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A speleothem δ 18O record from Xiaobailong cave in southwest China characterizes changes in summer monsoon precipitation in Northeastern India, the Himalayan foothills, Bangladesh, and northern Indochina over the last 252 kyr. This record is dominated by 23-kyr precessional cycles punctuated by prominent millennialscale oscillations that are synchronous with Heinrich events in the North Atlantic. It also shows clear glacial-interglacial variations that are consistent with marine and other terrestrial proxies but are different from the cave records in East China. Corroborated by isotope-enabled global circulation modeling, we hypothesize that this disparity reflects differing changes in atmospheric circulation and moisture trajectories associated with climate forcing as well as with associated topographic changes during glacial periods, in particular redistribution of air mass above the growing ice sheets and the exposure of the "land bridge" in the Maritime continents in the western equatorial Pacific.he Indian summer monsoon (ISM), a key component of tropical climate, provides vital precipitation to southern Asia. The ISM is characterized by two regions of precipitation maxima: a narrow coastal region along the Western Ghats, denoted by ISM A , with moisture from the Arabian Sea, and a broad "Monsoon Zone" around 20°N in northeastern India, denoted by ISM B , where storms emanate from the Bay of Bengal and whose rainfall variability is well correlated with that of "All India" rainfall (1). Multiple proxies obtained from Arabian Sea sediments have revealed the variability of summer monsoon winds on timescales of 10 1 to 10 5 y (e.g., refs. 2-6). Our understanding of the paleo-precipitation variability of ISM B remains incomplete, owing to the scarcity of long and high-resolution records. Here we present a 252,000-y-long speleothem δ 18 O record from Xiaobailong cave, southwest China and characterize variability in the ISM B precipitation on multiple timescales.Xiaobailong (XBL, "Little White Dragon") cave is located in Yunnan Province, southwestern China, near the southeastern edge of the Tibetan Plateau (103°21′E, 24°12′N, ∼1,500 m above sea level; SI Appendix, Fig. S1). Local climate is characterized by warm/wet summers and cool/dry winters. The mean annual precipitation of ∼960 mm falls mostly from June through September (∼80%) (SI Appendix, Fig. S2), indicating the summer monsoon rainfall dominates the annual precipitation at the cave site. The temperature in the cave is 17.2°C, close to local mean annual air temperature (17.3°C).Eight stalagmites were collected from the inner chamber (∼350 m from the entrance) of the cave, where humidity is ∼100% and ventilation is confined to a small crawl-in channel to the outer chamber. One hundred four 230 Th dates were determined on inductively coupled plasma mass spectrometers with typical relative error in age (2σ) of less than 1% (Methods and SI Appendix, Table S1 and Figs. S3 and S4). The ages vary monotonically with depth in the stalagmites (SI Appendix, Fig. S4...

Computer models suggest that the Holocene Optimum for East Asian summer monsoon precipitation occurred at different times in different regions of China. Previous studies indicate that this time-transgressive Holocene Optimum should have been experienced about 3000 yr ago in southern China. In this study we describe a section which allows us to test this timing directly. We have closely examined high-resolution eutrophic peat/mud sequences covering the past 18,000 cal yr at Dahu, Jiangxi, on the southern boundary of the mid subtropical zone in China. Late Pleistocene successions in the Dahu record indicate cooler and much wetter conditions relative to synchronous events in north-central China. Our results indicate that the Holocene Optimum occurred between ca. 10,000 and 6000 cal yr ago in southern China, consistent with the global pattern. Conditions were relatively dry and cold from 6000 to 4000 cal yr ago. Our data also support the conclusion that the last deglaciation to early Holocene in the south was much wetter, resulting in the formation of dense broad-leaved forests, which could have acted to moderate land temperature f10,000 to 6000 cal yr ago, yielding a stable early-Holocene climate. After 6000 cal yr, forest reduction led to unstable land temperatures, and possibly to a northerly shift of the subtropical high-pressure system. Whatever the mechanism, these changes resulted in decreased precipitation between 6000 and 4000 cal yr B.P. in southern China.