Characteristics of Hg concentrations and isotopes in terrestrial and marine facies across the end-Permian mass extinction

The end Permian mass extinction (EPME) is the greatest among the “Big Five” extinctions of the Phanerozoic, and is believed to have been triggered primarily by the Siberian Traps Large Igneous Province (STLIP). This hypothesis is supported by the temporal correlation of STLIP with the EPME by radiometric ages and Hg enrichments in sedimentary rocks. However, how signatures of volcanic Hg emissions are preserved in sediments, and how this may vary from deep basin to shelf to terrestrial successions, remain unclear. To examine variability in the Hg record in different environments, we systematically measured organic carbon isotopes, Hg concentrations and isotopes, major elements, and total organic carbon (TOC) contents from: 1) a terrestrial section at Chahe, 2) a marine-nonmarine transitional section at Jinzhong both in Guizhou Province, Southwest China. Results show that Hg versus TOC and Hg versus Al exhibit no correlation through the terrestrial section at Chahe, whereas there is positive correlation in the transitional Jinzhong section. These relationships indicate that organic matter and clay minerals would not affect Hg fluctuations on land, but dominates Hg sequestration in transitional settings. Thus, Hg/TOC along with Hg/Al ratios were able to be employed to recognize abnormal Hg deposition in Jinzhong, and Hg concentrations were utilized to examine records of Hg enrichments in Chahe. The studied sections all show synchronous changes between carbon isotopic perturbations and Hg deposition events with first Hg enrichment and associated negative carbon isotopic negative excursion coinciding with terrestrial ecological disturbance and with a second Hg enrichment corresponding to the largest carbon isotopic negative excursion and marine biological crisis. These trends across the Permian-Triassic boundary are also observed in other 13 sections around the world, indicating a common source that injected massive CO₂ and Hg, impacting the global C and Hg cycles. Hg isotope (Δ¹⁹⁹Hg values) of the first peak in Hg concentrations have similar characteristics in the terrestrial Chahe and transitional Jinzhong sections along with other sections (e.g., Shangsi and Chaohu in South China, Guryul Ravine section in India and Buchanan Lake section in Canada), all showing positive values, indicative of predominantly atmospheric-derived volcanic Hg. In contrast, Δ¹⁹⁹Hg values at the second Hg peak, corresponding to the marine extinction horizon in Chahe and Jinzhong, are negative. This relationship indicates an elevated involvement of terrestrial sourced Hg into the terrestrial or nearshore aquatic realm due to increased weathering after the collapse of terrestrial ecosystem. Our study suggests that the STLIP could have two pulses of volatile eruptions that released massive CO₂ and Hg, resulting in two pulses of Hg enrichments coupled with negative carbon isotopic excursions coinciding with end Permian terrestrial and marine extinction events.