Lipid distribution in the lake Ennamangalam, south India: Indicators of organic matter sources and paleoclimatic history

We have undertaken a comprehensive study on sediments (catchment, surface and core) and modern vegetation samples in and around lake Ennamangalam, south India with focus on the reconstruction of past climate during mid to late Holocene epoch using lipid biomarkers (n-alkanes) distribution and carbon isotopic composition of organic matter (δ 13COM). The retrieved ca. 1.65 m long well dated sediments from the lake Ennamangalam span the past ca. 4800 cal BP. The n-alkane indices such as Paq, carbon preference index (CPI), terrestrial vs. aquatic ratio (TAR) have been used to infer the changes in source of organic matter and to reconstruct past environmental changes. The end-member δ 13C value based calculation on abundances of C3 and C4 vegetation using δ 13COM value provided an erroneous estimation of past vegetation due to lake eutrophication or organic matter alteration by microbial degradation. On the other hand, long-chain n-alkane was not affected by bacterial activity and can be used to infer terrestrial vs. aquatic productivity through the modified n-alkane indices. Symbiotic contribution from aquatic productivity was inferred as the Paq′ value always exceeds 0.2. Higher TAR′ value observed after ca. 3000 cal BP indicate increased precipitation in the region.