Cenozoic/Mesozoic variability of seawater sulfate concentrations

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Microbial sulfate reduction (MSR) is the fundamental process linking the biogeochemical cycles of carbon (C), oxygen (O), sulfur (S) and phosphorous (P). MSR respires approximately 85% of all sedimentary organic matter in marine sediments, and is thus a major control on atmospheric oxygen levels. MSR is also a key element for anaerobic methane oxidation, which critically controls the flux of methane (a major greenhouse gas) across the sediment water interface. MSR depends on sulfate as an electron acceptor, and variable sulfate concentrations affect this process in a non-linear way (Wortmann & Chernyavsky, Nature, 2007). While it is generally assumed that marine sulfate concentrations have been fairly stable through time (about 28mmol/l), I recently published the hypothesis that seawater sulfate concentrations fluctuated between 1 to 30 mmol/l throughout the last 130 Ma (Wortmann & Paytan, Science, 2012). Sulfate concentration changes of this magnitude will alter organic matter remineralization rates by a factor of three (Wortmann & Chernyavsky, 2007), affect microbial nitrogen fixation, and considerably modify the size of the marine phosphate reservoir, a major control on ocean fertility. Furthermore, it is likely that variable sulfate concentrations will influence the concentration of sulfate aerosols thus altering the radiative properties of the atmosphere (Wortmann & Paytan, 2012). Given the far reaching consequences of variable seawater sulfate concentrations, it is important to understand the robustness of this hypothesis. Here I propose to use a combination of modeling and fieldwork to evaluate whether alternative explanations are consistent with the geologic record and our understanding of ocean chemistry. These research questions are at the leading edge of the field and will result in high impact publications, furthering the international reputation of Canada's research competence. This proposal provides opportunities to train three Ph.D. and six M.Sc. students in data analysis, wet chemical laboratory methods, stable isotope analysis, computer programming, and international collaboration. These skills build the foundations for Canada's next generation of academic and business leaders.