The Xiamen Symposium on Marine Environmental Sciences (XMAS 2025)

The Toarcian Oceanic Anoxic Event (T-OAE, ca. 183 Ma) was a global carbon-cycle perturbation. A pronounced decrease in the carbon-isotope composition of exogenic carbon reservoirs during this event has been linked to the large-scale release of ¹²C-enriched carbon, perhaps from large-scale volcanism. The T-OAE was characterized by global warming, elevated atmospheric CO₂, enhanced weathering, mass extinction and widespread seawater deoxygenation. Mercury (Hg) concentrations and isotopes measured in sedimentary rocks have been widely used to track past volcanic activity, and mass independent fractionation (MIF) of odd-numbered Hg isotopes (represented by Δ¹⁹⁹Hg) can track changing sources of Hg in the biosphere. Here, we use high-resolution sedimentary Hg isotope data to track changing sources of Hg to seawater during the T-OAE from the Yorkshire (UK) section. Previously recognized abrupt negative shifts in organic carbon isotopes through the Yorkshire section have been attributed to rapid (millennial-scale) pulses of carbon release. Our Hg isotope data indicate that abrupt negative shifts in Δ¹⁹⁹Hg occurred coevally with these abrupt carbon isotope shifts, combined with increased Hg concentrations, likely reflecting marked but short-lived inputs of terrestrial Hg to the basin driven by transient increases in weathering/erosion. By contrast, there is no clear evidence of enhanced atmospheric deposition of Hg from volcanism. The geologically synchronous coupling between carbon release and enhanced terrestrial fluxes that we observe contribute to an emerging view that hydrological changes can occur extremely rapidly in response to warming.
 

T-OAE, mercury isotope, continental weathering, carbon cycle