Millennial-scale climate variability of the Asian summer monsoon over the last 690,000 years: insights from cave records

The Asian summer monsoon (ASM) is an important component of the global climate system. Cave oxygen isotope (δ¹⁸O) records from the region have well characterized the ASM millennial-scale climate variability (MCV) over the last 640 ka, with especially detailed insights for the most recent 60 ka, but little is known about ASM variability beyond the U-Th dating limit of ∼640 ka. Furthermore, questions remain regarding the climatic significance of millennial-scale ASM variability recorded among various climate archives, particularly in the context of the "orbital-scale paradox". Here, we present high-resolution and U-Pb dated cave δ¹⁸O records from two Chinese caves spanning 690-600 ka BP (before present, where present = 1950 CE). These records reveal coupling between millennial-scale ASM weakening, North Atlantic cooling, and Antarctic warming, essentially mirroring the pattern observed during the last 640 ka, despite a potential change in the Atlantic meridional overturning circulation (AMOC) caused by a switch in the freshwater route to the North Atlantic at ∼640 ka BP. Comparisons of MCV amplitudes among different ASM proxies show remarkable disparities, suggesting that each proxy reflects different aspects of the ASM. In our records, declines in summer insolation repeatedly triggered millennial-scale weak ASM events near the mid-precession band, associated with AMOC weakening, rather than only at interglacial terminations. Additionally, our analysis highlights the critical roles of atmospheric CO₂ and global ice volume conditions in shaping the ASM variability during ∼688-635 ka BP.