Modeling of methane bubbles released from large sea-floor area: Condition required for methane emission to the atmosphere

Abstract

Massive methane release from sea-floor sediments due to decomposition of methane hydrate, and thermal decomposition of organic matter by volcanic outgassing, is a potential contributor to global warming. However, the degree of global warming has not been estimated due to uncertainty over the proportion of methane flux from the sea-floor to reach the atmosphere. Massive methane release from a large sea-floor area would result in methane-saturated seawater, thus some methane would reach the atmosphere. In this study, we discussed the possibility of the methane release from a large sea-floor area to the atmosphere focusing on methane saturation in the water column necessary for a methane bubble to reach the atmosphere. Using a one-dimensional numerical model integrated over time, we predict methane bubbles and methane concentration in the water column under the condition of continuous methane input from the sea-floor to the water column. We found that some methane bubbles reach the atmosphere even when the methane saturation fraction in the water column is much lower than 100%. We compared the methane input from the sea-floor required for a methane bubble to reach the atmosphere to the amount of methane in the sediment in the form of methane hydrate and free gas. In most cases, our results suggest that the typical amount of methane in the sediment (i.e., typical hydrate fraction of ~ 2% and free gas of two-thirds of the amount of hydrate) is significantly lower than the required minimum methane input. It is, therefore, suggested that, except in the case of an extraordinary methane flux, the massive quantity of methane bubbles released from sea-floor gas hydrate would not reach the atmosphere directly but would be dissolved in the seawater. With regard to global warming due to human activities, the release of methane bubbles due to methane hydrate decomposition may not be enough to significantly accelerate total global warming. In the case of metamorphic methane release during PETM, there is the possibility that the released methane resulted in methane-saturated seawater, allowing some methane to reach the atmosphere.