青藏高原秋里南木湖原生硼砂、芒硝韵律层的发现及气候环境意义

Discovery and Its Significance of the Alternation of Mirabilite and Borax in Qiuli'nanmu Lake,Tibet

提要湖泊沉积物中沉积韵律是个普遍现象,但盐类矿物组成的韵律现象比较少见。盐类矿物多出现在干旱的环境中,芒硝是干冷环境下沉积的盐类矿物。本文主要根据芒硝层厚度的变化,结合原生硼砂、粘土、孢粉、碳酸盐等环境指标,讨论了秋里南木湖27.8～4.97kaBP的气候环境演化并进行了区域对比,为青藏高原是气候变化敏感区的观点提供了盐类矿物学方面的证据。研究认为,27.8～20.5kaBP湖区气候相对温暖潮湿,20.5～11.4kaBP气候寒冷,其中20.5～17.5kaBP气候极端干冷,为末次盛冰期在秋里南木湖的反映,17.5～17.1kaBP、15.9～15.2kaBP和14.1～13.4kaBP气候条件略微好转;11.4～10.1kaBP气候温暖,10.1～7.5kaBP气候寒冷,其中8.1～7.9kaBP的冷气候为"8.2kaBP冷事件"的反映;7.5～4.97kaBP气候相对温暖,但波动频繁。原生硼砂的存在说明,27.8ka以来秋里南木湖水温度不高,湖水较浅,水动力条件稳定。

The Qiuli＇ nanmu Lake is a dried saline lake in central Tibet with many thick mirabilite layers. Saline minerals are suggested to be products of dry and cold environment compared with clays. A detailed mineralogical investigation was carried out with mainly thick mirabilite and thin borax layer and thin clay layer altemately. The stratigraphic variation in the mineralogy of salt lake sediments is an indicator of paleoclimatic fluctuations. Based on the variations of mineralogy, pollen and carbonate ,it is suggested that the climate was warm and wet in 27.8-20.5 ka BP,cold and dry in 20.5-11.4 ka BP, warm and wet 11.4-10.1 ka B, cold and dry in 10.1-7.5 ka BP, warm but changed frequently in 7.5-4.97 ka BP. The Last Glacial Maximum may be appeared in 20.5-17.5 ka BP. The warm pluses were at 17. 5-17.1 ka BP,15.9-15.2 ka BP and 14.1-13.4 ka BP. The ＂8.2 ka BP cold event＂ appeared at 8.1-7.9 ka BP. Borax, a mineral appeared in low temperature, steady and shallow brine water, suggested the same environment during 27.8-4.97 ka BP in Qiuli＇ nanmu lake. The changes of summer monsoon led to the climatic conditions in the area. This work gives a mineralogical evidence for the view that Tibet is a sensitive area of climatic changes.