Carbon isotope variations on ancient carbonate platforms: The roles of organic carbon burial and sea level

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2013-05

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The Ohio State University

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Abstract

Positive delta 13C trends in ancient carbonate rocks are commonly interpreted to reflect increases in the burial of organic carbon in sediments. An alternative model to explain positive delta 13C trends in shelf carbonate rocks is related to mixing with open ocean water during transgression that floods the carbonate platform. Rock samples from two age-correlated, stratigraphic sections from the Arbuckle Mountains, Oklahoma and Clear Spring, Maryland of Middle to Upper Ordovician age were measured and sampled for delta 13C and delta 18O. The delta 13C curves reveal 3-4 per mil positive shifts in the upper Darriwilian to lower Sandbian stages (C. sweeti to gerdae conodont zones). Lithological analysis indicates transgressive events (shallow to deeper marine) associated with the positive delta 13C shifts in the Appalachian foreland basin but indicates a transgressive-regressive sequence in the upper portions of the Arbuckle Mountains. Plots of delta 13C versus delta 18O reveal minimal covariance, potentially invalidating diagenetic explanations for the trend. Because delta 13C does not show an obvious correlation with water depth or diagenesis, these results suggest that the positive delta 13C positive trends of both sections may be closely related to global changes in organic carbon burial.

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sequence stratigraphy, carbon isotopes, sea level, Ordovician, paleoceanography, North America

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