Franziska Günther

Reduced early Holocene moisture availability inferred from δD values of sedimentary n-alkanes in Zigetang Co, Central Tibetan Plateau

An 885-cm-long laminated sediment core was retrieved from Zigetang Co, a non-glacial meltwater-fed lake in the central Tibetan Plateau, and analyzed for n-alkanes as well as their hydrogen isotopes to trace the regional climatic and environmental history spanning the last 13.8 cal. ka BP. The short-chain n-alkanes C15/16/17, likely derived from aquatic algae, plankton, and photosynthetic bacteria, dominate the n-alkane composition in this lake. This unusual distribution pattern might be attributed to the meromictic lake system that is characterized by a rapid salinity increase and an abrupt decrease in dissolved oxygen in the lake water with depth, as well as anoxic conditions at the bottom. n-Alkane indicator ratios (e.g. carbon preference index (CPI), average chain length (ACL), and aquatic/terrigenous ratio (ATR)) and δD values reveal that higher effective moisture availability at Zigetang Co occurred in the middle (5.8–2.7 cal. ka BP) rather than the early Holocene. This contradicts palaeoclimate records from neighboring lakes (e.g. Nam Co, Siling Co, and Paru Co). We suggest that in the Zigetang Co catchment where glacial meltwaters are not available, the temperature-induced evaporation would outweigh the monsoonal precipitation and, therefore, result in lower effective moisture displayed by the increasing δD values during the early Holocene. The local recycling of air masses could also have an important impact on the lake level and moisture availability and cannot be excluded. Our record provides further evidence for the complex relationship of insolation-induced temperature, evaporation, and precipitation affecting the regional climate changes on the Tibetan Plateau.

Rapid northward shift of the Indian Monsoon on the Tibetan Plateau at the end of the Little Ice Age

Variations in the Indian Monsoon (IM) and Westerlies (WS) significantly affect the climate on the Tibetan Plateau (TP) and have widespread ecological and socioeconomic impacts on the whole of Asian society. So far, however, the rate and magnitude of changes in the IM have still remained unclear. Here we report for the first time that the IM rapidly shifted northward at the end of the Little Ice Age (LIA). We used sediment proxies for humidity and moisture sources from the Taro Co lake, which is located in the transition zone between the WS and IM. Our comprehensive survey of climate records for the TP and its peripheral mountain ranges revealed that the northern boundary of the IM (i.e., the southern boundary of the WS) lay along the southern slope of the Gandise Range (~29.5° N) in the late LIA. In contrast, it passed quickly over the Gandise Range by at least 1.5° in latitude at the end of the LIA. Our results suggest that this rapid climatic shift was potentially triggered by the counteracting effects of blocking by the TP and its marginal orography, which hindered the northward movement of the IM, and the pulling thermal gradient of the TP.