly followed by the mantle (see previous discussion). interpreted to represent a high-190Pt component from the core, Work on this analytically challenging system, which began in the and later work on Archean komatiites identifi ed similar fea- 1980s, showed that the mantle has remained relatively nonradio- tures (Puchtel et al., 2005). Alternative proposals for correlated genic, lying within the range of chondritic meteorites, but the 187Os/188Os and 186Os/188Os in OIBs include Pt- and Re-rich mate- very high Re/Os ratios of continental crust produced very high rials that could refl ect ancient crustal components, or ancient 187Os/186Os ratios over time (Allègre and Luck, 1980; Luck and pyroxenite in the mantle, although Brandon and Walker (2005) Allègre, 1983). Correlations between Os and O isotopes in OIBs suggested that such materials are unlikely to be common compo- have been interpreted to refl ect ancient subducted sediments nents in OIBs. Recently, Luguet et al. (2008) documented 186Os (Lassiter and Hauri, 1998), or assimilation of hydrothermally enrichments in pyroxenites, eclogites, sulfi des, and Pt-rich alloys altered oceanic crust during ascent of magmas to the surface in peridotites, providing support for the argument that 186Os may (Gaffney et al., 2005). In contrast, Escrig et al. (2005) interpreted not be a unique tracer of core components in OIBs. high 187Os/187Os ratios at Fogo Island to record assimilation of lower continental crust during opening of the Atlantic Ocean. ORGANIC GEOCHEMISTRY: INVESTIGATING The large Re/Os fractionations that are produced dur- CARBON COMPOUNDS PRESERVED IN ROCKS ing melting of peridotite have been used to trace the evolution of the lithospheric mantle. Walker et al. (1989) fi rst proposed The past 50 yr have witnessed exponential growth in our this approach for cratonic peridotite xenoliths, where they cal- appreciation of the ubiquity and diversity of organic compounds culated Os model ages that refl ect Re depletion “events” that that are preserved in sediments and sedimentary rocks. A number may record the time of stabilization of the lithospheric mantle. of these substances, or their derivatives formed through diage- Compilations of Os isotope data from both whole-rock samples netic alteration processes, were discovered in sediments before and sulfi des from cratonic peridotite xenoliths commonly show they were recognized as natural products in living organisms very nonradiogenic 187Os/188Os ratios that may be interpreted as (Ourisson and Albrecht, 1992; Ourisson et al., 1979). Techno- Re depletion ages between ca. 2.5 and 3.0 Ga (Carlson et al., logical innovation combined with the quest for an understanding 2005; Pearson and Wittig, 2008). Although Os model ages pro- of fossil fuel composition, formation, and occurrence were the vide insight into the evolution of the lithospheric mantle not initial drivers of research on sedimentary organic matter in the available by other radiogenic isotope systems, complexities such 1960s and 1970s (Kvenvolden, 2006). As the possibility of gain- as sulfi de breakdown, metasomatism, and mantle-melt interac- ing evolutionary insights by way of “molecular paleobiology” tions can cloud interpretations (Rudnick and Walker, 2009). In became more clear, the driving force for organic geochemistry contrast to the nonradiogenic Os isotope compositions that have shifted (Eglinton, 1970; Peterson et al., 2007). In more recent been used to infer Re depletion histories, Shirey and Richardson times, it has been recognized that organic molecules in the air, (2011) recently reported high 187Os/188Os ratios in sulfi de inclu- in water, and in sediments carry in their chemical structures and sions in diamonds of 3 Ga age or younger, which they interpreted stable isotopic compositions a myriad of environmental and cli- to refl ect eclogites that originated as subducted oceanic crust that mate signals. Investigating these records on both human and geo- became incorporated in the subcontinental lithospheric mantle. logic time scales has now become the prime focus of research in Radiogenic (high) 187Os/188Os ratios in orogenic arc lavas organic geochemistry (Eglinton and Eglinton, 2008; Gaines et have been interpreted in two ways. Some workers have inferred al., 2009). such compositions to refl ect subducted sediments (Alves et al., 1999, 2002), although other workers have argued that the low Os The Quest to Understand the Origins of Petroleum and content of crustal material is unlikely to shift the Os isotope com- Other Fossil Fuels positions of the subarc mantle, instead interpreting the radiogenic Os isotope compositions as refl ecting assimilation of young, The invention of computerized mass spectrometers, in com- mafi c lower crust (Chesley and Ruiz, 1998; Hart et al., 2003; bination with high-resolution capillary gas chromatography, Jicha et al., 2009). The importance of the second interpretation was a critical early technical development, and this led to the lies in the potential ability of the Re-Os isotope system to trace discovery that petroleum was composed of thousands of dis- interaction with arc crust, which is essentially invisible using crete organic compounds. In the 1960s, geochemists working other isotopic systems such as O, Sr, Nd, Hf, and Pb isotopes, in the fossil fuel industry, in collaboration with academic scien- and which has implications for estimating net crustal growth in tists, also demonstrated that sedimentary kerogen and bitumen, orogenic arcs. the precursors of petroleum, could be correlated to commercial Turning to the very long-lived 190Pt-186Os system, Walker et deposits of oil and gas using combinations of organic compound al. (1995) fi rst proposed that core material might be identifi ed distributions and isotopic patterns (Hunt et al., 2002; Peters et al., in OIBs, often suggested to refl ect mantle plumes that originate 2005). They also convincingly demonstrated that buried organic at the core-mantle boundary (e.g., Hawkesworth and Schersten, matter was predominantly of biological origin (e.g., Hills and 2007). Brandon et al. (1999, 2000) observed correlated enrich- Whitehead, 1966), and that progressive changes in its chemical spe500-08 1st pgs page 18

18 Johnson et al. composition correlated with burial depth and geothermal heating alence of triterpenoids is now known to be associated with the rates, thereby providing a tool with which to assess the nature increased abundance of fl owering plants in the late Mesozoic and and distribution of coal, petroleum source rocks, and natural gas Cenozoic (Moldowan et al., 1994; Murray et al., 1998; Simoneit (Seifert and Moldowan, 1980, 1978). Transport of organic matter et al., 1986). Recently, it has become clear that biogeographical in clays opened a new mechanistic understanding to its preser- aspects of plant occurrence and dispersal are refl ected in sedi- vation and accumulation in source rocks, and a way in which mentary hydrocarbons (Dutta et al., 2011). In the marine realm, organic carbon burial could be understood in the context of the micropaleontology informs us that the Paleozoic dominance of principles of sequence stratigraphy (Creaney and Passe, 1993; green and red clades of planktonic algae gave way to the modern Hedges and Keil, 1995). dominance of chlorophyll a + c plankton, an observation that is Diagenetic processes, mediated by microbes, water, and directly correlated with the prevalence of their respective steroi- reduced sulfur compounds, render complex biomolecules into dal and acyclic isoprenoidal hydrocarbons in marine sediments both simpler hydrocarbons and much more complex material and oils (e.g., Knoll et al., 2007; Rampen et al., 2007; Sinninghe (kerogen) through polymerization and cross-linking (Adam Damsté et al., 2004; Volkman, 2003). A practical application of et al., 1993; Kohnen et al., 1989). Integration of geochemical the evolution of complex organisms and changes in the composi- understanding with sedimentary geology and basin modeling tion of ocean plankton through time has been development of ultimately resulted in a more informed understanding of hydro- age-diagnostic biomarkers that can place constraints on the tim- carbon systems, including resource and risk assessment (Hunt, ing of deposition of petroleum source rocks (e.g., Grantham and 1996; Tissot and Welte, 1978). Wakefi eld, 1988; Holba et al., 1998). As with studies of algae, sensitive GS-MS (gas Development of Molecular Paleontology chromatography–mass spectrometry) methods developed in the last few decades have provided the means with which to study Paleontologists have long recognized that microbial life has biomolecules that are unique to animal phyla. Cholesterol is the dominated life on Earth for most of its history, yet microscopic predominant membrane sterol of animals, and there is little else fossils comprise an infi nitesimally small fraction of the biomass in the way of other preservable molecules that could constitute a that has been present on Earth, and the vast majority of microbes robust “metazoan” biomarker. Basal invertebrate phyla, defi ned leave no visible trace at all. It was recognized early, however, that as those least removed from their single-celled ancestors, include microbial lipids derived from their cell walls, membranes, and sponges, cnidarians, and echinoderms, and these are the excep- pigments are preserved, and, in fact, form the dominant organic tions in that they have a wealth of complex biochemistries, possi- molecules that can be found preserved in sedimentary rocks. In bly refl ecting defenses against predation. Demosponges are well Figure 8, we summarize some of the important organic mole- known for their biosynthetic capacity to produce distinctive ter- cules used in geochemistry, organized according to contemporary penoids, including sterols that have a rare 24-isopropycholestane molecular phylogeny. The concept of “biomarker chemostratig- skeleton (Bergquist et al., 1991). 24-Isopropylcholesterols have raphy” has emerged as a means for using these molecules to chart been found in the calcisponges, hexactinellid sponges, or choano- evolutionary innovation, mass extinctions and radiations, chemi- fl agellates, and these form a unicellular sister group to sponges cal events in the ocean, and climate change (Gaines et al., 2009). (Love et al., 2009). Hydrocarbon derivatives of the distinctive C30 For example, the rise of complex multicellular life in the Neopro- sponge sterols are prevalent in sedimentary rocks and petroleums terozoic, the transitions in ocean plankton through the Phanero- of Cryogenian (Neoproterozoic) to Early Cambrian age, and they zoic, and the advent and radiation of land plants are among a few are thought to refl ect an acme in the abundance of demosponges of the biological innovations that have been, and continue to be in sedimentary environments of this time (Love et al., 2009; studied using biomarkers. McCaffrey et al., 1994). The molecular fossil record suggests that After several decades of study, it is now clear that major demosponges made their fi rst appearance in the Cryogenian, and paleontological mileposts are accompanied by corresponding this is concordant with molecular clock estimates of metazoan biosynthetic innovations that are recorded in molecular fos- divergence times (Peterson et al., 2007). sils. Colonization of the continents by plants, for example, was It is now recognized that biomarker hydrocarbons can be enabled by invention of a range of structural biopolymers, includ- diagnostic for certain kinds of paleoenvironmental circum- ing lignin and cellulose. Plants built defenses against desiccation stances (Fig. 9). Water-column redox stratifi cation, elevated and predation utilizing cuticles made of waxy hydrocarbons salinity, marine versus nonmarine sedimentation, and clas- for the former, and an array of sesqui- (C15), di- (C20), and tri- tic versus carbonate environments are some of the conditions

