Na-Ca-Cl-SO 4-type submarine formation waters at the seikan undersea tunnel, Japan. chemical and isotopic documentation and its interpretation

Abstract

Formation waters were collected from the submarine volcanogenic Kunnui formation about 200 m below the sea floor and zero to 4 km off the shore in shafts of the Seikan Undersea Tunnel between Honshu and Hokkaido, Japan. The concentration of Cl − ranges from 20 to 500meq/l, more than 90% of the concentration in seawater. Stable isotopic and chemical evidence indicate that the waters having Cl − less than 70–80 meq/1 are essentially of meteoric origin, whereas those of higher Cl − contents are mixtures of the meteoric formation waters and seawater. Aquifers of the mixed formation waters overlie those of the meteoric formation waters. The former seem to have expanded into the latter by infiltration of seawater as a result of excavation of the tunnel. Sulfur and oxygen isotopic ratios of dissolved sulfates in the meteoric formation waters are +32 -+ 25 per mil and +9 –+15 per mil, respectively, and approach the values of dissolved oceanic sulfate as the proportion of the seawater component in the mixed formation waters increases. Thus, two sources of sulfate exist; one is gypsum in the Kunnui formation which is highly enriched in 34S and 18O compared to the other source, modern marine sulfate. Both the meteoric and mixed formation waters are rich in Na +, Ca 2+, Cl − and SO 4 2− and extremely depleted in Mg 2+ and K +. They are highly enriched in Ca 2+ and SO 4 2− compared to simple mixtures of meteoric and seawater. The observed chemical composition of the formation waters of Cl − less than 150 meq/1 can be satisfactorily reproduced by a model calculation, if we assume: 1. (1) the formation waters are in cationic exchange equilibrium with montmorillonite-rich tuffs and 2. (2) they are saturated with gypsum and calcite.