2020 年 54 巻 4 号 p. 243-254
To better understand the activities of subsurface microbes producing and oxidizing methane in terrestrial regions and the hydrological factors controlling the activities, we conducted geochemical and microbiological studies on the Holocene mud beneath the Kanto Plain. The 13C- and 14C-tracer experiments indicates that the methane oxidation activity far exceeded the methane production activity in the sediments, which is consistent with the predominance of anaerobic methane-oxidizing archaea (ANME)-1 in the archaeal population. Depth profiles of sulfate and sulfide ions in the pore water and the hydrogen and oxygen isotopic compositions of the pore water in the sediments indicates that surface meteoric water had recently infiltrated into the muddy sediments and a reduction of the sulfate derived from the meteoric water had occurred in the upper part of the sediments. Meanwhile, the molar ratios of methane to ethane plus propane and the carbon isotopic compositions of methane showed that methane in the Holocene mud was microbial in origin. Because the methane was dissolved in the low-salinity pore water, which had almost completely replaced the original paleo-seawater, the methane production activity would have occurred after the replacement. The pore water in the lower part of the sediments with low hydrogen isotopic compositions may have been derived from waters recharged under colder climate, such as the Last Glacial Maximum. The small size of the pores in the muddy sediments restricts the flow and migration of microbes. The carbon isotopic relationship between archaeal lipids and dissolved carbon dioxide fits the isotopic fractionation associated with carbon fixation and lipid biosynthesis by methane-producing archaea rather than that by methane-oxidizing archaea, which suggests that the dominant ANME-1 may have produced the dissolved methane under a low sulfate condition before the infiltration of meteoric water into the sediments.