terpenoids (C30) and resinitic compounds for signaling and for that can be encoded into fossil hydrocarbon distribution pat- warding off insect predators. Distinct differences in the chemi- terns. A prime example lies in the insights provided by organic cal defenses utilized by conifers (Gymnosperms) and fl owering geochemical proxies into biogeochemical processes occurring plants (Angiosperms) can be recognized through the presence of during oceanic anoxic events (OAEs), and especially those distinctive diterpenoids in Paleozoic and early Mesozoic rocks associated with mass extinction events. The work of Holser and petroleums (e.g., Noble et al., 1985). An increase in the prev- and others (Holser, 1977; Holser et al., 1989) fi rst recognized spe500-08 1st pgs page 19

Five decades of advances in geochemistry 19 shifts in the isotopic compositions of C, O, and S in marine anoxygenic phototrophs that use sulfi de as an electron donor for sediments, interpreted to record the disruption of ocean chem- photosynthesis, and these produce distinctive light-harvesting istry and consequent biological mass extinction. Biogeochem- carotenoid and chlorophyll pigments that may be recorded in ists have subsequently observed concomitant signals in organic preservable structural features (Grice et al., 1996; Sinninghe molecules, redox-sensitive trace elements, and other geochemi- Damsté and Koopmans, 1997; Summons and Powell, 1987). cal proxies. One recurring trend is seen in the prevalence of Diagenetic reduction and stabilization of these compounds are biomarkers derived from the green sulfur bacteria (Chlorobi) enhanced under the strongly reducing (euxinic) conditions that in sediments that were deposited during OAEs. Chlorobi are are favored by the green sulfur bacteria.

Figure 8. A “tree of life,” based loosely on the nucleotide sequences of small subunit ribosomal ribonucleic acid (RNA), to illustrate life’s three domains and the diverse structures of lipids that are characteristic of organisms at the domain level and, in some cases, phylum level. For example, most eukaryotes utilize sterols, and, in a few cases such as diatoms, dinofl agellates, and demosponges, there are particular sterols that are very specifi c. The membrane lipids of bacteria and eukaryotes incorporate hydrocarbon chains that are linear or branched in relatively simple ways, and these are mostly linked to glycerol via ester linkages. Archaea, on the other hand, build equivalent structures using isoprenoidal chains, and they are linked to glycerol with ether bonds. Many of these features are preservable and can be observed in organic matter extracted from sedimentary rocks. Improved understanding of the way in which lipid chemistry relates to physiology, phylogeny, and environment, and deduction of the pathways by which organic molecules become preserved in sediments have been prime accomplishments of organic geochemists in the past 50 yr. GDGT—glycerol dibiphytanyl glycerol tetraethers. spe500-08 1st pgs page 20

20 Johnson et al.

Figure 9. An example of the way in which marine water-column redox structure can be deduced from the preserved remains of photosynthetic pigments. The carotenoid β-carotene is an accessory pigment common to oxygen-producing phototrophs such as algae and bacteria and is, there- fore, produced in, and diagnostic for, oxygenated environments. In contrast, isorenieratene is produced by brown strains of the Chlorobi, which are obligately anaerobic photoautotrophic bacteria and which thrive in low-light, sulfi dic environments, typically where the oxycline is as deep as 60 m. Chlorobactene is produced by the “green” forms of the Chlorobi, which require higher light intensities and are generally found where the oxycline shoals to 20–40 m depth. Okenone is characteristic of the purple sulfur bacteria (Chromatiaceae), which require sulfi de and even higher light intensities. This pigment can be diagnostic for highly restricted environments such as hypersaline lagoons and where oxyclines are at 20 m or less. All these pigments can also be found in redox-structured microbial mats. It is probably no coincidence that the hydrocarbon derivatives of these pigments abound in sediments and petroleum from the Mesozoic and other warm intervals of Earth’s history and particularly when narrow seaways and lack of ice resulted in sluggish circulation and ready maintenance of deep-ocean anoxia.

Geologists and paleontologists have long debated the origin was a contributing factor to the biological mass extinction in the of the Permian-Triassic extinction, both the cause and the bio- marine realm as well as on land. Co-occurring with the evidence logical response, and work in molecular paleontology in the last for euxinia, molecular fossils shed light on other aspects of this decade has provided important insights. Studies of a core drilled event, including transitions in ocean plankton (Cao et al., 2009), into the end-Permian type section at Meishan, China, show that and a massive terrestrial weathering event (French et al., 2012; Chlorobi carotenoids are particularly abundant throughout the Sephton et al., 2005; Xie et al., 2007). last few million years of the Permian and into the Early Triassic, Although many theories abound, no consensus has been implying that shallow-water euxinic conditions were protracted reached on the processes that instigated the end-Permian mass at the type section of the greatest mass extinction of the geologi- extinction (Erwin, 2006). Evidence that the event had its roots in cal record (Cao et al., 2009; Grice et al., 2005). Examination of ocean chemistry is consistent: Anoxia with accompanying effects other Permian-Triassic transition sections confi rms the pattern of hypercapnia, ocean acidifi cation, and sulfi de toxicity could and suggests that euxinic conditions were pervasive globally, all have contributed to the loss of marine life, and, arguably, to as far as can be discerned from continental margin sediments of loss of life on land (Holser, 1977; Knoll et al., 1996; Twitchett the Tethys, Panthalassic, and Boreal Oceans (Hays et al., 2007, et al., 2001; Wignall and Hallam, 1992). Numerous researchers 2012). These observations are consistent with other isotopic and also point to volatile release during emplacement of the Siberian inorganic proxies for a global and intense oceanic anoxic event Traps lavas and intrusions (Payne and Clapham, 2012). These during the Permian-Triassic transition. Independent modeling authors argued that gases liberated during the volcanism itself, also suggests that toxic hydrogen sulfi de would have been upwell- augmented by additional C and S volatiles released from affected ing from the deep ocean onto continental shelves and entering the sediments, can account for most of the end-Permian paleonto- atmosphere (Kump et al., 2005), implying that sulfi de toxicity logical and geochemical observations, and this is consistent with spe500-08 1st pgs page 21

