The large decline in carbonate 8238U from a PETM section at Tingri (South Tibet) was driven by local sea-level changes, not global oceanic anoxia

Uranium isotope compositions (8238U) and Th/U in carbonates are being explored as paleoredox proxies to study global oceanic anoxia. However, the impact of changes to local depositional environments that might cause a decline in 8238U and an increase in Th/U has not been thoroughly investigated. Here we report a decline in 8238U (from +0.1 to -0.3%0) and an increase in Th/U from shallow-marine carbonates during the Paleocene-Eocene thermal maximum at Tingri (south Tibet). Changes of 8238U and Th/U are not fully coupled. The decline in 8238U coincided with a local sea-level fall, whereas the increase in Th/U occurred mainly during a sea-level rise. The decline in 8238U likely results from changes in redox conditions of pore waters and in primary carbonate mineralogy. The increase in Th/U is ascribed to reduced authigenic U(IV) accumulation. Our results suggest that changes in local depositional environments can cause behavior of 8238U and Th/U resembling that induced by expanded global oceanic anoxia. This finding calls for caution to consider local factors before applying 8238U and Th/U as global paleoredox proxies. (c) 2023 Elsevier B.V. All rights reserved.