Leakage of magmatic–hydrothermal volatiles from a crater bottom formed by a submarine eruption in 1989 at Teishi Knoll, Japan

• We detected minor CH4 and CO2 addition to seawater through the crater bottom.
• δ13C and Δ14C values indicate CH4 and CO2 of hydrothermal or magmatic origin.
• 3He/4He ratios show the magmatic volatile leakage through the submarine crater.
• Very weak volcanic volatile emission has continued 23 years after the eruption.
• Trace dissolved components in seawater are useful for submarine volcano monitoring.

A submarine eruption occurred off the Izu Peninsula of Japan on 13 July 1989, forming Teishi Knoll, which has a diameter of 450 m and a height of ca. 10 m above the surrounding 90–100 m deep seafloor. Immediately after the eruption, intense gas release was observed from two vents in the crater. The gas bubbling gradually decreased and apparently ceased in 1990. Given that no survey has been undertaken to examine volatile release from the crater of Teishi Knoll, we collected seawater samples at three different sites from just above the crater bottom on 17 July 2012, in order to detect signs of magmatic volatile release. Seawater samples from the crater bottom have dissolved CH4 contents and δ13C values higher than those of shallower (50–100 m deep) seawater samples. Total inorganic carbon contents from the bottom seawater samples are also higher, and δ13C and Δ14C values lower than those of shallower seawater samples. These data indicate the addition of minor CH4 and CO2 of hydrothermal or magmatic origin to the bottom seawater from the crater. 3He/4He ratios and total organic carbon data are also consistent with the leakage of magmatic fluids. The most prominent CH4 and CO2 anomalies were observed at the site located closest to one of the bubbling gas sites of the 1989 eruption. As such, volcanic gas emissions still continue today at extremely low levels, 23 years after eruption of this monogenetic volcano. The monitoring of ultra-trace amounts of chemical components in seawater is a prospective method to monitor temporal changes in magmatic activity at such submarine volcanoes.