245 / 2025-04-15 19:41:33

Constraining the Rate of Ocean Deoxygenation Leading to the End-Permian Mass Extinction: Toward a Multi-Isotope Tracer Approach

Permian-Triassic mass extinction,Uranium isotopes,Cerium isotopes,Thallium isotopes,Ocean anoxia

Theme 4: Microbial and Biogeochemical Evolution through Time > Session 4c: Reconstructing Biogeochemical Cycles in Deep Time: Assumptions, Bias, Constraints, and Directions

The end-Permian mass extinction (EPME), which occurred approximately 252 million years ago, represents the most severe biotic crisis of the Phanerozoic, resulting in the mortality of more than 81% of marine species. While the expansion of oceanic anoxia has been commonly cited as a proximate cause of the EPME, the rate of ocean deoxygenation in the latest Permian oceans remains poorly quantified. In this study, we integrate multiple Permian-Triassic records from widely spaced sections—Meishan, Dajiang, and Lengqinggou in South China, and the Abadeh section in Iran—to construct high-resolution δ²³⁸U, δ¹⁴²Ce, and ε²⁰⁵Tl profiles. These profiles provide new quantitative constraints on the rate and extent of ocean deoxygenation during the EPME. The sections were temporally correlated using a combination of δ¹³C records and conodont biostratigraphy.

The correlated sections demonstrate excellent agreement, with an average δ²³⁸U value of -0.23‰ before the End-Permian Mass Extinction (EPME), which drops to -0.80‰ during the EPME interval and stabilizes around -0.55‰ into the earliest Triassic. An inversion mass balance model indicates that global anoxia expanded from approximately 1% to 6% during the EPME. Thallium isotope records reveal an average ε²⁰⁵Tl value of -5.4 ± 3 ‱ (2SD, n=7) during the end-Permian, decreasing to a minimum of -7.2 ‱ before rising to -2.0 ‱ prior to the EPME and remaining stable around this value into the earliest Triassic. A detailed comparison of the δ²³⁸U and ε²⁰⁵Tl profiles shows that the positive excursion in ε²⁰⁵Tl occurred approximately 71 kyr before the δ²³⁸U signal, suggesting that global ocean deoxygenation began ~71 kyr earlier than previously indicated by δ²³⁸U studies. Furthermore, considering the differences in residence time and isotopic behavior in the oceans, the comparisons among the δ²³⁸U, δ¹⁴²Ce, and ε²⁰⁵Tl profiles document that anoxic waters affected both shallow marine environments and deep seafloors. Since over 95% of marine species inhabit shallow marine environments, our study offers a more comprehensive understanding of the connection between marine deoxygenation and the EPME event.