PROLONGED AND RECURRENT GLOBAL SEAFLOOR ANOXIA IN THE EARLY TRIASSIC FROM URANIUM ISOTOPIC EVIDENCE

The end-Permian extinction and prolonged Early Triassic recovery of marine ecosystems have been attributed in part to marine anoxia. However, the spatial and temporal extent of anoxic waters during Early Triassic time remains poorly understood. To better constrain the evolution of seawater conditions, we present a record of δ^238/235U and uranium concentrations collected from the Great Bank of Guizhou, a Late Permian to Late Triassic isolated carbonate platform in the Nanpanjiang Basin, South China. The isotopic composition and concentration of uranium are independent constraints on paleore­dox conditions and can be used as indicators for the global extent of ocean anoxia. Our δ^238/235U results demonstrate that two large negative excursions of up to ~-0.4‰ occurred in the Induan and in the Spathian, before stabilizing in the Middle Triassic at Late Permian values. Uranium concentrations mirror the isotopic trends, reaching sustained minima of less than 0.2 ppm that correspond to the most negative isotopic values. By placing these observational constraints on a box model of the geological uranium cycle, we calculate that up to half of the continental shelves may have been affected during the two pulses of bottom-water anoxia.

The expansion, contraction, and re-expansion of extreme low-oxygen conditions could explain many unresolved aspects of the prolonged recovery of marine ecosystems. The recurrence of widespread anoxia during Spathian time may have interrupted the recovery of marine organisms that began in the more oxic waters of the Smithian. These episodes of significant and prolonged bottom-water anoxia coincide with the most negative δ¹³C values, suggesting that Early Triassic perturbations to the global carbon cycle were tightly coupled to changes in ocean redox chemistry.