Temporal change in chemical composition of seawater around Nishinoshima volcano according to volcanic activity

Nishinoshima volcano on the Izu-Ogasawara volcanic front began intense volcanic activity in November 2013 after approximately 40 years of quiescence. The Japan Coast Guard investigated the chemical compositions of the seawater around Nishinoshima to monitor temporal and spatial variations related to the volcanic activity. Seawater sampling was conducted by the S/V Shoyo and the autonomous survey vehicle (ASV) Manbo II in June-July 2015 and by S/V Shoyo in May 2016. There were no remarkable azimuthal dependence of variation in pH values and the concentration of F, Cl and SO4 of the seawater samples collected in 2015 and 2016. Most pH values of the seawater samples collected in 2015 were about 8.0 and significantly lower than the pH of the reference seawater sampled far from Nishinoshima. On the other hand, the pH values of all the seawater samples collected in 2016 were about the same value as that of the reference point. This suggests that the influence by the acidic hydrothermal water emissions from Nishinoshima appeared over a wide area around Nishinoshima in 2015 and that the influence in 2016 was reduced.