DO URANIUM ISOTOPES OF EPICONTINENTAL BLACK SHALES REFLECT GLOBAL OCEANIC CONDITIONS?

Variations of ²³⁸U/²³⁵U ratios (δ²³⁸U) have been used to quantify the extent of marine anoxia in the geologic past. During times of ocean anoxia, δ²³⁸U values of black shales systematically shift toward lighter δ ²³⁸U and lower U concentrations in response to an increase of the size of anoxic/euxinic sink relative to other sinks.

The Late Paleozoic cyclothems are unique in that the depositional environments and the model for organic matter preservation are distinctly different from modern day analogues. In the modern ocean extensive organic-carbon rich sediments are deposited in two distinct depositional environments: coastal upwelling zones and silled basins that are characterized by stagnant, oxygen-poor conditions. However, the benthic anoxic conditions of the black shale intervals of the Late Paleozoic cyclothems were driven by the lateral advection of preconditioned, oxygen-depleted Panthalassic ocean waters through the Permian Basin underneath a strong pycnocline.

Three lines of evidence indicate that the paleoceanographic conditions of this “superestuarine circulation” had a significant basin-scale impact on the U isotopic composition of the black shale interval of the Hushpuckney Shale (Swope Formation). First, moving upsection, δ²³⁸U shifts from -0.21‰ to +0.50‰ and then back to lighter values, in positive correlation with variations in N isotope values. The δ¹⁵N changes have previously been interpreted as intensification and weakening of denitrification due to rapid sea-level rise and sea-level stabilization during the deposition of the cyclothems. The δ²³⁸U-δ¹⁵N correlation suggests that the U and N systems were both influenced by local water mass conditions, rather than global changes. Second, the δ²³⁸U shift occurs over only a few centimeters, representing only a few thousand years according to current understanding of cyclothems. This is much less than the residence time of U in the modern ocean. Third, the shift toward heavier δ²³⁸U corresponds with enhanced euxina in the basin. This is opposite to the shift expected from this proxy due to a global shift toward euxinic conditions. These results indicate that interpretations of the δ²³⁸U record of ancient black shales must take local paleoceanographic conditions into account, as local effects might obscure the global signal.