Gilleaudeau to examine lower Mississippian black shales

Geoffrey J. Gilleaudeau, Assistant Professor, Atmospheric, Oceanic and Earth Sciences, is set to begin a project next year that will provide valuable new insight on the uranium isotope system, which has emerged as a premier tool for deciphering Earth’s oxygenation history. Scientists now see uranium isotopes in ancient sedimentary rocks as a powerful proxy for the oxygenation history of Earth’s surface environments. Proper quantitative interpretation of uranium isotope data hinges, however, on an understanding of isotope fractionation associated with uranium removal to sediments under different redox conditions.

Whereas oxygenated environments are plentiful in the modern ocean, studying contemporaneous uranium isotope behavior under a range of low-oxygen conditions is difficult given the paucity modern anoxic basins. For example, it is not possible to simultaneously test uranium isotope fractionation under oxic, sulfidic, and iron-rich (ferruginous) marine conditions using modern analogues.

For this project, the researchers propose a novel “ancient analogue” approach for understanding uranium isotope behavior. Iron speciation and trace metal data indicate that coeval Lower Mississippian (Tournaisian) shales of the Appalachian Basin were deposited across a strong redox gradient, from oxic conditions proximal to the clastic wedge, to ferruginous conditions in the basin trough, to sulfidic conditions towards the basin-bounding sill. In addition, coeval shale of the Williston Basin (Upper Bakken Formation) was deposited under hypersulfidic conditions as evidenced by zinc and vanadium hyper-enrichment. Taken together, these deposits represent the perfect test case for uranium isotope behavior because a full range of redox conditions–oxic, ferruginous, sulfidic, and hypersulfidic–are recorded in coeval marine units.

Gilleaudeau and his collaborators will employ two undergraduate students to analyze 160 Tournaisian shale samples for uranium isotopes.

The researchers will receive $55,000 from the American Chemical Society for this project. Funding will begin in September 2020 and will end in late August 2022.

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