Major volcanic eruptions linked to the Late Ordovician mass extinction: Evidence from mercury enrichment and Hg isotopes

The Late Ordovician mass extinction (LOME) was the second most severe Phanerozoic biodiversity crisis. While environmental deterioration and oceanographic changes associated with the Hirnantian glaciation have been frequently invoked as potential extinction drivers, recent evidence for a large igneous province eruption at that time has challenged this prevailing view. As such, the triggering and killing mechanisms of the LOME remain debated. Here we report mercury (Hg) concentrations, isotopic compositions, Hg/total sulfur (Hg/TS), and Hg/total organic carbon (Hg/TOC) in Late Ordovician limestone/black shale alternations from two successions in South China and Laurentia that straddled the paleoequator. Our results, in both areas, show multiple Hg enrichments before and during the LOME, suggesting a global, or at least a widespread increase in environmental Hg loading. The initial Hg enrichments occur in the mid-upper Katian units and are followed by additional Hg anomalies in the lower Hirnantian strata that coincide temporally with the first pulse of the LOME. Extremely high levels of Hg, Hg/TS, and Hg/TOC, with maximum values of 737 ng g−1, 633 ng g−1 Hg/wt% TS, and 167 ng g−1 Hg/wt% TOC, respectively, represent ~3–13 × background values, indicating increased Hg input to the ocean. The absence of mass-independent fractionation of Hg isotopes in the Hg-enriched intervals suggests a volcanic source for the observed Hg anomalies. The temporal coincidence of Hg anomalies with the extinction horizon in both continents suggests that extensive and widespread volcanism may have had global climatic and ecological impact, and was a primary trigger for prolonged and synergetic deterioration of Late Ordovician environment such as climate changes, ocean acidification, and anoxia, causing the LOME.