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《Acta Geosicientia Sinica》 2002-04
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U-Series Chronology of Lacustrine Deposits from the Nam Co Lake, North Tibet Plateau

MA Zhibang ZHAO Xitao ZHU Dagang WU Zhonghai (Institute of Geology and Geophysics,CAS,Beijing,100029) (Institute of Geomechanics,CAS,Beijing,100081)  
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.
【Fund】: 中国地质科学院地质调查项目 (编号DKD2 0 0 10 0 6与DKD990 10 0 1)资助
【CateGory Index】: P597
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