A comparison of n-alkane contents in sediments of five lakes from contrasting environments

Abstract

In paleolimnology studies, the trajectories of n-alkane abundances and distributions in dated sediments are widely used indicators of changing environmental and ecological status in the overlying water over time. However, the factors contributing to the variable n-alkane profiles of lake systems are not yet fully understood. Here, a comparative study was conducted to examine natural and anthropogenic influences on the sediment n-alkane profiles of five representative lakes in China. Core samples from the deep oligo- mesotrophic lakes Fuxian and Lugu (FX-1 and LG-1 cores), the shallow eutrophic lakes Taibai and Changdang (TB-1 and CD-1 cores) and the alpine Lake Heihai (HH-1 core) were used. While there were striking differences in n-alkane abundances and distributions between the cores, the results suggested that the n-alkanes in all five studied lakes were derived mainly from biogenic sources, rather than from petroleum contamination. The short-chain (<C20) even carbon n-alkanes, assumed to derive from non-photosynthetic bacteria, were especially prevalent in FX-1, reaching 158.3–284.4 μg/g TOC, which might be a result of high levels of bacterial productivity in the deep (average 89.6 m) water column of Lake Fuxian. The contribution of bacterial production diminished in the other four lakes, with <C20n-alkanes declining to between 21.4 and 60.5 μg/g TOC on average in the studied cores. In the two shallow lakes, abundances of short-chain (<C20) odd carbon n-alkanes were greatly elevated in sediments corresponding to the eutrophic phase; for example, with maximum values of 136.3 μg/g TOC recorded in TB-1. This nutrient-driven effect was less apparent in deep lakes. The >C21n-alkane profiles of different cores varied significantly. Abundances of long-chain n-alkanes (C29, C31 and C33) in HH-1 were approximately 3–7 times greater than in other cores. Coupled with the lowest Paq (average 0.29) values recorded in the study, this indicates a predominantly terrestrial plant origin for the >C21n-alkanes, most likely the dense alpine meadow which surrounds the lake. In other cores, however, aquatic macrophytes appear to be more important contributors of the n-alkanes, a result that corresponds with the relatively low density of terrestrial plants in those catchments.