Sediment biomarker profiles trace organic matter input in the Pichavaram mangrove complex, southeastern India

The nature and distribution of lipid biomarkers (n-alkanes, n-alkanols, sterols and triterpenols) were investigated in five dated sediment cores from the Pichavaram mangrove-estuarine complex in order to: 1) identify the organic matter (OM) sources and its preservation and 2) trace recent changes associated with coastal processes and anthropogenic activities. The mangrove sediment extracts have higher biomarker concentration (22.6 ± 13.3 μg/g dry weight) than the estuarine extracts (6.42 ± 4.92 μg/g dry weight). Triterpenols are dominant biomarkers in both estuarine and mangrove sediments, and constitute > 50% of the total lipid extracts. The presence of n-alkanols (n-C26,28,30) and abundance of phytosterols (stigmasterol and β-sitosterol) and triterpenols (taraxerol, β-amyrin, germanicol, and lupeol) indicate that mangrove vegetation is the primary source of sedimentary OM. The high abundance and unimodal distribution of the long-chain n-alkanes (mainly n-C25,27,29), and high values of Terrestrial Aquatic Ratio (TAR 2.4 to 41) and Carbon Preference Index (CPI > 8) indicate dominance (and better preservation) of higher plant derived immature OM in mangrove sediments. In contrast, the weak dominance of high molecular weight n-alkanes, low CPI (0.75 to 0.90) and TAR (1.9 to 5.7) values, and the presence of high C27 and C28 sterols indicate that phytoplankton/algal derived OM is more pronounced in estuarine sediments. Diagenetic changes in sedimentary OM indicate that some of the lipid fractions are reactive, and as a result, they degrade more rapidly than bulk OM. These compounds follow first order decay kinetics, and concur with the downcore diagenetic changes in coastal areas. Resistance to degradation among the different lipid classes show the trend: n-alkane > sterol > triterpenol > n-alkanol. Finally, the study indicates that less freshwater discharge from the Coleroon River is causing a gradual change in mangrove vegetation. There is less input of OM derived from mangrove vegetation into recent sediments. Biomarker trends also indicate that less salt tolerant Rhizophora spp. are gradually replaced by more salt tolerant vegetation consisting of Sueda spp. and Aviccenia spp.