Massive Emission of Thermogenic Mercury from Black Shales During the Emplacement of Large Igneous Provinces

Large Igneous Provinces (LIPs) have a critical influence on the environment and the life in Earth history. Mercury (Hg) is a powerful proxy for tracing LIPs linked to environment and life. Anomalous Hg enrichment in sedimentary rocks is typically considered as a sign of volcanic inputs, but other Hg sources, such as the release of Hg from organic-rich sediments by sill intrusions, have also been proposed as plausible causes for Hg enrichment. However, the effects of sill intrusion into surrounding sediments on Hg release and the subsequent re-distribution and re-accumulation of Hg in sediments is poorly constrained. Here we report Hg concentrations (THg), speciation and isotopic compositions of two sections in the Mesoproterozoic Xiamaling Formation, where black shales are intruded by diabase sills. We found one section shows nearly complete loss of Hg in the hornfelsic contact aureole (HCA) (THg ~3 ppb, ~90% loss), accompanied by a progressive positive shift in δ202Hg toward the sill (from -1.52‰ and -0.41‰), reflecting thermal degassing and hydrothermal leaching as dominant removal pathways. In contrast, the other section shows much less Hg loss in the HCA (average THg ~16 ppb, ~50% loss), with only minor δ202Hg variations (between -0.68‰ to -0.24‰). We suggest that the contrasting Hg behaviours between the two sections are primarily controlled by the availability of Fe-bearing minerals or sulfides during the cooling stage, which facilitated re‑deposition of Hg. Mass balance calculations preliminary estimate that at least 98-209 kilotons of Hg was liberated from black shales during sill intrusions. Thermogenic Hg released from black shales significantly contributes to anomalous Hg enrichment and shifts of Hg isotopes in pelagic sediments during the emplacement of continental LIPs. Our result determines the loss of Hg from black shales and highlights the production of Hg in sedimentary basins emplaced by LIPs. The massive release of toxic Hg and greenhouse gases could significantly affect life and climate.