Gamma Ray Doses in Water Around Sea Floor Hydrothermal Area in the Southern Mariana Trough

The dose rates in sea water in the Southern Mariana Trough sea floor hydrothermal area were investigated. The dose rate in sea water was less than detection limit (about 0.1 mGy/year) near the sea surface while it was around 2 mGy/year at the sea floor of hydrothermal area, where those levels of dose rates will be negligible for ESR (electron spin resonance) dating of barite of sulfide deposits. During the dive by a submarine vehicle, there were three locations at which the level of dose rate increases more than ten times as much as the usual level. One was very close to a hydrothermal vent, but no hydrothermal activities were found near the other locations.