Five decades of advances in geochemistry 21 the timing and tempo indicated by the most recent geochrono- and represent compounds that were biosynthesized prior to the logic studies (Kamo et al., 2003; Mundil et al., 2010; Shen et Great Oxidation Event (GOE) at ca. 2.3–2.4 Ga. The presence al., 2011). Despite continued debate on the ultimate trigger, most of steroids in older than 2.5 Ga rocks supports a raft of isotopic authors agree that the geochemical and geologic evidence for a and trace-element proxies, some of which have been discussed bolide impact (Becker et al., 2001) is unclear (Farley and Muk- already, which indicate that trace amounts of oxygen were being hopadhyay, 2001). In this context, it is also noteworthy that the produced (e.g., Anbar et al., 2007; Czaja et al., 2012) and respired organic geochemical signals that accompany the Cretaceous- (Eigenbrode and Freeman, 2006) in the surface oceans well in Paleogene impact and mass extinction event bear no resemblance advance of the GOE. Further drilling, using ultraclean drilling to those seen at OAEs (Sepulveda et al., 2009). protocols and contamination tracers, is currently under way in There are other examples where biomarker research in the the Pilbara craton, Australia, and this is expected to shed further last 20 yr has helped us to understand oceanic anoxia. The Chlo- light on ocean-atmosphere redox conditions in the Neoarchean. robi carotenoid signal is one of several organic geochemical fea- tures that are consistently observed during intervals of enhanced Elucidating Paleoenvironmental Records black shale deposition during the Paleozoic and Mesozoic Eras (Koopmans et al., 1996; Pancost et al., 2004). Isorenieratane, One of the most important discoveries in organic geochem- chlorobactane, and the aryl isoprenoids derived from them, along istry in the last 50 yr has been the recognition of some classes of with C, S, and N isotope and other biomarker anomalies that are organic molecules that encode signals for past climate regimes. indicative of anoxia, euxinia, and water-column stratifi cation, Of key importance are the proxies for sea-surface temperature comprise a recurring signal that is seen in both clastic and car- (SST) based on the degree of unsaturation in long-chain ketones bonate sediments deposited during Mesozoic OAEs (Kuypers (C37 alkenones) from marine algae (the Uk37 index; Brassell et al., 2004; Schouten et al., 2000b). Many of these sediments, et al., 1986), and the proportions of isoprenoidal ether lipids such as the Kimmeridge Clay Formation, are also prolifi c source (dubbed glycerol dibiphytanyl glycerol tetraethers or GDGT) rocks for petroleum deposits that carry the same signal (Van derived from archaea (the Tex86 index; Schouten et al., 2002). In Kaam-Peters et al., 1998a, 1998b). The prevalence of organic- soils and paleosols, bacterial (non-isoprenoidal) ether lipids have rich and petroleum-prone sediments that were deposited during been proposed to provide information on both temperature and the Mesozoic Era comprises the bulk of the world’s oil-in-place pH (Weijers et al., 2006). Signifi cant effort has been expended in inventory, refl ecting pervasive development of narrow rift basins elucidating the specifi c organisms responsible for these proxies, and seaways, greenhouse conditions, and sluggish ocean circula- as well as the physiological basis of the sedimentary temperature tion during breakup of the Pangean supercontinent (Klemme and records that are encoded by alkenones and ether lipids. In the Ulmishek, 1991). case of the former, prominent marine haptophyte algae such as Studies of molecular fossils in Precambrian sedimentary Isochrysis galbana, Emiliania huxleyi, and Gephyrocapsa oce- rocks have been pursued in earnest since the 1980s, and this work anica, living primarily in the surface mixed layer and sometimes has shown that such rocks exhibit an array of novel molecular extending to the thermocline (Ohkouchi et al., 1999), are widely and isotopic features that are not seen in younger sedimentary recognized as the primary biological sources in the oceans (Volk- sequences (Grantham and Wakefi eld, 1988; Grosjean et al., 2009; man et al., 1995, 1980). Given the biogeographical trends and Klomp, 1986; Summons et al., 1988). Unprecedented distribu- temperature preferences of alkenone-producing algae, it appears tions of steroids, for example, can be traced to the proliferation likely that the alkenones record lipids that were derived from a of green and red algae, as well as sponges, and this accords with combination of cold- and warm-water adapted strains. Nutrient other geochemical evidence for the Neoproterozoic oxygenation status, salinity, growth rate, growth stage, and temperature all of the atmosphere and ocean (Canfi eld et al., 2007; Fike et al., infl uence the distributions of alkenones in cultured haptophytes, 2006) and paleontological evidence of the proliferation of com- yet empirical calibrations of the Uk37 indices to core tops, and plex life (Erwin et al., 2011; Knoll et al., 2004). Euxinic condi- to cultures, appear to be very robust over a wide range of tem- tions have been inferred from geochemical studies of Mesopro- peratures, environmental variables, and spatial scales (Prahl et terozoic strata, and this is refl ected in carotenoid molecular fossil al., 2003; Rosell-Melé et al., 1995; Sikes and Volkman, 1993). distributions in the same rocks (Brocks et al., 2005). Steroid and The C37 alkenones occur in sediments that predate the appearance triterpenoid hydrocarbons have also been recovered from much of contemporary alkenone-producing haptophytes, and, because older rocks (Dutkiewicz et al., 2006; George et al., 2008; Wald- of their apparently close evolutionary relations and similarity to bauer et al., 2009). Some of this work has been challenged on the fossil forms, this has potentially opened application of the proxy basis of C isotope data obtained by nano-SIMS (Rasmussen et al., to ancient sedimentary records on multi-million-year time scales. 2008), suggesting that the recovered biomarkers instead refl ect Paleotemperature reconstructions into deep time, however, will contamination. In contrast, studies of fresh drill cores recovered have to be viewed cautiously until more is known about the phys- from the Kaapvaal craton that were targeted to address this issue iological function of C37 alkenones in haptophyte algae. What is (Waldbauer et al., 2009) suggest that complex terpenoids, and most impressive about the alkenone SST records is the degree particularly the steroids, could be indigenous to the samples of concordance with independent isotopic records, including spe500-08 1st pgs page 22

22 Johnson et al.

O isotope variations recorded in ice cores and foraminifera (e.g., continuous-fl ow approaches that were analogous to GC-MS and Herbert et al., 2001). It has become clear that multiple approaches LC-MS (liquid chromatography–mass spectrometry; see also are needed to understand and constrain each proxy in terms of the previous discussion). Carbon was the fi rst and most obvious ele- robustness of paleotemperature records and the effects of lateral ment targeted for compound-specifi c isotope analysis (CSIA), sediment redistribution (Mollenhauer et al., 2011). and the initial results immediately revealed the diverse origins In contrast to the alkenones and their sedimentary records as of sedimentary organic molecules (Freeman et al., 1990; Hayes expressed in Uk37 data, the temperature proxy based on archaeal et al., 1990). Analogous methods for H (Sessions et al., 1999), N ether lipids (Tex86) is less well constrained with respect to the (Macko et al., 1997), and S (Amrani et al., 2009) soon followed. precise sources of the isoprenoidal GDGTs and the controls on It is now common to measure multiple isotopic compositions of their production. The Tex86 proxy emerged from the discovery individual compounds such as chlorophylls, as well as the geo- of abundant archaeal plankton communities that populate oceans, porphyrins and maleimides that are derived from them (Chikarai- lakes, and other aquatic environments, in addition to recognition shi et al., 2005). In turn, the technical capability to make precise that membrane-spanning GDGTs contain variable numbers of isotopic measurements on these elements exposed the dearth of cyclopentane rings that could be isolated from particulate organic knowledge about the ways in which these biosynthetic processes, matter and the underlying sediments (Delong et al., 1998). A organismic physiology, and environmental parameters infl u- particular problem, however, is that diverse Crenarchaeota and ence the isotopic abundances of individual molecules (Hayes, Euryarchaeota that inhabit both the water column and the sedi- 2001; Laws et al., 1997, 1995; Popp et al., 1998; Sessions et al., ments produce some of the same compounds that comprise the 1999; Smith and Freeman, 2006). The impact of developing and proxy. In addition, other studies show that the prevalence, and perfecting tools and techniques for compound-specifi c isotope production, of GDGT often takes place at great depth (Ingalls analyses has been profound. The protocols and logic are now et al., 2006; Sinninghe Damsté et al., 2002), frequently near or routinely used in archaeology, paleoclimatology, paleohydrology within oxygen minimum zones. Nevertheless, evidence suggests ecology, and forensics, and geochemists have been the instigators that the distributions of GDGT in core-top sediments are similar of many of these applications (Lichtfouse, 2000). to those of (epipelagic) marine Crenarchaeota living in the top Radiocarbon measurements of individual organic molecules 100 m of the water column, an observation consistent with the have also led to new insights into the age structure of fossil relatively robust core-top calibrations of SST with Tex86 (Wuch- lipid assemblages, and their transport from source to sedimen- ter et al., 2005). Numerous studies point to an origin for archaeal tary sinks (Pearson et al., 2001). Thus, in contemporary envi- ether lipids in ammonia-oxidizing Marine Group 1 Crenarchaeota ronmental settings, it is possible to discern the age relations of (Schouten et al., 2008). Evidence has been presented to show that multiple paleoclimate proxies (Mollenhauer et al., 2003), thereby Tex86 values agree with foraminifer proxies that indicate rapid resolving processes such as winnowing and lateral advection of ocean warming during the late Paleocene thermal maximum organic matter (Kusch et al., 2010; Mollenhauer et al., 2011). The (Zachos et al., 2006). Paleotemperature records extending into robustness, scope, and fi delity of organic SST proxies based on Mesozoic time are considered possible (Schouten et al., 2003), algal alkenones and archaeal ether lipids have been signifi cantly but there are few avenues available for independent verifi cation. increased through knowledge gained from compound-specifi c An important consequence that fl ows from the discovery of iso- radiocarbon measurements (Eglinton and Eglinton, 2008). prenoidal GDGT has been the realization that the Crenarchaeota are exceptionally important components of the marine carbon Which Organisms Produce All These Sedimentary Lipids? and nitrogen cycles (Delong, 2009; Ingalls et al., 2006). As interest turned to understanding the origins and diage- Using Stable Isotopes to Better Understand the Origins netic pathways by which organic molecules became fossilized, of Biomarkers there was a great expansion in our knowledge of the diversity of lipids in sediments before, in fact, their biological precur- One of the continuing challenges of organic geochemistry sors were identifi ed in living organisms (Ourisson et al., 1979; has been to determine the sources of individual biomarker mol- Rohmer, 2010; Rohmer et al., 1980). The emergence of another ecules, because few compounds are thought to exclusively derive analytical tool, high-performance liquid chromatography cou- from a single organism or represent a single biogeochemical pro- pled to mass spectrometry (HPLC-MS), enabled identifi cation cess. Sterols, for example, are made or used by almost all eukary- of increasingly large and complex polar lipid molecules such otes, from microbes to mammals. Despite their varied chemical that, today, geochemists have focused on tracking the distribu- structures, sterols are not as diverse as the taxonomic distribu- tions of membrane-spanning lipids (Schouten et al., 2000a), tions that they represent. One way to constrain the origins of mol- intact polar lipids (Sturt et al., 2004), and complex biohopanoids ecules is through their stable isotope compositions. Initial work (Talbot et al., 2008) from organisms through the water column used bulk measurements (e.g., Hare et al., 1991), but a desire for and into sediments. Intact polar lipids can be proxies for living improved specifi city and precision drove development of tools organisms and, often in combination with deoxyribonucleic acid that allow isotopic measurements at the molecular level using (DNA), have been used to map out biogeochemical processes spe500-08 1st pgs page 23

