The purpose of this study is to clarify the vertical prokaryotic distribution in groundwater in a terrestrial subsurface sedimentary environment with a geological complex. Six groundwater samples were collected from a coastal 1200-m-deep borehole in which digging strata deposited between 2.3 and 1.5 Ma in Horonobe, Hokkaido, Japan. The studied succession was divided into three vertical zones that were geochemically differentiated according to their chloride contents and water-stable isotopes. The upper zone (UZ; shallower than 500 m) primarily contained fresh water supplied by penetrating meteoric water, the connate water zone (CWZ; deeper than 790 m) contained paleo-seawater, and the diffusion zone (DZ; 500–790 m depth) located between UZ and CWZ. Fluctuations in the prokaryotic density and constituents were observed across these three zones. The prokaryotic density decreased from UZ toward DZ, and the density of DZ was two orders of magnitude lower than that of UZ and CWZ. High prokaryotic activity was observed in CWZ below DZ. The upward expansion of prokaryotic distribution from CWZ, where high prokaryotic potential expressed by biomass can be maintained almost equivalent to that in the marine environment, probably occurred on a geological timescale from 80 ka to 1.3 Ma, as shown by the groundwater age of DZ. The DZ is a zone where the geochemistry has changed drastically owing to the mixing of penetrating meteoric water and the diffusion of deep paleo-seawater, preserving a unique subsurface environment. This chemically mixed zone might be considered as a buffering zone for prokaryotes to prevent the expansion of prokaryotic density and activity provided by diffusion and their in situ growth from both above and below the zones, which is expected to be maintained over a geological timescale. This zone is considered important for using subsurface space in the deep subsurface environment of the island arc.

本研究的目的是阐明具有地质复杂性的陆地地下沉积环境中地下水中原核生物的垂直分布。六个地下水样本是从日本北海道幌延市沿海 1200 米深的钻孔中采集的，该钻孔中的挖掘地层沉积时间为 2.3 至 1.5 Ma。研究的序列被分为三个垂直区域，根据氯化物含量和水稳定同位素在地球化学上进行区分。上部带（UZ；浅于500 m）主要含有由渗透大气水供给的淡水；原生水带（CWZ；深于790 m）含有古海水；扩散带（DZ；深500-790 m） ）位于 UZ 和 CWZ 之间。在这三个区域中观察到原核生物密度和成分的波动。原核细胞密度从UZ向DZ递减，DZ的密度比UZ和CWZ低两个数量级。在 DZ 以下的 CWZ 中观察到高原核活性。原核生物分布从CWZ向上扩展，以生物量表示的高原核潜力可以维持几乎与海洋环境中相同的原核潜力，可能发生在80 ka至1.3 Ma的地质时间尺度上，如DZ地下水年龄所示。 DZ 是一个由于渗透的大气水和深层古海水的扩散而导致地球化学发生巨大变化的区域，保留了独特的地下环境。该化学混合区可被视为原核生物的缓冲区，以防止扩散所提供的原核生物密度和活性的扩大及其在该区域上方和下方的原位生长，预计这种情况将在地质时间尺度上维持。该区域被认为对于利用岛弧深层地下环境中的地下空间很重要。