Biogeochemical evidence for freshwater periods during the Last Glacial Maximum recorded in lake sediments from Nam Co, south-central Tibetan Plateau

摘要

Improved knowledge of deglaciation processes during the termination of the Last Glacial Maximum on the Tibetan Plateau can provide important information for understanding deglaciations in climate-sensitive high-altitude ecosystems. Little, however, is known about this time interval because most lacustrine sediment records from the Tibetan Plateau are younger than 19,000 years. This study focused on a lake sediment record from Nam Co, south-central Tibetan Plateau, covering the interval from similar to 23.7 to 20.9 cal ka BP. We analysed the distribution and compound-specific hydrogen isotope composition (delta D) of sedimentary n-alkanes, as well as the bulk sediment TOC, TN, delta C-13(org) and delta N-15 composition, to infer lake system development. Pronounced changes in environmental conditions between similar to 21.6 and 21.1 cal ka BP, as well as between 23.1 and 22.5 cal ka BP (Greenland Interstadial 2), were inferred from increased aquatic n-alkane amounts and decreased delta D (n-C23) values within these time intervals, respectively. Freshwater inputs, which most likely resulted from enhanced glacier melting, caused these changes. Our results suggest that mountain glacier retreat on the Tibetan Plateau started earlier than previously assumed. The required energy for thawing was probably provided by temperature changes caused by reorganization of atmospheric circulation, which has also been recorded in Greenland ice records.