Organic geochemistry of hydrothermal petroleum generated in the submarine Wakamiko caldera, southern Kyushu, Japan

Abstract

Four sediments impregnated with hydrothermal petroleum were dredged from the shallow seafloor (≈ 200 m) of the submarine Wakamiko caldera in northern Kagoshima Bay, southern Kyushu, Japan. Their organic geochemical parameters were studied. The hydrothermal petroleums were characterized by higher Pr/n-C17 and Ph/n-C18 ratios and lower phenanthrene/methylphenanthrene ratios than hydrothermal oils known from comparable seafloor hydrothermal systems around the world. The former characteristic is interpreted to reflect the large contribution of unaltered terrigenous organic matter because of the shallow water depth and land proximity of the caldera. The latter characteristic indicates high maturity rather than simple pyrolysis of the source organic matter. Furthermore, the input of components from thermally-unaltered sediment and mild biodegradation are evident in the oil compositions. Among the collected samples, differences in hydrocarbon compositions between lithified sediments and normal shallow sea muds were notable. The lithified sediments had higher maturity levels for the sterane and triterpane distributions of the petroleums. Higher BeP/BaP [benzo(e)pyrene/benzene(a)pyrene] ratios suggest that petroleums in the lithified sediments are more altered, because the ratio reflects the extent of secondary oxidation and/or thermal loss of BaP once formed. These organic geochemical parameters suggest that the lithification sediments are derived from a significant depth below the seafloor and had erupted recently onto the seafloor. The frequent eruptions would enhance organic maturations and lithification of sediment and transport of subsurface sediment to the seafloor.