Source characterisation of Sedimentary organic matter in mangrove ecosystems of northern Kerala, India: Inferences from bulk characterisation and hydrocarbon biomarkers

Abstract

Surface sediment samples from five mangrove ecosystems along the Kerala coast, north of Kochi were analysed for elemental ratios, stable isotopes of carbon and hydrocarbon biomarkers (n-alkanes, n-alkenes and hopanes) to assess the sources and early diagenesis of sedimentary organic matter. The elemental and isotopic ratios along with sedimentary chlorophyll a content, suggested substantial contribution of planktonic organic matter apart from mangrove/terrestrial derived organic matter. The sources of organic matter showed variability in relation with the geographical setting of mangrove systems as well as with seasons. The n-alkanes in mangrove sediments consist of organic matter derived from varying contributions of planktonic, bacterial and terrestrial sources. The distributional features of n-alkanes with low molecular weight (<C20) indicated the preponderance of algal derived organic matter, whereas the unresolved complex mixtures for low molecular n-alkanes indicated active microbial reworking. The n-alkane derived molecular indices enable the effective source appropriation of different end members in the study region, while diagenetic alteration products of lipids such as n-alkenes, branched alkanes and n-alkylcyclohexanes along with hopanes are indicative of organic matter diagenetic pathway in sediments. The n-alkenes in the study area probably derived from decarboxylation reactions of unsaturated fatty acids and other terrestrial inputs. The higher input of mangrove/terrestrial derived organic matter is evident by the average chain length data. Information obtained using biomarkers will allow to discriminate organic carbon sources in these mangrove ecosystems.