Time-series measurements of the colored volcanic vent seawaters during a tidal cycle in Nagahama Bay, Satsuma Iwo-jima Island, Kagoshima, Japan

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Satsuma Iwo-jima Island, located 40 km south of Kyushu Island, Japan, bears a shallow-water hydrothermal system. The coastal seawaters are stained ivory and red colors attributing to the presence of suspended aluminous and ferric precipitates produced by the mixing of volcanic fluids and seawaters. In Nagahama Bay, which is partly isolated from the pelagic realm, the seawaters appear dark reddish brown, attributing to the concentrated ferric precipitatess. In order to understand how the ferric precipitates behave, we focused on characterizing the relationship between the ferric precipitates and the diurnal tidal cycle in the bay. Using a multiple sensor, we measured the seawaters from the surface to the bottom of the bay for two weeks, in order to sample one complete tidal cycle, the spring and neap tides in 2007. We accumulated data on pH, depth, and turbidity. The reddish brown waters exhibited both daily and lunar cycle changes in turbidity corresponding to the tidal inflow of pelagic seawaters. The waters lost ferric precipitates during the high tide within a day, and during the spring tide when there was a major difference in sea level between high and low tides. During under an influence a Typhoon passing through the region, however, the waters appeared significantly cloudier with suspended particles than on normal days due to sediment re-suspension.
2009-03-02