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Physical Chemistry

Geologic Sulfur Cycle Reexamined

Sulfur-based biogeochemistry is more variable than previously recognized

by Jyllian Kemsley
July 23, 2012 | APPEARED IN VOLUME 90, ISSUE 30

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Credit: Shanan Peters/U Wisconsin
Sedimentary rock from Texas shows layers of anhydrite (CaSO4, light bands) interspersed with dolomite (CaMg(CO3)2, dark bands).
09030-scicon-sedimentaryrockcxd.jpg
Credit: Shanan Peters/U Wisconsin
Sedimentary rock from Texas shows layers of anhydrite (CaSO4, light bands) interspersed with dolomite (CaMg(CO3)2, dark bands).

Earth’s sulfur cycle is more varied and has greater effects on marine chemistry, the carbon cycle, atmospheric oxygen levels, and climate than previously realized, according to two reports in Science (DOI: 10.1126/science.1220656 and 10.1126/science.1220224). Sulfate from dissolved minerals such as an­hydrite (CaSO4) or from oxidative weathering of pyrite (FeS2) gets carried by rivers into oceans, where microbes may use organic carbon to convert sulfate to S2– and O2, which is released to the atmosphere. Scientists had believed that oceanic sulfate concentrations varied slowly during the past 500 million years. In one of the new studies, however, researchers used sulfur isotope data to show that marine sulfate concentrations fluctuated substantially with tectonic events roughly 125 million and 50 million years ago. In the other study, sulfate deposition in sedimentary rock layers was also found to be more variable during the past 500 million years than previously understood. Together, the studies indicate that sulfur has a large role in regulating ocean pH, nutrient bioavailability, and atmospheric O2, as well as in climate through the production of sulfate aerosols.

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