Petrology and organic geochemistry of peat samples from a raised bog in Kalimantan (Borneo)

Two peat cores, from the margin and centre of a tropical raised bog from the Sebangau River near Palangkaraya in Kalimantan, Indonesia, were investigated to illustrate the changes during peatification using coal petrographic and organic geochemical techniques. These methods included a maceral analysis on polished peat blocks under reflected white and blue light excitation using the brown coal maceral nomenclature. Soxhlet extraction was carried out and an estimate of the humic composition was undertaken on the residues. GC and GC/MS analysis of the alkane and aromatic fractions were also made to identify biomarkers present in the peat. Results of this study show that peats from the margin of the raised bog were more decomposed than peats from the centre. Tissue content, extract yield and alkaline insoluble/acid soluble “humic acid” fraction support these results. Not only were there differences between the two locations of the peat cores, but also differences arose with respect to depth in both cores: the basal peats were more decomposed than the peats from the upper layers. This was a result of their differing environments of deposition, i.e. the basal peats were deposited under relative mesotrophic conditions and a more seasonal climate, whereas the peats from the upper layers were deposited under acidic and oligotrophic conditions. The biomarker composition of the alkane fraction included long chain n-alkanes from n-C 23 to n-C 35, with an odd over even predominance, and interestingly n-alkanes from n-C 36 to n-C 40 with an even over odd predominance peaking at n-C 38. Pentacyclic triterpenoids were mainly represented by 17α, 21β (H)-homohopane and hop-17 (21)-ene. Biomarkers in the aromatic fraction included polyaromatic triterpenoids derived from α- and β-amyrin.