Five decades of advances in geochemistry 23 in sediments (e.g., methanotrophy; Orphan et al., 2002) and the Meteorites were initially the only extraterrestrial samples avail- water column (e.g., Sinninghe Damsté et al., 2005), as well as to able for geochemical analysis, and these have now been supple- help defi ne the extent of the deep subsurface biosphere (Lipp et mented by samples returned from the Moon, a comet, an asteroid, al., 2008; Rütters et al., 2002). and the solar wind. In addition, new kinds of meteorites, includ- Advances in identifi cation of the diverse types of organic ing samples from Mars and the Moon, have been recognized in molecules in rocks have been concomitant with efforts to the last fi ve decades. Signifi cant strides have also been made in increase our understanding of the ways in which organisms pro- geochemical analyses by remote sensing. duce these lipids (biosynthetic pathways), the evolution of these pathways, and the array of functions performed by lipid mol- Extinct Radionuclides and Early Solar System Chronology ecules in cells (physiological roles). This is one of the current frontiers of organic geochemistry, because, very often, the taxo- Short-lived radionuclides that were present in the solar neb- nomic relations are often ambiguous between molecules of inter- ula have decayed away, but their presence can be inferred from est in the environment or rock record and their precursor organ- their radiogenic daughter isotopes. The fi rst extinct radionuclide isms. Based on knowledge derived from cultured taxa, we know found in meteorites, 129I, was discovered in 1961, and others that hopanoids, for example, are distributed very unevenly across followed in the next 50 yr. Additional extinct nuclides are now the bacterial domain, but the reasons for this remain mysterious recognized to have been present in the solar nebula, including (Rohmer et al., 1984). Precisely how key compounds are pro- 10Be, 26Al, 41Ca, 53Mn, 60Fe, 107Pd, 146Sm, and 182Hf. In some cases, duced is only now coming to light, and efforts to deduce this have extinct radionuclides can provide high-resolution chronometers led to the discovery of a completely unknown isoprenoid biosyn- for early solar system events (Nyquist et al., 2009). The 146Sm- thesis pathway in bacteria (Rohmer, 2003; Rohmer et al., 1993); 142Nd system has already been discussed here in regard to early such fi ndings can have societal implications because they may terrestrial rocks. lead to new classes of antibiotics. The revolution in genomics One of the most useful extinct nuclides is 26Al, which decays has, therefore, given geochemists a new tool for identifi cation of to 26Mg with a half-life of ~730,000 yr. Its former existence was important biosynthetic genes and gene families, thereby enabling fi rst proven when Lee et al. (1977) analyzed plagioclase and other culture-independent approaches to be more nuanced in providing minerals in a refractory Ca-Al inclusion (CAI) from the Allende an understanding of the sources and function of biomarker lipids. meteorite, and they determined that plagioclase had a large 26Mg By way of example, we know that pentacyclic hopanoids excess (relative to nonradiogenic 24Mg). Because CAIs are the and tetracyclic steroids are biosynthetically and evolutionarily oldest solar system materials, based on their 207Pb-206Pb ages, related (Rohmer, 2010; Summons et al., 2006) because they are their 26Al contents are taken to refl ect those at the beginning of derived from the common acyclic precursor squalene. Genomic the solar system, and other measurements are compared to the databases can be queried for both hopanoid and steroid cyclase 26Al/27Al ratio and 207Pb-206Pb age of CAIs for chronology. The genes, the sequences of which carry information on their evolu- 26Al value refl ects production by stellar nucleosynthesis, and its tionary heritages (Fischer and Pearson, 2007). No longer are bio- occurrence in the solar nebula is commonly attributed to “seed- synthetic capabilities solely based on studies of cultured organ- ing” by a nearby supernova. Refi nements in SIMS and ICP-MS isms because their genomes and natural environmental samples techniques have permitted measurements of 26Al-26Mg systemat- can now be queried directly for the biosynthetic capabilities of ics in samples that have low Al contents, including chondrules community members (Fischer et al., 2005; Pearson et al., 2009). and differentiated meteorites. An example is illustrated in Fig- The unexpected occurrence of 2-methylhopanoids in cultures of ure 10, which shows different 26Al/27Al ratios for chondrules in a purple nonsulfur bacterium has led to a raft of new knowledge several classes of chondrites, and this implies that chondrules about the genes responsible for hopanoid biosynthesis (Welander formed several million years after CAIs. et al., 2012) and the capacity to construct mutants lacking genes The 182Hf-182W system, with a half-life of 8.9 m.y., has also of interest (Welander et al., 2010), pointing to the involvement proved to be very useful in dating early solar system events. Early of these complex molecules in ameliorating stress (Doughty et measurements by N-TIMS (negative thermal ionization mass al., 2009; Welander et al., 2009). In summary, molecular paleon- spectrometry) were diffi cult, but this chronometer was made tology that is informed by individual and community genomes truly accessible when MC-ICP-MS analysis became available is providing signifi cant insights into the sources of organic mol- (Halliday et al., 1996). Both Hf and W are refractory elements ecules that may be preserved in the rock record, which in turn and so usually occur in chondritic proportions. Because, how- provides fundamental constraints on the evolution of the biosyn- ever, Hf is lithophile and W is siderophile, the 182Hf-182W system thetic pathways used by life. can date metal-silicate fractionation events such as core forma- tion. Such ages, in turn, constrain the time of planet accretion. GEOCHEMISTRY ON OTHER WORLDS For example, the estimated age of Earth’s accretion (50% mass), based on this chronometer, is 30 to >100 m.y. after CAIs (Halli- The last 50 yr have been a golden age for exploration of day, 2004). More rapid accretion ages for asteroidal bodies were our solar system, and geochemistry has played a dominant role. determined by Kleine et al. (2009). spe500-08 1st pgs page 24

