August 2009 Geology and GSA Today media ...(ersity of North Carolina-Chapel Hill Cha...)

Sulfur isotope ratios (34S/32S) in marine limestones tell us a great deal about sulfur and oxygen cycles in the ocean at the time that those limestones were deposited. Sulfur isotopes in marine limestones are recorded in two ways: (1) as carbonate-associated-sulfate, or sulfate that is substituted for the carbonate ion in calcium carbonate; and (2) as pyrite. Carbonate-associated sulfate is an oxidized form of sulfur and is interpreted as a proxy for sulfate dissolved in seawater at the time of limestone deposition, while pyrite is a reduced form of sulfur that is used as a proxy for sulfide in sedimentary porewaters at the time of limestone deposition. The primary way that sulfate is reduced to sulfide in seawater is through bacteria, a process known as bacterial sulfate reduction. Because bacteria are more efficient at converting isotopically lighter sulfate (32SO4) to sulfide, sulfide -- and the pyrite that it forms -- tends to be lighter, on average, than the original sulfate. Additionally, experiments have shown that the more sulfate there is in seawater, the greater the isotopic offset between the sulfate and sulfide will be. In modern seawater, which has relatively high sulfate concentrations compared with the geologic past, the offset will typically be between 20 and 50 parts per thousand. As sulfate concentrations in seawater decline, the isotopic offset between sulfate and sulfide declines toward zero. Geologists use the amount of this offset, as recorded in carbonate-associated sulfate and pyrite in marine limestones, to reconstruct sulfate levels in the ancient ocean and, therefore, oxygen levels in the ancient atmosphere (since seawater sulfate is formed primarily from the oxidation of sulfides on land). Ries et al present a very intriguing record of sulfur isotopes from carbonate-associated sulfate and pyrite within a well-preserved, 10 million year li
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