腾冲青海湖沉积物磁学特征对印度季风变化的指示与响应

Indian summer monsoon (ISM) is driven through the couple of bi-hemisphere system. The sensible heating in the Indian continent and the latent heating from the sub-tropical Indian Ocean, as well as the Mascarene high system, dominate the monsoon variability in the different time scales. However, the pattern of Indian monsoon in the orbital time scale for the last glacial period is different with the varying style of Antarctic temperature and North Hemisphere ice volume, and the phase relation between Indian monsoon proxies and forcing factors also remains some uncertainties . Meanwhile, in the millennial time scale, how bi-polar temperature and AMOC (North Atlantic Meidional Overurning Circulation) influence the Indian monsoon variabilities is still argued. The lacustrine sediments simultaneously respond to the Indian monsoon cycles which modulate the precipitation variations in Tengchong, Yunnan province of the Western-south China during cycles of the climate changed from cold to warm, then are profit to analysis the Indian monsoon pattern during the last glacial period. In this proposal, we investigate the Qinghai lake sediments in Yunnan province to construct the millennial resolution Indian Monsoon proxies and discuss the couple model between the Southern and Northern Hemisphere temperature changes, as well as the North Atlantic meridional overturning circulation forcing the Indian monsoon varying. First of all, the chronology of the stratigraphy will be constructed combined the AMS14C ages of the terrestrial plants, OSL dating, tephra layer chronology and the paleomagnetic events, excursions and secular variations. Then, the effective climate proxies will be achieved including the environmental magnetism parameters (such as the ratio of hematite to goethite, which is a reasonable precipitation proxy), the detritus grain size (representing the lake level changes) and the stable isotope of the organic carbon and nitrogen. Thus, a multi-proxy record of the ISM variability using the principal component analysis will be constructed to discuss the Indian monsoon pattern and its forcing both in orbital and millennial time scale.

北半球的感热和南印度洋潜热形成的大陆低压系统及其南印度洋高压系统，是驱动印度季风在轨道尺度变化的重要因素。然而~100 kyr以来，印度季风在与两极冰量、温度变化的模式与相位差等方面，存在一定的模糊性。同时在千年时间尺度，南半球温度变化与北大西洋环流(AMOC)是否共同调控了印度季风的变率，仍然存在疑问。申请项目以位于印度季风区域的封闭火山湖泊—腾冲青海湖沉积物为对象，综合地球磁场长期变化地层学和多样品AMS14C、光释光、火山灰同位素年代数据，建立高分辨率的地层年代框架。以特征磁性矿物的比值代表的湖区干旱-湿润指数、以碎屑沉积物粒度代表的湖泊水动力变化及其湖平面升降和有机质碳、氮稳定同位素代表的湖区生态系统变化为环境参数，提取末次冰期以来千年分辨率的有效印度季风指标。讨论印度季风与南、北半球冰量和温度变化的相位差关系，进而分析在千年时间尺度，南、北极温度变化和AMOC对印度季风的调控。

项目以位于西南季风区的腾冲青海湖沉积物为研究对象，在湖泊中系列钻孔短柱状样品和两根直达基岩的长钻孔样品，综合应用沉积学、古地磁学和地球化学方法，建立反映湖水平面升降、湖泊温度和降雨变化以及湖区周围风化作用的指标，探讨了湖泊记录的印度季风变化规律及其地球磁场长期变化模型。首先沿着湖泊汇水面积区域采集表层土壤样品，测试其沉积物粒度和环境磁学参数，有湖泊沉积物进行的参数进行对比，认为湖泊的碎屑物质自汇水面积区域进入湖泊之后，没有经历显著的变化，碎屑物质的变化主要反映了搬运水动力和风化作用强度。同时在紧邻湖泊的高黎贡山从低海拔一直到山顶，分层采取表层土壤剖面，建立了土壤剖面风化物质的磁学参数与温度变化的定量关系，为利用湖泊沉积物中的磁学参数定量重建古温度变化提供标准。在由AMS14C和古地磁学方法建立的综合年代框架基础上，湖泊沉积物细粒物质含量变化表示的湖平面自110 kyr以来经历了四个阶段的变化,，即100 kyr之前的低湖面，100-28 kyr的高湖面，28-15 kyr的低湖面和全新世早中期的高湖面。末次冰期期间的高湖面，反映了腾冲地区强烈的印度季风带来丰沛的净降雨量。与湖平面的变化相比，湖泊记录的水体温度、由氢同位素反映的降雨变化和湖水平面的波动在总体趋势一致的同时，在千年时间尺度存在一定差异，温度对湖水平面的影响更为显著。湖泊周围风化作用的强度反映了由温度和降雨综合作用的湿度的变化，在30 kyr之前，风化作用的强度与温度反相关，而在30 kyr之后，则于温度正相关。风化作用强度的变化，与南极温度和印度洋表层海水温度表现出明显的相关性，当南极和印度洋水体温度较低时，风化作用反而加强，反映了南极温度降低，通过印度洋压力梯度差异，潜热释放，导致陆地区域季风的加强。 腾冲地区记录的地球磁场变化，表现出强烈的非偶极子场特征，与北美东海岸沉积物记录的相对强度呈现一定的反相位，说明北美和欧亚两个不同的地磁Flux Lobe对区域地球磁场的影响。