USING 238U/235U RATIOS IN CARBONATES AS A PALEOREDOX INDICATOR: VARIATIONS ACROSS THE PERMIAN-TRIASSIC BOUNDARY

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.