Roger and colleagues have had two companion papers come out this Thanksgiving Day in Science Magazine. The collective message is that marine sedimentary pyrite sulfur isotopes (canonically a proxy for ocean oxidation state) are dominantly controlled by local environmental conditions (e.g., sedimentation rate, porosity, permeability, organic matter abundance, reactive iron abundance). This much was already suspected based on some decades-old papers and, since 2017, a body of work by Virgil Pasquier and others. What is new here is that methods have been developed to attribute mechanism to the local controls on pyrite sulfur isotopes.
Roger and colleagues at Washington University in St. Louis developed a method to isolate individual pyrite grains from sediment samples and measure their sulfur isotope ratios by Secondary Ion Mass Spectrometry. This method allows the extraction of information about metabolism (i.e., the magnitude of isotopic fractionation associated with microbial sulfate reduction) and ‘system openness’ (the degree of Rayleigh distillation of sulfate in sediment pore waters) from individual sediment samples. Ultimately, the latter factor was found to almost exclusively control variations in bulk pyrite sulfur isotopes between glacials and interglacials in a Pleistocene core from the Mediterranean.
Itay Halevy wrote the companion paper, which describes a state-of-the-art numerical model simulating pyrite formation in marine sediments. The model is applied to all published marine pyrite sulfur isotope ratios from modern sediments, and the sedimentary record. Itay argues that almost all bulk pyrite sulfur isotope data likely dominantly reflect changes in system openness (which can also be a function of parameters likely bottom water O2 and sulfate concentrations, in addition to the aforementioned local sedimentary parameters). The paper is a treasure trove of information and arguably one of the most important advances in the sulfur isotope world since the turn of the millennium.
See Roger’s paper here, and Itay’s paper here. See write-up of the papers (by Talia Ogliore at Washington University in St. Louis) here.
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