重庆新崖洞XY6石笋4.5ka以来高分辨率δ^(18)O、δ^(13)C记录的气候变化

A 4500-YEAR HIGH-RESOLUTION CLIMATIC RECORD FROM A STALAGMITE IN XINYA CAVE,CHONGQING,CHINA

对采自重庆市东北部奉节县新崖洞XY 6石笋进行ICP-M S-U系测年表明,该石笋生长时期为4.5~0.25 ka。石笋在4.5~4.1ka、3.2~2.5ka、1.85~1.5ka期间相对偏负的1δ8O和1δ3C值记录了3个相对湿润的时期;其中以4.5~4.1ka时期δ18O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1δ8O和1δ3C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。XY 6石笋1δ3C与1δ8O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。δ13C与δ18O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。

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.