Effects of paleoseawater composition on chemical and isotopic exchange at mid-ocean ridges

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The chemical composition of the oceans holds clues to important aspects of the history of climate, life, and mountain building on the Earth. Although we now know that the amount in seawater of certain elements like calcium, magnesium and sulfur, which are minor, but significant, constituents of seawater, have changed by large amounts and cyclically over many millions of years, we still do not know what causes these changes. One process that can contribute to, and also be affected by, the changes is the circulation of seawater through the fractured oceanic crust near mid-ocean ridges. This research is aimed at understanding how changes in dissolved S, Mg, and Ca in seawater affect the large geothermal systems that stretch around the globe on the ocean floor. Better understanding of could lead to breakthroughs in our understanding of why Earth's climate oscillates over long time periods and how mid-ocean ridge geothermal systems transfer chemical elements between the deep Earth and the oceans. The research involves laboratory experiments that reproduce the conditions and processes deep within hot oceanic crust and models how seawater circulates through the rocks and changes as chemical reactions occur. In terms of broader impacts, the experiments will provide thermodynamic and kinetic data that can be used in geothermal resource development and in studies of water-rock interaction that occur in other contexts, such as nuclear waste storage. New data on Sr partitioning in secondary minerals could have broad applicability to studies of hydrothermal/geothermal systems. Additional impacts include support of analytical and experimental research facilities at that benefit students at the University of California and many other scientists and students from other institutions. The educational value of the project is substantial in that it connects geological processes with oceanography and climate, and integrated knowledge of chemistry, hydrology and geology.Mid-ocean ridge hydrothermal systems are a major contributor to the ocean's geochemical cycle. The chemical exchange that occurs as seawater circulates through basalt and gabbro of varying temperatures affects the balance of ions in seawater and influences the carbon cycle through the coupling of dissolved Ca and Mg and carbonate sedimentation. Understanding the geologic history of the oceans and the relationships between seawater chemistry and climate requires that chemical exchange between seawater and oceanic crust at and near mid-ocean ridges be understood, not only at present, but back in time through Earth's history. Differences in the chemical composition of seawater in the geologic past could have had a significant effect on the seawater-basalt exchange in seafloor hydrothermal systems, especially at times when seawater Mg and SO4 were lower, and Ca and Sr were higher than at present. Consideration of these differences could cause major changes in the way we interpret records from past oceans, in particular the Sr isotopic evolution of seawater. It may also change how we view the causes and consequences of changes in seawater composition through Earth history. There is evidence that Sr isotopic exchange between seawater and ocean floor rocks changed substantially during periods when the Mg/Ca and Ca/SO4 ratios of the ocean were different than they are today. Hence modern seafloor hydrothermal systems may not be representative of most of the Phanerozoic. To better understand how seawater Mg-Ca-SO4 chemistry affects MOR hydrothermal experiments using reactors and mineral/fluid compositions that mimic seafloor hydrothermal systems will be run at varying fluid compositions. Run products will be analyzed geochemically and the results will be incorporated into the water-rock interaction modeling code TOUGHREACT, a hydrological modeling code that will be modified to accommodate conditions of seafloor hydrothermal systems.