Increasing lake levels on the central Tibetan Plateau since 1920 CE inferred by a sub-fossil chironomid record from Shen Co

Lakes on the Tibetan Plateau are especially vulnerable to global warming and increasing temperatures but are also sensitive to changes in the atmospheric circulation such as the Westerlies and the Asian Summer Monsoon, which are main drivers of precipitation on the Plateau. Shallow lake environments in such high-altitudinal areas, which are not directly influenced by meltwater supply, are excellent study sites to determine changes in precipitation and evaporation. Here, we present a 300-year high-resolution chironomid record from the high-altitude (> 4,733 m a.s.l.), saline (9 g L-1) and shallow (~ 5 m water depth) lake Shen Co (N 31° 00’, E 90°29’), situated on the southern part of the central Tibetan Plateau. We combined chironomid assemblages with sedimentological, mineralogical and geochemical data from a short sediment core (37.5 cm) to detect hydrological changes since 1830 CE. Our study revealed three different periods in Shen Co: (1) from 1830 until 1920 CE sediments were void of chironomids, suggesting dry conditions leading to low lake levels, high salinity resulting from low runoff and high evaporation rates, supported by increasing Mg/Ca and Sr/Rb ratios of the sediments; (2) a humid phase characterized by the appearance of Acricotopus indet. morphotype incurvatus from 1920 until 1950 CE, indicating rising lake levels caused by higher runoff and decreased evaporation, also supported by sediment analysis with increasing TOC and Ti as well as a decreasing Ca/Ti ratio; and (3) a continuous water level rise from the 1950s onwards with a lake level maximum as well as high macrophyte growth since the beginning of the 21th century, supported by the dominance of Procladius and the phytophilic taxon Psectrocladius sordidellus-type. We compared our results with paleoclimate records from the Tibetan Plateau, based on e.g. ice core δ18O, pollen, tree rings, indicating warmer and wetter climate conditions on the central Tibetan Plateau during the last ~ 100 years. Our findings highlight that chironomid records from shallow lake environments are excellent indicators of lake level variations as well as changes in macrophyte vegetation.