藏东南林芝晚更新世咸水湖

doi:10.1360/N972015-01109

CORC > 北京大学 > 地球与空间科学学院

	藏东南林芝晚更新世咸水湖; Late Pleistocene salt-water lake in the Linzhi area, southeastern Tibetan Plateau
	胡燕 ; 季建清 ; 徐芹芹 ; 王峰伊 ; 杨晨 ; 钟大赉
刊名	科学通报
	2016
关键词	西藏东南古湖泊 ~(14)C年龄黏土矿物结晶学特征古盐度
DOI	10.1360/N972015-01109
英文摘要	雅鲁藏布大峡谷直白以上地区分布着一系列湖相沉积阶地, 这些阶地代表的古湖泊存续年代及成因特征反映了当时大峡谷地区的地质地理环境, 特别是河流体系的演化. 采用沉积学方法, 对大峡谷入口处鲁霞地区一套湖泊沉积阶地中部与底部各选取一段剖面, 连续采集黏土沉积物, 对中部剖面样品进行~(14)C定年测试与校正, 得到45~31 ka的年龄区间. 采用X射线粉晶衍射技术(X-ray diffraction, XRD)测定两段剖面中黏土各矿物组成: 2 μm以下矿物中伊利石含量占绝对优势, 次为绿泥石, 蒙脱石和高岭石含量很少. 利用XRD图谱分析伊利石结晶学特征, 发现均具有很低的化学指数(chemical index, CI)和积分宽度(integral breadth, IB), 说明沉积的伊利石富Fe-Mg、结晶程度极好, 产出于物理风化盛行的环境. 结合样品硼元素含量, 使用Couch公式恢复湖泊古盐度大于1‰, 在4.5‰~8.5‰之间, 属于咸水湖范围, 并且两段剖面从底部到顶部盐度持续升高. 鲁霞地区湖泊阶地黏土矿物特征与古盐度揭示出: 在45~31 ka的晚更新世, 甚至更晚时期, 现今雅鲁藏布江河道发育位置存在一个古湖泊. 古湖属于封闭型咸水湖泊, 缺少外流水系, 湖泊沉积的黏土矿物形成与物理风化过程相关, 可能是湖盆周围冰川作用产物.; A series of lacustrine terraces distributed upstream of Zhibai village in the Yarlung Tsangpo Gorge have been identified during the past tens of years. The precise age and formation of the paleolake provide evidence of the geological and geographical environment of the Yarlung Tsangpo Gorge in the past, especially the evolution of the river systems. A lacustrine terrace in Luxia village in the Linzhi area was the object of this study, because its well-preserved sediments provide ideal samples for sedimentological research. Continuous sediment samples were collected from two sections of the terrace, one in the middle part and the other at the bottom. This research combines field geological investigation, radiocarbon dating and analysis of chemical and mineral compositions to study the sedimentary features of the lacustrine terrace and to reconstruct the salinity and classification of the paleolake. Fourteen samples were collected from the middle sampling profile and were dated by accelerator mass spectrometry radiocarbon dating. The measurements were conducted in the BETA Analytic radiocarbon dating laboratory in Miami and the measurements were calculated using Oxcal 4.2. The radiocarbon dating results yield an age range of 45-31 ka, during the Late Pleistocene. All the 27 samples from the two sampled sections were determined to consist mostly of clay minerals based on scanning electron microscopy observations. Analysis of the X-ray diffraction spectrum of the clay minerals demonstrated that illite is the predominant constituent, followed by chlorite, and the contents of smectite and kaolinite are very small. The low numbers of the chemical index and integral breadth of illite indicate that the illite in the samples is rich in Fe and Mg and has perfect crystallinity, leading to the conclusion that the clay minerals in the lacustrine sediments were formed mainly through physical weathering processes. Geochemical analysis of the samples shows that the concentration of boron is comparatively high, from less than 40 ��g/g at the bottom of the profile to 75 ��g/g at the top. The boron concentration of clay minerals can be correlated with the paleosalinity of the waterbody in which the clay minerals were deposited. Of the different methods to calculate paleosalinity based on the boron-salinity relationship, the validity of the Couch Formula has been established over the past several decades. The Couch Formula accounts for the different capacities of boron absorption of different kinds of clay minerals, and the method is applicable to a considerable salinity range, between 0�� and 70��. In this study, we calculated the paleosalinity of the lake using the Couch Formula, which yielded a paleosalinity of more than 1��, ranging from 4.5�� to 8.5��. In addition, the paleosalinity showed an increasing trend from the bottom of the sampling profile to the top. Salinity plays an important role in understanding the chemical features of a water body, and a salinity of 1�� is commonly used to distinguish fresh-water from salt-water lakes. The paleolake in the Linzhi area was a salt-water lake. We hereby conclude that a salt-water lake, which existed from at least 45 to 31 ka during the Late Pleistocene, was present upstream of the Yarlung Tsangpo Gorge. The salt-water paleolake was a landlocked body of water that lacked an external outflow. The clay minerals deposited in the paleolake were formed in a physical-weathering-dominated environment. The results of this study indicate that the drainage networks of the Southeastern Tibetan Plateau may have been substantially different from those of the present day. ? 2016, Science Press. All right reserved.; 国家自然科学基金; EI; 中文核心期刊要目总览(PKU); 中国科技核心期刊(ISTIC); 中国科学引文数据库(CSCD); 22; 2512-2523; 61
语种	中文
内容类型	期刊论文
源URL	[http://ir.pku.edu.cn/handle/20.500.11897/494477]
专题	地球与空间科学学院
推荐引用方式 GB/T 7714	胡燕,季建清,徐芹芹,等. 藏东南林芝晚更新世咸水湖, Late Pleistocene salt-water lake in the Linzhi area, southeastern Tibetan Plateau[J]. 科学通报,2016.
APA	胡燕,季建清,徐芹芹,王峰伊,杨晨,&钟大赉.(2016).藏东南林芝晚更新世咸水湖.科学通报.
MLA	胡燕,et al."藏东南林芝晚更新世咸水湖".科学通报 (2016).