西藏纳木错深水湖芯反映的8·4ka以来气候环境变化

ENVIRONMENTAL CHANGES REFLECTED BY CORE SEDIMENTS SINCE 8.4ka IN NAM CO,CENTRAL TIBET OF CHINA

利用地震剖面仪和回声测深仪调查了纳木错大部分湖区的水深和沉积物厚度分布,并在湖区东部水深60m的湖盆利用PISTON采样器获得一个332cm长的钻孔。对该钻孔进行了12个AMS14C年代测定,并进行了日历年龄校正和“碳库”效应估算,根据沉积物平均粒径建立了整个钻孔的深度-年代模式。对该钻孔进行了有机碳、总氮、正构烷烃、粒度、元素地球化学、碳酸钙和矿物等环境指标的分析,结果显示8·4ka以来湖区环境变化可以分为明显的3个阶段。早期约8400～6400aB·P.,以温度缓慢下降为主,但在8100～7800aB·P.出现一次显著的冷干事件;中期为6400～2900aB·P.,其开始显示了温暖湿润的环境特点,在6000aB·P.左右出现最强的暖湿特征后其后期转向冷干,在3000～2900aB·P.达到寒冷和干旱的低谷,反映了新冰期时期的温度强烈下降和降水的分配不均;晚期从2900aB·P.到现在,尽管温度出现回升,但总体上显示了向冷干波动变化的趋势,期间第一次降温在1800～1600aB·P.,之后温度回升后在600～300aB·P.再次下降,前者反映了公元初期的降温,后者则是小冰期的反映。

The Tibetan Plateau is a sensitive area to global changes due to its high elevation and frail surface ecosystem. This huge plateau induces and enhances the Asian monsoon that influences the plateau itself, East China, and even the whole Asia. Pursuing the changing history of monsoon and its impact in this environment-sensitive area is a key content for understanding the rules of climatic changes. Here, we present the initial results from a core sediment from Nam Co, Central Tibet to elucidate the Holocene environment changes in this area. By using echo-seismic profiler and hydro-echo-sounder, we investigated the water depth distribution of the most areas of the lake. One 332-cm long PISTON core was successfully taken at the east part of the lake at the site of 60m water depth. 12 AMS ^14C dating data from top to bottom established a good time sequence of this core. TOC, TN, n-alkanes, grainsize, Fe/Mn, Sr/Ba, and CaCO3 content were analyzed with the sample intervals from lcm to 10cm. Results showed that since ca. 8400aB. P. the environmental change in this region can be divided into three stages. The early stage is from ca. 8400 ～6400aB. P. that is characterized by slight shifting from warm to cold with a cold-dry process during 8100 ～ 7800aB. P. The middle stage was from 6400 ～ 2900aB. P. , during which the climate changed from warm-humid to cold-dry. However, around 2900aB. P. the climate once was slightly cold but strongly dry. The late stage was from 2900aB. P. to present that showed a general tendency toward cold-dry. The first temperature decline was around 1700 ～ 1500aB. P., following which the temperature was once increased. But the temperature decreased again around 800 ～ 300aB. P., which might be the results of the Little Ice Age.