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