Characteristics of sedimentary organic matter and vascular plants in tropical brackish Bolgoda Lake, Sri Lanka: Implications for paleoecology and chemotaxonomy

Abstract

Tropical brackish ecosystems are characterized by mangrove-dominant communities which are well-adapted to extreme and unique environmental conditions. Organic matter (OM) derived from adjacent vegetation contributes to the chemical composition of sediments. Sedimentary OM can thus be used for paleoecological studies. Surface and core sediments of Bolgoda Lake were analyzed to identify sources of OMs using C, N, S elemental analysis and gas chromatography-mass spectrometry. C/N ratios of raw sediments and protokerogen derivatives illustrate the preferential degradation of mixed OM in surface sediments and mangrove muds. The n-alkanes distributions show that vascular plants are predominantly preserved in sediments due to resistance to microbial decomposition and available biomass in the watershed. Taraxerone provides the most important chemotaxonomical information to identify mangrove-dominant sediments. Ternary n- C27, n- C29 and n- C31 alkanes relationships show that mangrove-dominant sediments are enriched in n-C31along with significant amounts of n-C29. Mangrove and herbaceous plants (grasses and/or floating plants) can be preferentially dispersed in tropical brackish sediments. The relative proportions of n-C27, n- C29 and n- C31n-alkanes can be applied to identify different growth habitats of a particular landmass. The mangrove biomarker (pentacyclic taraxerone) and visualized long-chain n-alkanes proportions (i.e., n-C27, n- C29 and n-C31) can thus be used as chemotaxonomical indicators to reconstruct the paleoecological characteristics of tropical brackish vegetation.