VARIATION OF THE INDIAN MONSOON DURING THE PERIOD OF 50~11 KA: INFERRED FROM SPELEOTHEM RECORDS IN YUNNAN PROVINCE, SOUTHWESTERN CHINA

Thirty-nine ICP-MS 230Th-234U dates show that three stalagmites, collected from two adjacent caves in Yunnan Province, SW China, were deposited between 50 and 11ka B.P. and that two of them grew contemporaneously between 18 and 15 ka B.P. Considering dating errors and resolution differences, the d¹⁸O variation of two stalagmites are similar during the interval of contemporaneous growth. Moreover, R2 values between d¹⁸O and d¹³C for each stalagmite are low, showing no evidence for kinetic fractionation.

Meteorological records and d¹⁸O analysis of precipitation reveal that monsoon circulation strongly affects the amount and isotopic composition of precipitation at the city of Kunming, Yunnan Province. Seepage water in the caves may be mainly recharged in the rainy season (May to Oct.), because more than 85% of annual precipitation falls during this period. The amount of rainfall during the rainy season anti-correlates with the d¹⁸O values of the precipitation, while temperature does not correlate significantly with the d¹⁸O values. The large range of the d¹⁸O values suggests that rainfall amount£¬which is affected by the monsoon intensity£¬may be an important control. Nevertheless, we cannot exclude the effect of temperature since it affects the fractionation between deposited calcite and drip water.

The d¹⁸O record shows the Indian Monsoon varied significantly on century to millennium time scales during the last glacial period, resembling cave Monsoon records further to the east and D-O cycles from Greenland ice cores. As opposed to records further east, the long-term trend does not appear to follow summer insolation at the cave site, suggesting that in this region, the Indian Monsoon may respond nonlinearly to insolation forcing when its intensity is affected by Eurasian snow cover and ice sheet extent. The close linkage between D-O events and d¹⁸O variation indicates a positive correlation between high northern latitude temperature and moisture transmission from low to high northern latitudes. During the YD, our high resolution d¹⁸O record has many high amplitude oscillations distinct from the long cold period recorded in Greenland. In comparison with other records during the YD, our record is consistent with the idea that fluctuation in precipitation within the YD chronozone is related to low-latitude forcing.