2009 Portland GSA Annual Meeting (18-21 October 2009)

Paper No. 4
Presentation Time: 8:45 AM


BRENNECKA, Gregory A., School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287, HERRMANN, Achim D., Barrett Honors College and School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287, SALTZMAN, Matthew R., School of Earth Sciences, The Ohio State University, 275 Mendenhall Laboratory, 125 South Oval Mall, Columbus, OH 43210 and ANBAR, Ariel D., School of Earth and Space Exploration and Department of Chemistry & Biochemistry, Arizona State University, Tempe, AZ 85287-1404, brennecka@asu.edu

The Permian-Triassic boundary (PTB) represents the largest mass extinction event in Earth’s history. Evidence for the existence of widespread ocean anoxia spanning the PTB provides a kill mechanism for ~95% of marine species, but the cause, exact timing, and extent of the anoxia continue to remain controversial. Novel work using the 238U/235U ratio of carbonates deposited during times of known ocean anoxia could help clarify these issues. Development of 238U/235U as a paleoredox proxy for shallow water environments (i.e., carbonates) would compliment existing proxies constrained to deep basinal units.

Recent work has demonstrated that 238U/235U fractionation is induced by low-temperature redox changes and is measureable in samples from the natural environment. Due to the long residence time of U, 238U/235U ratios are homogenous in seawater as supported by previous measurements. Uranium is a highly soluble element under oxidizing conditions, but under suboxic and anoxic conditions soluble U(VI) is reduced to insoluble U(IV) and removed from the water column. Reduced sediments, such as black shales, preferentially sequester isotopically heavy uranium. Times of ocean anoxia and increased removal of isotopically heavy uranium would drive the 238U/235U of seawater isotopically lighter. Similarly, a decrease in black shale deposition would cause isotopically heavier seawater values. If the 238U/235U ratio of seawater is in fact recorded by contemporaneously deposited carbonates, as work on modern corals has shown, then the 238U/235U ratio on ancient carbonates may act as an archive for the redox state of ancient oceans.

Profound paleoceanographic events like global deep ocean anoxia in the Late Permian would increase the flux of reduced sedimentation, therefore altering the 238U/235U composition of the global ocean. Samples from the Confusion Range (Utah, USA) spanning the late Permian to Early Triassic were measured for 238U/235U ratios. We present evidence of an abrupt shift (+0.5‰ δ238U) in 238U/235U of carbonate samples from Induan to Olenekian age. Taken at face value, this would suggest a dramatic decline of global anoxia in the early Triassic. This study represents an early step towards utilizing uranium isotopes as a paleoredox proxy for shallow water environments.