TERRAPUB Geochemical Journal

Geochemical Journal, Vol. 51 (No. 3), pp. 263-275, 2017

Deep incursion of seawater into the Hiroshima Granites during the Holocene transgression: Evidence from 36Cl age of saline groundwater in the Hiroshima area, Japan

Yuki Tosaki,* Noritoshi Morikawa, Kohei Kazahaya, Hitoshi Tsukamoto, Yoko S. Togo, Tsutomu Sato, Hiroshi A. Takahashi, Masaaki Takahashi and Akihiko Inamura

Research Institute of Earthquake and Volcano Geology, Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology (AIST), AIST Tsukuba Central 7, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8567, Japan

(Received August 25, 2016; Accepted December 27, 2016)

Abstract: We investigated the influence of long-term sea-level changes on the groundwater flow system of a mid-latitude coastal granite area. Saline/brackish groundwaters were sampled from deep boreholes in the Hiroshima area, southwestern Japan. The origin of these waters is seawater (but not modern), as indicated by Br/Cl ratio, δD-δ18O and 3H concentration. The 36Cl/Cl ratios of the seawater component of sampled water are similar to the present seawater value. The calculated 36Cl ages are not more than 40 ka, indicating that the saline/brackish waters are derived from an incursion of seawater after the Last Glacial Maximum (LGM). Such a ``young" seawater component is distributed within the estimated seawater inundated area during the Holocene Jomon transgression (ca. 6 ka). From the distribution of the saline/brackish waters along profiles from the present and the estimated 6 ka coast, we conclude that seawater infiltrated and reached to ∼1,700 m during or after the Jomon transgression. This in turn suggests that any older saline waters were flushed out during the last glacial period, which could have been facilitated by the development of a large regional groundwater flow system. The following factors are considered to be important for the seawater incursion in crystalline rock areas: (1) permeability of the pathway (fractures), (2) the density contrast between seawater and the groundwater (freshwater) at the time of transgression, and (3) meteoric flushing during the glacial period.
Key words: coastal granite area, 36Cl age, deep saline groundwater, groundwater flow system, long-term sea-level change

*Corresponding author E-mail: yuki.tosaki@aist.go.jp

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