NSFOCE-BSF: Foraminiferal Na/Ca: a new proxy for reconstructing Cenozoic seawater composition

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Variations in the composition of seawater have important implications for Earth climate and biological processes in the ocean because of their relation to the carbon cycle. Specifically, the history of calcium concentration in the ocean over the Cenozoic (past ~65 million years) is tightly linked to chemical weathering of silicate and carbonate rocks on land and the precipitation and dissolution of carbonate minerals in the ocean, processes that control long-term changes in atmospheric carbon dioxide concentration and thus strongly influence global climate. Given the current acceleration of biogeochemical cycling due to the increase in greenhouse gases, these past processes may provide insights to future conditions that have no modern analogs. This research seeks to develop a new method of reconstructing variability in calcium in the oceans using comparison to seawater sodium concentrations, which are thought to be constant on timescales of many tens of millions of years. The project is funded through a special opportunity for U.S.-Israeli collaboration in ocean sciences. The project will train a U.S. post-doc and two Israeli graduate students who will spend time at both labs and will interact with the two principal investigators. The project also involves several undergraduate students who will be trained in various laboratory activities. Specifically, this research will provide a new method to reconstruct Cenozoic seawater calcium concentrations ([Ca]) using foraminiferal Na/Ca. Culture experiments (led by the Israeli team) and core top calibrations (led by the U.S. team) will be undertaken to further validate preliminary observations and generate records of Na/Ca and other elemental ratios in benthic foraminifera as means for reconstructing Cenozoic seawater chemistry. Culture experiments of benthic (Operculina ammonoides, Ammonia tepida and Amphistegina lobifera) and planktonic (G. ruber and G. siphnifera) foraminifera will be used to determine the distribution coefficients of Na/Ca and the other elements discussed above at different seawater [Ca] and under variable and constant calcite saturation conditions. The results from these experiments and core top calibrations will be used to generate a complete Na/Ca record for the last 55 Myr.