Passive acoustic monitoring of tidally controlled bubble phase transitions at the seabed

Seabed fluid seepage is a dynamic process that modulates gas and heat transfer between the Earth’s interior and the ocean. In submarine volcanic settings, short-term changes in hydrothermal seep behaviour can provide tangible indicators of volcanic activity. However, such signals are rarely, if ever, observed directly. Here, we show that passive acoustic monitoring can resolve tidally controlled variability in condensable bubble dynamics at the Calypso hydrothermal vent field in the Bay of Plenty, New Zealand. Over three days of acoustic recording, we detect an exceptionally large number of acoustic emissions. We demonstrate they are vapour-rich bubble sounds generated by boiling fluids within unconsolidated sediments. Systematic changes in sound frequency of bubbles condensing in sub-cooled water reveal cyclic transitions in fluid phase. Therefore, tidal loading may constrain high-temperature fluid expulsion, directly modulating seep intensity and heat flux. These results establish passive acoustics as a powerful high-resolution monitoring tool of transient sub-seabed processes in hydrothermal systems. This study provides a new window into the physics of condensable bubbles in extreme environmental conditions, with implications for volcanic hazard assessment.