Intensification of the lacustrine methane cycle during the Toarcian Oceanic Anoxic Event

The Toarcian Oceanic Anoxic Event (T-OAE, ca. 183 Ma) was one of the most intense perturbations of Earth’s System of the last 250 million year. It was associated with the large-scale emission of 12 C-enriched carbon, global warming, and increased organic carbon burial. Although the T-OAE and its impact on climate and biogeochemical cycles are well-documented for the marine realm, the impact on continental biogeochemical cycles that could provide powerful positive feedback mechanisms that exacerbate the initial perturbation is poorly understood. Here we show that the stable carbon isotopic compositions (δ 13 C) of bacterial lipids (hopanoids) became very depleted during the T-OAE in the Ordos Basin, a large inland lake in northern China. We interpret these data as reflecting a transient increase in aerobic methane consumption in the lake due to increased methane production in the lake sediments. The surplus depletion in hopanoid 13 C coincides with the appearance of biomarker evidence for photic zone euxinia, indicating a link between lakes’ water column stratification / deoxygenation and intensification of the methane cycle. Our results indicate that the T-OAE had a direct impact on terrestrial biogeochemical cycles that could have provided a positive feedback, enhancing and prolonging its duration.