PERTURBATIONS TO THE GLOBAL SULFUR CYCLE DURING OCEAN ANOXIC EVENTS

Understanding the causes and consequences of ocean anoxic events (OAEs) has been at the forefront of studies in paleoceanography for the last several decades. The primary interest in these events stems in large part from the geochemical and paleontological impact they had on the biosphere, with possible parallels to the recent expansion of coastal hypoxia in the modern ocean. The Mesozoic Era is noted for numerous OAEs that are diagnostically expressed by widespread organic carbon deposition and coeval positive carbon isotope excursions. Recent work has shown that coupled with the carbon isotope record are parallel positive sulfur isotope excursions in carbonate-associated sulfur (CAS)—a proxy that can faithfully record the isotope composition of seawater sulfate. The parallel excursions of the Toarcian OAE (~183 Ma) and OAE II (~93.5 Ma) suggest large-scale coupled burial of organic carbon and pyrite sulfur. Despite these similarities, the details of the excursions—e.g. magnitudes and durations—differ significantly and likely reflect differences in the forcing parameters of the two events. The sulfate isotope curve for the Toarcian OAE recovers to baseline levels after ~8 Myrs, while OAE II recovers about three Myrs faster. Preliminary modeling results for the marginal Proto-North Atlantic and Tethys during OAE II suggest that sulfate concentrations are lower than the 5-8 mM concentrations estimated for the Toarcian OAE. We have used box modeling to explore the factors behind the perturbations to the carbon-sulfur cycles expressed in our data. For example, increasing ΔS during the events—the isotopic difference between marine sulfate and buried pyrite—substantially lessens the required increase in pyrite burial needed to drive the sulfur isotope excursion. Such a scenario is likely, given the expansion of euxinia and subsequent syngenetic pyrite formation with characteristically light isotopic values. Quantitative consideration of these cycles is paramount to constraining the budgets of not only carbon and sulfur, but also oxygen and other elements and thus is key to identifying the mechanisms behind these OAEs.