利用湖泊沉积物稳定同位素揭示全新世以来西昆仑山冰川变化及其影响因素

Tibetan Plateau (TP) is referred to “The Asian’s Water Tower”, because the glaciers on the TP are source water for major rivers in the East and South Asia, which bring freshwater to feed more than 1/3 of the world’s population. In 2007, IPCC suggested that the glaciers on the TP would disappear in 2035, but IPCC announced their suggestion was not based on solid evidence later. In fact, it is not clear how the TP glaciers respond to climate changes. Therefore, it is necessary to investigate the mechanism for the glacier variation under various climate background. This proposal aims to reconstruct the changes in TP glaciers using lake sediment which could provide relatively continuous records for glacier changes. Two lakes that located south of Guliya glacier in the Western Kunlun Mountain are proposed to investigate. Lake Guozha Co, a freshwater lake, is mainly fed by the meltwater from Guliya glacier, and Lake Bangda Co, a hypersaline lake, is mainly fed by precipitation. We propose to reconstruct oxygen isotope variation of lake water using ostracod, hydrogen isotope variation of lake water using short-chain lipid compounds. The water isotope variation at the precipitation-fed Bangda Co would reflect the local climate changes, while the water isotope variation at the meltwater-fed Guozha Co could reveal the input of meltwater because that the isotope ratios of meltwater are usually depleted than lake water. Commonly used proxies in lake sediments, including total organic content (TOC), dry bulk density (DBD), grain size, magnetic susceptibility (MS), and XRF scanning will also be measured, which would provide further evidence to the glacier variation. We will calculate the quantity of meltwater input to the Guozha Co using a Craig-Gordon model, using the lake water isotope, precipitation isotope ratios that based on leaf wax hydrogen isotope ratios, temperature records based on bGDGTs and alkenones. We will combine the mean annual temperature records (bGDGT), summer temperature records (alkenones), precipitation isotope ratios (leaf waxes) and the meltwater records to investigate the response of glacier melt to climate changes.

青藏高原冰川为东亚和南亚提供了重要的淡水资源，但由于对不同气候背景下冰川如何变化缺乏深入研究，目前青藏高原冰川未来变化趋势仍不明确。本项目选择位于西昆仑山地区古里雅冰帽南侧郭扎错（冰融水补给）和邦达错（降水补给）的全新世沉积物岩芯进行对比研究，重建湖水稳定同位素变化，根据两个湖泊湖水稳定同位素记录差异重建古里雅冰帽消融历史，并利用常规指标对其验证。同时利用湖泊沉积物中生物分子标志物重建西昆仑山地区年均气温和降水同位素记录，研究全新世以来该地区气候变化机制，并通过同位素模型估算古里雅冰帽消融的定量-半定量变化，进而研究西昆仑山地区冰川消融的主要影响因素。本项目创新性地对比冰融水补给湖泊与降水补给湖泊的湖水同位素变化差异反映冰川消融连续变化，不仅可揭示全新世以来西昆仑山地区冰川变化影响因素，也可为预测青藏高原冰川未来变化趋势提供理论基础。

本项目聚焦青藏高原西北部两个不同补给类型湖泊：邦达错和郭扎错。邦达错主要由降水补给，受区域气候变化影响，因此应用邦达错沉积物可以重建区域的气候变化。郭扎错主要受西昆仑山冰川融水补给，郭扎错沉积物记录可以反映叠加在气候之上的冰川融水的变化。在邦达错，利用生物标志化合物，包括支链甘油二烷基甘油四醚类化合物（branched glycerol dialkyl glycerol tetraethers, bGDGTs）、类异戊二烯GDGTs (isoprenoid GDGTs, iGDGTs)、叶蜡化合物氢同位素等可以重建青藏高原气候记录。通过郭扎错沉积物的细粒碳酸盐氧同位素、粒度、元素等多指标定量重建全新世西昆仑山冰川融水记录。结合冰川融水与气候变化记录，探讨全新世以来西昆仑山冰川消融的主要影响因素。本项目主要获得以下认识：.1、通过古地磁长期变记录对比得到邦达错与郭扎错沉积物的年代框架，并与放射性碳年龄对照，得到郭扎错沉积物的碳库年龄。结果显示碳库年龄远大于已发表的青藏高原碳库年龄，并且碳库年龄随时间变化。.2、邦达错记录显示，在青藏高原西北部西昆仑地区，早全新世期间，青藏高原西北部地区气候温暖湿润；中全新世期间，该地区气候寒冷干燥，重建的年均温记录显示中全新世温度比其它时期低~5℃；晚全新世期间，印度季风对该地区的影响较弱，而西风环流对该地区的影响逐渐增强。该地区年均温及降水在晚全新世呈现逐渐升高趋势。.3、利用郭扎错沉积物定量重建了全新世西昆仑山冰川融水变化记录。结果显示西昆仑山冰川融水量在早全新世保持最大值，从中全新世至最近1ka冰川融水量持续下降，最近1ka冰川融水量开始升高。.4、郭扎错沉积物反映的西昆仑山冰川融水变化与重建的年均温以及降水记录在早-中全新世具有可比性。西昆仑山全新世冰川融水变化可能受控于夏季积温变化。最近1ka的冰川融水增加说明，夏季太阳辐射/温度的降低固然引起冰川消融能力减弱，但在冬春降水增多冰川积累增加的情况下，总的冰川消融量仍然可能增加。