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

Euxinic conditions, which are defined by the presence of sulfide in the water column, were common in ancient oceans. However, it is not clear how the presence of sulfide in the water column affects the balance between rates of sulfide oxidation and sulfate reduction, which plays a major role in regulating the net redox state of the ocean-atmosphere system. Euxinia could lead to higher rates of sulfide oxidation because sulfide may diffuse more rapidly into the oxic zone in solution than in sediment. Alternatively, sulfide oxidation could be inhibited by low overall availability of suitable oxidants in euxinic settings. Here, we constrain rates of sulfide oxidation versus sulfate reduction in four euxinic water columns in coastal ponds by modeling the evolution of the concentration and sulfur and oxygen isotope compositions of sulfate from post-hurricane, well-oxygenated conditions to modern, euxinic conditions. The results of the one-dimensional, depth-dependent models of water column sulfate geochemistry indicate that the fraction of sulfate reduced that is subsequently reoxidized is low (0.11-0.42) in euxinic systems relative to the modern well-oxygenated ocean (0.75-0.90). This implies that sulfide reoxidation rates are low in euxinic systems because of oxidant limitation and physical transport. Low fractional sulfide reoxidation in euxinic systems has important implications for exploring how oxygen levels in the ocean and atmosphere have changed through Earth history. We use a marine sulfate isotope box model to explore how low reoxidation rates in euxinic systems affect marine sulfate sulfur and oxygen isotope records. Model results indicate that marine sulfate sulfur and oxygen isotope compositions increase during the expansion of euxinia with patterns that are distinct from other isotopic changes to the marine sulfate reservoir. Thus, marine sulfate sulfur and oxygen isotope box models can be applied to ancient isotope records in order to evaluate the expansion of euxinia in ancient oceans because sulfide oxidation rates are low relative to sulfate reduction rates.