High-resolution record of multiple organic carbon-isotope excursions in lacustrine deposits of Upper Sinemurian through Pliensbachian (Early Jurassic) from the Sichuan Basin, China

The Sinemurian-Pliensbachian boundary event (ca. 193 Ma) is recorded as a global perturbation of the carbon cycle, as evidenced by a large negative carbon-isotope excursion recorded in many marine sedimentary successions. Whereas multiple lines of evidence testify that the Sinemurian-Pliensbachian boundary event was associated with environmental and climatic changes, sea-level oscillations, and biotic turnovers in marine settings, the record and effects of the event on continents are poorly known. In this paper, we report a high-resolution δ13Corg record and palynological data from the Lower Jurassic lacustrine succession of the Sichuan Basin that allow a prominent 8‰ negative carbon-isotope excursion to be identified at the Sinemurian-Pliensbachian transition. We therefore interpret this perturbation as the expression of the Sinemurian-Pliensbachian boundary event in the Sichuan Basin, and we propose a correlation with the marine realm. Facies evolution illustrates that the Sinemurian-Pliensbachian boundary event coincided with a phase of expansion of the lacustrine systems. Palynological analyses indicate a sharp shift from arid to humid climate conditions coincident with the carbon-isotope perturbation that supports a scenario of lake expansion driven by increased rainfall. In contrast to observations in the Sichuan Basin, where deep lake conditions persisted across the Sinemurian-Pliensbachian boundary event, a global drop in the sea level is documented at the onset of the isotope perturbation. This suggests that eustatic oscillations due to increased continental water storage in lakes and aquifers in the context of a wetter climate phase may have been associated with the early stages of the Sinemurian-Pliensbachian boundary event.