The Evolution of Methane Vents That Pierce the Hydrate Stability Zone in the World's Oceans

We present a one-dimensional model that couples the thermodynamics of hydrate solidification with multiphase flow to illuminate how gas vents pierce the hydrate stability zone in the world's oceans. During the propagation phase, a free-gas/hydrate reaction front propagates toward the seafloor, elevating salinity and temperature to three-phase (gas, liquid, and hydrate) equilibrium. After the reaction front breaches the seafloor, the temperature gradient in the gas chimney dissipates to background values, and salinity increases to maintain three-phase equilibrium. Ultimately, a steady state is reached in which hydrate formation occurs just below the seabed at a rate necessary to replace salt loss. We show that at the Ursa vent in the Gulf of Mexico, the observed salinity and temperature gradients can be simulated as a steady-state system with an upward flow of water equal to 9.5 mm yr-1 and a gas flux no less than 1.3 kg m-2 yr-1. Many of the world's gas vents may record this steady-state behavior, which is characterized by elevated temperatures and high salinities near the seafloor.