--> Geochemical Evidence of a Large Shift in Redox Conditions and Long Term-Deep Burial Oxidation by Radiolysis Associated with Elevated Organic-Matter Content and Gamma-Ray Intensity in the Paleozoic New Albany Shale, USA

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Geochemical Evidence of a Large Shift in Redox Conditions and Long Term-Deep Burial Oxidation by Radiolysis Associated with Elevated Organic-Matter Content and Gamma-Ray Intensity in the Paleozoic New Albany Shale, USA

Abstract

The New Albany Shale (NAS) was deposited in a marine setting in the cratonic Illinois Basin through Devonian and Mississippian time. The NAS is a source of hydrocarbons and contains high organic carbon (Corg) and pyrite. Data are presented here from sequentially extracted fractions of sulfur for the Ellsworth, Clegg Creek, and Camp Run members of the NAS. Sulfur geochemistry of whole-rock samples, metal concentrations, and visual microscopy of organic particles (macerals) were used (i) to understand the depositional environment of the NAS, (ii) to investigate linkages between shale intervals with high gamma ray and shifts in sulfur isotopic compositions (δ34S), and (iii) to explore using proportions of metals as indicators of depositional environments. Members of the NAS are characterized by depletion of 34S in pyrite and reflect sulfide production by bacterial sulfate reduction in open-system conditions. The Camp Run Member and lower portion of the Clegg Creek Member are characterized by similar δ32S of pyrite. Isotopic data gradually change from the middle portion of the Clegg Creek Member into the Ellsworth Member and indicate increasing variability of depositional conditions. Extreme depletion in 34S for sulfides in some samples reflects alternating oxic/suboxic or anoxic conditions with shifts in benthic redox conditions and associated opportunities for microbial disproportionation of intermediate sulfur species in accumulating fine-grained sediment. Systematic variation in δ34S for large grains of pyrite was discovered during repeated chrome-reduction extraction, suggesting overgrowths of pyrite under changing microbial ecologies during diagenesis. Corg, acid-insoluble sulfur, degree of pyrtization, maceral compositions, and metal-ratio paleoredox indices suggest that the Camp Run Member and lower portion of the Clegg Creek Member were deposited under poorly oxygenated and moderately reducing conditions in sediment compared to the upper portion of the Clegg Creek Member and the Ellsworth Member. The presence of extractable sulfate in Camp Run and Clegg Creek members resulted from gradual oxidation of sulfides in the subsurface over hundreds millions of years. The overlying Ellsworth Member is dominated by sulfate, likely originated from seawater and was trapped as carbonate-associated sulfate. Correlation between δ34S, gamma ray intensity, and uranium concentration are consistent with abiotic oxidation of pyrite by radiolysis in the Clegg Creek Member.