青藏高原兹格塘错沉积记录的全新世水位变化事件及其原因初步研究

A preliminary study of the Holocene lake level changes and their causes derived from the sediment record of Zigetang Lake,Tibetan Plateau

兹格塘错是一个内陆封闭型湖泊,位于藏北高原腹地,处于西南季风作用边缘地带。由于流域内无冰川分布,湖泊补给主要靠大气降水,因此水体的扩张与收缩能够直接反映西南季风的变化。半干旱气候地区湖泊沉积物碳酸盐含量能够敏感地反映水体的扩张与收缩。兹格塘错沉积物碳酸盐含量高,与可溶盐(氯离子与硫酸根离子)含量变化结合能更好地揭示水体的演化阶段。分析结果表明,在9.3～8.9calkaBP、8.3～7.8calkaBP、5.0～4.7calkaBP、4.0～3.8calkaBP和3.1～2.7calkaBP碳酸盐含量出现大幅度下降,指示湖泊淡化、水位升高。但在3.8calkaBP左右碳酸盐含量和可溶盐(氯离子与硫酸根离子)含量同时出现峰值,指示出湖泊水体盐度升高、水位出现下降;近1calkaBP以来,碳酸盐与可溶盐含量都呈现逐渐下降趋势,表明湖泊水体逐渐淡化、水位缓慢上升的过程。但近100a来可溶盐含量上升,指示出湖泊水体的不断浓缩和水位下降过程,这和近百年的气候暖干化过程是一致的。兹格塘错沉积岩心碳酸盐含量全新世以来5次极低事件,有力地证明青藏高原西南季风在全新世期间的不稳定性。

The Zigetang Lake is an enclosed lake,located in the hinterland of the North Tibetan Plateau and within the marginal region of southwest monsoon.There is no distribution of glacier in the catchment,and the precipitation controlled by the southwest monsoon is the main water supply for the Zigetang Lake.Therefore,the expansion and contraction of lake water area reflects the changes of southwest monsoon directly.The carbonate concentration of enclosed lake sediments in semi-arid region indicates the changes of the lake level sensitively.The carbonate concentration in the sediment core of Zigetang Lake has generally been in high value but there were several abrupt lowering stages during the Holocene period,which could reveal the evolution stages of the lake level better when associated with the changes of soluble salt（Cl-and SO^2-4）concentration in the sediment core.The analysis results for a 740 cm-long core drilled in the central Zigetang Lake show that this core has completely covered the whole Holocene period and that the carbonate concentration decreased greatly during 9.3-8.9 cal ka BP,8.3-7.8 cal ka BP,5.0-4.7 cal ka BP,4.0-3.8 cal ka BP,and 3.1-2.7 cal ka BP,respectively,indicating the desalination of the lake and the rise of the lake level.The high value of carbonate and soluble salt（Cl-and SO2-4）content around 3.8 cal ka BP indicates the increase of salinity and the decline of the lake level.The content of carbonate and soluble salt decreased gradually since 1 cal ka BP,which indicates the gradual desalination of the lake and the slow rise of the lake level.However,the increase of soluble salt content in the past 100 years indicates the contraction of lake water area and the decline of the lake level,which is in accord with the warming and drying process of the climate.The fluctuations of southwest monsoon during the Holocene can be derived from the five extremely low value events of carbonate concentration in the sediment core from the Zigetang Lake.