库赛湖沉积物中正构烷烃的分布特征及古环境意义

Distribution of N-Alkanes in Sediment Core and Implications of Paleoenvironments of Kusai Lake

正构烷烃作为一种常见的生物标志物,其结构和组成的改变可记录气候环境的变化。位于青藏高原北部的库赛湖是研究古气候、古环境的重要载体,为了探讨库赛湖地区正构烷烃的分布特征及其古环境指示意义,在库赛湖采集了一根长约4m的沉积柱岩心。结果表明,库赛湖沉积柱中正构烷烃的碳数范围在C_16-C_33之间,奇碳优势明显,整体呈现以高碳数占优势的组成特征,揭示库赛湖沉积物中正构烷烃主要来源于陆源高等植物。库赛湖沉积柱中所有样品的主碳峰（C_max）都是C_31,进一步显示陆源输入中以草本植物输入为主。据正构烷烃参数,库赛湖地区过去2 500cal a BP来的环境气候变化具3个阶段。阶段1（2 420～2 000cal a BP）,总正构烷烃（TNA）质量分数呈下降趋势,碳优势指数（CPI）呈上升趋势,说明阶段1气候较冷,nC_27/nC_31和P_aq呈先降低后回升的变化趋势,说明湿度先降低后略有回升;阶段2（2 000～850cal a BP）,有机物变化趋势不明显,CPI值呈下降趋势,说明气候较暖;阶段3（850cal a BP至今）,nC_27/nC_31值和P_aq增加,CPI值降低,说明该阶段气候整体较暖湿。

N-alkanes, as one of common biomarkers, can record environmental and climatic changes on the basis of variations of their structures and compositions. Kusai Lake locates in the northern part of Tibet Plateau and is sensitive to climate change. In order to reconstruct the history of the environmental changes in this area, a 4 m sediment core was obtained and a systematic study of the n-alkanes in the sediments was performed. Results showed that n-alkanes ranged from C_16 to C_33 in Kusai Lake, and were dominated by long-chain n-alkanes. The odd-over-even predominance of the n-alkanes suggested that they mainly origi- nated from terrestrial higher plants. The C_max was C_31 in each sample, which further suggested that woody plants were the domination in terrestrial higher plants. Based on the n-alkane and loss on ignition （LOI）, we proposed that Kusai Lake experienced three phases during the past 2 500 cal a BP. The increase of the carbon preference index （CPI） and total n-alkane concentrations （TNA） in phase 1 （2 420--2 000 cal a BP） corresponded to an overall cold period at the early stage. Both the ratio of nC_27/nC_31 and P_aq（aquatic plant n-alkane proxy） showed decreasing and then increasing trends, indicating the increasing humidity af- ter a decline. The decrease of CPI and the relatively small change of the organic matter indicated warm conditions during phase 2 （2 000-850 cal a BP）. During the third phase （850 cal a BP to present）, both nC_27/nC_31 and Paq increased while CPI decreased, indicating a warm and humid climate.