中文摘要：

有机碳同位素已经被广泛的用于湖泊环境变化研究，特别是沉积物中的单体有机分子化合物被认为主要来源于陆地植物，其碳同位素组成可以很好地定量重建陆地C4／C5植被变化和相应的气候变化信息。然而，最近的系列研究结果表明：青海湖中的水生生物也可以产生长链正构烷烃（C27和C29），并且在湖泊沉积物中含量较高，这些水生植物产生的长链正构烷烃的碳同位素组成十分接近陆地C4植物。这些结果提示我们，湖泊内生生物的贡献可能会影响沉积物正构烷烃碳同位素古环境变化意义的解读。特别是通过湖泊长链正构烷烃碳同位素记录重建陆地C4／C3植被变化时，如果不考虑内源生物的贡献，估算的结果可能过高或过低的估计了历史时期陆地C4植物的丰度，以致得出不真实的气候／环境变化信息。

英文摘要：

The carbon isotope been considered main composition of total organic matter especially for long chain n-alkanes in lake sediments has derived from terrestrial plant and could be used as a reliable quantitative indicator to track changes in the terrigenous contribution of plants with C3 and C4 photosynthetic pathways. However, our recent studies showed that many submerged aquatic plants in Lake Qinghai also produce abundant of long chain n-alkanes （ C27 and C29） of which the carbon isotope values are very like to the values of terrigenous C4 plants. In addition, a large portion of long chain n-alkanes （C：7 and C29） in lake sediments are contributed by submerged aquatic plants. These new discoveries indicated that effects of aquatic plants on carbon isotope values of long chain n-alkanes in lacustrine sediments may have long been underestimated. Thus, the use of δ13C values of sedimentary C27 and C29 n-alkanes for tracing terrigenous vegetation composition may create a bias toward significant overestimation/ underestimation of the proportion of terrestrial C4 plants. Furthermore, the information of climatic or environmental changes deduced from variation in plant-type may also be unreasonable.