末次冰期东海嵊山岛黄土粒度端元分析及其环境意义

Grain Size End Member Characteristics and Paleoclimatic Significance of Loess Deposit in Shengshan Island during the Last Glacial Period

【目的】东海岛屿晚更新世风尘黄土堆积是我国黄土在东部长三角及外延海域重要的拓展,深化这一区域风尘黄土沉积的物源特征和后期演化问题研究,对于认识晚更新世区域古环境变化尤为重要。【方法】在OSL测年和磁化率测试基础上,基于东海岛屿典型风尘黄土剖面(嵊山岛黄土)的粒度分析,开展了粒度组成的端元分析,探讨了其蕴含的古环境信息。【结果与结论】嵊山岛黄土沉积可提取出3个粒度端元(EM1~EM3),进一步分析发现EM1可能代表通过高空西风气流传输的远源黏粒组分,EM2可能代表以浮尘形式远距离悬浮搬运并随风力减弱而沉降堆积的细粉砂组分,EM3可能代表冬季风驱动下的近地表短距离搬运堆积的粗粉砂组分。粒度端元分析结果表明,高空西风输送的黏粒端元组分和远距离悬浮运移的浮尘端元组分可能是沉积剖面主要粉尘来源;综合年代学结果、百分频率磁化率和颗粒中值粒径等参数分析,认为东海嵊山岛黄土沉积过程在MIS3阶段先后经历暖湿—冷干—暖湿的旋回波动;并且,所记录的MIS3阶段气候波动以及MIS2阶段末次冰盛期和冰消期的古环境变化同深海氧同位素和古海平面变化趋势相对应,说明东海陆架区域该时期环境演变与全球性气候波动相协同。

[Objective]The Upper Pleistocene eolian loess accumulation in islands of the East China Sea is an important expansion for loess in China in the eastern Yangtze River delta and its extended sea area.The better study of the eolian loess provenance and its evolution in this region is particularly important for understanding regional paleoenvironmental changes.[Methods]Based on the analyses of optically stimulated luminescence(OSL)dating and magnetic susceptibility,parametric end-member analysis was used to clarify the grain size data for Shengshan Island loess and its paleoenvironmental effect.[Results and Conclusions]Three end-members(EM1-EM3)were identified:EM1 is the clay component transported by the westerly circulation;EM2 is the fine silt component transported over long distances,suspended in the form of floating dust that was deposited when the wind force weakened;and EM3 is the coarse silt component carried by winter winds for short distances and deposited near the land surface.It was inferred from the analysis that EM1 and EM2 were the main sources of dust in the loess profile on Shengshan Island.Combined with the optically stimulated luminescence(OSL)results,the variations of frequency susceptibility,median particle size,Rb/Sr values and the fluctuations of the end-members in the loess profile revealed that the Shengshan Island loess accumulated in marine isotope stage MIS2 experienced paleoclimatic fluctuations from warm and humid to cold and dry,and then to warm and humid in MIS3.The end-member properties also responded to the paleoenvironmental changes during the last glacial maximum and deglaciation period of the MIS2 stage.Climatic fluctuations recorded by the loess deposits also correspond to the trends of deep-sea oxygen isotopes and ancient sea-level changes.Thus the environmental evolution in the shelf areas of the East China Sea were closely related to global paleoclimatic variations.