Astronomical calibration and carbon isotope stratigraphy of the Lower Jurassic Daanzhai Member in the Sichuan Basin: Implications for the cause of T-OAE

Abstract

The Early Jurassic Toarcian Oceanic Anoxic Event (T-OAE) was a critical climate-environmental perturbation. It was associated with global warming and widespread deposition of organic carbon-rich sediments in oxygen-depleted marine and lake settings. 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. Provided that continental strata are well-suited for capturing cyclic signals driven by orbital forcing, we combined time series analysis of gamma ray logs and global correlation of carbon isotope stratigraphy to construct a high-resolution continental astronomical timescale (ATS) for the Da'anzhai Member in the Sichuan Basin of South China Block, which is one of the most well-studied lacustrine sedimentary records during that time. The results show that the T-OAE interval in the Paleo-Sichuan Lake spans ∼360 kyr (182.88 to 182.52 Ma) and can be further divided into three phases: Onset (∼198 kyr), Body (∼80 kyr), and Recovery (∼75 kyr). The Multi-Taper Method analysis of tuned organic carbon isotope reveals that precession forcing influenced carbon cycle variations during the T-OAE. Moreover, our results indicate that organic carbon mass accumulation rates in the Paleo-Sichuan Lake were 2–5 times higher than the background level, further supporting the hypothesis that the lake could play a crucial role in regulating carbon cycles in a regional context. By calibrating marine and continental strata, this study provides a temporal window for reconstructing global T-OAE chronologies and carbon cycle perturbation patterns.