An Evaluation of the Long-Term Stagnancy of Porewater in the Neogene Sedimentary Rocks in Northern Japan

Abstract

A groundwater dating for very old porewater using 36 Cl and 4 He was applied to the Koetoi and Wakkanai formations distributed in the northernmost part in Japan. Measured 36 Cl/Cl in the Koetoi Formation was 2.6 ± 2.0 × 10 −15 and that in the Wakkanai Formation was 8.1 ± 2.5 × 10 −15 . These values are similar to 36 Cl/Cl in situ secular equilibrium calculated from chemical compositions of core suggesting that Cl − ions and porewater have remained in the formations for much longer than half-life of 36 Cl . He concentration in porewater ranged from 1.1 × 10 −6 to 2.6 × 10 −5 (cc STP g w −1 ) and it is much higher than water saturated with air indicating that both formations contain very old porewater. However, the possibility of mixing of young water was indicated because He concentration was lower than that calculated by multiplication of in situ He production and time after the uplift. This possibility was also supported by Cl − , D, and 18 O data. After combining information on 36 Cl/Cl, 4 He, and D and 18 O, it was inferred that the porewater in the deep part of the Wakkanai Formation might have been stagnant since the uplift. The porewater in the Koetoi Formation and the shallow part of the Wakkanai Formation were found to be affected by young surface water.

Characterization of sediment and granite hosted deep underground research laboratories reveals diverse microbiome functions, limited temporal variation and substantial genomic conservation

“…Alternatively, it is possible that microbial dispersal occurred under conditions that differ from the present. H and O stable isotopic measurements indicate that meteoric water was intruded into the Horonobe subsurface during glaciation (likely the Last Glacial Stage, between 12,000 and 70,000 years ago), when hydraulic gradients were higher than at present (Teramoto et al, 2010; Ishii, 2018; Nakata et al, 2018; Mochizuki & Ishii, 2022). Intrusion of meteoric water probably ceased when hydraulic gradients decreased due to sea level rise (Hanatani et al, 2010).…”

Assessment of Hydraulic Connectivity of Fractures in Mudstones by Single‐Borehole Investigations

“…However, meteoric water admixing in the Koetoi Formation and the DI < 2 domains of the Wakkanai and Palfris Formations involved surface water younger than a few tens of thousands of years (Kunimaru & Metcalfe, ; Nagra, ; Murakami et al, ; Vomvoris et al, ). The intrusion of such young surface water to depths of several hundreds of meters is difficult to explain by diffusion process alone, and the intrusion likely occurred via advection along fractures (Nakata et al, ; Pearson et al, ). Such intrusion indicates flow path networks connected to the surface, which supports the high hydraulic connectivity of fractures estimated from the derivative analyses.…”

“…The groundwaters sampled at the Horonobe and Wellenberg sites are characterized by Na‐Cl groundwater (>1 Ma) at deeper levels (>100–500 m depth in the Horonobe area; >∼400 m depth in the Wellenberg area) and shallow groundwater, which likely formed by the mixing of surface meteoric water and Na‐Cl groundwater (Figure ; Hama et al, ; Nagra, ; Nakata et al, ; Teramoto et al, ; Vomvoris et al, ). 14 C is detected in the shallow groundwater (Kunimaru & Metcalfe, ; Murakami et al, ; Nagra, ), suggesting the admixture of surface waters younger than a few tens of thousands of years.…”

“…The Na‐Cl groundwater is composed of altered fossil seawater that has reached isotopic equilibrium with the surrounding rock (clay/silica minerals) through long‐term water‐rock interactions (Matsumoto, ; Mazurek, ; Ueda et al, ), resulting in δ 2 H values of −30‰ to −20‰ in the Koetoi and Wakkanai Formations (Ishii et al, ; e.g., Figure c) and −45‰ to 3‰ in the Palfris Formation (the aqueous fluid inclusions in Alpine quartz and calcite veins, as reported by Mazurek ()). Although the fossil seawater in the formations has been significantly diluted by the dehydration of clay/silica minerals over time (Matsumoto, ; Mazurek, ; Ueda et al, ), young external water (e.g., surface water) is not detected in the fossil seawater in the deeper levels, based on isotope analyses (Mazurek, ; Nakata et al, ; Teramoto et al, ; Vomvoris et al, ). The groundwater of the three formations also includes a large amount of dissolved methane (Mazurek, ; Miyakawa et al, ).…”

Study on discrimination methodology to identify the origin and area of influence of deep-seated fluids in terms of dissolved chemicals

“…)よりも高いことか ら，マントル由来の物質が混合していると考えられている [9]．有馬温泉以外にも有馬温泉に似た水質を持つ地下水が 報告されており，有馬型熱水や有馬型塩水とも呼ばれてい る．火山の影響がないと考えられる状況で有馬温泉に似た 化学組成をもつ水の事例として，兵庫県の宝塚温泉 [9][10][11]， 大阪府南部の石仏の湧水 [8,11]，紀伊半島の温泉や湧水 [12,13]，四国 [14]，島根県の津和野地域の湧水 [15]，長野県の鹿 塩温泉 [16][17][18]，大分県の大分平野の温泉 [19]が報告されて いる． 海道 [27,[30][31][32][33][34][35]，東北地方日本海側 [29,30,[36][37][38][39]]，関東地方 [31,[40][41][42]]，宮崎県 [30,43] 素・水の安定同位体比を Table 4，炭化水素ガス等の組成を δD(‰)…”