Glacial-interglacial lake hydrochemistry in step with the Pleistocene Indian summer monsoon at the southeastern Tibetan Plateau

The lake state and chemistry are sensitive to climate change at short timescales, but it is unclear how lake hydrochemistry responds to monsoonal climate, especially over glacial -interglacial cycles, owing to lack of welldated and continuous sedimentary records. The types and geochemical compositions of authigenic carbonates from lacustrine sediments can be used to reconstruct past hydrochemistry. Yet, little is known about the significance and influencing factors of the Mg/Ca and Sr/Ca ratios of authigenic carbonates over glacial -interglacial cycles. Here, trace -element data of a 666 m long sediment sequence from the Heqing basin, a paleo-lake situated at the southeastern margin of the Tibetan Plateau, provide detailed glacial -interglacial hydrochemical conditions and their association with Indian summer monsoon (ISM) evolution during the entire Pleistocene. In interglacials, variation in both Mg/Ca and Sr/Ca ratios can serve as proxies for salinity as a result of precipitation of low-Mg calcite, constrained by enhanced ISM intensity and catchment weathering product. Instead, in glacials, high Mg/Ca and Sr/Ca ratios reflect high -Mg calcite and aragonite precipitation under low lake level conditions. There is a nonlinear response of the Sr/Ca ratio of lacustrine sediment to salinity variation when high -Mg calcite and aragonite precipitate in a hydrologically closed and evaporative lake. Based upon the sedimentary Sr/Ca and Mg/Ca ratios in the Heqing paleo-lake, the lake hydrochemistry during the Pleistocene experienced obvious glacial -interglacial ISM cycles at the life span of the lake itself: (1) in the old interval (2640-1840 ka), the salinity was low with cycles at the early stage of lake development owing to strong ISM; (2) in the middle interval (1840-920 ka), the lake was at a balance state and its salinity was more constrained by weathering solute, rather than ISM; and (3) in the young interval (920-130 ka), the salinity fluctuated with large amplitude over glacialinterglacial cycles and subsequent increase paced the weakened ISM. This study highlights the sensitivity of the types of lacustrine authigenic carbonates and lake chemistry to ISM changes over glacial -interglacial cycles.