Testing Early Triassic timescales based on sulfur-isotope cyclostratigraphy

The distribution of time among the stages and substages of the ∼5-m.y.-long Early Triassic Epoch remains debated, with two competing chronological models: the short-Smithian timescale (∼0.5−1.2 m.y.), based mainly on radiometric dating of zircons, and the long-Smithian timescale (∼1.7−2.1 m.y.), based mainly on time-series analysis of various proxy records. Here, we evaluate these competing models based on a synthesis of biostratigraphy, chemostratigraphy, and cyclostratigraphy from three marine sections in South China: Qiyueshan, Jianshi, and Daxiakou. These sections yield similar records of carbonate carbon-isotopic (δ13Ccarb) excursions and high-frequency variability in both δ13Ccarb and carbonate-associated sulfate sulfur-isotope (δ34SCAS) profiles, which together record five discrete cycles in the Dienerian Substage and six cycles in the Smithian Substage of the Lower Triassic. Cyclostratigraphic analysis of δ34SCAS, informed by a biostratigraphically and radiometrically constrained geochronological framework, links high-frequency variation in the δ34SCAS and δ13Ccarb records to short-eccentricity (∼100-k.y.) orbital forcing, yielding duration estimates for the Dienerian and Smithian substages of ∼500 ± 100 k.y. and ∼600 ± 100 k.y., respectively, which is more consistent with the short-Smithian timescale. This finding illustrates the value of integrated studies that combine zircon U-Pb dating and astrochronology within a unified biostratigraphic and carbon-isotopic chemostratigraphic framework to establish a robust temporal framework for the Early Triassic.