An easier approach for helium isotope flux estimation in a submerged caldera

Wakamiko Caldera is an isolated basin located in the northern part of Kagoshima Bay. The underwater caldera is known for the active fumaroles that continuously discharge volcanic substances into the overlying water. Due to the limited exchange of waters in the caldera with the surrounding area, the water column gets stagnant and stratified from spring to autumn. Previous studies have shown that overturning occurs in the winter. Based on new data obtained by field observation in Wakamiko Caldera, we show that during winter, vertical mixing “resets” the helium isotope profile. After winter, as the water column regains its stratification, fluids with mantle helium slowly build up near the seafloor, resulting in the high ³He/⁴He ratio previously observed. We now use the temporal variation of ³He/⁴He ratio in different seasons to calculate the flux of ³He. Using this method, we estimate the ³He flux at approximately 0.013 mol/year.