Large-scale volcanogenic Hg enrichment coincided with the Sinemurian-Pliensbachian boundary event (Early Jurassic)

The Sinemurian-Pliensbachian boundary event (ca. 192.5 Ma) is an Early Jurassic era phase of perturbation of the global carbon cycle that coincided with environmental changes recorded worldwide. The causal mechanisms behind the Sinemurian-Pliensbachian boundary event remain unclear. Here, we present and discuss geochemical investigations, including δ13Corg, mercury Hg abundance, and isotopic composition and elemental analyses conducted across the Sinemurian-Pliensbachian transition in rocks from marine deep-water (Western Tethys, Italy) and continental (Sichuan Basin, China) settings. In both, an Hg enrichment coincides with a negative δ13Corg perturbation that can be correlated to the Sinemurian-Pliensbachian boundary event. Furthermore, in marine rocks, Δ199Hg across the Sinemurian-Pliensbachian boundary event displays near-zero values and variation patterns that are consistent with inputs of volcanogenic Hg into the atmosphere/ocean system. This evidence supports the currently scattered but consistent reports from other sedimentary basins, and indicates that Hg enrichments at the Sinemurian-Pliensbachian boundary event are not local, which points to a prolonged phase of volcanism as a likely cause of the carbon cycle perturbation during the Sinemurian-Pliensbachian boundary event and the associated environmental and climate changes. We propose that this volcanism could be related to the formation of thinned ocean-continent transitional crust during rifting that led to the breakup of Pangea during the Sinemurian-Pliensbachian transition.