24 Johnson et al.

Nebular Condensation

Many, although not all, of the solids that now comprise solar system bodies once condensed from nebular vapor. Early attempts to determine the condensation sequence in a cooling gas of solar composition involved simple calculations, performed long before digital computers were available. The equilibrium condensation behavior of elements in a nebular gas was rigor- ously modeled by Grossman and Larimer (1974) and other work- ers since then (e.g., Petaev and Wood, 1998). The canonical con- densation sequence, showing condensing phases and the fraction of each element condensed at various temperatures, is illustrated in Figure 12. Based on experimental determinations of entropy, enthalpy, and heat capacity, equations of state describing the Figure 10. Ion microprobe measurements of initial 26Al/27Al, and thermodynamic stabilities of a host of possible minerals under corresponding ages relative to Ca-Al inclusion (CAI), in chon- various conditions can be calculated. Liquids are not stable at the drules from primitive meteorites (unequilibrated ordinary chon- drites and several classes of carbonaceous chondrites). Figure is low pressures appropriate for the solar nebula. Some minerals in from McSween and Huss (2010; used with permission). UOC— the condensation sequence do not condense directly, but form or unequilibrated ordinary chondrite. adjust their compositions by reactions of previously condensed phases with the gas. This condensation calculation was done for the 23 elements with the highest cosmic abundances. Generally, Mass-Independent Isotope Fractionation and Early Solar thermodynamic data for trace elements are lacking, so chemi- System Conditions cal analyses of trace elements in high-temperature minerals of chondritic meteorites guide our understanding of their condensa- One of the fi rst mass-independent fractionations (MIF) for tion behavior. Accordingly, in these models trace elements are stable isotopes was observed for O measured in CAIs in mete- allowed to condense as simple metals, oxides, or sulfi des, which orites (Clayton et al., 1973), as illustrated in Figure 11. For are assumed to dissolve into appropriate major mineral phases. 17O/16O-18O/16O variations, mass-dependent fractionation defi nes The validity of condensation calculations has been supported by a line with a slope of ~0.5. Subsequent analyses of O isotopes studies of refractory inclusions (CAIs) in chondrites, which have in bulk meteorites showed that igneous meteorites (achondrites) bulk chemical compositions consistent with those calculated for defi ned distinct fractionation lines parallel to that of Earth. Bulk the fi rst 5% of condensable matter (Davis and Richter, 2004). chondrites of different classes also plot in distinct areas of the Moreover, the same minerals that comprise these inclusions are diagram. As a consequence, O isotopes have become a very use- predicted to have been the earliest condensed phases (Fig. 12). ful criterion for meteorite classifi cation, and they place strong There remains some controversy, however, about whether the constraints on processes that operated early in the history of the CAIs are actually condensates or refractory residues from evapo- solar system (Clayton, 2004). ration (the reverse of condensation). The MIF trend for CAIs in Figure 11 was originally inter- Condensation calculations have also been done at different preted as a mixing line between solar system gas (plotting on nebular pressures and using nonsolar gas compositions (Ebel and or above the terrestrial fractionation line) and exotic grains that Grossman, 2000; Wood and Hashimoto, 1993; Yoneda and Gross- contained pure 16O (plotting on an extension of the CAI line). A man, 1995). Parts of the solar nebula may have been enriched in newer explanation is that the isotopic variations in CAIs arose dust, which, when vaporized, could have yielded nonsolar vapor, from self-shielding during photodissociation of CO, a major possibly allowing more reduced phases or even liquids to con- nebular gas (Clayton, 2002). Because 16O is abundant, there were dense. These condensation calculations could also apply to other large differences between the amounts of C16O, on the one hand, stars, e.g., red giants, which can have nonsolar compositions. and C17O and C18O on the other. The abundant C16O thus became optically thick far from the Sun; C17O and C18O remained opti- Organic Matter from Space cally thin, and so were dissociated to a greater extent by radiation. This produced an inner zone in the nebula that was enriched in The molecular and isotopic chemistry of extraterrestrial 17 18 16 O and O, which reacted with H to make H2O. The O-depleted organic matter is another area where technical innovation has water then reacted with dust or condensates to form nebular sol- spurred improved appreciation for the nature and diversity of ids. In this model, the Sun, representing the nebular gas, must be organic compounds formed in the interstellar medium and the 16O-rich. The fi rst measurement of O isotopes in the solar wind processes that alter them prior to their delivery to Earth in mete- returned to Earth by the Genesis mission (McKeegan et al., 2011) orites (Derenne and Robert, 2010) and interstellar dust particles is consistent with this model. (Flynn et al., 2004). The low-molecular-weight compounds spe500-08 1st pgs page 25

Five decades of advances in geochemistry 25

tein amino acids (Cronin and Pizzarello, 1997). Moreover, the l- enantiomeric excess of some compounds such as isovaline in the Murchison and Orgueil meteorites appears to be related to the extent of aqueous processing, suggesting that it could refl ect amplifi cation of a small initial isovaline asymmetry. If correct, this would be inconsistent with the theory that ultraviolet (UV) circularly polarized light was the primary source of l-enrichment in amino acids. It seems possible, therefore, that early life on Earth had access to molecular building blocks with the left hand- edness that characterizes the amino acids of all life today (Engel and Macko, 2001; Glavin and Dworkin, 2009). Such results dem- onstrate the importance of “off-world” geochemistry to inform- ing us about terrestrial evolution.

Lunar Geochemistry Figure 11. Oxygen isotopes (relative to SMOW) measured in Ca-Al inclusion (CAI) from a carbonaceous chondrite defi ne a mass-independent fractionation trend, distinct from the terres- The Moon is a geochemical experiment conducted under trial mass-dependent trend. Figure is modifi ed from Clayton et vastly different conditions than Earth. The geochemical explo- al. (1973). SMOW—standard mean ocean water. ration of the Moon began with the return of samples by Apollo astronauts in 1969, which extended over an exciting 4 yr period. Depletions in volatile elements and enrichments in refractory ele- identifi ed include many classes of biologically important mol- ments measured in these rocks support the giant impact hypoth- ecules, for example, amino acids (Kvenvolden et al., 1970), esis for the Moon’s formation (Taylor et al., 2006). Vaporization hydroxyacids and dicarboxylic acids (Cronin et al., 1993; Law- of portions of the target (Earth) and the impactor, followed by less et al., 1974; Peltzer et al., 1984), as well as nucleobases incomplete condensation in Earth orbit, can account for the frac- (Martins et al., 2008). Isotopic data and pyrolysis studies suggest tionation observed in volatile and refractory elements. Measured that distinct processes are involved in formation of small mol- depletions of siderophile elements in lunar rocks are consistent ecules and macromolecular material (Sephton et al., 2004; Yuen with models that indicate preferential incorporation of the silicate et al., 1984). Perhaps the most profound, enigmatic, and contro- mantles of the impactor and target into the Moon, accompanied versial fi nding was that some amino acids were not racemic, as by accretion of the impactor’s core into Earth’s core. had long been considered fact (Engel and Macko, 1997; Engel The revelation that a magma ocean once existed on the Moon et al., 1990; Engel and Nagy, 1982). This discovery, originally (Wood et al., 1970) has profoundly changed planetary science, dismissed as due to terrestrial contamination, became accepted and now wholesale melting has been proposed for a number of once it was demonstrated also to be a feature of some nonpro- bodies, including early Earth. REE analyses came of age during

Figure 12. A model of the condensa- tion sequence for a cooling gas of solar composition at 10−4 atm pressure. Con- densed minerals are labeled in italics, and curves show the fraction of each element condensed as a function of temperature. REE—rare earth element. Figure is modifi ed from Grossman and Larimer (1974). spe500-08 1st pgs page 26

