Monitoring the magmatic activity and volatile fluxes of an actively degassing submarine caldera in southern Japan

The sudden eruption of submerged calderas can potentially pose imminent danger to nearby communities. Hence, it is crucial to closely monitor the changes in volatile effluents in these environments. Unlike subaerial volcanoes, underwater volcanic activity is difficult to observe. Wakamiko, a submerged caldera in southern Japan shares a magma source with the active subaerial volcano, Mt. Sakurajima. Following years of inactivity, the volcanic eruptions in Mt. Sakurajima became more frequent in 2009; however, it is undetermined how the neighboring Wakamiko caldera has behaved in relation to this upsurge in activity. In this study, we assess the state of hydrothermal venting inside Wakamiko following recent disturbances in Mt. Sakurajima. Based on gas bubbles, seawater, and sediment pore water measurements of samples obtained in 2015, it appears that acidic fluids enriched in 3He continue to be discharged to the seafloor. Excess 3He in the water column shows no apparent change since 2010; however, the helium isotopic ratio of the magmatic source continues to be elevated at ∼7.2 Ra. This ratio for the magmatic component is higher than previously observed in 1986 (∼6 Ra). The difference could be related to changes in the degassing activity as well as the mantle composition of the magmatic source. Using the new data from sediment pore water, we estimate the diffusive fluxes of 3He and 4He through the seafloor at 16.05 atoms cm−2 s−1 and 1.52 × 106 atoms cm−2 s−1, respectively. We also calculated the vent 3He flux at 3.38 × 104 atoms cm−2 s−1. With the use of the 3He flux, we calculated the diffusive CO2 flux from Wakamiko at 1.41 × 106 mol yr−1 and the vent flux at 3.72 × 108 mol yr−1. The available data that we have are still limited and a more frequent and longer observation is necessary to deduce the temporal changes in Wakamiko and its relation to the volcanic activity of Mt. Sakurajima.