Deconvolving microbial and environmental controls on marine sedimentary pyrite sulfur isotope ratios.

Reconstructions of past environmental conditions and biological activity are often based on bulk stable isotope proxies, which are inherently open to multiple interpretations. This is particularly true of the sulfur isotopic composition of sedimentary pyrite (δ 34 S pyr ), which is used to reconstruct ocean-atmosphere oxidation state and track the evolution of several microbial metabolic pathways. We present a microanalytical approach to deconvolving the multiple signals that influence δ 34 S pyr , yielding both the unambiguous determination of microbial isotopic fractionation (ε mic ) and new information about depositional conditions. We applied this approach to recent glacial-interglacial sediments, which feature over 70‰ variations in bulk δ 34 S pyr across these environmental transitions. Despite profound environmental change, ε mic remained essentially invariant throughout this interval and the observed range in δ 34 S pyr was instead driven by climate-induced variations in sedimentation.