西藏纳木错及邻区全新世气候与环境变化的地质记录

Geological Records of Climate and Environment Changes during the Holocene in the Nam Co Lake and Its Adjacent Areas

综合分析了全新世期间内的多种与气候和环境变化密切相关的地质记录,结果表明该区全新世期间的气候变化可划分为3个阶段:1)约11 8～8 4kaBP期间,处于微温期和升温期,气候相对温和稍湿.2)8 4～4 0kaBP期间,为全新世气候最适宜时期或大暖期.该期间的平均气温可能比现今高约5℃,降水量比今多100～200mm.3)4 0kaBP以来,气候整体较为干冷.纳木错湖面发生持续下降,其最大下降幅度可达11 4m.冰川进退和湖面波动表明,该期间内的气候波动过程分别与新冰期和小冰期相对应,其中又各包含了3次明显的冷期,其中新冰期期间的最低年平均气温可达-6℃左右.约1970年以来,区域气候向暖湿方向转化,造成念青唐古拉山西布冰川后退约120～200m,纳木错湖面上涨了约2m.

The Nam Co Lake and its adjacent areas, including the Nam Co Lake in the north, the Nyainqêntanglha Range in the middle and the Damxung-Yangbajain basin in the south, are located in the central Tibetan Plateau. Based on the data of ^(14)C, TL, OSL and U-series dating, the geological evidence from lacustrine deposits, sporo-pollen analysis of lake terraces T_(1), glacier fluctuation, paleo-cryoturbation, palaeosol and palaeo-carbonaceous sediments indicates that the variation of palaeovegetation, palaeoclimate and palaeoenvironment during the Holocene in the Nam Co Lake and its adjacent areas can be divided into the three stages: (1) During 11'8～8'4 ka BPClimate was relative warm with a few wet phases in the previous period. The lower section of the Nam Co Lake terrace T_(1) formed. Lake level fluctuation and carbonaceous sediments were not prominent in this stage. Vegetation was Artemisia steppe with a few Pinus and Betula trees around the lake, and alpine Cypressaceae meadow on Damxung s adjacent areas. (2) During 8'4～4'2 ka BPThe Holocene Megathermal occurred in the stage. The warmest and wettest climate during the Holocene occurred. The middle-upper part of the Nam Co Lake terrace T_(1) formed. Vegetation was coniferous and deciduous broad-leaf mixed forest or forest-steppe with Pinus, Betula and Artemisia around the lake. At present, the similar vegetation is generally distributed in the area, where temperature and precipitation is higher than that around the lake. It indicates that the mean annual temperature and precipitation in the stage were respectively ～5℃ and ～100～200 mm higher than that at present around the lake. The shorelines show that the lake level rose to about 11'4 m higher than the lake level at present. In addition, black-gray carbonaceous fossil soil and river terrace T_(1), ～4 m above the river level, in Damxung-Yangbajain basin formed, and the extent and deposited rate of carbonaceous sediments increased. (3) Since 4'2 ka BPClimate was colder and drier than that during 8'4～4'2 ka BP. Vegetation was alpine Cypressaceae meadow in the mountains near Damxung. Level of the lake dropped and terraces T_(1) of the lake formed. The lake-level experienced 10～19 fluctuations between 0 m and 11'4 m above lake level. At the same time, the Xibu Glacier in the Nyainqê ntanglha Range had experienced 12 fluctuations. There were 5 fluctuations occurring during the Neoglaciation, 6 fluctuations during the Little Ice Age and 1 fluctuation since 1970 AD. Many cryoturbations, occurring in the late-Pleistocene and early-middle Holocene sediments, formed during Neoglaciation. Sand-wedge discovered in the cryoturbations of Riabu imply that the annual mean temperature was approximately 6 ℃ below zero during the Neoglaciation. Though climate was cold in the stage, it has become obvious warmer since 1970 AD. Owing to the glaciers in the Nyainqêntanglha Range prominently retreating, the level of the lake has risen about 2 m since 1970 AD.