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The Biogeochemistry of Black Shales Following the Neoproterozoic, Mid-Cryogenian (Sturtian) Snowball Earth Event of Australia


The dynamic transition following the termination of the mid-Cryogenian snowball Earth glaciation fostered widespread deposition of black shales in the Australian region. Organic-rich shales and siltstones interbedded with diamictites from the Centralian Superbasin of Australia, and Tasmania record marine geochemical conditions during the recovery from the Sturtian (mid-Cryogenian) glaciation. Glacial termination and transgression fostered a resumption of biogeochemical cycles, a rapid productivity recovery that led to post-glacial anoxic conditions and black shale deposition. Nitrogen isotope data, and a high flux of phosphorus from post-glacial weathering suggest that the ocean may have been exceedingly nutrient-rich, resulting in the high fractional burial of organic carbon inferred by the carbon isotope record. Extreme 34S-enrichments in sedimentary pyrites (up to +50‰) demonstrate that sulfate reduction was highly efficient and acted to maintain low sulfate conditions over a long duration. Carbon isotope depletion with increasing paleodepth suggests that more significant contribution of organic matter from anaerobic prokaryotes utilizing 13C-depleted carbon substrates (dissolved inorganic carbon or methane) and is consistent with low and variable sulfate cocentrations. The post-glacial productivity response could have been the result of short-term post-glacial meltwater induced stratification, or the result of long-term limitation of gas exchange in an ice-covered ocean. Slow oxidation of the post-glacial Centralian Superbasin is indicated by a gradual decrease in d15N values, a decrease in the rate of rise in d13C values and a decrease in total organic carbon content.