Cosmogenic 10Be and 26Al dating of palaeo-lake shorelines on Tibet

The Tibetan Plateau is covered by lakes between 74-98°E and 28-40°N which have a present total surface area exceeding approximately 30,000 km2. Most of these high altitude lakes are surrounded by conspicuous sequences of regressive shorelines as remnants of elevated lake levels in the past. Some remnants would be erased during repeated increase-decrease of lake levels. However, the highest shorelines would survive the lake level changes and record the time when the lakes reached their largest sizes. We have dated, using in situ produced cosmogenic nuclides 10Be and ^{26}Al, lacustrine shorelines around 8 lakes on Tibet. Two lakes located at the south of Yarlung Zangbo river were overflowed at high lake levels; the lake Cuocuolong became closed at 3.4-3.7 ka BP and were shrunken over 120 m vertically since then. The other six lakes are developed closely in Qiangtang, north of Yarlung Zangbo river. The exposure ages of lacustrine shorelines are over 150 ka for Dangreyong and siling lakes, and this is the first report of such old lake shorelines on Tibet. Our results show that high lake shorelines occurred at the early part of MIS 3 for Dajia lake, at Last Glacial Maximum (LGM) for Dangxiong lake and after LGM for Zharinanmu lake. From the limited data we have not seen synchronous high lake levels on Tibet.