Kaustubh Thirumalai, Steven Clemens, Judson Partin

Earths orbital geometry exerts a profound influence on climate by regulating changes in incoming solar radiation. Superimposed on orbitally-paced climate change, Pleistocene records reveal substantial millennial-scale variability characterized by trends, tipping points, and rapid swings. However, the extent to which orbital forcing modulates the amplitude and timing of these millennial variations is unclear. Here we isolate the magnitude of millennial-scale variability (MMV) in two well-dated records, both linked to precession cycles (19,000-23,000-year periodicity): atmospheric methane and Chinese speleothem δ¹⁸O, where the latter is commonly interpreted as a proxy for Asian monsoon intensity. At the millennial timescale (1,000-10,000 years), we find a fundamental decoupling wherein precession directly modulates the MMV of methane but not that of speleothem-δ¹⁸O. We hypothesize that mid-to-high latitude insolation modulates the MMV of atmospheric methane, but feedbacks internal to the Earth-climate system modulate the strength of millennial-scale monsoonal circulation.