Resolving mercury cycling and the role of volcanism during the Toarcian Oceanic Anoxic Event

The emplacement of the Ferrar large igneous province has been implicated as the ultimate driver of the Early Jurassic Toarcian Oceanic Anoxic Event (T-OAE). Mercury (Hg) systematics, alongside other lines of evidence, have been used to support this assumption, but controversy exists over the relative roles of volcanic versus terrestrial Hg inputs. Here, we investigate the Hg record in two cores that document a bathymetric transect across the lacustrine Sichuan Basin, China. Both cores are characterized by Hg accumulation during the Toarcian Oceanic Anoxic Event. However, observed negative correlations between Hg concentrations and geochemical indicators of water column sulfide availability suggest modification of primary Hg enrichments via redox-driven loss of Hg from the sediments. In addition, Hg isotope systematics show differing signals between the shallow and deep cores, indicative of increased inputs from terrestrial and atmospheric sources, respectively. These results suggest that regional factors exert a major control on Hg enrichments in sediments, which must be considered when utilizing Hg systematics to evaluate volcanic activity. However, our approach highlights that lacustrine systems do document an important role for atmospheric Hg deposition during the Toarcian Oceanic Anoxic Event, confirming that large-scale activity of the Ferrar large igneous province was a key driver of this major environmental perturbation.