Constraining marine anoxia under the extremely oxygenated Permian atmosphere using uranium isotopes in calcitic brachiopods and marine carbonates

The redox chemistry change in ancient oceans has profoundly shaped the evolutionary trajectories of animals. Uranium isotopes (delta U-238) in marine carbonate sediments have widely been used to place quantitative constraints on the oxygenation state of the oceans through geological history. However, syndepositional and post-depositional diagenesis impose a positive and variable delta U-238 offset in the carbonate sediments relative to contemporaneous seawater, leaving uncertainties on quantification of anoxic seafloor areas in the past. Studies from modern settings suggest that Low-Magnesium Calcite (LMC) in articulate brachiopod shells are diagenetic resistant materials that may faithfully record the delta U-238 value of ancient seawater. However, this notation has not yet been validated in geological records. Here, we test this hypothesis by analyzing paired brachiopod shells and the host carbonate rocks from several Permian stratigraphic successions in South China. Forty-nine articulate brachiopod fossil shells and twenty-six host carbonate rocks from the Early Permian to the Late Permian were investigated. We performed a rigorous screening to monitor the diagenetic process that might have influenced the delta U-238 of the shells using a scanning electronic microscope, cathodoluminescent microscopy, and major and trace elements. We estimate that 53% of the shells preserve the primary seawater delta U-238 signals. Using the screened delta U-238 values of shells and bulk carbonates, we suggest there were four episodes of expanding marine anoxia during the Permian that are temporally coincident with periods of volcanism. The first two oceanic anoxic events occurred at a time when atmospheric pO(2) levels were predicted to be higher than today. The last two events are temporally coincident or immediately preceding the end-Guadalupian and the end-Permian mass extinctions, respectively, confirming previous suggestions that marine anoxia was a critical factor in driving marine animal extinctions. (c) 2022 Elsevier B.V. All rights reserved.

Automated summary

The redox chemistry change in ancient oceans has had a significant impact on the evolutionary trajectories of animals. Uranium isotopes in marine carbonate sediments have been used to quantify the oxygenation state of the oceans throughout geological history. However, diagenesis processes can introduce uncertainties in these measurements. This study examines the potential of using articulate brachiopod shells as diagenetically resistant materials to accurately record the ancient seawater conditions. Results show that approximately 53% of the shells preserve the primary seawater signals. The study also reveals four episodes of expanding marine anoxia during the Permian, which are temporally correlated with periods of volcanism. The last two anoxic events coincide with or precede the end-Guadalupian and end-Permian mass extinctions, confirming the importance of marine anoxia in driving marine animal extinctions.