Past and Future Methane Loss From Arctic Gas Hydrate Based on Pore-Fluid Overpressure

Observations from the shallow shelf region of the Beaufort Sea indicate several sites with known or inferred gas hydrate. This gas hydrate was formed on land beneath permafrost during glacial periods when sea level was lower and has been dissociating since submergence initiated warming several thousand years ago. Current anthropogenic warming is substantially smaller, and shorter in duration, than the ~ 17°C warming due to submergence of the shelves. Thus, we do not expect the total methane output from the Arctic shelves to be dramatically altered by global warming (at the present time). We instead characterize the past long-term effect of warming in the Arctic by modeling the gas hydrate response to sea level change. Insights gained from past warming offer clues to the future response of gas hydrate to anthropogenic warming. Furthermore, the model establishes a baseline for gas hydrate release, necessary to identify the contribution from anthropogenic warming. A key parameter for assessing the long-term response to warming is the overpressure of the sediments, both within and below the hydrate stability zone. We attribute this overpressure to the phase change from hydrate to free gas bubbles and interpret this to mean that the rate of conversion is faster than the rate of pressure dissipation due to fluid flow. The low permeability of the frozen permafrost sediments above the hydrate stability zone may be an important factor in setting these rates. We use measured profiles of overpressure from several wells in the Beaufort Sea to place limits on the gas hydrate volume and the rates of dissociation. By utilizing these limits, we can improve the prediction of the hydrate response to future warming.