Geochemical characteristics of n-alkanes in sediments from oligotrophic and eutrophic phases of five lakes and potential use as paleoenvironmental proxies

Abstract

Profiles of n-alkane proxies in sediment cores have been frequently used to detect historical trends in the nutrient status of lakes. However, the results obtained from these proxies are occasionally confusing because the situations in which a given n-alkane proxy might make a valid indication have tended thus far to be elusive. The current study compared geochemical characteristics of n-alkanes in sediments from the oligotrophic (before 1950) and relatively eutrophic phases (after 1980) of five representative lakes in order to assess their suitability and reliability as proxies for lake trophic state. The results from Lakes Liangzi, Changdang, Taibai and Erhai demonstrated that short-chain odd carbon n-alkane abundances were significantly higher (P < 0.05) in sediments deposited after 1980 than in those dating to before 1950, whereas TAR (terrigenous: aquatic ratios) and ACL15–33 (average chain length of C15–C33n-alkanes) were significantly lower (P < 0.05) in the more recent sediments. These results are compatible with generally accepted variation seen in lakes that have undergone eutrophication. In Lake Fuxian, however, short-chain odd carbon n-alkane abundances, TAR and ACL15–33 were all higher in sediments after 1980 than in sediments before 1950, although the variation was only significant (P < 0.05) for ACL15–33. These results, especially the unexpected trends in TAR and ACL15–33, mean these cannot be relied upon as proxies for eutrophication in this lake. The anomaly is likely to be related to the predominantly low phytoplankton productivity and OM input in Lake Fuxian, coupled with the availability of OM from submerged macrophytes in the relatively eutrophic phase. Interestingly and importantly, two new proxies CPI15–20 ((n-C15 + n-C17 + n-C19)/(n-C16 + n-C18 + n-C20) alkanes) and n-C17/n-C16 alkane, which are indicative of OM input from phytoplankton over non-photosynthetic bacteria, were significantly higher (P < 0.05) in post-1980 sediments than in those dating to before 1950 in all five studied lakes. These two are thus shown to be widely applicable for recording lake eutrophication. Their presence in Lake Fuxian can be explicitly attributed to the absence of influence from submerged macrophytes. Moreover, as ratio results of n-alkanes with similar carbon number, these two proxies are less influenced by processes of OM diagenesis due to comparable degradation of n-alkanes in sediments. The combined results suggest that these previously overlooked proxies may be of considerable value to studies of lake trophic history.