Thiophenes in the Cretaceous/Tertiary boundary sediments at Kawaruppu, Hokkaido, Japan

抄録

One C₂₅ highly branched isoprenoid (HBI) thiophene, one C₂₅ HBI alkane, and five polynuclear aromatic thiophenes were detected in the Cretaceous/Tertiary (K/T) boundary sediments at Kawaruppu, Hokkaido, Japan. The C₂₅ HBI thiophene was found only in the sediments below the boundary claystone but not in the claystone and sediments above it. On the other hand, the C₂₅ HBI alkane was present in all the sediments. However, its concentration was low below and over the lower two thirds of the claystone, but increased in the upper one third. This distribution pattern resembles that of the longer chain n-alkanes (Mita and Shimoyama, 1999a). The depth profiles of the thiophene and alkane concentrations possibly reflect the massive extinction of organisms. Among the polynuclear aromatic thiophenes, dibenzothiophene and benzo[b]naphto[1, 2-d]thiophene are relatively abundant. Their concentrations were roughly constant in the sediments below the horizon at the upper one third of the boundary claystone. Their concentrations began to increase at the one third horizon towards the 40 cm horizon (26 cm above the top horizon of the claystone) and then steeply decreased in the sediment above the horizon. A nearly constant ratio of 4- to 1-methyldibenzothiophene found in the sediments above, within, and below the boundary claystone indicates that diagenesis has occurred evenly over the sediments. The depth profile of the dibenzothiophene concentrations resembles that of the reported sulfide contents (Kajiwara and Kaiho, 1992) suggesting that polynuclear aromatic thiophene formation is related in some way to sulfide content at deposition.