Proglacial lake response to Late-Holocene glacial fluctuations in Southeast Tibet

Abstract

Global warming leads to drastic glaciers shrinkage worldwide, hence affecting the global water balance. Small mountain glaciers are the most widespread types of glaciers but have received less attention compared to larger ones, despite their importance for the regional hydrological cycle. To better understand the forcing underlying their dynamics, we investigated the glacial activity in the Hengduan Mountains in the SE Tibetan Plateau over the last 2000 years by analysing sediments from the Yunzhu proglacial lake. Five sediment cores were retrieved and dated using short-lived radionuclides, 14C, and analyses of Earth's magnetic field palaeosecular variations. A multiproxy analysis was performed on sediment cores, including geochemical, sedimentological, and hyperspectral analyses. As the sediment was relatively homogeneous and poor in organic material, geochemical ratios and pigment analysis were used to track the evolution of terrigenous inputs and lake photosynthetic activity. Sedimentation is dominated by clastic inputs, which are traced by K flux, while the authigenic components are traced by chlorophyll pigments. During the Little Ice Age (LIA), anoxic conditions prevailed at the bottom of the lake, leading to the preservation of millimetric laminae in the sediment and a bloom of purple sulphur bacteria, visible through abundant bacteriochlorophyll a pigment. We attribute this to persisting ice cover preventing water column mixing over most of the year. During cold periods such as the LIA or the Dark Ages, the relative amount of terrigenous inputs increased at the expense of lake bioproductivity. We interpret the observed erosion increases during cold periods as the consequence of glacial activity, consistent with other sparse glacial records available on the Tibetan Plateau. We show that glacial extent was mainly controlled by temperature in the past two millennia rather than by precipitation, yielding potential widespread melting in the next decades as well as more irregular water supply downstream.