Shen Yongping, Xu Daoming
CoNag Lake and ZigeTangco Lake are two bigger modern lakes in Amdo area, Tibet.The former is an opening lake basin and the later is closed one. The records of lacustrinedepositional stratigraphy,pollen data,hydrochemical analysis and 14C datings from theCoNag Lake and the ZigeTangco Lake cores and from sediments cropping out in the basinsby the Sino-German Joint Expedition to Tibet 1989 give information on major changes inboth regional environments and local hydrology.Both lake basins are tectonic-glaciated basins and the lacustrine deposits are generallythinner and younger. The lakes might have only existed since 35 ka B.P.. The underlying stra-ta is Tertiary fluviallacustrine sediments with glaciated deformational structures. In extensivelake period ofearly stage, two lakes were connected together, the lake level was 130-160 mmore higher than these of present. After that, the unifed lake had been separated and the leveldeclined gradually by a reversal event toward dry. However, a rapid lake-level rises with rela-tive stable level height ocurred in the ZigeTangco Lake at ca.20-17 ka B.P.. After ca, 17 kaB.P., an abrupt dropping of the lakelevels started reappeaing with the long-term trend to-ward dry. The salinity of the ZigeTangco Lake water trended to increase and the sedimentspacked with calcium ocurred. And ten rimsand bars around the ZigeTangco Lake show thateach lakelevel declined event leads to drier conditions than the preceding one, whereas succes-sive humid pluses have lower and lower amplitude.
