枪勇冰川冰前湖沉积物记录的全新世冰川与气候变化

Paleoglacier variations in the Tibetan Plateau have been reconstructed mostly by dating of moraine-ridge sequences. However, the moraine-stratigraphic reocrds are always incontinuous and evidences of older glacier advances can be eroded by larger and younger advances. Meanwhile, several classic climatic records always be used to infer the foring machanisms of past glacier changes, without considering the spatial differences of climatic changes.Thus,independent records of past temperature and precipitation in the proglacial areas are needed. Dlaciolacustrine sediment records have been more regularly used to obtain continuous and high-resolution glacier records in Euroean and North America than in the Tibetan Plateau, where mutiproxy reconstructions based on proglacial lake sediments are still rare.In this study,we will use the glaciolacustrine sediment records of Qiangyong Co in the southern Tibetan Plateau to obtain continuous and high-resolution records of glacier,temperature and precipitation.Radiocarbon dating results of pollen concentrates and macrofossil plants will be used to obtain the ages of long-core sediments. We use grainsize, branched GDGTs, hydrogen isotope of n-alkanes and other proxies to reconstruct glacier, temperature and precipitation changes during the Holocene in this area. Based on mutiproxy comparison and regional comparison, we try to reveal the response patterns of the glacier in this region to climatic forcings. This study will provide base data to understand the forcing mechanisms of glacial and climatic changes of the whole Tibetan Plateau.

目前青藏高原古冰川变化重建主要依赖于冰碛物序列，然而冰川地貌证据通常是不连续的，而且后期更大规模冰进可能侵蚀掉以前的冰进证据。其次，该地区古冰川进退驱动因素的解译通常依靠与少数几个经典古气候记录（如冰芯与石笋）对比，忽略了气候变化可能的空间差异性，因而亟需在冰缘区多地点开展独立的古温度、古降水重建研究。冰前湖沉积物已经被广泛用于重建欧美地区连续的高分辨率古冰川与古气候变化，然而基于青藏高原冰前湖沉积物的古冰川、古温度、古降水的多指标重建工作还较缺乏。本项目以青藏高原南部枪勇错冰前湖沉积物为研究对象，利用孢粉浓缩物、植物残体14C测年等手段确定岩芯年龄，基于粒度、支链GDGTs和叶蜡氢同位素等指标重建连续的古冰川变化及古温度、古降水记录。通过同岩芯多指标对比及区域对比，揭示该地区古冰川变化的气候驱动机制，探讨局地因素、外部驱动因子的可能影响，为理解整个青藏高原古冰川与古气候变化机制积累数据。

本研究选择青藏高原南部枪勇冰川的冰前湖作为研究对象，开展了湖泊基本特征考察和沉积物研究工作。研究发现，该湖的水下地形、湖水理化性质、湖水同位素及湖水悬浮物粒度特征均表现出冰前湖的特殊性，与青藏高原其它类型湖泊有明显不同。本研究通过对该湖QYL09-4岩芯的多方法（植物残体、孢粉浓缩物、全有机质）放射性碳测年对比，发现陆生植物残体是青藏高原湖泊较为可靠的测年材料，并提出一个重建古冰川融化强度的新指标（“老孢粉效应”，即沉积物孢粉年龄与沉积物真实年龄的差值）。依据该研究提出的概念模型，现代孢粉通过大气干湿沉降进入冰川积累区，并随着冰川流动逐渐变成老孢粉储存在消融区的老冰之中，冰川的加速融化会释放更多的老孢粉进入冰前湖，导致“老孢粉效应”值增加。本研究重建了该地区近2500年的冰川融化历史，与达索普冰芯氧同位素记录以及青藏高原、欧洲的古冰川记录等具有较好的一致性，说明北半球温度、西风环流活动可能是高原季风区冰川晚全新世百年尺度波动的主控因素。对比发现，近代的冰川融化强度达到过去2500年以来最强，超过了历史上的中世纪暖期和罗马暖期，这意味着近代全球变暖已对青藏高原冰川构成了极大威胁。该项目的核心成果以第一标注发表在地学著名期刊Geology杂志上。