青海湖碳酸盐氧同位素环境记录再认识

青海湖是我国内陆最大的闭流型水体,地处东亚季风和西风的交汇影响区,对区域降水的改变等气候变化反应敏感,其水位变化历史是研究区域季风环境演变极其宝贵和重要的环境档案。青海湖Q14B孔岩芯介壳δ18Oc变化曲线自1991年发表以来,受到国内外同行的广泛关注和继续探讨。依据近年来青海湖气候与环境演变研究的最新研究结果和个人对闭流型湖泊同位素地球化学的认识,对介壳δ18Oc变化曲线进行了重新判读并得出以下结论:14.5~10.5 ka B.P.,青海湖区气候已逐渐从干冷向温湿过渡,季风降水逐渐增加;10.8~10.5 ka B.P.,青海湖处于碳酸盐滩湖环境,湖水深度从几米演变到接近干涸;10.5~9.5 kaB.P.,季风降水增加;9.5~8 ka B.P,湖水位从此前的接近干涸演变到此间的2~8 m,δ18Oc值跌落到一个较低的位置;8~3.5ka B.P,气候条件相对稳定,湖水不断蒸发引起重同位素的富集;3.5~0ka B.P,湖水处于同位素稳定阶段。研究结果还显示,δ18Oc值的短期波动与湖泊水位短期变化关系密切且明显,即水位高低分别对应δ18Oc的低值与高值。δ18Oc值的长期变化与湖泊水位长期变化关系不明显,水位较浅时,二者几乎无关联;水位较深时,水位的长期缓慢下降自然会导致δ18Oc逐渐攀升,而水位的长期缓慢上升也可以伴随δ18Oc逐渐攀升。

Oxygen Isotope Records of Carbonates from Lake Qinghai as Environmental Archives: Re-explanation

Lake Qinghai,the largest closed water body in China,influenced by both the eastern Asian monsoon and westerlies,is sensitive to climatic changes such as regional precipitations.The history of the lake-level changes is an important environmental archive for studying regional monsoon evolution.Since its publication in 1991,the δ18Oc curve of the core Q14B of Lake Qinghai has been attracting extensive attentions and continuous discussions.New data have been published with regard to climatic and environmental evolution of the lake in the last decade.The following conclusive remarks are based mainly on an attempt to reevaluation of the δ18Oc record.Climatically,the period of 14.5 to 10.5 ka B.P.was a transitional stage from cold and arid to warm and wet conditions,during which monsoon precipitations increased gradually.The lake level was from several meters to playa status between 10.8 and 10.5 ka B.P.,followed by an increasing amount of monsoon precipitation between 10.5 and 9.5 ka B.P.During the period of 9.5-8 ka B.P.,the lake level changed from previous playa to 2-8 m,and δ18Oc became very negative.The climatic regime remained stable from 8 to 3.5 ka B.P.,which resulted in the evaporative enrichment of δ18O in the lake water reservoir.From 3.5 to 0 ka B.P.,lake water was in an isotopic steady stage.Results also suggest that the short-term fluctuation of δ18Oc is mainly caused by short-term lake-level changes,positive and negative excursions reflecting lower and higher levels respectively.Long-term changing trend of δ18Oc is not clearly correlated with long-term lake-level changes.When the lake level is low,δ18Oc shows no correlation with the lake level.When the lake is deep,the long-term slow decline of lake level will certainly lead to gradual increase in δ18Oc,but the long-term rising of lake level can also accompany the gradual enrichment of δ18Oc.