SULFUR ISOTOPES AS PALEOENVIRONMENTAL INDICATORS: EVOLUTIONARY IMPLICATIONS

Canfield (1998) postulated widespread euxinic conditions in the deep oceans during much of the late Paleoproterozoic and the Mesoproterozoic (~ 1800 – 1000 Ma). Simple modeling suggests this was unavoidable if atmospheric O₂ was < 1/3 of the present level and if biological productivity was at all comparable to that of modern oceans.

The S isotope system provides evidence in support of the “Canfield Ocean” scenario. In the geologic record, sedimentary D³⁴S (d³⁴S_sulfate - d³⁴S_sulfide) rarely exceeds 45 ‰ until after ~ 1000 Ma. Because S isotope fractionation > 45 ‰ appears to require oxidation of sulfide to S^o followed by bacterial disproportionation to produce highly ³⁴S-depleted sulfide, ocean oxygenation less extensive than today’s is indicated until after 1000 Ma (reviewed in: Canfield and Raiswell, 1999). Additionally, S isotope data from sulfides and sulfates in individual basins suggests a low-sulfate ocean between ~ 1800 and 1000 Ma (e.g., Shen et al., 2002).

This scenario has implications for Proterozoic biogeochemistry and evolution. Many biologically-important elements would have been scarce in euxinic oceans. These include Fe and Mo, both of which are important for biological N fixation and NO₃^- assimilation. In contrast Fe was probably abundant in Archean oceans, which were likely anoxic but not euxinic, while Mo is the most abundant transition metal in modern, oxygenated oceans. Therefore, it follows from Canfield's hypothesis that the Proterozoic N cycle was strongly perturbed. N-limitation of primary production seems likely. Eukaryotes, unable to fix N, and limited in their ability to take in NO₃^- in a low-Mo ocean, would have been particularly disadvantaged. This may explain the limited diversification of eukaryotes until late in the Eon.

These implications of expanded Proterozoic ocean euxinia underscore the importance of a detailed understanding of S isotope systematics during microbial S transformations.