Paleoclimate of Southwestern China for the past 50,000 yr inferred from lake sediment records

Long sediment cores (12.5 and 13.5 m) from two lakes in Yunnan Province wer e used to infer the paleoclimate of southwest China over the past 50,000 yr , During the Holocene and marine isotope stage (MIS 3), bio-induced carbona te precipitation and organic matter (OM) production was high, suggesting wa rm temperatures and high primary productivity. In contrast, sediment inorga nic carbon (IC) and organic carbon (OC) concentrations were low in last gla cial deposits from 38,000 to 12,000 cal yr B.P., indicating cool temperatur es and low productivity. The 50,000-yr record has alternating peaks of carb onate and coarse-grain (>38 mu m) quartz that reflect warm, moist interglac ial or interstadial conditions alternating with cold, dry glacial or stadia l conditions, respectively. Spectral analysis of the carbonate and quartz s ignals reveals power concentrated at periods of 7200 and 8900 cal yr, respe ctively, that may reflect a nonlinear climate response to precessional forc ing at a time of reduced eccentricity modulation (McIntyre and Molfino, 199 6). Oxygen isotope values of calcite from Yunnan lake cores indicate the su mmer monsoon was weak during the last glaciation from 50,000 to 12,000 cal yr B.P. The summer monsoon intensified between 12,000 and 8000 cal yr B.P., but weakened gradually in response to insolation forcing during the mid-to -late Holocene. Our results support the Overpeck ef al. (1996) model that p osits a weak summer monsoon during the last glaciation that responded nonli nearly to insolation forcing when its intensity was affected by Eurasian sn ow cover and ice-sheet extent. The summer monsoon intensified and responded linearly to seasonal insolation forcing in the Holocene when ice volume di minished. (C) 1999 University of Washington.