Sulfur isotope fractionation derived from reaction-transport modelling in the Eastern Equatorial Pacific

Abstract

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 phosphorus. Similar to carbon reduction, sulfate reduction is associated with a strong isotope fractionation process that allows 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 Ocean Drilling Program (ODP) Site 1226 (Leg 201). We find that the upper 100 m 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. Because large parts of deep oceans are characterized by exceedingly low sulfate reduction rates, this process may be widespread and possibly explain why pyrite S-isotope data suggest that the average S-isotope fractionation is around −50‰, rather than the theoretically predicted value below −70‰. Supplementary material: Supplementary figures and model results are available at 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