Oxygen and sulfur isotopes in sulfate in modern euxinic systems with implications for evaluating the extent of euxinia in ancient oceans

Abstract:

Variability in the fraction of biogenic sulfide that is reoxidized back to sulfate has played a role in regulating redox budgets and oxygen levels in the ocean-atmosphere system throughout Earth history. In the modern, well-oxygenated ocean, 75-90% of sulfide produced by microbial sulfate reduction is reoxidized back to sulfate. At present, the areal extent of seafloor overlain by euxinic (i.e., anoxic and sulfidic) water is very low (<0.6%). However, geological evidence suggests that euxinia was much more common in ancient oceans. In theory, the presence of sulfide in the water column could induce higher reoxidation rates, as the delivery of oxidants is less transport limited in solution than when the chemocline is in sediments. In order to better understand these sorts of systematics and place isotopic constraints on this cycling, we present sulfur and oxygen isotope geochemistry in four modern euxinic systems. To interpret these data, we further develop a one-dimensional, depth-dependent geochemical model to estimate sulfide reoxidation rates in euxinic systems and evaluate model results in the context of geochemical and isotopic information. From these results, we determine that the percent of sulfide that is reoxidized back to sulfate is quite low (~11-42%). Given the proximity to overlying, fully oxygenated waters, this suggests that vertical exchange is significantly muted. This is in some sense surprising, but also consistent with the observation that sulfide is indeed allowed to accumulate in the bottom waters. We explore how water column density stratification and lateral transport influence reoxidation rates in these coastal ponds. We further pursue how these results can be extrapolated to the global ocean of times past.