高分辨率石笋记录的近700年来滇西南夏季风降水变化

SUMMER MONSOON PRECIPITATION VARIABILITY DURING RECENT 700 YEARS RECORDED IN A HIGH-RESOLUTION STALAGMITE FROM SOUTHWESTERN YUNNAN,CHINA

基于云南西南部司岗里洞一支长约168 mm石笋(编号SGL1)的7个230Th年龄和325个δ^(18)O数据,建立了1322~1605 A.D.和1700~1927 A.D.时段平均分辨率达2 a的夏季风降水演化序列。该序列显示存在百年尺度的δ^(18)O偏正阶段,即1322~1605 A.D.和1700~1927 A.D.时段,指示当时夏季风降水持续减少,这与印度季风区石笋所记录的结果一致,但区别于东亚季风区石笋记录的小冰期后期(1700~1900 A.D.)季风逐渐增强的特征,表明小冰期后半段亚洲季风区石笋记录的区域干湿变化空间差异显著。显著的多年代际振荡是SGL1石笋序列最为突出的特征,在石笋生长稳定的1322~1540 A.D.时段,高分辨率氧同位素序列记录到9个持续时间约10~20 a、平均振幅达1.4‰的干旱事件,这些年代际尺度干旱事件在东南半岛及南亚地区的高分辨率记录中均有不同程度的体现。SGL1石笋氧同位素序列与太阳总辐照度记录呈显著的正相关关系,说明太阳活动可能是小冰期季风气候百年尺度变化的主要驱动因子,但二者呈负耦合关系,即太阳辐射增强(减弱),对应区域季风降水减少(增多)。小冰期早期(1322~1540 A.D.),SGL1石笋记录的年代际干旱事件与太平洋年代际振荡(PDO)暖相位和大西洋多年代际振荡(AMO)冷相位相对应,其显著的15 a、24 a和56 a周期也类似于PDO的主导周期,说明该区多年代际气候变化主要受PDO调控;功率谱分析结果还显示SGL1序列具有显著的2.6~3.2 a和6 a周期,暗示该时期区域年际尺度气候变化可能受ENSO和(或)印度洋偶极子(IOD)的影响。

High-resolution and precisely dated climate records are necessary for further investigating the climate variability during the Little Ice Age(LIA).Here,we present a high-resolution oxygen isotope(δ^(18) O)time series from Sigangli Cave to characterize the climate changes during the LIA.Sigangli Cave(23°32’N,99°33’E;1200 m a.s.l.)is located in Cangyuan County,southwestern Yunnan,China.Regional climate of the cave is dominated by Indian summer monsoon(ISM)with moistures mainly coming from the Bay of Bengal.Mean annual temperature and precipitation in this region recorded by Cangyuan Meteorological Station are 17.7℃and 1750 mm(1981~2010 A.D.),respectively.Most of the precipitation(about 87%)occurs during the summer monsoon season from May to October.Stalagmite SGL1 was collected in the innermost chamber of the cave with 168 mm in height and 60 mm in diameter.It is composed of gray-white,densely crystallized pure calcite.A clear hiatus was observed on the polished surface at 35~36 mm from the top.Total of 7230 Th dating were performed on MC-ICPMS.Totally 350 subsamples for stable isotope measurements were drilled(325 along the vertical growth axle and 25 along horizontal layer at 5 depths for"Hendy Test"),and analyzed with MAT 253 coupling with Gasbench-Ⅱ.The SGL1δ^(18) O time sequence with an average resolution of 2 years covers the intervals of 1322~1605 A.D.and 1700~1927 A.D.Its variabilities are interpreted as the variation of regional monsoon precipitation.The SGL1δ^(18) O values vary from-9.16‰to-6.84‰,with an average value of-8.01‰,and exhibit centennial-scale fluctuations with increasing values from 1322 A.D.to 1605 A.D.and from 1700 A.D.to 1927 A.D.,indicating the persistently decreasing trends in summer monsoon precipitation.These decreasing trends inferred from SGL1 broadly coincide with other stalagmite records from ISM region within the limitation of dating error.However,the increasing trend during interval of 1700~1927 A.D.was observed in some cave records from East Asian summer monsoon(EASM)region.The existence of inconsistent monsoon precipitation behaviors in Asian monsoon region on centennial scale reveals the complexity of monsoonal climate variation during the LIA.The most prominent feature of the SGL1 sequence is a series of decadal-scale oscillations punctuating on longterm trend.There are nine drought events(numbered W1~W9)with the duration of 10~20 years and average amplitude of 1.4‰can be identified during 1322~1540 A.D.,in which the time resolution ofδ^(18) O sequence is up to 1~2 years.These drought events coincide well with drought events logged in tree-ring based Palmer Drought Severity Index(PDSI)record from southern Vietnam.Within them,drought event W1~W2 and W3~W4 correspond to so called AngkorⅠandⅡDrought,respectively,the later was suggested as a key factor to explain the demise of Angkor Period.A more remarkable drought event,with an amplitude of 2.1‰inδ^(18) O,was identified in the interval of 1730~1820 A.D.This decadal-scale drought events were also recorded in other climate records from Asian monsoon region.The SGL1δ^(18) O sequence is positively correlated with the total solar irradiance(TSI)on multi-centennial scale,suggesting that solar activity plays an important influence on monsoon precipitation in southwestern Yunnan through the LIA.During the interval of 1322~1540 A.D.with well dating and high resolution,our record agrees with index of Pacific Decadal Oscillation(PDO)and is similar in part with Atlantic Multi-decadal Oscillation(AMO).Most of the drought events during this time periods are corresponded to warm phases of PDO,indicates that PDO plays importance role on climate on decadal scale in study region.Spectral analysis results show significant cycles of 15,24 and 56 years,which are similar with dominant periodicities of PDO,further supporting the PDO impact.Statistically significant periodicities of 2.6~3.2 and 6 years had also been identified,hinting influence of EI Ni1 oSouthern Oscillation(ENSO)and/or Indian Ocean Dipole(IOD),on sub-annually scale climate in this region.