西藏纳木错地区约120ka BP以来的古植被、古气候与湖面变化

Palaeovegetation, Palaeoclimate and Lake-level Chang since 120 ka BP in Nam Co, Central Xizang

西藏纳木错湖相沉积的U系和^(14)C测年结果表明,湖泊沿岸的6级湖岸阶地及拔湖约48～139.2m的高位湖相沉积形成于约120ka BP以来的晚第四纪。本文根据该套湖相层的发育和其中的孢粉组合特征对纳木错地区约120ka BP以来的湖面变化与古植被、古气候变迁进行了探讨。结果表明,纳木错地区晚更新世以来经历了频繁的湖面波动、气候的冷暖与干湿变化以及森林-草原与草原植被的交替演化。其总体特征是:约115.9ka BP时,纳木错湖面最高。在116～78ka BP期间,该区气候温和凉爽或温和偏湿,植被以疏林草原与森林草原或森林的交替出现为特征,湖面经历了较大幅度的波动,但基本保持在拔湖140～88m之间。在78～53ka BP期间,该区气候干冷,植被以疏林草原为主,湖面大幅度下降,并在拔湖约36～48m之间波动。约53～32ka BP期间,气候转为温暖偏湿或温暖湿润,湖面波动于拔湖约15～28m之间,波动较为频繁。与阶地的发育相对应,该时期内包含了3次明显的暖期和湖面波动过程,区域植被主要以松、蒿、桦为主,为含一定量的冷杉的森林。其中36ka BP左右气候最温暖湿润,区域内可能出现针叶林或针阔混交林。约32～12ka BP期间,该区气候最为干冷,古植被以草原和疏林草原为主,湖面再次发生较大幅度的下降,最低可至拔湖约8m处,但通常维持在拔湖?

Nam Co is the biggest and highest lake (4718 m a. s. 1. ) in the central Xizang (Tibet) Plateau. The survey of level shows that there are 6 lake terraces and highstand lacustrine sediments from 48 m to 139. 2 m above lake level. Age results of U-series and 14C indicates that the highstand lacustrine sediments, lake terraces of Nos. 6, 5, 4, 3, 2 and 1 were formed in ca. 115. 9~71.8ka BP, 53.7 ka B. P. , 41. 2 ~ 39. 5 ka BP, 36. 1 ~35. 2 ka BP, 32. 3 ka BP, 28. 2~11. 2 ka BP and 11. 2~4. 2 ka BP, respectively. Based on the data of age results, terraces, lacustrine sediments and sporopollen analysis, the paleovegetation, paleoclimate and lake-level fluctuation of Nam Co since approximate 120 ka BP are discussed in this paper. The change of paleovegetation, paleoclimate and lake-level fluctuation of Nam Co since the Last Interglacial Stage can be divided into the five stages: (1) In 116 ~ 78 ka BP, lake-level fluctuated between 140 m and 88 m above lake level. The climate was warm and slightly moist. The vegetation was forest-steppe and steppe with sparse-forest. (2) In 78 ~ 53 ka BP, lake-level strikingly dropped and fluctuated between 48 m and 36 m above lake level. The climate was colder and drier than that earlier. The vegetation was steppe with a few trees. (3) In 53~32 ka BP, lake-level fluctuated between 15 m and 28 m above lake level. The climate was warmer and wetter than that of the present and there were 3 stages of warm climate corresponding to the lake-terraces of Nos. 5, 4 and 3. The vegetation was forest and forest-steppe. Particularly, in the period of 36 ?35 ka BP, coniferous and deciduous broad-leaf mixed forest occurred. It shows, in the period, the climate was the warmest and wettest during the Last Glacial Stage and also was warmer and wetter than that of the present. (4) In 32 ~12 ka BP, lake-level strikingly dropped and fluctuated between 17m and 12m above lake level. The climate was cold and dry. The vegetation was steppe or steppe with sparse trees. The lowest lake-level is 8 m above lake level in the period. (5) In 11. 8~4. 2 ka BP, lake-level fluctuated between 2 m and 9 m above lake level. The climate was warm and wet. The vegetation was forest and steppe with a few trees. Especially, coniferous and deciduous broad-leaf mixed forest occurred during 8. 4 ~ 4. 2 ka BP and the climate is the warmest and wettest during the Holocene. The lowest lake-level is under present lake level in the period. A comparison of the climate change and lake-level fluctuation of Nam Co with the data of oxygen isotopic curve from the Guliya ice core in the West Kunlun Mountains and the ice core of GRIP2 shows the cooling and warming change of climate in the Nam Co area correspond to the climate change of the Tibet Plateau and the northern hemisphere. Especially, the striking lake-level dropping revealed by lake terraces formed is broadly associated with the Henrich events.