CONSIDERATIONS ON THE USE OF IRON, CARBON AND SULFUR PALEOREDOX PROXIES IN LOW SULFATE PROTEROZOIC OCEANS: AN EXAMPLE FROM THE MESOPROTEROZOIC TOUIRIST FORMATION, MAURITANIA

Organic-rich black shales of the Mesoproterozoic (1.1 Ga) Touirist Formation, Mauritania, record deposition in shallow-water epeiric seas that covered the West African craton. Shales were sampled stratigraphically through 40 meters of the Tourist Formation and analyzed for Fe-speciation, total organic carbon (TOC), and pyrite S in order to evaluate water column oxygenation and the potential for either ferruginous or sulfidic conditions. We find that the ratio of highly reactive iron to total iron (Fe_HR/Fe_T; a proxy for anoxia) fluctuates between <0.1 and >0.9 within 40 meters of strata, indicating high frequency oscillation between oxic and anoxic depositional conditions. Interestingly, through this same 40 meters we find that ratios of pyrite iron to total iron (Fe_py/Fe_T; a proxy for euxinia = anoxia plus hydrogen sulfide) are largely >0.8, suggesting dominantly euxinic conditions. Finally, when examining the ratio of TOC to pyrite S, we find values consistently above 2.8, which are typically interpreted as deposition under sulfate-deficient, potentially freshwater, depositional conditions. These seemingly conflicting results call into question the extent to which well-established, empirical proxies for paleoredox can be applied indiscriminantly to: 1) Proterozoic oceans that were low in sulfate and potentially limiting to bacterial sulfate reduction and pyrite formation, or 2) epeiric sea settings with limited communication with open ocean environments, which may fundamentally affect the behavior of C-S-Fe biogeochemical cycles. Using these standard proxies, a combination of low sulfate availability and Fe-limitation within euxinic settings would cause sediments to appear oxic by limiting pyrite formation. Interestingly, in Touirist Formation shales, >50% of total S can not be accounted for by pyrite formation, suggesting substantial organically-bound S. Data are consistent with an environment where reduced S is available, but pyrite formation is severely limited. Although C-S-Fe paleoredox proxies have provided remarkable insight into the redox structure of a number of deeper-water Proterozoic successions, it is clear that we must evaluate their utility under a broader range of depositional conditions if we are to determine a detailed redox structure of the Proterozoic water column.