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

Abstract

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 (OCpetro) 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 OCpetro 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 OCpetro 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.