An intensified lacustrine methane cycle during the Toarcian OAE (Jenkyns Event) in the Ordos Basin, northern China

The Toarcian Ocean Anoxic Event (T-OAE, also called the Jenkyns Event (ca. 183 Ma)), was marked by a negative carbon isotope excursion (CIE) representing one of the most intense carbon cycle perturbations of the last 250 million years. This early Toarcian CIE (Toa-CIE) was driven by a large-scale release of 13C-depleted carbon, which triggered global warming and accelerated weathering rates. Although the carbon cycle perturbation and its associated environmental effects are well-documented in the marine realm, the impact on lacustrine biogeochemical cycles that could exacerbate the perturbation via positive feedbacks is poorly understood. Here we provide novel data from the Anya section in the Ordos Basin, which was a large endorheic paleo-lake in northern China. We show that the stable carbon isotopic compositions (δ13C) of bacterial lipids (hopanoids) became as low as –60 ‰ during the main phase of Toa-CIE. We interpret these data to reflect a transient increase in aerobic methane consumption in the ancient lake due to increased methane production in the lake sediments. The 13C-depletion in hopanoids is followed by the appearance of biomarker evidence for photic zone euxinia, indicative of an interplay between the sulphur cycle and methanogenesis/methanotrophy. Our results highlight that the carbon cycle perturbation of the T-OAE had direct impacts on lacustrine biogeochemical cycling, and these could have acted as a positive feedback on the Toa-CIE, with the warming-induced production and release of methane enhancing greenhouse warming. Paleo-lakes, therefore, could have played a major role in T-OAE carbon cycling and have helped in prolonging the duration and amplifying the magnitude of the Toa-CIE.