青藏高原大湖期

The Last Greatest Lakes on the Xizang (Tibetan) Plateau

青藏高原湖盆中古湖岸线分布广泛 ,从最高湖岸线的分布确定的大湖期湖泊面积 ,一般比现代湖泊面积大数倍至十多倍。据高原不同地区的十多个湖泊沉积测年数据分析 ,大湖期的年代大致相近 ,在 40～ 2 5ka BP居多 ,有的可能延续至 2 0 ka BP,与深海氧同位素 3阶段、末次冰期间冰段相当。该时期高原环境特别湿润。大湖期的形成与该时期亚洲夏季风特别强盛有关。

The paleo lake shorelines and lake deposits are widely distributed in the lake basins on the Xizang (Tibetan) Plateau. Deduced from the highest lake shorelines, the lakes area during the last Greatest Lake Period (GLP) was several to tens times larger than their present area. However, there are still divarication on its age, e.g. some thought GLP was formed in the early and middle Holocene, some thought GLP in the Last Glacial Interval, also some thought in the early middle period of Pleistocene. There is no final conclusion related to the last GLP on the total plateau. According to the analyses of the lake deposits and 14 C dating from 12 lakes such as Tianshuihai Lake, etc., the GLP on the Xizang Plateau was formed generally from 50 ka BP to 30~20 ka BP, even some appeared in a little newer time. Because of the difference of climatic environment and lake landform in different areas, there are also differentiation on the end age of GLP and ratio of paleo and present lake area. The GLP in the northern part of the Plateau and the middle west part of Qiangtang Plateau approximately ended at 25 ka BP while the ratio was bigger between paleo and present lake area. It was a little later to the closed time of the lakes in the south and east part of the Plateau. Some big lakes even were exterior lakes in the late Holocene while the ratio was smaller between paleo and present lake area. The GLP that represented a special warm humid stage in Xizang area generally appeared from 40 to 25 ka BP. This corresponded to the third stage of marine oxygen isotope records and Last Glacial Interval. During this period, the lake areas were bigger and lakes levels were higher than those in the best period of the Holocene. Reflected by the relics of high lake level, the climate in the Xizang area was abnormally warm in the Last Glacial Interval compared with the records of environmental change from Antarctica and Greenland. This was also documented by the records from ice core and lake deposits, even reflected in the Tengger Desert to the north of the Plateau and north China area. It may be related to two factors: one was that the incidence solar radiation between 50°N 30°S was higher than that in the high latitude area in GLP; the other was that the powerful Australia high pressure might get across the equator line in that period. This caused the strong summer monsoon on the Plateau that brought plenty of water content from the south to the north interior land.