The Xiamen Symposium on Marine Environmental Sciences (XMAS 2025)

Understanding orbital-scale monsoon variability in the Tethys region is essential for comprehending the broader climatic and biotic dynamics during the Triassic, which was characterized by a prevailing megamonsoon system, elevated CO₂ levels, and witnessed life recovery following the Permian-Triassic mass extinction (PTME). Here, we investigate the terrestrial sedimentary record of the Middle Triassic Yanchang Formation, Ordos Basin, in order to explore the hydrologic evolution and the possible linkage with orbital-scale monsoonal variability in the northeastern Tethys. By analyzing the lithology, astrochronology, and U-Pb dating age of a drilling core, we show that this sequence, spanning from ~246 to ~240 Ma, documented lake expansion cycles were paced by a grand orbital eccentricity cycle, exhibiting a cyclical pattern of ~3.3 million years (Myr). Climate model simulations corroborate a monsoon-dominated hydrologic regime over the Ordos Basin, thus supporting a close linkage between monsoon precipitation and the lake-level changes in this basin. Our empirical and modeling results, in conjunction with other geological records, indicate that the ~3.3-Myr cyclicity in monsoon variability is primarily driven by greenhouse gases, such as CO₂, rather than solar insolation on eccentricity timescales. Moreover, over a ~3.3-Myr eccentricity timescale, the close association between monsoon intensification and the emergence of marine and terrestrial biotas in the northeastern Tethys underscores the significance of eccentricity-paced monsoon variability in shaping biotic habitats. These findings offer valuable insights into the mechanisms governing eccentricity-scale monsoon variability in a hothouse world, as well as its impacts on post-extinction biotic recovery.

climate change