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

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, delta D-delta O-18 and H-3 concentration. The Cl-36/Cl ratios of the seawater component of sampled water are similar to the present seawater value. The calculated Cl-36 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 similar 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.