20 Sedimentary Geochemical Characterization of Aquatic Oxygen Dynamics

Tuesday, 14 March 2023
8:00 AM - 12:00 PM
Room 110

LATE DEVONIAN BASIN REDOX CONDITIONS REVEALED BY URANIUM GEOCHEMISTRY

Paper number: 20-9
Program: Theme Sessions
Time: 10:35 AM – 10:55 AM

Authors

Abstract

Typically, in black shales, uranium (U) contents display a linear relationship to total organic carbon (TOC). This association is a result of the mechanisms responsible for the incorporation of U into the sediment during the deposition and remineralization of organic matter. As such, the U content in black shales has been used as a proxy for redox conditions and organic carbon burial. However, some shales display uncharacteristically low U content despite having high TOC content. Other shales have large enrichments of U relative to TOC. Here we examine the U to TOC ratios and U-isotope composition (δ238U) of three Late Devonian-Early Mississippian shales: the Woodford Shale, the Cleveland Shale, and the Bakken Shale. The low average U/TOC ratio and the relatively high δ238U values of the Cleveland Shale suggest deposition along the basin margin with periods of reduced bottom water oxygenation, likely due to fluctuations in the location of the pycnocline. Higher U/TOC ratios and higher δ238U characterize the Woodford Shale which suggests deposition in an unrestricted setting with intermittent sulfidic conditions. In the Bakken Shale cores, high U/TOC ratios, and exceptionally high δ238U values indicate intense metal draw-down into sediments under persistently sulfidic waters. These three nearly time-equivalent shales, while lithologically similar, have distinct geochemical compositions which indicate decidedly different depositional environments.

Geological Society of America Abstracts with Programs. Vol. 55, No. 1

doi: 10.1130/abs/2023SC-385228

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