Lacustrine redox variations in the Toarcian Sichuan Basin across the Jenkyns Event

The Early Jurassic Jenkyns Event (~183 Ma) represents a major environment perturbation event, characterized by the negative carbon isotope excursion during the early Toarcian. Reconstruction of redox conditions, especially in the mega-lakes, across the Jenkyns Event is of significance to understand the biogeochemical dynamics on land at the time. Here, we report iron speciation and trace metal data from the two drill cores (LQ104X and X3 wells) with different depositional environments in the lacustrine Sichuan Basin of SW China. Results show that the water column in the Sichuan Basin experienced a significant shift of redox conditions in the Toarcian. Frequent development of anoxic-ferruginous conditions, interspersed with significant euxinic episodes across the Jenkyns Event. Conversely, during the pre/post-Jenkyns Event interval, dominantly oxic conditions, with short-lived anoxic-ferruginous or euxinic conditions, developed in the water column of the Sichuan Basin. We then proposed three possible mechanisms interdependent in nature that could have induced the water-column redox variability in Toarcian lakes: (1) lake stratifications, (2) enhanced methane release from the lake floor under low sulfate conditions that exhausted bottom water oxygen, and (3) lake eutrophication. We further anticipated that a close link between redox shift and ecological stress in the Sichuan Basin across the Jenkyns Event.