西藏纳木错晚更新世以来的湖泊发育

The Development of Nam Co Lake in Tibet since Late Pleistocene

位于藏北高原东南部的纳木错是西藏面积最大 (192 0km2 )的湖泊和海拔最高 (4718m)的大湖。 19条剖面的水准测量结果表明 ,在纳木错沿岸 ,发育了拔湖 1.5～ 8.3m、8.3～ 15 .6m、14 .0～ 19.9m、18.7～ 2 5 .8m、2 6 .0～ 36 .9m和 38.3～4 7.6m等 6级湖岸阶地和拔湖 4 8m以上 (最高至 139.2m)的高位湖相沉积 ;在拔湖 2 7m以下 ,发育多达 8～ 30条的湖岸堤 ;而一条明显的湖蚀凹槽则集中出现在拔湖 17.5m～ 19.8m的高度上 ,与纳木错和仁错的分水垭口的高度相当。纳木错沿岸7个剖面中的 12个和邻近湖泊的 3个富含碳酸盐的湖相或湖滨相沉积的铀系全溶样品的等时线年龄测定结果表明 ,高位湖相沉积形成于 90 .7± 9.9kaBP至 71.8± 8.5kaBP的晚更新世早期 ,第六、五、四、三和二级阶地分别形成于 5 3.7± 4 .2kaBP、4 1.2± 4 .7～ 39.5± 3.0kaBP、35 .2± 3.0kaBP、32 .3± 4 .4kaBP和 2 8.2± 2 .8kaBP的晚更新世中晚期 ,而与湖蚀凹槽相当的湖滨相沉积则稍早于 2 9.3± 2 .7kaBP。因此 ,本文将纳木错的发育划分为 90～ 4 0kaBP间的羌塘古大湖 ,4 0～ 30kaBP间的外流湖和 30kaBP以来的纳木错等 3大阶段。在古大湖阶段 ,包括纳木错、色林错等藏北高原东南部的一大批现代大中型湖泊 ,是互相连通的一个大湖 ,?

Nam Co, located in the southeast part of the North Tibetan (Qiangtang) Plateau, is the largest lake (1 920 km 2) and the highest large lake (4 718 m a. s. l.) in Tibet. Leveling of 19 profiles along the shores of Nam Co shows that there are 6 lake terraces of 1.5～8.3 m, 8.3～15.6 m, 14.0～19.9 m,18.7～25.8 m, 26.0～36.9 m and 38.3～47.6 m respectively and high lacustrine sediments from 48 m to 139.2 m above the lake level. Below 27 m a.l.l., 8 to 30 shore levees and an erosional trough were disco vered, which are at the height of 17.5～19.8 m, as high as the water divide between the Nam Co basin and the Ren Co Jiuru Co basin. 12 carbonate bearing sediment samples of lacustrine and beach facies from 7 profiles along the shores of Nam Co and 3 samples from the adjacent lakes were dated by an isochron technique of total sample dissolution. U series dating shows that the high lacustrine sediments were deposited in the early Late Pleistocene from 90.7±9.9 to 71.8±8.5 ka B P, the terraces of Nos. 6, 5, 4, 3,and 2 were formed in the middle late Late Pleistocene of ca. 53.7±4.2 ka B P, 41.2±4.7～39.5±3.0 ka B P,35.2±3.0 ka B P, 32.3± 4.4 ka B P and 28.2±2.8 ka B P, respectively, and the beach conglomerate as high as the lake eroded trough, a little older than 29.3±2.7 ka B P. Based on the above mentioned data, this paper divides the lake development of Nam Co into three stages: ① the early middle Late Pleistocene Qiangtang great palaeolake before 40 ka B P, ② outflow lake between 40 and 30 ka B P and ③ Nam Co since 30 ka B P. In the stage of great palaeolake, a series of lakes, such as Nam Co, Siling Co and other big and moderate lakes in the southeast Qiangtang Plateau were joined together into a great lake whose range might overstep the water divide between the inland lakes and the Nujiang River system, but no lakes were linked with each other by rivers. The great palaeolake is named the 'East Qiangtang Lake' which might be connected with the other great palaeolakes in central-southern and southwestern Qiangtang Plateau, forming a united great 'Qiangtang Palaeolake'.