Rhenium isotopes reveal enhanced rock organic carbon oxidation over the Toarcian Oceanic Anoxic Event

Weathering plays a central role in the geological carbon cycle. Silicate mineral weathering is invoked as a stabilizing feedback on CO₂ emissions, for example from volcanism during the emplacement of Large Igneous Provinces. However, modern-day studies show weathering can emit CO₂ during oxidation of rock organic carbon (OC_petro) in sedimentary rocks and function as a positive feedback on climate warming. Here we measure the rhenium isotope composition (δ¹⁸⁷Re) of Early Jurassic marine sediments to explore how OC_petro oxidation rates changed during warming across the Toarcian Ocean Anoxic Event (T-OAE). We find a 0.22 ± 0.10‰ decrease in δ¹⁸⁷Re values during the T-OAE, with mass balance modeling showing this can be explained by increased OC_petro weathering intensity on land associated with 6-7 °C of global warming. We estimate this could have delivered 7600-20,490 PgC to the oceans and atmosphere, demonstrating that chemical weathering does not simply act as a stabilizing feedback during hyperthermal events.