Enhanced hydrological cycling and continental weathering during the Jenkyns Event in a lake system in the Sichuan Basin, China

Knowledge of past hydrological cycling and continental weathering under greenhouse climate conditions helps understand extreme climatic events that are currently occurring. Previous studies of marine strata have reported enhanced hydrological cycling and continental weathering during the Jenkyns Event (JE, also known as the Toarcian Oceanic Anoxic Event; ca. 183 Ma). We undertook a sedimentological and geochemical investigation of semi-deep to deep lacustrine deposits that correspond to the Jenkyns Event (i.e., the Da'anzhai Member in the well LQ104X) in the Sichuan Basin, China. Statistical analysis of sedimentological data demonstrates that the tempestities (bioclastic limestones, discrete shell beds, and shell laminae, lenses, and aggregates in mudstones) and hyperpycnites (sandstones and siltstones with normally and inversely-normally graded beddings) occurred in the Jenkyns Event, which is indicative of enhanced hydrological cycling. Hyperpycnites associated with flooding events developed in the upper part of the Jenkyns Event. Geochemical data suggests an intensified continental weathering during the Jenkyns Event. However, weaker chemical weathering recorded in the upper part of the Jenkyns Event compared with the lower part. The enhanced hydrological cycling increased the transportation during the upper part of the Jenkyns Event, which weakened the chemical weathering in the provenance area and resulted in low CIA (chemical index of alteration). The correlation with sea surface temperatures implies that elevated temperatures during the Jenkyns Event lead to enhanced hydrological cycling, increased frequency of storms and floods, and intensified continental weathering.