Alkyl and polynuclear aromatic thiophenes in Neogene sediments of the Shinjo basin, Japan.

Abstract

Alkyl and polynuclear aromatic thiophenes were detected in sedimentary rocks of a stratigraphic sequence from Miocene to Pliocene in Shinjo basin, Japan. C25 Highly branched isoprenoid (HBI) thiophenes, dibenzothiophene, C1- and C2-alkyldibenzothiophenes, and benzo[b]naphto[1, 2-d]thiophene were rather predominantly present. The C25 HBI thiophenes consist of three isomers including a pair of diastereomers. They were present in sediments from the Nakawatari to the upper part of the Kusanagi Formations and absent at the middle and lower parts of the Formation. The depth profiles of their concentrations do not coincide with those of the C25 HBI alkane concentration and bulk sulfur content in the same sediment samples. It is postulated that the C25 HBI thiophenes and the C25 HBI alkane were produced by the reaction of C25 HBI alkadiene and C25 HBI polyene, respectively, with reduced sulfur during early diagenesis. The variation in composition of the C25 HBI thiophene isomers was observed in the sediments, suggesting that they were derived from different kinds of C25 HBI alkadienes. The concentrations of the polynuclear aromatic thiophenes showed an increasing trend downwards the upper to middle Kusanagi Formation. Among these thiophenes dibenzothiophene and benzo[b]naphto[1, 2-d]thiophene concentrations varied from 10 to 730 and 14 to 210 pmol g-1, respectively. The depth profiles of their concentrations were different from that of the C25 HBI thiophenes. It is likely that the polynuclear aromatic thiophene formation started from different precursors followed by different processes from those of the C25 HBI thiophenes.