Redox Cycling In the Greenhouse Ocean: Exploring Rapid Sulfur Isotope Variation In the Middle Ordovician

Carbon-isotopes in the Paleozoic record contrasting periods of isotopic stability and isotopic volatility that is hypothesized to result from changes in the availability of essential bionutrients (i.e. phosphorous and nitrogen) associated with changes in ocean circulation driven by greenhouse-icehouse climate conditions (Saltzman 2005). In this scenario, greenhouse conditions are characterized by globally stagnant oceans, deep-water anoxia, reduced productivity, and limited carbon-isotope variability. By contrast, icehouse conditions are characterized by vigorous circulation, well-ventilated deep oceans, potential for sustained productivity and burial, and increased carbon-isotope variability. The effects of changing oceanographic circulation should be recorded in the marine sulfur-isotope record as well, particularly during greenhouse times, when the extent of bottom water anoxia may play a primary control on organic carbon supply, sulfate reduction, and pyrite burial.

The Middle Ordovician (Arenig-Llanvirn) San Juan (Argentine Precordillera) and Table Point (Western Newfoundland) formations provide an excellent starting point for exploring potential effects of oceanic circulation on marine sulfur-isotope records. The San Juan and Table Point formations represent deposition in shallow and mid-shelf to deep-shelf environments, respectively. Sulfur-isotope curves for both formations, constructed via isotopic analysis of carbonate-associated sulfate (CAS) show short-term (20-30m) isotopic shifts of up to 6‰ that are superimposed over a longer-term isotopic signal. Isotopic values are independent of lithology and depositional environment, suggesting that short-term isotopic variation may represent a global phenomenon related to greenhouse oceanographic conditions. We suggest that short-term sulfur-isotope variation recorded in these units reflects transient changes in the extent of oceanic bottom-water anoxia, resulting in variable redox cycling (BSR and sulfide oxidation) in deep-ocean environments. Here we present new data regarding reactive iron content, sulfur-isotope composition of sedimentary pyrite, and oxygen-isotope composition of trace sulfate to explore potential changes in redox cycling on initial C- and S-isotope datasets.