中文摘要：

对采自重庆市东北部奉节县新崖洞XY6石笋进行ICP—MS—U系测年表明，该石笋生长时期为4．5～0．25ka。石笋在4．5～4．1ka、3．2～2．5ka、1．85～1．5ka期间相对偏负的δ^18O和δ^13C值记录了3个相对湿润的时期；其中以4．5～4．1ka时期δ^18O最偏负，之后突然偏重，表示了中全新世气候适宜期的快速结束；之后东亚夏季风在波动中逐步减弱。该石笋没有明显记录到发生在1．1～0．9ka的“中世纪暖期”事件。在4．1～3．2ka、2．5～1．85ka、0．5～0．25ka期间相对偏正的δ^18O和δ^13C值代表3个相对冷干的时期，0．5～0．25ka是欧洲小冰期事件在本地区的表现。该地区气候在1．5～0．5ka相对平稳，从前半段的相对暖湿过渡到后半段的相对冷干。XY6石笋δ^13C与δ^18O具有非常一致的变化趋势，可能暗示了该地区为“暖湿、冷干”的水热配套模式，也表明本地区植被对气候变化的响应是快速的，并没有明显的滞后期。δ^13C与δ^18O变化峰值的差异表明，在东亚季风区植被变化不仅取决于夏季降水的多寡，可能更主要的是受制于有效湿度的变化。

英文摘要：

A 145mm long stalagmite （XY6） from Xinya cave in northeastern Chongqing, a site under influence of both East Asian monsoon and Indian monsoon, has been ICP-MS ^230Th/U dated. Depositional characteristics and five ^230Th/U dates show that the stalagmite deposited over the past 4500a but ceased about 220a ago, with an average growth of 0. 035mm/a ranging from 0. 016mm/a to 0. 068mm/a. The white gray zones deposited faster, whereas dark brown zones grew slower. We have analyzed 102 samples for δ^18O and δ^13C, revealing decadal-to-centennial climate changes during the past 4500a. The δ^13C fluctuated from -4‰ to -10‰ while the δ^18O fluctuated from -6‰ to -8‰ （PDB）, showing positive correlation on decadal-to-centennial scales. Speleothem δ^18O in eastern China including our study area can be used as a proxy of summer monsoon strength, with lighter values indicating stronger summer monsoon and higher precipitation. Speleothem δ^13C, on the other hand, can be used as a proxy of paleo-vegetation, with lighter values indicating higher C3/C4 plant ratios and good vegetations, and vice versa. The positive correlation of δ^18O and δ^13C may indicate warm-wet and cold-arid paleoclimate patterns in this area. Based on the δ^18O and δ^13C variation, the paleoclimate in the past 4500a can be divided into 3 warm-wet periods： 4500～4100a, 3200～2500a, 1850～1500a and 3 cold-arid periods： 4100～3200a, 2500～1850a, 500～50a, respectively. A sharp increase of ^18O occurred around 4100 a B.P. and kept relatively heavy values after then, perhaps indicating a decrease of the Summer Monsoons since 4.1ka. The enriched ^18O values between 500～250a might reflect cold-dry conditions in this area corresponding to the Little Ice Age.