Sedimentary environments and preservation biases limit sulfur isotope fractionation observed in pyrite, despite large microbial fractionations

Abstract:

Microbial S isotope fractionations close to the thermodynamic fractionation between sulfate and sulfide (~70‰) are encountered only at the slowest rates of sulfate reduction in laboratory cultures. In turn, the slowest laboratory reduction rates overlap with only the highest values of cell-specific sulfate reduction rates measured in marine sediments. This chain-of-logic implies that sulfate-reducing microbes in the marine sedimentary biosphere fractionate S isotopes at magnitudes close to the thermodynamic limit. Recent observations from sulfate-poor environments indicate that fractionations are large even at micromolar sulfate concentrations, in agreement with model predictions of near-thermodynamic S isotope fractionation at these sulfate concentrations as long as cell-specific sulfate reduction rates are low. Despite the expectation of large microbial fractionations, pyrite in both modern marine sediments and Phanerozoic sedimentary rocks records apparent fractionations ranging from 0 to more than 70‰.

We suggest that the observed range of modern marine and geologic apparent fractionations recorded in pyrite does not reflect variability in intrinsic microbial behavior, but an early diagenetic modulation of large microbial fractionations, which are pinned to the thermodynamic limit by the low natural rates of sulfate reduction. With a diagenetic model developed in this study, we show that the entire range of apparent fractionations is possible with microbial fractionations at the thermodynamic limit. Apparent fractionations depend on a variety of physical parameters of the sedimentary environment like sedimentation rate, porosity, and organic matter content, most of which correlate with water depth. These findings, in combination with knowledge about the preservation potential of sediments deposited at different depths, make predictions for the observed geologic range of apparent fractionations, and ways in which it differs from the range in modern marine sediments.