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

摘要

We report high-resolution, high-precision delta Ca-44/40, delta Sr-88/86, and Sr-87/Sr-86 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 delta Ca-44/40 and delta Sr-88/86 excursions and invariant Sr-87/Sr-86 ratios in the <60 kyr timeframe between the main extinction horizon and the PTB. The Meishan delta Ca-44/40 and delta Sr-88/86 records are generally shifted to higher values and display both synchronous and asynchronous trends relative to Dajiang. In addition, Sr-87/Sr-86 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, delta C-13, and delta O-18), 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 delta Ca-44/40 and delta Sr-88/86 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 delta Ca-44/40 and delta Sr-88/86 values without affecting Sr-87/Sr-86 ratios. (C) 2019 Elsevier Ltd. All rights reserved