Abstract
The Middle Triassic is thought to have had a greenhouse paleoclimate with a few short humid phases. However, the timing of these humid events, and the extent to which orbital forcing influenced the evolution of climate, are unclear. Here, a cyclostratigraphic analysis has been carried out based on the integrated study of magnetic susceptibility, elemental chemistry and lithofacies from two shallow-marine carbonate platform sections in Guizhou, South China. Combined, these sections (Pohong and Yongyue) span the late Anisian to early Carnian. Spectral analyses of magnetic susceptibility, Fe/Al and P through the studied sections show significant cycles of ~85-m, ~7-m and ~ 4-m. Based on independent age constraints and sedimentation rate modelling, these cycles are interpreted as orbital cycles that reflect 405-kyr long-eccentricity, ~33-kyr obliquity, and ~ 20-kyr precession forcing, respectively. Tuning of the records to the stable 405-kyr long-eccentricity parameter in each section yields a tuned astronomical time scale spanning ~7.4 ± 0.2 Myr, which covers from ca. 244.4 to 237.0 Ma and includes the entire Ladinian Stage. Magnetic susceptibility and Fe/Al data from the studied sections show intervals with relatively high values in the middle-late Anisian (ca. 244.0 to 242.6 Ma), and the latest Anisian to early Ladinian (ca. 241.8 to 240.9 Ma). These geochemical changes, combined with lithological changes, indicate enhanced terrigenous flux caused by the acceleration of hydrological circulation under a warmer and more humid climate. In addition, P data records two sharp falls that appear broadly coeval with the previously postulated late Ladinian ecological crisis. Our new astrochronology also reveals that ~1.2-Myr obliquity amplitude modulation cycles are expressed in the Middle Triassic. The regional sea level changes inferred from sequence stratigraphy and sedimentary noise modelling can be correlated with Tethyan and global sea level change. This work demonstrates the utility and power of sedimentary noise modelling in shallow marine environments for inferring past sea level dynamics. It also furthers constrains the timing of putative Middle Triassic climatic and ecological events.
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