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

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 fresh-water 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 suscepti-bility, the delta D 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 delta D values during the Late Pleis-tocene, suggesting the predominance of the Westerlies and limited moisture delivery by the ISM, which was associated with cooling at high northern latitudes. The fast delta D depletion indicates that ISM penetrated into TP since similar to 10.5 ka BP (before present, the present is 1950 CE), then the gradually increased delta D values suggested the ISM attenuated in phase with summer insolation during early to middle Holocene. The negative deviation of 8D (> 8 parts per thousand, 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 delta D record from Ngamring Co with independent paleotemperature records indicates the control of summer tem-perature/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 similar to 0.2 ka BP is likely to be indicative of the fate of other glaciers on the TP under ongoing global warming. However, inde-pendent reconstructions of temperature and direct glacier dynamics from the same site are needed to validate our hypothesis.

Automated summary

The study of sedimentary leaf waxes' compound-specific hydrogen isotope record in the Ngamring Co area of the upper-middle Yarlung Tsangpo River in Tibet covering the past 18 ka reveals the significant influence of the Indian Summer Monsoon and glacier meltwater on local hydroclimatic changes. The research emphasizes the impact of the Indian Summer Monsoon on the Tibetan Plateau and highlights the contribution of glacier meltwater to the hydroclimatic changes in the Yarlung Tsangpo watershed.