Spatiotemporal disparity of volcanogenic mercury records in the southwestern Neo-Tethys Ocean during the Permian–Triassic transition

The Permian–Triassic (P-Tr) transition marks a vital period in Earth's history, characterized by major environmental perturbations and the largest mass extinction event of the Phanerozoic, with volcanic activities playing a key role. Previous investigations of mercury (Hg) anomalies in over 50 marine and terrestrial sections spanning the P-Tr boundary (PTB) have suggested a predominant volcanogenic influence. However, there remains ongoing debate regarding the exact timing and primary sources of these anomalies in different regions. In this study, we present stratigraphically high-resolution (∼7× higher compared to the previous work in the same section) Hg records from the shallow marine strata of the Qubu section, located at the Himalayan Tethys Zone of southern Tibet, Southwest China. Our analysis reveals peak Hg concentrations of approximately 80 to 100 ng/g and Hg/TOC ratios of 111 to 263 (ppb/wt%) at the uppermost Permian. Notably, new measurements of Hg isotopes, characterized by ∼0‰ of Δ¹⁹⁹Hg values, provide unambiguous evidence supporting the prevailing volcanic influence. Our results are consistent with similar observations of Hg anomalies in proximal shallow-marine sections around the Neo-Tethys Ocean, including those in northern India and western Australia. However, we found that relatively shallower marine settings (shelf, lagoon or inshore) tend to exhibit Hg spikes in the latest Permian, whereas deeper sections (outer-shelf or deep carbonate ramp) show peaks in the Early Triassic. Since Hg anomalies for all the sections have been verified to be volcanogenic based on their near-zero values of Δ¹⁹⁹Hg, the discrepancies among them concerning timing and water depth may be attributed to prolonged volcanic influences extending into the Triassic period. Our findings underscore the complexity of sedimentary Hg records and further raise questions about the spatiotemporal consistency of Hg anomalies during the P-Tr transition. Additionally, the most negative Δ¹⁹⁹Hg value (−0.20‰) in the uppermost black shale in the Qubu section likely resulted from photic zone euxinia consistent with globally developed P-Tr shallow-marine anoxic conditions, while other low values of Δ¹⁹⁹Hg with low Hg concentrations were derived from some moderate terrestrial influx.