Mercury anomalies across the end Permian mass extinction in South China from shallow and deep water depositional environments

Life on Earth suffered its greatest bio-crisis since multicellular organisms rose 600 million years ago during the end-Permian mass extinction. Coincidence of the mass extinction with flood basalt eruptions in Siberia is well established, but the exact causal connection between the eruptions and extinction processes in South China is uncertain due to their wide spatial separation and the absence of direct geochemical evidence linking the two. The concentration and stable isotope analysis of mercury provides a way to test these links as its concentration is thought to be tied to igneous activity. Mercury/total organic carbon ratios from three Permian-Triassic boundary sections with a well-resolved extinction record in South China show elevated values (up to 900 ppb/wt.% relative to a background of <100 ppb/wt.%) that exactly coincides with the end-Permian mass extinction horizon. This enrichment does not show any correlation with redox and sedimentation rate variations during that time. Hg isotope mass-independent fractionation (Δ199Hg), with sustained positive values, indicate a predominant atmospheric-derived signature of volcanic Hg in deep-shelf settings of the Daxiakou and Shangsi sections. In contrast, the nearshore environment of the Meishan section displays a negative Δ199Hg signature, interpreted to be related to terrestrial Hg sources. Such temporal differences in Δ199Hg values shed new light on Hg geochemical behavior in marine settings, and also on the kill mechanisms associated with volcanism that were responsible for biotic mortality at the end of the Permian.