Orbitally-paced climate change during the Carnian Pluvial Episode

Abstract

Global climate change has profound implications for human survival and prosperity. The Carnian (early Late Triassic, ∼233 Ma) was a time of global environmental perturbations, climatic change, and biotic turnover, commonly known as the Carnian Pluvial Episode (CPE). However, the understanding of possible triggering mechanisms of the CPE is still partially untraveled due to the lack of a high-resolution chronostratigraphic framework. Here we present an astrochronology of episodic negative carbon isotope excursions (NCIEs) observed in the shallow marine sediments of the Bagong Formation in the Qiangtang Basin (Tibetan Plateau, China) to explore the role of orbital forcing during the CPE. This study marks the first identification of five episodic NCIEs in the Carnian strata of the Bagong Formation. These NCIEs correspond with five distinct periods of increased detrital influx. The coupling relationship between each phase of NCIE and related terrigenous input indicates that episodic NCIEs may be influenced by pulses of active continental weathering. Mercury isotope is contemporaneous with the active volcanism that triggered the onset of the NCIEs; however, the impact of volcanism in the Qiangtang Basin was weak and almost non-existent at this time. An anchored floating astronomical timescale (ATS) for the episodic NCIEs is established, revealing evidence for a 405 kyr eccentricity in high-resolution gamma ray data series using time series analysis. A ∼13.16 Myr-long ATS of the Bagong Formation is developed by astronomical tuning of gamma ray logs to the stable 405-kyr long-eccentricity cycles. This floating ATS takes the U-Pb zircon as its anchor point and establishes an anchored floating ATS of the Bagong Formation from 233.16 ± 1.37 Ma to 220.4 ± 1.1 Ma. The innovative astrochronology approach, employing a sedimentary noise model, has successfully reconstructed sea-level changes during the Late Triassic period. These findings align well with the documented global sea-level fluctuations of the same era. The antiphase relationship of the filtered ∼1.2 Myr cycles between the sedimentary noise model sea-level curve and the obliquity modulation cycles demonstrates that the ∼1.2 Myr modulation cycles may be the main driver of sea level changes during the Late Triassic. The ∼1.2 Myr obliquity modulation maxima correlate well with the high sea level, episodic NCIEs, global warming, and marine life crisis, suggesting that obliquity forcing could have played a prominent role during the CPE. Our results reveal that the orbital forcing enhanced the hydrological cycle during the CPE, which provides a broader perspective of the CPE-related to the astronomical forcing.