Geochemical modeling suggests that large volcanic intrusions are responsible for major changes in oceanic sulfate over the past 120 million years, according to a study. Sulfate is an abundant ion in seawater that also plays a critical role in Earth's carbon, nitrogen, and iron cycles. Thomas A. Laakso and colleagues investigated anomalous increases in the seawater sulfate concentration during the Eocene period, around 50 million years ago, and during the Cretaceous period, around 100 million years ago. The researchers developed a model of historical seawater sulfate concentrations and its isotopic composition over time. The model suggests that the large, stepwise increases of the ratio of 34S to 32S, two stable sulfur isotopes, are due to outgassing of sulfur from large releases of magma into sediments primed with sulfur-rich sediment. Two of these large igneous provinces mapped to the appropriate time periods. In the model, this volcanic activity is coupled with changes in the efficiency of sulfur burial over long periods. According to the authors, the findings provide a quantitative look into the changes in one of the ocean's most abundant oxidizers and chart long-term patterns in sulfur biogeochemistry.
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ARTICLE #19-21308: "Volcanic controls on seawater sulfate over the past 120 million years," by Thomas A. Laakso, Anna Waldeck, Francis A. Macdonald, and David Johnston.
MEDIA CONTACT: Thomas A Laakso, Harvard University, Cambridge, MA; e-mail: laakso@fas.harvard.edu
Proceedings of the National Academy of Sciences