Microbial sulfate reduction in subseafloor sediments regulates a significant portion of the marine organic matter burial flux. Over secular timescales, sulfate reduction is the fundamental process connecting the biogeochemical cycles of sulfur, carbon, oxygen, and phosphorous. Similar to carbon reduction, sulfate reduction is associated with a strong isotope fractionation process that enables us to track this process through time. It depends on a variety of factors and it has been argued previously that systematic differences between shallow and deep-sea environments might explain secular changes in the marine S-isotope ratio. However, observational data of in-situ fractionation from deep-sea areas are scarce. Here we use a reaction-transport model to analyse the S-isotope fractionation during microbial sulfate reduction in the interstitial water of ODP Site 1226 (Leg 201). We find that the upper 100 metres below seafloor are best explained with fractionation around -76‰, whereas below this depth the degree of fractionation drops to around -42‰. We propose that this shift is caused by changes in the ratio of the rate of microbial sulfate reduction relative to the rate of abiotic sulfide oxidation. Since large parts of deep oceans are characterised by exceedingly low sulfate reduction rates, this process may be widespread and possibly explain why pyrite S-isotope data suggests that the average S-isotope fractionation is around -50‰, rather than the theoretically predicted value below -70‰.

Supplementary materials: supplementary figures and model results as .csv files are submitted with this article https://doi.org/10.6084/m9.figshare.c.5976772.

Thematic collection: This article is part of the Sulfur in the Earth system collection available at: https://www.lyellcollection.org/cc/sulfur-in-the-earth-system

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