中新世女川珪質堆積物中のイソプレノイドチオフェン

The isoprenoid thiophenes with 20 carbons (3-methyl-2-(3.7,1 l-trimethyldodecyl)-thio-phene and 3-(4,8,12-trimethyltridecyl)-thiophene) are significantly detected in Miocene Onnagawa siliceous sediments (sample 82408) from Oomagari, Akita Prefecture, Japan. Associated kerogen is also rich in organosulphur (S 7.7% and atomic S/C 0.063) and can be classified as Type II-S kerogen. The isoprenoid thiophenes are possibly formed abiogenically by reactions of H2S with several phytadiene isomers derived from dehydration and reduction of phytol in the very early stage of diagenesis. The isoprenoid thiophenes are not always detected in contemporaneous siliceous and non-siliceous sediments. The unique presence of isoprenoid thiophenes in Onnagawa siliceous sediments from Oomagari is due to the followings: (1) the sample is anomalously poor in clay minerals having active iron which removes HIS. (2) the marked contribution of diatom-derived organic matter rich in chlorophyll-derived phytol as a precursory compound of isoprenoid thiohenes, and (3) the anoxic condition favorable for the formation of abundant H2S as source of organosulphur.