西藏纳木错湖相沉积的铀系年代学研究

U-Series Chronology of Lacustrine Deposits from the Nam Co Lake, North Tibet Plateau

纳木错是西藏海拔最高的大湖 ,位于藏北内流区东南部。研究中应用铀系全溶样品的等时线技术所测定的纳木错西北岸 3个剖面的 8个富含碳酸盐湖相沉积物样品的年龄。其实验程序是 :先按矿物的粒径将样品分成若干子样品 ,全溶子样品并分别测试它们的2 3 0 Th/ 2 3 2 Th和2 3 4 U/ 2 3 2 Th比值 ,然后通过年龄等时线来校正由碎屑物带来的初始钍的影响 ,从而避免了常用的稀酸淋滤法难于验证的同位素分馏问题。测试结果显示 ,在等时线上所有数据都有良好的线性关系 ,表明所获得的年龄数据是合理的。由此可以初步认为 ,纳木错拔湖 4 7.5m以上的高位湖相沉积 ,形成于 90 .7± 9.9～ 71.8± 8.5kaBP间的晚更新世早期 ;而在拔湖 4 2 .8m、2 7.7m、2 3m、17m和 10 .7m的第六、五、四、三和二级阶地 ,则分别形成于 5 3.7± 4 .2kaBP、4 1.2± 4 .7kaBP、35 .2± 3.0kaBP、32 .3± 4 .4kaBP和 2 8.2± 2 .8kaBP左右的晚更新世中晚期。

Nam Co, the largest lake (1 920 km 2) and the highest large lake (4 718 m a.s.l.) in Tibet, is located in the southeast part of the North Tibet Plateau. Along the shores of Nam Co, late Quaternary lacustrine deposits are widely distributed. They are mainly carbonate bearing sand, silt and clay. In order to provide a better chronology for palaeoclimatic reconstruction of this region since late Pleistocene, the authors collected 8 lacustrine deposit samples from 3 profiles along the northwest shore of Nam Co for U series dating.However, many carbonates deposits, especially those of continental origin, are extremely impure and contain detrital material composed largely of silicate minerals, which is attributed to such factors as incorporation of detrital particles into the carbonate lattice, the poorly crystalline nature of some pedogenic carbonate (which facilitates post depositional U and Th exchange) and travertine, soil caliche and lacustrine sediments. These impure carbonates are heterogeneous mixtures of at least three recognizable phases: (i) the original host or matrix material, mineral grains, and rock fragments of various sizes; (ii) the non carbonate authigenic materials of different ages, such as clay minerals, zeolites, and opaline silica; (iii) the authigenic calcium carbonate component. Clearly, bulk analysis of a whole sample cannot yield a meaningful age. In order to date carbonates with U series method, Luo and Ku report total sample dissolution isochron techniques (e.g., TSD scheme) for 230 Th dating of evaporite sediments, and assume that these samples have different U/Th ratios but an identical 230 Th/ 232 Th ratio when they are deposited. In this study, the authors ground the whole sample (lacustrine deposits) to submillimeter sizes to effect homogenization and obtained the subsamples by passing the powders through sieves with opening sizes of 125 to 250 meshes. At least three subsamples were obtained from each sample, and they were treated with the total sample dissolution for radiochemical analysis. With the above mentioned total sample dissolution isochron techniques, the ages of eight lacustrine deposits from the Nam Co in northern Tibet Plateau were dated. The ages of these samples are 90.7±9.9 to 71.8±8.5 ka B P,53.7±4.2 ka B P,41.2± 4.7 ,35 ka B P,35.2±3.0 ka B P, 32.3±4.4 ka B P and 28.2±2.8 ka BP for the horizons of 47.5 m, 42.8 m, 27.7 m, 23 m, 17 m and 10.7 m (T 6～T 2) above the lake level, respectively. For the Nam Co, the isochron ages are in line with the observed stratigraphic order. It is perhaps surprising that such a good agreement was obtained in the work. It is thus concluded that the method determining the age of authigenic U is suitable for other Late Quaternary deposits and has a great potential for the dating of semi arid and arid continental environment.