کد مقاله | کد نشریه | سال انتشار | مقاله انگلیسی | نسخه تمام متن |
---|---|---|---|---|
4467024 | 1622242 | 2012 | 26 صفحه PDF | دانلود رایگان |
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.
► We use carbon and sulfur isotope profiles to model Ordovician marine C–S cycling.
► We examine marine redox conditions after the Late Cambrian C-isotope excursion—SPICE.
► Rapidity of S-isotope change requires smaller than modern marine sulfate reservoir.
► S-isotope variation is best explained by oxidation of reactive hydrogen sulfide.
► Oceanic euxinia persisted into the Ordovician despite the earlier global SPICE event.
Journal: Palaeogeography, Palaeoclimatology, Palaeoecology - Volumes 313–314, 1 January 2012, Pages 189–214