神农架全新世东亚季风演化及其热带辐合带控制*

文章基于湖北神农架山宝洞3支石笋的13个²³⁰Th年龄和505个氧同位素数据,建立了全新世8.45~0.46kaB.P.东亚季风降水序列,其长期演化趋势与33°N夏季太阳辐射能量变化曲线基本一致。神农架山宝洞与阿曼Qunf洞和贵州董哥洞的石笋高分辨率δ¹⁸ O记录整体相关(r 分别为0.75和0.94), 说明全新世东亚季风、印度季风系统的演化主要受控于同一驱动机制,即北半球夏季太阳辐射控制下赤道热带辐合带逐渐南移,导致亚洲季风降水持续减弱。功率谱分析表明:5ka以来山宝洞石笋记录具有显著的550a周期旋回,与树轮Δ¹⁴ C和北大西洋温盐环流周期基本一致。

THE EAST ASIAN MONSOON CLIMATE CHANGES AT MT. SHENNONGJIA AND ITS RELATION TO SHIFT OF INTERTROPICAL CONVERGENCE ZONE DURING THE HOLOCENE

The Shanbao Cave(31°40'N,110°26'E,1902m above sea level) is located almost at the top of Yanziya, 40 km NW away from the Songbai Town, Shennongjia Natural Conservation, Hubei Province. Most of peaks in this area are higher than 1500m and generally become lower and lower from SW to NE. Mean annual precipitation and temperature are 1500~2000mm at 1500~2000m above sea level and 12.1℃ at Songbai Town (930m above sea level). Three stalagmites(SB6, SB26, SB27)were obtained from different positions of the Shanbao Cave. They appear to be cylinders and are mostly composed of pure, compact calcite minerals, and stalagmite SB6, SB26 and SB27 are 90mm, 260mm and 150mm long, respectively. A high-resolution oxygen-isotope record from a thorium-uranium-dated stalagmite from the Shanbao Cave reflects variations of monsoon precipitation for the period from 8.5ka to 0.5ka (1ka=1000cal.aB.P.). Thirteen samples for ICP-MS dating were conducted at the Isotope Laboratory of Geology and Geophysics Department, Minnesota University, USA. The reported error is in ±2σ. A total of 505 samples for δ¹⁸ O measurements have been analyzed with on-line automated carbonate preparation system (KIEL CARBONATE DEVICE) linked to Finnigan MAT-253 ratio mass spectrometer at the Isotope Laboratory of College of Geography Science, Nanjing Normal University. The long-term trend of the Shanbao Cave record correlates the summer insolation at 33°N latitude. As mentioned in many previous studies, heavier δ¹⁸ O values are associated with diminished monsoon precipitation, and vice versa. By comparing high-resolution δ¹⁸ O record of stalagmite from the Shanbao Cave (Hubei, China) with that from the Qunf Cave (Oman), and the Dongge Cave (Guizhou, China), we find out that the average value of stalagmite δ¹⁸ O decreases northward with an increase in latitude. Furthermore, the long-term trends from three stalagmite records agree each other well during the Holocene (the correlation coefficient is 0.74 and 0.94, respectively), which indicates that long-term changes of the two sub-systems of Asian Monsoon have been controlled by a common forcing mechanism. As suggested by many previous studies, the broad decline in Asian monsoon intensity throughout the most of the Holocene was directly controlled by the orbitally-induced lowering of summer insolation affecting gradual southward migration of Intertropical Convergence Zone (ITCZ). Power spectral analysis of the SB26 δ¹⁸ O record shows a statistically significant centennial periodicity centered on 526 years. This periodicity is close to the significant period of the Δ¹⁴ C record, the North Atlantic thermohaline circulation, and previous findings from spectral analysis of other Chinese speleothems (DA and D4 stalagmites from Dongge Cave). Our data, together with other Chinese work and southern Oman stalagmite δ¹⁸ O record, support the idea that solar changes are partially responsible for changes in Holocene East Asian Monsoon (EAM) intensity during the past 5ka.Thus, our reconstruction of the history of East Asian Monsoon strength and precipitation will provide a basis for evaluating and understanding of a possible dynamic mechanism of the Asian Monsoon at orbital and millennial time scales.