Geochemical evidence for different peat sources in the Siak estuary and along the east coast of Sumatra, Indonesia

The distribution pattern of selected biomarkers extracted from samples of outcropping peat from the Siak river, its estuary, the coast around Dumai and the island of Bengkalis have been investigated by gas chromatography (GC) and combined gas chromatography/mass spectrometry (GC/MS), and compared with samples of eroded peat washed ashore (re-deposited) on the coastline of Sumatra. Geochemical analyses of n-alkanes, n-alkanols, n-alkanoic acids, steroids and triterpenoids show that outcropping and re-deposited peats have different chemical compositions. The outcropping peats around Dumai, Bengkalis and in the Siak River estuary contain high concentrations of the pentacyclic triterpenoid taraxerol, a typical constituent of the mangrove species Rhizophora. A comparison with the lipid composition of leaves from the fringing mangrove species (Avicennia alba, Sonneratia alba and Rhizophora apiculata showed that only R. apiculata contains significant amounts of taraxerol. Taraxerol was completely absent from the leaves of A. alba and S. alba. This suggests that the peats outcropping around Dumai, Bengkalis and in the Siak estuary must be formed by a dominant input of mangroves of the Rhizophora family. The n-alkane distribution patterns of the outcropping peats near Dumai and the Siak estuary are similar to those of the surrounding mangrove vegetation with a maximum at C31 and a strong predominance of odd over even carbon numbers in all samples. Biomarker analysis of eroded peats washed ashore along the coastline around Dumai and the Siak estuary shows a different lipid composition with high amounts of the triterpenoids friedelin, ?-amyrin and ?-amyrin. These compounds are typical biomarkers for tropical forest vegetation as found along the Siak River and for peats eroding at upstream river banks. The n-alkanol distribution patterns of re-deposited peats also indicate a different origin for their organic matter. Peats re-deposited in the Siak estuary lack the C30 n-alkanol that it typical of coastal peats, suggesting an upstream source.