Rhythms of Methane Emission: Long-term Observations on Mud-volcano Eruptions

Submarine mud volcanoes are important sources of methane to the water column. It has been estimated that the submarine volcanos emit 27 Million tons of methane, which is about 5% of the global emission. However, these numbers are uncertain as the temporal variability of their mud and methane emissions is unknown. Methane emissions were previously proposed to result from a dynamic equilibrium between upward migration and consumption at the seabed by methane-consuming microbes. Here we show non-steady state situations of vigorous mud movement that are revealed through variations in fluid flow, seabed temperature and seafloor bathymetry. Time series data for pressure, temperature, pH and seafloor photography were collected over 431 days using a benthic observatory at the active Håkon Mosby Mud Volcano. We documented 25 pulses of hot subsurface fluids, accompanied by eruptions that changed the landscape of the mud volcano. Four major events triggered rapid sediment uplift of more than a meter in height, substantial lateral flow of muds at average velocities of 0.4 m day^-1, and significant emissions of methane and CO₂ from the seafloor. We calculated that to drive these eruptions at least 10 times more gas is emitted that previously assumed. The contribution of submarine mud volcanoes to the global methane budget may thus strongly underestimated.