Pristene, phytene, and sterenes in immature Horonobe diatomaceous sediments as potential indicators of paleo-temperature and depositional environment

Amo, Miki*; Suzuki, Noriyuki*; Shinoda, Toshio*; Ratnayake, N. P.*; Takahashi, Kazuharu

Diagenetic degradation and transformation of steroid and acyclic isoprenoid hydrocarbons in immature Horonobe diatomaceous mudstones of Wakkanai and Koetoi Formations were studied using the borehole core samples from HDB-3 and HDB-4. Late Miocene to Pliocene marine diatomaceous mudstones in Horonobe, Hokkaido, Japan are rich in steroid hydrocarbon such as sterenes, steradienes and sterane and isoprenoid hydrocarbons such as pristenes, phytenes, pristane and phytane. The present study attempts to constrain the diagenetic pathway of these compounds in a thick sedimentary sequence of immature mudstones. Detailed geochemical analyses for Horonobe diatomaceous mudstones revealed the timing of transformation and the compositional change of isoprenoid hydrocarbons during diagenesis. Potential maturity parameters based on ratios of sterenes to steranes, pristene to pristane, and phytene to phytane are proposed in the present study. Possible potentials of these compounds as indicators of depositional environment for immature sediments are also investigated in the present study.