Orbital eccentricity and internal feedbacks drove the Triassic megamonsoon variability

The evolution of the Triassic megamonsoon was closely linked to Earth's orbital variations. Despite recognizing secular orbital cycles as a fundamental pacemaker of the megamonsoon, the driving mechanisms remain unclear. Here, we use data-model synthesis to study orbital-scale megamonsoon variability during the Middle Triassic (~ 246-239 Ma). By integrating high-resolution reconstructions of hydrologic fluctuations, obtained from lithological and magnetic susceptibility data series in the lacustrine sediments of the Ordos Basin (Northeast Tethys), with the climate simulations, we identify monsoon cycles in the ~ 20, 100, and 405 kyr Milankovitch bands. Comparisons with other records further reveal an additional eccentricity-related ~ 3.3 Myr orbital cycle in monsoon variabilities, temperature oscillations, carbon cycles, and sea-level changes. Earth system models show the effects of orbital configurations and atmospheric CO₂ concentrations on megamonsoon dynamics, implying threshold responses to solar radiation and the impacts of temperature and sea-level fluctuations on long-term megamonsoon variability. These findings improve our understanding of the interplay between astronomical forcing and feedbacks in shaping orbital-scale monsoon dynamics.