Intensified continental weathering and reductive surface runoff during the Triassic−Jurassic transition

The end-Triassic mass extinction (ETE) is characterized by profound declines in biodiversity in both marine and terrestrial ecosystems, widely accepted to have been triggered by the Central Atlantic Magmatic Province (CAMP). In response to the CAMP, global continental weathering intensity and terrestrial redox conditions at the extinction interval may have substantially changed, but these are not fully constrained yet. Here we report high-resolution Fe (δ56Fe) and Zn (δ66Zn) isotope data on sedimentary successions of terrestrial open systems across high-latitude and low- to middle-latitude continental Triassic−Jurassic transition (TJT) sections. Both sections recorded a positive δ56Fe excursion and an increase in chemical index of alteration along with fluctuating δ66Zn values during the ETE−TJT period, indicating enhanced continental weathering. The notable increase of δ56Fe with lower total Fe2O3 contents suggests a reductive continental weathering condition considering that Fe isotope fractionation during silicate weathering requires loss of fluid-mobile, reduced Fe(II). Notably, the positive δ56Fe excursion was concurrent with and proportional to the increasing concentration of polycyclic aromatic hydrocarbons from wildfires in corresponding coal layers, implying that the surface runoff during intensified weathering may have been reduced by biochar from CAMP-triggered wildfires. The reductive terrestrial runoff might have influenced the surface ocean across the TJT to some extent.