青藏高原东北缘高山湖泊记录的全新世气候与环境变化

The Northeastern margin of the Tibetan Plateau lies in the marginal region of modern Asian summer monsoon. The high mountains in this region are the headstreams of the inland rivers flowing into the arid regions in northwest China. Therefore the climate and environmental changes in this region is of vital importance to the variations of water resource in NW China. However, the spatio-temporal variations of the regional paleoclimate documented by lake records appear to be quite complicated, mechanism of which is currently not clear. In addition, the interactions of the two circulation systems, the westerlies and the monsoon, and their contributions to the regional precipitation have not been studied. Considering the above scientific issues, this research project select an alpine lake, Lake Haha in NE Tibetan Plateau as the research site. Comprehensive drainage investigation, lake coring, chronology, high-resoluion analyses on proxies and regional comparison of paleoclimate records will be carried out in this research. Emphasis will be put on the precipitation-related proxies, such as the high-resolution XRF scanning for element composition of the core sediment and the stable oxygen isotopes of authigenic carbonates of both fine-grained carbonates and ostracod shells. On the basis of regional comparison of the precipitaion-related proxies, we aim to extract the information on the interactions of westerlies and monsoon circulations and their contributions to the regional precipitation during the mid- to late Holocene when the Asian summer monsoon began to weaken. This research will benefit both the scientific knwowledge on the history of the two atmospheric circulations and the management of regional water resource in NW China.

青藏高原东北部高山地区是我国西北干旱区几条主要内陆河的发源地，区域气候与环境变化对内陆地区水资源变化影响意义深远。该区现有全新世以来的气候记录时空变化过程较为复杂，其机理尚不明确，全新世以来季风和西风环流对区域降水的影响和贡献缺乏深入研究。项目选择高原东北缘高山湖泊哈拉湖为研究对象，通过流域综合考查、湖泊岩芯沉积物年代学、代用指标分析测试，恢复区域全新世以来的气候和环境历史，侧重运用岩芯高分辨率XRF元素扫描、自生碳酸盐稳定同位素等与降水有关的指标，结合现代环流格局和区域降水同位素空间变化特征，提取影响区域降水的环流信息，并通过区域湖泊与石笋同位素记录对比，探讨季风和西风环流在不同时段的空间格局和对该区域降水的影响，为掌握影响该区降水的两个环流系统的空间变化历史积累基础资料，也为干旱区水资源变化和管理提供科学依据。

青藏高原东北部现代季风边缘区末次盛冰期以来气候变化历史和季风与西风的相互作用过程尚不清楚。本项目在海拔4180米的哈拉湖深水区（65米）获得末次盛冰期以来连续的高分辨率湖泊沉积记录，流域现代过程调查明确了代用指标的环境气候意义。岩芯沉积物全有机质（TOC）年代显示该湖存在碳库效应，在1300年左右。合理评估碳库效应后建立了岩芯沉积物的年代序列。哈拉湖深水区沉积物颗粒以细颗粒为主，2至16微米粒径的颗粒含量超过50%，该粒级含量是湖泊入湖径流量、水位的综合反映；有机质碳同位素反映了受湖泊水位控制的沉水植物和浮游植物贡献的比例；碳酸盐含量受流域Ca离子输入控制，降水量多输入湖泊Ca离子多的时期，碳酸盐含量高；碳酸盐氧同位素受控于湖泊水体的同位素，偏正或偏负指示了夏季降水和冬季降水的贡献，因此可以用来指示季风（夏季）和西风环流（冬季）降水所占的比例。综合哈拉湖代用指标，末次盛冰期以来，湖泊经历了盛冰期（22~20 ka BP）的低水位、冰消期的逐渐升高（20~12.8 ka BP）、H1（冰筏事件）和新仙女木冷期（YD）湖泊水位又下降、全新世早起快速升高（11.6~10 ka BP），10 ka BP时达到类似目前的水位，之后整个全新世湖泊水位维持在接近目前水平，波动较小。该湖的水量平衡受入湖水流量和区域蒸发量控制，在高海拔、寒冷地区二者可能达到平衡，因此或水位波动较小。在入湖水量中，夏季降水和冰雪融水（冬季降水到来年夏季融化）的比例控制了湖泊水体的同位素组成和沉积的碳酸盐含量。全新世开始（11.6 ka BP）到6 ka BP之间，夏季降水占主导，同位素偏正，指示了源于亚洲季风的（经历局地再蒸发）水汽降水贡献，6ka BP以后，夏季降水逐渐减少，转变为以冬季降水（西风）为主，并于1500年前目前的状态。项目研究结果为理解现代季风边缘区过去气候变化历史中亚洲季风和西风环流相互作用的过程提供了重要证据，也为理解高寒地区水文循环和水量平衡研究提供基础资料。