Refining the Early Jurassic Astronomical Time Scale: New Constraints from the Da’anzhai Member Cyclostratigraphy, Sichuan Basin.

The Early Jurassic Toarcian Oceanic Anoxic Event (T-OAE) represents a profound global hyperthermal and carbon cycle perturbation. The timescale of this event is essential for understanding global environmental and biological covariations. Previous cyclostratigraphic studies based on marine strata have yielded contrasting durations (300-1200 kyr), whereas recent high-precision geochronology suggests that this event lasted approximately 288 kyr, which highlights the limitation of the existing cyclostratigraphic framework. Therefore, it is crucial to establish a new high-resolution age timescale for the T-OAE to reconcile cyclostratigraphic work with radioisotopic geochronology. Here, we present an integrated study combining cyclostratigraphy and carbon isotope stratigraphy from the lacustrine Da’anzhai Member in the Sichuan Basin, South China. Our time-series analysis establishes a high-resolution continental Astronomical Time Scale (ATS), constraining the T-OAE interval to ~360 kyr (182.88-182.52 Ma) in the Sichuan Basin. This interval is further resolved into three phases: Onset (~198 kyr), Body (~80 kyr), and Recovery (~75 kyr). Furthermore, we find that organic carbon mass accumulation rates in the Paleo-Sichuan Lake were 2-5 times higher than background levels during the T-OAE, highlighting the significant role of lacustrine basins in regional carbon sequestration. This study provides a critical temporal framework for reconciling marine-continental correlations during the Mesozoic hyperthermal.