Microbial Sulfur Fractionation in Natural Springs - Implications for Archean Seawater Chemistry

Net fractionations recorded in laboratory studies are not necessarily reflective of a microbial community under natural conditions. We examined the control of sulfate concentration on isotope fractionation by microbial communities in natural sulfur spring systems.

Our results show that Δδ³⁴S is not significantly affected by temperature, however there is a strong log-linear relationship (r²=0.72) with [SO₄]. Based on a Δδ³⁴S =10‰ cutoff to define minimal fractionation, this equates to a sulfate value of 2340 μM. Our results based on natural systems thus show a significant divergence from previous laboratory based study, and suggest that concentrations of sulfate in the Archean ocean may have been an order of magnitude higher than previously thought.