26 Johnson et al. the Apollo program, as discussed earlier, and the complementary mic rays. Orbital measurements provide global coverage and thus REE patterns of anorthosite from the highlands crust and basalts are particularly useful in understanding geochemical processes from the maria (Fig. 13) provide the most persuasive evidence for at a planetary scale. Global maps of the distribution of Fe and a lunar magma ocean (Taylor and Jakes, 1974). In this model, the Th, which are particularly sensitive to this technique, have been thick anorthositic crust was formed by fl otation of plagioclase, employed to distinguish lunar terranes based on their geochemi- which produced a positive Eu anomaly in the crust, and this was cal characteristics (Jolliff et al., 2000). The petrology of each balanced by the Eu depletion that was produced by olivine and terrane has been interpreted unambiguously by comparison with pyroxene that accumulated to form the mantle, a characteristic the Fe and Th contents of returned lunar samples. Other results, that was inherited by later mare basalt melts. The Eu anomaly made possible by measuring the neutron fl ux from the surface, in lunar rocks was enhanced by the Moon’s low oxidation state, included the discovery of H at the lunar poles associated with which increased the proportion of Eu2+, allowing it to partition cold, permanently shadowed craters believed to contain water ice into plagioclase. The last dregs of the magma ocean were rich (Feldman et al., 2001). in Fe, Ti, and incompatible trace elements, including K, REEs, The Mars Exploration Rovers Opportunity and Spirit car- and P (the “KREEP” component), which became sandwiched ried Alpha Particle X-Ray Spectrometers (APXS) that analyzed between the crystallized crust and mantle. Gravitational overturn the chemical compositions of hundreds of rocks and soils on the of this dense, Fe- and Ti-rich layer resulted in mixing of KREEP surface of Mars (Brückner et al., 2008; Gellert et al., 2006). By into magmas that subsequently intruded the anorthositic crust. measuring characteristic X-rays produced by alpha particles and The discovery of KREEP was initially made through geochemi- X-rays emitted from a radioactive source, the APXS was able cal analyses of Apollo samples (Taylor et al., 2001). to analyze all of the major elements and a few minor and trace elements. These studies were a critical part of the classifi cation Geochemistry by Remote Sensing of rocks encountered during rover traverses, and they provided constraints on the processes that formed these materials. Oppor- Geochemical analyses no longer are restricted to the labora- tunity analyzed sedimentary rocks that contained high contents tory. Here, we consider two examples of geochemistry by space- of S, Cl, and Br, interpreted to refl ect evaporation of salt-laden craft that have altered our perceptions of what is possible using brines, demonstrating that liquid water was once abundant on remote sensing. Mars (Squyres et al., 2004). On the other side of the planet, The Gamma Ray Spectrometer on the Lunar Prospector Spirit analyzed a variety of ancient basaltic rocks (Fig. 14) that orbiter analyzed the abundances of a handful of elements (Fe, have compositions that are distinct from those of younger Mar- Ti, K, Mg, Al, Ca, Si, Th, and H; Lawrence et al., 1998; Pretty- tian basaltic meteorites, buttressing the argument that although man et al., 2006), utilizing characteristic gamma-ray emissions heterogeneous, Mars is fundamentally a basalt-covered world produced by radioactive decay or by reactions initiated by cos- (McSween et al., 2009).

Stardust in the Laboratory

Tiny motes of stardust, condensates that formed around dying stars or farther out in the interstellar medium, were dis- covered in chondritic meteorites (Lewis et al., 1987), after a long search beginning in the 1960s. These diamond nanoparti- cles were isolated by dissolving chondrites in a series of harsh acids, and at each step tracking the isotopically anomalous Xe they contained. The grains are thought to have been implanted when the outer layers of giant C-rich stars were sloughed off and condensed as diamond, and were subsequently implanted with distinctive nuclides produced when the star exploded as a super- nova. Approximately 20 types of presolar grains, including sili- con carbide (Tang and Anders, 1988) and graphite (Amari et al., 1990), as well as oxides, nitrides, and silicates, have now been found. All are distinguished by their exotic isotopic compositions (Tang and Anders, 1988). The challenging elemental and isotopic analyses of stardust Figure 13. Chondrite-normalized rare earth ele- grains, which range from a few nanometers to a few micrometers ment (REE) patterns for lunar anorthosite and mare basalt. The complementary europium anom- in size, illustrate the great progress made in micro-analytical alies for crust and mantle-derived rocks support techniques. An example, using SIMS-analyzed C and N isotopes the magma ocean hypothesis. to fi ngerprint the sources of presolar silicon carbide grains, is spe500-08 1st pgs page 27

Five decades of advances in geochemistry 27 shown in Figure 15. More importantly, the isotopic compositions toward trying to understand how cycles are linked. Many ele- of presolar grains provide “ground truth” for stellar nucleosyn- mental cycles require monograph-length treatment, and the fact thesis models, directly linking cosmochemical measurements in that there is a journal devoted solely to biogeochemical cycling the laboratory to astrophysical theory. These data also (1) provide (Global Biogeochemical Cycles) refl ects the sustained interest in information on capture cross sections for neutrons, the capture of geochemical cycles. which makes heavier elements, (2) demonstrate where theoreti- In geochemical cycles, the reservoirs (mass, M) and fl uxes of cal models are inadequate to describe the internal structures of individual elements (or groups of closely related elements) into

stars, (3) identify neutron sources for the s-process, and constrain and out of the system (mass/unit time, Fin and Fout) are quantita- the scale of mixing in supernovae (Nittler, 2003; Zinner, 2004). tively accounted for over geological time (t). Accordingly, assess- Presolar grains also fundamentally changed the way we think ments of geochemical cycles are essentially a mass balance of the about the solar system’s formation. Their widespread occurrence element of interest on some appropriate physical scale over some in primitive meteorites demonstrates that a hot nebula did not appropriate duration, and to a great extent are the natural conse- vaporize all preexisting solids, overturning a view that prevailed quence of quantifying the overall rock cycle (Gregor, 1992) and 50 yr ago. hydrological cycle (Fig. 16). Geochemical cycles are considered to be either “exogenic,” for those operating on or near Earth’s INTEGRATING THE PICTURE: surface (hydrosphere, atmosphere, biosphere ± sediments), typi- GEOCHEMICAL CYCLES cally on relatively short time scales (<~104–107 yr), or “endo- genic,” for those operating within the interior of Earth (oceanic The concept of geochemical cycles dates to the late nine- and continental crust, mantle, core ± sediments), typically on teenth century and was well established (i.e., began to appear relatively long time scales (>~104–107 yr). The physical inter- in textbooks) by the mid-twentieth century (e.g., Goldschmidt, face between the endogenic and exogenic parts of an element’s 1954). The modern phase of quantifying global geochemi- geochemical cycle typically occurs in soils and the sedimentary cal cycles arguably dates from the pioneering work of Garrels cover. Characterization of complete global cycles requires inte- and Mackenzie (1971, 1972), who integrated major-element gration of both endogenic and exogenic cycles as witnessed, for chemistry (including C, S, and Cl) into a steady-state recycling example, by the relatively recent recognition that the C cycle is model for the evolution of the sedimentary mass. As described in greater detail in the following, geochemical cycles are often com- plex and inter-related, and considerable effort has been directed

Figure 15. Carbon and nitrogen isotopic ratios in presolar silicon carbide grains, measured with an ion microprobe. Most grains from Figure 14. A geochemical classifi cation diagram for volcanic stars on the asymptotic giant branch (AGB) of the Hurtzsprung- rocks comparing the compositions of Martian rocks and soils Russell diagram plot above the solar composition (dashed lines). derived from them in Gusev crater, analyzed by the Spirit rov- Supernovae grains have lower 14N/15N, and rare grains from no- er’s Alpha Particle X-Ray Spectrometers (APXS), with labo- vae, which are powered by explosive hydrogen burning, plot in the ratory analyses of Martian meteorites. Figure is modifi ed from lower-left quadrant. The stellar sources of other grains are not cur- McSween et al. (2009). rently known. Figure is modifi ed from Zinner (2004). spe500-08 1st pgs page 28

28 Johnson et al. affected by signifi cant levels of microbial activity deep within the 1960s–1980s, considerable effort went into determining the parts of Earth’s crust (e.g., Hazen et al., 2012), in addition to apparent mean oceanic residence times of the elements (τ), a con- mantle sources. cept fi rst introduced by Barth (1952), as well as documenting the Depending on the way in which the physical and/or tempo- balance between the masses of the various elements that entered ral scales are defi ned, geochemical cycles may be either open or the oceans from rivers, and those exiting the oceans through closed, where the distinction is a function of whether or not exter- sedimentation (e.g., Drever et al., 1988). The ensuing research nal inputs and/or outputs take place within the defi ned reservoirs. identifi ed important additional sources of elements to the oceans Those concepts govern whether or not a geochemical cycle is in (e.g., basalt-seawater hydrothermal interaction) that had not been steady state (i.e., dM/dt = 0; Fin = Fout), leading to the further con- previously recognized, additional reservoirs for elements (e.g., cept of residence time (τ = M/F), which in turn is equivalent to pore waters, altered basalts, estuaries, continental shelves), and the inverse of the fi rst-order rate constant (τ = 1/k) for simple lin- numerous mineral and biogeochemical reactions, leading to ear cycles, thus providing information about the response times a far more complete understanding of marine chemistry (e.g., (i.e., kinetics) of the system (Lasaga and Berner, 1998). Broecker and Peng, 1982). One problem that attracted early attention was the issue of By evaluating geochemistry in the framework of geochemi- elemental cycling through the oceans. This interest was initiated cal cycles, the fundamental processes that infl uence elemental largely by the pioneering work of Sillèn (1961), who demon- distributions (geological, geophysical, chemical, biological, strated that seawater chemistry could be in a steady-state con- temporal) are better addressed (Lerman and Wu, 2007). A good dition, controlled by equilibrium reactions between atmospheric example of the value of this approach in identifying previously gases and marine carbonate and silicate minerals. Up until the unrecognized processes and reservoirs is the well-known “miss- early 1950s, the conventional wisdom was that the oceans’ salt ing sink” issue for carbon (Broecker, 2012). During the 1990s, had accumulated over the entirety of geological time. During the short-term (exogenic) C cycle had been quantifi ed suffi ciently

