Sedimentation rate variations of the proglacial lake (Qiangyong Co) and its implications for glacial fluctuations over the past century, southern Tibet, China<sup>*</sup>

Abstract

青藏高原上分布着大量的大陆性冰川，其对区域及全球气候变化响应极其敏感.工业革命以来，随着全球升温速率加快（特别是北半球），青藏高原部分地区的冰川在近百年显著退缩.冰前湖沉积物是最直接的冰川变化记录载体之一，但其沉积速率如何响应冰川及气候变化，能否反演冰川进退过程却知之甚少.本文依据²¹⁰Pb和¹³⁷Cs限定藏南冰前湖枪勇错QY5沉积岩芯的年龄，计算出不同深度沉积物的沉积速率，且与前人（QY-3）的沉积速率进行对比，揭示了近百年来枪勇错流域冰川变化历史及其与气温之间的关系.结果表明，枪勇错QY5近百年来的平均沉积速率为0.21 cm/a，比湖心（QY-3）快2倍左右，但两者的变化基本同步，高沉积速率对应温度上升期，是冰川退缩的直接响应：（1）1900-1960年，枪勇错沉积速率整体增加且变幅较大，与1890-1950年之间西藏温度波动式升高相对应，反映枪勇冰川总体处于退缩状态；（2）1960-1985年，沉积速率低且变幅较小，同期气温下降，枪勇冰川退缩程度相对较低且保持平稳；（3）1985年以来，枪勇错沉积速率呈上升趋势，是全球增暖下冰川显著退缩的直接响应.在短时间尺度内冰前湖沉积速率所揭示的枪勇冰川变化主要受控于温度，降水量对冰川变化的影响较小，但冰川对温度变化的响应滞后5~10 a.由于全球变暖和冰川对温度响应的滞后，在未来几十年高原冰川的融化速率可能会加快，亚洲水塔将面临着新的挑战.;Continental glaciers on the Tibetan Plateau are sensitive to regional and global climate change. Since the industrial revolution, with the acceleration of global warming (especially in the Northern Hemisphere), glaciers in most parts of the Tibetan Plateau have shrunk significantly over the past century. The sediments in proglacial lakes are one of the direct record carriers of glacial fluctuations, but little is known about how their sedimentation rate (SR) responds to glacier and climate changes, particularly under the background of global warming. In this study, the ²¹⁰Pb and ¹³⁷Cs activities were used to constrain the age of a core (QY5) from Qiangyong Co in southern Tibet and to calculate the SRs of the core sediments at different depths. By comparing the SRs of the core (QY5) with those of the QY-3 core from central Qiangyong Co, the history of glacial fluctuations and its relationship with air temperature during the last century were revealed. The results show that the average SR of upstream of Qiangyong Co (QY5) is 0.21 cm/a, about double of the centre of the lake (QY-3), but with the same patterns. High SRs are corresponded to warm temperatures, as a direct response to the glacial retreat within the Qiangyong catchment: (1) from 1900s to 1960s, the SR of Qiangyong Co increased and fluctuated greatly as a result of the glacial retreat, corresponding to increased temperature in Tibet between 1890s and 1950s; (2) during 1960s-1985 when temperature decreased, the SR was low and stable, indicating stable Qiangyong glacier; (3) since 1985, the SR in Qiangyong Co increased gradually, as a direct response to glacier retreat under the acceleration of the global warming. Over a short time scale, the change of the Qiangyong glacier revealed by the SR in the proglacial lake is mainly controlled by air temperature, rather than by precipitation. However, there is a 5 to 10 years delay of glacier retreat to increased temperature. Due to global warming and the delayed response of glacier retreat to temperature, the melting rate of the Qiangyong glacier may accelerate in the next decades, and the Asian water tower will face a new challenge.