Biogeochemical profiles in deep sedimentary rocks in an inland fore-arc basin, Central Japan

Microorganisms thrive in the deep subsurface, but their extent and distribution, type and rate of metabolism are not well constrained, at least in part, owing to geochemical, lithological, hydrological and tectonic variations. We obtained deep sedimentary rocks from a tectonically stable inland basin in central Japan by drilling with minimized microbial and geochemical disturbance. Along with hydrogeological and microbial characterizations, geochemical properties of drilled cores from depths between ~ 200 and ~ 350 m were investigated. Except for a depth of 202 m, nitrate and/or nitrite were present in pore-water at all depths investigated in the present study. In most of the pore-water samples, the levels of dissolved Fe and H2S were low. Additionally, the presence of microbially reducible Fe(III) was not evident in most of the core samples. Based on the acetylene blockage method, it was revealed the denitrification potential was high at depths of 302 and 351 m where pore-water was depleted in nitrate and enriched in nitrite, while the denitrification potential was relatively low at depths of 300 and 340 m where nitrate and nitrite were both detected in pore-water. Potential rates of methane production via CO2 reduction were higher than those via acetate fermentation in the 302- and 350-m deep core samples. In the silty sandstone interval at depths of 340 and 351 m where permeability was relatively low, isotopically light methane of microbial origin was slightly enriched with sodium and chloride in pore-water. As a moderately mineralized fluid similarly enriched in sodium and chloride is entrained in the deeper subsurface with a depth over 1200 m around the drilling site, the increase in dissolved aqueous species is attributed to the remnant of the mineralized groundwater after the circulation of meteoric groundwater. The ubiquity of type of the sedimentary rocks suggests that the biogeochemical processes presented in this study might not be limited to the terrestrial sedimentary basin in central Japan.