Sulfur isotope evidence for widespread euxinia and a fluctuating oxycline in Early to Middle Ordovician greenhouse oceans

Despite marine geochemical records indicating widespread oxygenation of the biosphere in the terminal Neoproterozoic Era, Late Cambrian records point to the persistence of deep-water anoxia and potential for development of euxinic conditions. The Late Cambrian SPICE (Steptoean Positive Carbon-Isotope Excursion) event, however, is a globally recognized chemostratigraphic marker that likely represents significant organic carbon burial and subsequent liberation of oxygen to the biosphere. Here, we present high-resolution carbon and sulfur isotope profiles from Early to Middle Ordovician carbonate rocks from the Argentine Precordillera and Western Newfoundland to constrain oceanic redox conditions in the post-SPICE world. Marine C-isotope profiles record relatively stable behavior (excursions < 3‰) that is characteristic of greenhouse climates. Marine S-isotope profiles record short-term (< 106 yr), rhythmic variation superimposed over a longer term (~ 107 yr) signal. Substantial isotopic heterogeneity between average S-isotope values of different sections (15–25‰) suggests the Ordovician marine sulfate reservoir was not well mixed, indicating a low marine sulfate concentration (likely < 2 mM or less than 10% modern). Short-term variation (7‰ excursions over 1 Myr) is consistent with a small sulfate reservoir size and is best explained by the rhythmic oxidation of a deep-water reactive HS− reservoir. Greenhouse intervals, such as that represented by the Ordovician ocean, are often associated with deep-water anoxia, and the presence of a persistent, deep water HS− reservoir that is fed through bacterial sulfate reduction (BSR) is not unexpected. A broadly sympathetic relationship between carbon and sulfur isotope systems over long time scales (~ 107 yr) suggests that the extent of deep-ocean euxinia was moderated by changes in organic productivity, which fueled BSR and production of reduced sulfide species. By contrast, short-term (< 106 yr) sulfur isotope variation appears to be decoupled from the marine carbon-isotope signal. We suggest that this apparent decoupling reflects a combination of elevated pCO2 during greenhouse times—which acts to dampen C-isotope response—and relatively small-scale fluctuations in organic productivity that affected the position of the marine oxycline and the balance of HS− production and reoxidation.