Figure 16. Representation of the pre-industrial (A.D. 1750) global carbon cycle (adapted from Sundquist and Visser, 2003). Boxes show major reservoirs, with carbon mass listed in units of Pg (1015 g), and arrows show major fl uxes of carbon (listed in units of Pg/yr). DIC—dissolved inorganic carbon. spe500-08 1st pgs page 29

Five decades of advances in geochemistry 29

to recognize that ~15%–20% of the CO2 delivered to the atmo- and Canfi eld, 2012). As discussed already, C is certainly the most sphere by fossil fuel combustion could not be accounted for by studied redox-sensitive element that is cycled between reduced the recognized major C reservoirs (atmosphere, accounting for and oxidized forms today on Earth, driven largely by photosyn-

~50%, and seawater, accounting for ~30%–35%). This led to a thesis. This in turns drives atmospheric O2 contents through the major research effort to identify the missing sink, which in turn extent of C burial and thus plays a central role in the long-term resulted in recognition of the terrestrial biosphere (previously O cycle. This infl uences, for example, the long-term cycling of considered relatively minor) as a major reservoir for exogenic C. Fe, one of the most important redox elements in rocks, given its high abundance in the terrestrial planets. The redox couple with The Biological Connection Fe was studied extensively in the 1970s, with a major focus on banded iron formations (BIFs), and later, the relation between The importance of biological activity in controlling the the Fe biogeochemical and marine evolutions. Evaluation of the cycling of a wide range of elements has been long appreciated geochemical cycles of redox-sensitive trace elements such as Cr (e.g., Vinogradov, 1943). Attempts, however, to both defi ne and and Mo rose in prominence in the 1980s and 1990s. quantify global biogeochemical cycles involving elements that One of the major issues addressed in studies of redox bio- are essential for biological activity have only occurred somewhat geochemical cycling over the past 50 yr has been the history of

recently, though they are of great geochemical interest (e.g., Gar- atmospheric O2 over geological time (Holland, 1962). As briefl y rels and Lerman, 1981). Biogeochemical cycles consider some noted already, a prominent approach to understanding the his-

or all of a wide array of biologically relevant elements in an inte- tory of atmospheric O2 on Earth has been study of both mass- grated manner, including those that are a signifi cant part of living dependent and mass-independent fractionation (MIF) of stable tissue and skeletons (C, O, H, N, P, S, Ca, Si), those that may be S isotopes, and these records are compared, along with C and less involved in living matter but are important in redox processes Fe isotopes, in Figure 18. The range in d34S values for sulfi des allowing for energy transfer (Fe, Mn, in addition to N and S), and in marine sedimentary rocks generally increases with decreas- a long list of minor and trace elements that may be necessary ing age, an observation that has also been long recognized (e.g., for metabolism (e.g., biolimiting) and/or substitute for major ele- Canfi eld, 2001). The general increase in the highest d34S values ments of biogeochemical importance (e.g., Fe, Mn, Mg, Ba, Ge, measured for sulfi des is broadly taken to record an increase in B, Mo, V, Zn, and even the REEs). the d34S values of seawater sulfate, and a recent compilation of The C cycle (and its role in biogeochemical cycles) is of this can be found in Canfi eld and Farquhar (2009). It is almost intrinsic interest to geochemists due to the role of C in all bio- universally accepted that the range in d34S values for sulfi des logical activity and its importance in the geological record as a common rock-forming constituent (limestone, dolomite, carbo- naceous sediment) and as a natural resource (fossil fuels). The fact, however, that C has received by far the most attention of any of the geochemical cycles, and indeed may be the most intensely studied geochemical problem of the past 50 yr and more (Ber- ner, 2004; Broecker, 2012; Des Marais, 2001), is mainly due to

the observation that atmospheric CO2 concentrations have sys- tematically risen since the industrial revolution at a rate that is unprecedented for the Phanerozoic, due mainly to the burning of fossil fuels (Fig. 17). It is now widely recognized that perturba- tions within the C cycle (temporal variations in atmospheric CO2) represent a dominant control on the changes of Earth’s current climate, as well as both short-term and long-term paleoclimate. The recent abrupt increase in atmospheric levels of this impor- tant greenhouse gas, due mainly to human activity, is also now understood to play a central role in infl uencing recent global cli- mate change and increases in mean global surface temperatures Figure 17. Monthly mean concentration of carbon dioxide, mea- (Berner, 2003). sured as parts per million molecules of CO2 in total molecules of dry air, measured at the Mauna Loa Observatory, Hawaii (data from Redox Processes in Biogeochemical Cycles National Oceanic and Atmospheric Administration [NOAA] Web site (http://www.esrl.noaa.gov/gmd/ccgg/trends/), the so-called The recent focus on element cycling, including biogeochem- Keeling curve. The annual cycle observed in the record is a sea- sonal effect due to the much greater land mass (and vegetation) in ical cycles, is primarily driven by redox chemistry, both abiologic the Northern Hemisphere, resulting in greater CO2 uptake during or biologic, due to its importance in controlling the composition plant growth in the northern summer. These relations were fi rst rec- and evolution of the ocean-atmosphere system (e.g., Raiswell ognized by Charles Keeling (Keeling, 1960). spe500-08 1st pgs page 30

30 Johnson et al.

Figure 18. Temporal variations in (A) banded iron formation (BIF) deposition and atmospheric oxy- gen, (B) d13C values of Ca-Mg carbonates, (C) d34S values for sulfi des, (D) mass-independent S isotope fractionation, expressed as D33S, and (E) δ56Fe values; δ56Fe values are broken out as black shales (high-S, high-C contents), Ca-Mg carbonates, and BIF sam- ples. Green band represents period of maximum BIF deposition and immediately predates the increase in atmospheric oxygen contents. BIF deposition is from

Bekker et al. (2010). Atmospheric O2 curve is from Catling and Claire (2005), shifted to older ages based on disappearance of mass-independent S isotope frac-

tionation (Farquhar et al., 2010). Early pulses of O2 time band are based on previous studies (Anbar et al., 2007; Czaja et al., 2012; Duan et al., 2010; Kendall et al., 2010; Voegelin et al., 2010). The d13C data are from Shields and Veizer (2002). The d34S data for sulfi des and seawater sulfate are from Canfi eld and Farquhar (2009), and D33S values for sulfi des are from same source. The δ56Fe values are from sources cited in Johnson et al. (2008b), with additional data sources (Czaja et al., 2010, 2012; Heimann et al., 2010; Hof- mann et al., 2009; Planavsky et al., 2009; Steinhoefel et al., 2009; Tsikos et al., 2010; Valaas-Hyslop et al., 2008; Von Blanckenburg et al., 2008). All isotopic data refl ect bulk sample analyses. spe500-08 1st pgs page 31

Five decades of advances in geochemistry 31 refl ects various extents of bacterial sulfate reduction (Canfi eld, not generally interpret the Fe isotope compositions of such lithol- 2001, 2005). Turning to S-MIF, nonzero D33S values for marine ogies to be a direct proxy for seawater, instead favoring microbial sedimentary rocks are restricted to samples of ca. 2450 Ma age Fe cycling as an explanation for the Fe isotope variability. and older, and most studies interpret this to indicate very low The relative paucity of highly negative δ56Fe values after atmospheric oxygen contents (originally discovered by Farquhar the GOE (Fig. 18) has been interpreted to refl ect a decrease in et al. [2000a], and recently reviewed by Farquhar et al. [2010]), the footprint of microbial Fe3+ reduction as microbial sulfate although alternative explanations have been proposed (discussed reduction increased in its impact on redox cycling. Sulfate reduc- earlier herein). The transition from large D33S values to zero D33S tion would produce abundant sulfi de, which readily reacts with values at ca. 2450 Ma correlates with an increase in the range of Fe3+ oxides, decreasing Fe3+ availability to microbial reduction. d34S values, consistent with an increase in seawater sulfate con- The increase in δ56Fe values that generally corresponds with a tents and development of free oxygen in the atmosphere, which decrease in BIF deposition (Fig. 18) is consistent with a tran- in turn would enhance rates of bacterial sulfate reduction (Can- sition from an Fe-dominated redox cycle before the GOE to a fi eld, 2005; Farquhar et al., 2011). S-dominated redox cycle after the GOE. Collectively, the tempo- As discussed earlier, the d13C values for Ca-Mg carbonates ral record of BIF deposition and C, S, and Fe isotopes displays of Archean and Proterozoic age largely scatter closely about zero, remarkable coherence that can be well explained by expected with the exception of the 2.3–2.0 Ga Lomagundi excursion (Fig. changes in microbial redox cycling during the transition from an 18). Increased organic C burial seems the most likely explana- anoxic to oxic world. tion for the increase in d13C values for carbonates at this time, We close by considering redox cycling on another planetary

