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

By
Clark M. Johnson
Clark M. Johnson
Department of Geoscience, University of Wisconsin, Madison, Wisconsin 53706, USA, and National Aeronautics and Space Administration (NASA) Astrobiology Institute, Wisconsin Team
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Scott M. McLennan
Scott M. McLennan
Department of Geoscience, State University of New York at Stony Brook, Stony Brook, New York 11794, USA
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Harry Y. McSween
Harry Y. McSween
Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, Tennessee 37996, USA
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Roger E. Summons
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
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Published:
September 01, 2013

Many of the discoveries made in geochemistry over the last 50 years 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 fluorescence [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 five 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-fluid reactions, and planetary differentiation. Significant 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 and three-dimensional mapping. Rare earth elements, in particular, have proved to be invaluable tracers of magmatic, sedimentary, aqueous, redox, and cosmochemical processes, and siderophile trace elements form a basis for modeling many aspects of planetary accretion and early evolution. An anomalous amount of iridium at the 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 instrumentation since the early 1960s have greatly broadened this field of geochemistry. Early work outlined the stable H and O isotope fingerprints of natural waters and water-rock interactions, and stable C and S isotope studies defined the biological fractionations 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 isotope 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 distinctions between continental crust and mantle, setting the stage for later tracing of mass fluxes 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 variations 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 paleobiology. Despite great challenges in tying specific 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 numerous 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 five decades has been intimately linked to the chemistry of other solar system bodies and the universe beyond. We routinely rely on meteorite falls, interplanetary dust particles, and Moon rocks for a baseline for comparison to Earth, which has been extensively differentiated and repeatedly resurfaced. Sophisticated 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 radionuclides 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 first-order explanation 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 complex organic molecules to the early Earth, which likely influenced 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 solidified from magma oceans.

The individual subfields 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 continue 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.

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GSA Special Papers

The Web of Geological Sciences: Advances, Impacts, and Interactions

Marion E. Bickford
Marion E. Bickford
Department of Earth Sciences, 204 Heroy Geology Laboratory, Syracuse University, Syracuse, New York 13244-1070, USA
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Geological Society of America
Volume
500
ISBN print:
9780813725000
Publication date:
September 01, 2013

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