石笋记录的西南地区MIS4阶段夏季风的演化

The evolution of summer monsoon in Southwest China during MIS 4 as revealed by stalagmite δ18O record

基于重庆市丰都县羊子洞石笋Y-02的12个230Th年龄和503个氧同位素数据重建了中国西南地区60~73ka B.P.时段夏季风的演化历史。Y-02的δ18O记录显示中国季风区D/O18、D/O19a和D/O19共3个千年尺度夏季风增强事件与格陵兰间冰阶（GIS）18~19相对应。此外，Y-02记录显示：紧接着D/O18事件以后，夏季风强度突然减弱，而此时北半球高纬度地区正经历着明显的降温过程。Y-02记录捕捉到的这次东亚夏季风突然且短暂的一次减弱过程，可能是H6事件或者是H6事件季风最弱的时段。Y-02的δ18O记录显示，D/O18事件是一次较强烈的千年尺度季风增强事件，持续时间近千年，而D/O19事件在持续时间和强度上不如D/O18事件，这不同于格陵兰冰芯记录。中国季风区气候变化与格陵兰地区存在着如此紧密的联系进一步支持：东亚夏季风演化受到北半球高纬度地区气候变化的影响，但是对具体的气候事件响应可能存在不同的模式。

Yangzi Cave （29°47＇N, 107°47＇E）, belonging to the upper caves of Xueyu Cave group, is a longitudinal cave system developing along the monoclinic strata, and it is situated on the left bank of downstream of Dragon River, about 18 kilometers from new downtown of Fengdu, Chongqing, Southwest China. Yangzi Cave is more than 500m in length, composed of lower and wide passage and two halls, with some residual underground and flood muddy sediments therein. This region is dominated by the typical subtropical humid monsoon climate. The current mean annual temperature is 16~18℃ and the average annual rainfall is ca.1400mm. Stalagmite Y-02, collected in Yangzi Cave, is 78cm in length. In this paper, we present the data from the top 25.5cm. The shape of Y-02 is cylindrical and the color of surface is milky. The stalagmite was cut along the central axis. It is composed of pure calcite, and the structure of calcite crystals appears mighty dense. 12 subsamples, 150~200mg for each, were drilled parallel to the growth axis of Y-02.4 subsamples （Y-02-4, Y-02-5, Y-02-6 and Y-02-9）were dated with U-series method at the Laboratory of Taiwan University, and the rest of subsamples were dated at the Minnesota Isotope Laboratory, and errors are given with two standard deviation （±2σ）. Stable oxygen and carbon isotope of stalagmite Y-02 was analyzed at Geochemistry and Isotope Laboratory of Southwest University. Analysis was performed using a Delta-V-Plus Mass Spectrometer, combined with a Kiel IV Carbonate Device. Every seven samples were intercalated with one standard, NBS 19. Isotopic results are given with respect to Vienna Pee Dee Belemnite （VPDB）standard with one-sigma external error 〈 ±0.1 ‰ for δ18O and 〈 ±0.06 ‰ for δ13C. The results suggested that the age of Y-02 between the top and the depth of 25.5cm grew from 60~73ka B.P., and all age data are ranked in the order of stalagmite deposition, which illustrates the data are valid and trusted. There is no obvious sedimentary discontinuity in this period, which we can use linear interpolation to establish its time scale. In total 503 stable oxygen and carbon isotope samples of stalagmite Y-02 were analyzed. The average resolution of the δ18O series is about 25.8a. The δ18O record of Y-02 displays a significant millennial-scale change that correlates well in timing and duration with Dansgaard/Oeschger （D/O）events identified in high-latitude regions of the Northern Hemisphere. We reconstructed the evolution of the East Asian summer monsoon during the MIS 4 glacial period. Millennial-scale events activity appears particularly weak in the meantime, which is also the most obvious character of the δ18O record in Y-02. The Y-02 record also displays that the intensity of the Asian summer monsoon immediately weakened after the D/O event 18, then rapidly recovered in about 1ka later. After comparing the stalagmite Y-02 δ18O record with other records from the Asian monsoon area, we find that this is not an isolated phenomenon. We consider this event may be the part of Heinrich event 6 or the weakest monsoon episode of it. We believe that climate change in the northern hemisphere high latitudes is the dominant factor, and that changes in temperature in the Antarctic region will also have some effect on it.