Uranium isotopes as a potential global-ocean redox proxy: a test from the Upper Pennsylvanian Hushpuckney Shale (Kansas, U.S.A.)

Abstract:

Uranium (U) isotope variation in marine sediments has been proposed as a proxy for changes in average global-ocean redox conditions. Here, we investigate U isotope variation in the black shale (BS) member of the Hushpuckney Shale (Swope Formation) at two sites ~400 km apart within the Late Paleozoic Midcontinent Sea (LPMS) of North America, with the goal of testing whether sediment δ²³⁸U records a global-ocean redox signal or local environmental influences. Our results document a spatial gradient of at least 0.25‰ in δ²³⁸U within the LPMS, demonstrating that local (probably redox) controls have overprinted any global U-isotope signal. Furthermore, the pattern of stratigraphic variation in δ²³⁸U in both study cores, with low values (‒0.4 to ‒0.2‰) at the base and top and peak values (+0.4 to +0.65‰) in the middle of the BS, is inconsistent with dominance of a global-ocean redox signal because (1) the middle of the BS was deposited at maximum eustatic highstand when euxinic conditions existed most widely within the LPMS and coeval epicontinental seas, and (2) more extensive euxinia should have shifted global-ocean seawater δ²³⁸U to lower values based on mass-balance principles. On the other hand, the observed δ²³⁸U pattern is consistent with a dominant local redox control, with larger U-isotope fractionations associated with more reducing bottom waters. We therefore conclude that U was not removed quantitatively to euxinic facies of the LPMS, and that sediment U-isotope compositions were controlled mainly by local redox and hydrographic factors. Our results imply that U-isotope signals from epicontinental-sea sections must be vetted carefully through analysis of high-resolution datasets at multiple sites in order to validate their potential use as a global-seawater redox proxy.