Diagenesis and distribution of acyclic isoprenoid hydrocarbons in Late Miocene to Pliocene marine siliceous rocks from Horonobe (Hokkaido, Japan)

A sequence of Late Miocene to Pliocene siliceous mudstones from boreholes HDB-3 and HDB-4 from Horonobe (Hokkaido, Japan) is homogeneous and useful for the study of diagenetic degradation and transformation of sedimentary organic matter. Using these siliceous mudstones, the diagenetic change in acyclic isoprenoid hydrocarbons was investigated. The palaeotemperatures of the samples were estimated to be in the range 30–60 °C on the basis of the silica mineral transformation and the present day geothermal gradient in the region. The relative abundances of pristane to pristenes and phytane to phytenes increase drastically with depth. The remarkable changes in pristane/pristene and phytane/phytene occur in a temperature range of about 35–50 °C. The relative abundances could be potential parameters for palaeotemperature analysis of immature sedimentary rocks. Remarkable increases in pristane and phytane concentration with maturity show that, in addition to extractable free pristenes and phytenes, bound acyclic isoprenoids are the major source of pristane and phytane. Pristane/phytane and pristenes/phytenes are nearly constant through the entire boreholes, showing that the relative abundance of the precursor C19–C20 acyclic isoprenoids was determined at the time of deposition and/or in the early stage of diagenesis. Pristene/phytene, as well as pristane/phytane, could be useful parameters for assessing palaeodepositional environment.