Early Jurassic massive release of terrestrial mercury linked to floral crisis

Abstract

The Toarcian Oceanic Anoxic Event (T-OAE, ∼183 Ma) was marked by a pronounced negative carbon-isotope excursion, rapid global warming, ocean deoxygenation, and turnover of marine and terrestrial biota. The T-OAE has been linked to carbon (C) release from volcanism, but the mechanisms controlling the cycling of C, metals, and nutrients during the event are still not fully understood. Here we show that lacustrine strata (Anya section) through the T-OAE in the Ordos Basin (China) record large increases in Hg/TOC (up to 453 ppb/wt.% relative to a background of 23 ppb/wt.%), which are coincident with the sudden demise of spore producing plants and seed ferns and the acme of Classopollis pollen of the thermophilic Cheirolepidiaceae conifers. These changes occurred during the peak of the negative carbon-isotope excursion that marks the T-OAE in the Anya section. Hg-isotopes are a useful proxy for recognizing changes in Hg sources and fluxes (notably from atmospheric deposition and terrestrial biomass) in sediments and sedimentary rocks. The negative Δ199Hg and δ202Hg throughout the Anya section are consistent with a constant terrestrial Hg source in the studied interval. Hence, the recorded Hg enrichment suggests that terrestrial Hg loading increased during the T-OAE. Coupled with evidence for a large increase in hydrological cycling, weathering and runoff, our data indicate that the T-OAE floral crisis was associated with a massive release of terrestrial Hg to oceanic and terrestrial ecosystems. This work provides new insights into the links between toxic metal cycling and mass extinction during times of large-scale volcanism.