which in turn would drive further increase in atmospheric O2. body. The recent phase of Mars exploration has led to recogni- This would tend to increase seawater sulfate contents, providing tion of a long-lived dynamic sedimentary cycle on that planet. opportunities to increase the 34S/32S fractionations produced by This has generated interest in characterizing geochemical cycles microbial sulfate reduction due to “excess” sulfate; this would and evaluating the implications for surface processes (e.g., Grotz- increase the inventory of sedimentary sulfi des that have very neg- inger et al., 2011), an interest that undoubtedly will increase with ative d34S values (Fig. 18). Accompanying the rise in atmospheric the successful landing of the Mars Science Laboratory. Mars may 33 O2 would be a loss of S-MIF, refl ected in a shift toward zero D S represent a valuable end member for understanding near-surface values for sulfi des younger than 2.3 or 2.4 Ga in age (Fig. 18). geochemical cycles in general due to the absence of plate tecton- The largest Fe isotope excursion known in the rock record ics. There appears to be a long-term evolution of sedimentary occurs in the Neoarchean and Paleoproterozoic (Fig. 18). mineralogy, where a common occurrence of Noachian (older Because the vast majority of Fe in the crust has a δ56Fe value than 3.7 Ga) clay minerals gives way to a sulfate-rich mineralogy near zero, including low-C, low-S sedimentary rocks that have in the Hesperian (ca. 3.7–3.2 Ga), and fi nally to anhydrous ferric Fe contents similar to those of the average crust (e.g., Johnson et oxides in the Amazonian (younger than 3.2 Ga; Bibring et al., al., 2008b), deviations in the δ56Fe values from zero are signifi - 2006). Recent work on the ALH84001 meteorite has provided cant and generally rare in terms of the Fe mass balance of Earth. support for clay minerals on the surface of Mars prior to 4.0 Ga The zero to positive δ56Fe values for rocks older than 3.5 Ga in (Beard et al., 2013). As on Earth, where sedimentary mineralogy age, which to date mainly include BIFs, are generally accepted to (e.g., BIF) provides a proxy for geochemical evolution (Hazen 2+ refl ect partial oxidation of marine hydrothermal Fe aq, suggest- et al., 2008), this secular change in sedimentary mineralogy has ing that the amount of oxidant was limited (Dauphas et al., 2004), been interpreted to represent acidifi cation, oxidation, and desic- and recent Fe isotope work suggests that photic zone O2 levels cation of the Martian surface over geological time. Although the were <0.001% of present day at this time (Czaja et al., 2013). current atmosphere is very thin, most models call for a more sub- δ56 More controversial, however, is the strong decrease in Fe val- stantial early Martian atmosphere that was dominated by CO2. ues between ca. 3 and 2.5 Ga. Rouxel et al. (2005) and Anbar and Sulfur, however, is also enriched in Mars surfi cial deposits, and it Rouxel (2007) proposed that oxidation of marine hydrothermal now appears that some form of a S-cycle may have strongly infl u- 2+ Fe aq during BIF genesis, or oxide precipitation on continental enced surfi cial processes over much of geological time, leading shelves, produced negative δ56Fe values in seawater, which was to widespread low pH conditions (Halevy et al., 2007; King and directly incorporated into sulfi de-rich marine sedimentary rocks. McLennan, 2010; McLennan, 2012). As recently discussed by Czaja et al. (2012), extensive oxidation 2+ δ56 of Fe aq is a likely explanation for the low Fe values of Neo- CONCLUDING REMARKS archean Ca-Mg carbonates and, in fact, provides one of several lines of evidence for intermittent oxygenation of surface environ- We are not so bold as to predict what advances will occur in ments in the time leading up to the Great Oxidation Event (GOE) geochemistry in the next 50 yr, as we are quite sure that many of at ca. 2.3 or 2.4 Ga (Anbar et al., 2007; Kendall et al., 2010; the analytical and computational gains that have been key factors 2+ Voegelin et al., 2010). Oxidation of Fe aq, however, cannot easily to a number of geochemical advances were not envisioned by explain the spread in δ56Fe values for Fe-rich rocks such as BIFs, geochemists in the early 1960s. It was also probably not clear and Yamaguchi et al. (2005) and Johnson et al. (2008a, 2008b) do to these geochemists what the impact of plate tectonics would spe500-08 1st pgs page 32

32 Johnson et al. be, nor of obtaining samples from the Moon, or of in situ analy- Alves, S., Schiano, P., and Allègre, C.J., 1999, Rhenium-osmium isotopic ses of rocks and soils on Mars. In a shorter time frame than fi ve investigation of Java subduction zone lavas: Earth and Planetary Science Letters, v. 168, p. 65–77, doi:10.1016/S0012-821X(99)00050-3. decades, however, it seems likely that the trend toward interdis- Alves, S., Schiano, P., Capmas, F.O., and Allègre, C.J., 2002, Osmium isotope ciplinary research, where multiple large data sets are brought binary mixing arrays in arc volcanism: Earth and Planetary Science Let- to bear on geologic problems, is likely to expand. In addition, ters, v. 198, p. 355–369, doi:10.1016/S0012-821X(02)00524-1. Amari, S., Anders, E., Virag, A., and Zinner, E., 1990, Interstellar graphite in there will likely be huge gains in computational and data stor- meteorites: Nature, v. 345, p. 238–240, doi:10.1038/345238a0. age capacity, although analytical advances may be more muted, Amelin, Y., Lee, D.C., Halliday, A.N., and Pidgeon, R.T., 1999, Nature of the given the fact that many analyses are now limited by counting Earth’s earliest crust from hafnium isotopes in single detrital zircons: Nature, v. 399, p. 252–255, doi:10.1038/20426. statistics rather than electronics, although there is certainly room Amelin, Y., Lee, D.C., and Halliday, A.N., 2000, Early-Middle Archaean crustal for large improvements in terms of sampling effi ciency of many evolution deduced from Lu-Hf and U-Pb isotopic studies of single zir- instruments (what gets to the detector versus what is consumed). con grains: Geochimica et Cosmochimica Acta, v. 64, p. 4205–4225, A major unknown for the future of geochemistry, and of science doi:10.1016/S0016-7037(00)00493-2. Amrani, A., Sessions, A.L., and Adkins, J.F., 2009, Compound-specifi c δ34S in general, is the commitment to research by broader society, and analysis of volatile organics by coupled GC/multicollector-ICPMS: Ana- such issues will undoubtedly affect what is written in a similar lytical Chemistry, v. 81, p. 9027–9034, doi:10.1021/ac9016538. paper in the early 2060s. Anbar, A.D., 2004, Molybdenum stable isotopes: Observations, interpretations and directions: Reviews in Mineralogy and Geochemistry, v. 55, p. 429– 454, doi:10.2138/gsrmg.55.1.429. ACKNOWLEDGMENTS Anbar, A.D., and Rouxel, O., 2007, Metal stable isotopes in paleoceanogra- phy: Annual Review of Earth and Planetary Sciences, v. 35, p. 717–746, doi:10.1146/annurev.earth.34.031405.125029. We thank the organizing committee of the 125th anniversary cel- Anbar, A.D., Duan, Y., Lyons, T.W., Arnold, G.L., Kendall, B., Creaser, R.A., ebration of the Geological Society of America for the opportu- Kaufman, A.J., Gordon, G.W., Scott, C., Garvin, J., and Buick, R., 2007, nity to write this summary of progress in geochemistry. Johnson A whiff of oxygen before the Great Oxidation Event?: Science, v. 317, p. 1903–1906, doi:10.1126/science.1140325. and Summons acknowledge support from the National Aeronau- Andersen, D.J., Lindlsey, D.H., and Davidson, P.M., 1993, QUILF—A Pas- tics and Space Administration (NASA) Astrobiology Institute cal program to assess equilibria among Fe-Mg-Mn-Ti oxides, pyroxenes, during the time this review was prepared. McLennan acknowl- olivine, and quartz: Computers & Geosciences, v. 19, p. 1333–1350, doi:10.1016/0098-3004(93)90033-2. edges support from NASA. 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