Biomarkers as indicators of sedimentary organic matter sources and early diagenetic transformation of pentacyclic triterpenoids in a tropical mangrove ecosystem

摘要

Mangrove vegetation covers extensive areas along Brazilian tropical coasts and accumulates large amounts of organic carbon in the intertidal sediments. We have investigated the molecular biomarker composition of mangrove sediments from the Potengi estuary North-East Brazil. To characterise sources and fate of sedimentary organic matter (OM), a direct analytical pyrolysis approach was used. The sediment pyrolysates yielded homologous series of aliphatic compounds (n-alkane-alkene doublets, n-alkanoic acid and n-alkan-2-ones), triterpenoids and lignin-derived methoxphenols, indicating that most of the sedimentary OM input originates from terrestrial vascular plants. High abundance of long-chain n-alk-1-ene series with an even carbon number predominance peaking at C-28:1-most likely originated from local mangrove vegetation-is found co-eluted with the saturated odd carbon n-alkane series. The occurrence of lignin and n-alkan-2-one biomarkers in the sediments has provided parallel information about the input from vascular plants dominating the intertidal zone. This was further corroborated by the presence of the triterpenols (beta-amyrin, and germanicol), that provided chemotaxonomic information on mangrove derived leaf wax in sedimentary OM. The unsaturated triterpenoid including teraxerol acetate, olean-12-ene, olean-18-ene, oleana-11, 13(18)-diene, a-neoursa-3(5), 12-diene and aneooleana-3(5), 12-diene, were also observed in sediment pyrolysates. The presence of these unsaturated pentacyclic triterpenoids revealed an early diagenetic alteration of terrestrial OM in the sediments. This study has demonstrated that routine application of analytical pyrolysis may provide important insight into the sources of sedimentary OM and its diagenetic fate in mangrove intertidal sediments.