Coupled δ^44/40Ca, δ^88/86Sr, and ⁸⁷Sr/⁸⁶Sr geochemistry across the end-Permian mass extinction event

We report high-resolution, high-precision δ^44/40Ca, δ^88/86Sr, and ⁸⁷Sr/⁸⁶Sr records spanning the Permian-Triassic boundary (PTB) from the Meishan and Dajiang carbonate successions in south China. The goal of the study was to understand the behavior of Ca and Sr isotopes during a time period in Earth history characterized by severe biological and environmental perturbations, including a major mass extinction, flood basalt volcanism, ocean acidification, and sea level fluctuations. Dajiang displays negative δ^44/40Ca and δ^88/86Sr excursions and invariant ⁸⁷Sr/⁸⁶Sr ratios in the <60 kyr timeframe between the main extinction horizon and the PTB. The Meishan δ^44/40Ca and δ^88/86Sr records are generally shifted to higher values and display both synchronous and asynchronous trends relative to Dajiang. In addition, ⁸⁷Sr/⁸⁶Sr ratios at Meishan are significantly elevated and do not define a clear secular pattern. We reconcile diverse conceptual models for the dataset with the aid of supporting elemental and isotope measurements (e.g., Sr/Ca, δ¹³C, and δ¹⁸O), as well as sequential leaching experiments. The combined body of evidence indicates that Dajiang sediments experienced recrystallization and neomorphism under rock-buffered conditions that preserved primary signals. In contrast, the Meishan records display influences from both primary and secondary processes, including local differences in fractionation, fluid-buffered early marine diagenesis, and late stage diagenetic overprinting. The three isotopic records for Dajiang, and to some extent, the δ^44/40Ca and δ^88/86Sr records for Meishan, preserve information about the geochemistry of end-Permian seawater. We find that enhanced weathering of shelf carbonates during sea level fall provides the best explanation for rapidly decreasing seawater δ^44/40Ca and δ^88/86Sr values without affecting ⁸⁷Sr/⁸⁶Sr ratios.