Changes in atmospheric circulation and glacier melting since the last deglaciation revealed by a lacustrine δD record at Ngamring Co, the upper-middle Yarlung Tsangpo watershed

Abstract

Both the Indian Summer Monsoon (ISM) and glacier meltwater are suggested to significantly influence the modern discharge of the Yarlung Tsangpo River, which is the longest river of Tibet and is an important freshwater source for the local population as well as for the large populations in India and Bangladesh in the lower reaches. Understanding past hydroclimatic variations of the region is therefore important for predicting future changes under global warming scenarios. Here we present a continuous compound-specific hydrogen isotope record of sedimentary leaf waxes covering the past 18 ka (ka = 1000 yr) from Ngamring Co, located in the upper-middle Yarlung Tsangpo range, a typical monsoonal region, with the aim of documenting hydroclimatic changes since the last deglaciation. Compared with measurements of continuous scanning XRF and magnetic susceptibility, the δD at Ngamring Co is believed to reflect atmospheric circulation and glacial meltwater input rather than direct local humidity. In detail, the record indicates relatively enriched δD values during the Late Pleistocene, suggesting the predominance of the Westerlies and limited moisture delivery by the ISM, which was associated with cooling at high northern latitudes. The fast δD depletion indicates that ISM penetrated into TP since ~10.5 ka BP (before present, the present is 1950 CE), then the gradually increased δD values suggested the ISM attenuated in phase with summer insolation during early to middle Holocene. The negative deviation of δD (>80‰, abrupt depleted at 3 ka BP) since ~6 ka BP suggested glacier melting in the catchment and ended at ~0.2 ka BP, which emphasizes the glacial contribution to the hydroclimatic change in Yarlung Tsangpo watershed. Such depletion was also observed at Aweng Co in the western TP, which suggests that a glacier meltwater contribution to lakes and rivers may be a large-scale phenomenon on the TP. Comparison of the δD record from Ngamring Co with independent paleotemperature records indicates the control of summer temperature/insolation on glacier melting may not be the most important factor during the late Holocene, and instead we propose that the mean annual air temperature and glacier volume were more crucial to the melting process. If our inference is correct, the disappearance of a glacier from the Ngamring Co basin at ~0.2 ka BP is likely to be indicative of the fate of other glaciers on the TP under ongoing global warming. However, independent reconstructions of temperature and direct glacier dynamics from the same site are needed to validate our hypothesis.