Environmental control of carbon isotope variations in Pennsylvania black-shale sequences, Midcontinent, U.S.A. |
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| Authors: | Lloyd M Wenger Donald R Baker HMoses Chung Thane H McCulloh |
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| Abstract: | A reversal of the conventional carbon isotope relationship, “terrestrial-lighter-than-marine” organic matter, has been documented for two Pennsylvanian (Desmoinesian) cyclothemic sequence cores from the Midcontinent craton of the central United States. “Deep” water organic-rich phosphatic black shales contain a significant proportion of algal-derived marine organic matter (as indicated by organic petrography, Rock-Eval hydrogen index and
ratios) and display the lightest δ13C-values (max −27.80‰ for kerogen) while shallower water, more oxic facies (e.g. fossiliferous shales and limestones) contain dominantly terrestrial organic matter and have heavier δ13Ckerogen-values (to −22.87‰ for a stratigraphically adjacent coal). δ13C-values for extract fractions were relatively homogeneous for the organic-rich black shales with the lightest fraction (often the aromatics) being only 1‰, or less, more negative than the kerogen. Differences between extract fractions and kerogens were much greater for oxic facies and coals (e.g. saturates nearly 5‰ lighter than the kerogen).A proposed depositional model for the black shales calls upon a large influx of nutrients and humic detritus to the marine environment from the laterally adjacent, extremely widespread Pennsylvanian (peat) swamps which were rapidly submerged by transgression of the epicontinental seas. In this setting marine organisms drew upon a CO2-reservoir which was in a state of disequilibrium with the atmosphere, being affected by isotopically light “recycled-CO2” derived from the decomposition of peaty material in the water column and possibly from the anoxic diagenesis of organic matter in the sediments. |
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| Keywords: | black shale carbon isotope anoxic diagenesis CO2-recycling depositional environment |
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