8.4 ka以来纳木错湖芯介形类组合的环境变化意义

对青藏高原中南部纳木错长332cm的NMLC1孔湖芯研究发现，沉积物的介形类动物群计有6属15种。介形类属种生态特征和组合变化分析表明这些介形类对环境条件具有敏感性，其组合能够很好地反映过去环境变化的特征。结果表明，8．4ka以来具有三个不同的环境变化时期：早期在8400～6800aBP，湖泊由浅向深发展，环境具有相对冷湿的特征：中期在6800～2500aBP，湖泊深度逐渐加大，环境经历了暖湿-冷湿-冷干的变化．其中由冷湿向冷干的转化奠定了纳木错现今环境条件的基础：晚期在2500--9aBP，湖泊深度继续增加，这个时期的较早阶段，继续保持了前一时期的冷干特点，但湖水盐度可能开始增加．较晚阶段的冷干化加剧，陆面流水的活跃性大大降低。研究发现,纳木错NMLC1钻孔介形类黑色壳体的高峰值与介形类的最大生产量相一致，并且与沉积水动力条件增强相适应．指示了这些黑壳的产生与介形类的大量繁殖处于同一阶段．并且主要为异地搬运为主。钻孔中出现大量Candona幼虫壳体．其原因可能与沉积环境的水动力条件迅速改变有关。

Ostracodes Assemblages and Their Environmental Significance from Lake Core of Nam Co, Tibetan Plateau

A 332-cm long lacustrine core was drilled in the middle south of Tibentan Plateau.From the core,15 species of ostracodes belonging to 6 genera have been identified.According to the changes of the ostracodes assemblage and the ostracodes ecological characters,which are sensitive to the changed of environment,three stages can be distinguished as follows:The early stage was from 8400 to 6800 aBP,during which the temperature descended in fluctuation which matched with the unstable cooling event of earlier Holocene,the climate was cold-humid,and the lake depth changed from shallow to deep.The middle stage was from 6400 to 2500 aBP,during which the climate changed from warm-humid to cold-humid and then became cold-dry,and the lake depth was becoming deep step by step.Weather pattern shifting during this period,from wet-cold to dry-cold,had been modifying and finally built up modern environments in Nam Co.The late stage(2500 aBP-present) showed a trait of lake depth increasing.At the earlier time of this stage,the climate was still cold-dry,which had the same trend of the end of the middle stage,however,the salinity of the lake increased;at the later time of this stage,the climate showed a general tendency toward cold-dry,as well as the hydrological condition of surface flowing water became faint.The concurrence of black shells percentage and the biggest ostracodes content,as well as the increasing of sedimentary hydrodynamic condition,indicated a post-mortem reworking.The abundance of Candona juvenile shells reflected juvenile mortality as a result of the rapid onset of unfavorable conditions such as catastrophic hydrological condition change.