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 (<10 6yr), rhythmic variation superimposed over a longer term (~10 7yr) 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 <2mM or less than 10% modern). Short-term variation (7‰ excursions over 1Myr) 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 (~10 7yr) 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 (<10 6yr) sulfur isotope variation appears to be decoupled from the marine carbon-isotope signal. We suggest that this apparent decoupling reflects a combination of elevated pCO 2 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.
- Carbon isotopes
- Sulfur isotopes
ASJC Scopus subject areas
- Ecology, Evolution, Behavior and Systematics
- Earth-Surface Processes