中全新世7～6 ka东亚季风气候的高分辨率石笋记录

基于神农架青天洞石笋两个²³⁰Th年龄、582条纹层计数和183个氧同位素数据,重建了7～6ka B.P.期间平均3年分辨率的δ¹⁸O变化序列。持续约580年振幅达1.8‰的δ¹⁸O在平均值-9.66‰上下波动,指示了东亚季风降水强度的长期演化趋势和高频振荡信息,大致划分为4个百年尺度旋回。青天洞与阿曼石笋δ¹⁸O记录的对比表明,东亚季风和印度季风在百年或更短时间尺度上同相位变化,可能反映了两地季风气候受统一的印度洋/太平洋大尺度水汽循环控制。功率谱分析揭示出163、81、16和11年的周期,大致与树轮Δ¹⁴C百年至数十年尺度太阳活动变化周期相同。

High Resolution Stalagmite Records of East Asian Monsoon from 7 to 6 ka B.P. in Mid-Holocene

The Asian monsoon system,composed of the Indian and East Asian mosoon subsystems,is a dynamic component of the modern and paleoclimate system.Studies of different influence extent of solar radiation and coupled atmosphere-ocean system to Asian monsoon subsystems still remain complex.Some stuidies indicated the inverse phase variations between the Indian and East Asian mosoon,which is different to most of monsoon proxies results.Understanding the interactions among them and their sensitivity to internal feedback mechanisms requires high resolution proxies of monsoon climate. Qingtian Cave(110°22′E,31°30′N,1 630m above sea level) is located almost at 10 km NW apart Muyu Town,Shengnongjia Natural Reserve,Hubei Province.Here,most of peaks are higher than 1500 m and generally become lower from SW to NE.Mean annual precipitation and temperature are 1 500-2 000mm at 1 500-2 000 m above sea level and 15.5℃ in the cave(measured in Sep.2004).Two samples for ICP-MS dating were conducted in the Isotope Laboratory of Geology and Geophysics Department,Minnesota University,USA.The reported error is in 2σ.A total of 183 samples for δ¹⁸O measurements have been analyzed with on-line automated carbonate preparation system(KIEL CARBONATE DEVICE) linked to Finnigan MAT-253 ratio mass spectrometer at Isotope Laboratory of College of Geography Science,Nanjing Normal University.Reference standards have been used to ensure a precision better than 0.06‰ and results were reported to PDB standard. A 3-year-resolution stalagmite oxygen-isotope record from Qingtian Cave has been established based on ²³⁰Th dates and results of annual band-counting.The variation of oxygen isotope ratios with time,with amplitude as large as 1.8‰ in δ¹⁸O,reflects changes in the amount of monsoon precipitation for the period from 6.7 to 6.1 ka B.P.This high temporal resolution δ¹⁸O series enables us to reconstruct decadal-centennial scale variation of East Asian monsoon intensity persisting about 580 years in Mid-Holocene. Comparison of the stalagmite δ¹⁸O time series between QT9 from Qingtian Cave and Q5 from Oman,shows the close relationship on multi-decadal to centennial variations in monsoon precipitation,indicating that the East Asian monsoon intensity is in phase with Indian monsoon.This observation suggests that Indian and East Asian monsoons,two sub-systems of Asian Monsoon system,are controlled by common forcing mechanisms.Consequently,these two climate subsystems may be influenced by large scale water transporting from Indian and Pacific Ocean,reflecting the integrated Asian land,ocean and atmosphere interacting system.A comparison between the stalagmite δ¹⁸O record and tree-ring ¹⁴C provides strong evidence for solar forcing of East Asian monsoons on centennial even multi-decadal time-scales.Results of spectrum analysis of the stalagmite δ¹⁸O indicate statistically significant periodicities centered at 163,81,16 and 11 years.The 163-year cycle is close to the 148 years periodicity of the tree-ring ¹⁴C records,while 81 and 11 years are similar to Gleissberg and Sunspot periodicities respectively.This statistic analysis further reinforces our interpretation for the solar forcing of Asian monsoon.