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

Abstract

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.

Processes affecting molecular and stable isotope compositions of sediment gas in estuarine waters along the southern Baltic coast (Poland)

“…In our study we were unable to measure bubble diameters. However, for typical bubbles, 6-10 mm in diameter (McGinnis et al 2006;Ostrovsky et al 2008;Yamamoto et al 2009), a trajectory 10-30 m long is required to cause such a depletion in CH 4 . On the other hand, for bubbles 20 mm in diameter, reported as being representative of gassy sediments (Haeckel et al 2007), a dissolution of 10-67% CH 4 would occur at water depths between ca.…”

“…It seems that the dissolution of bubbles is enhanced by the low concentrations of dissolved CH 4 (CH 4diss ) in lake water (Yamamoto et al 2009). In the lakes studied, the concentrations of CH 4diss vary from 0.024 to 3.770 lmol L -1 (0.495 lmol L -1 on average) (unpubl.…”

“…These values supported the hypothesis on the potential effect of storm redeposition on air bubble entrapment in the sediments. Third, the N 2 and O 2 can invade ascending gas bubbles in a lake water column due to exchange between CH 4 /CO 2 and N 2 /O 2 dissolved in the column (Leifer and Patro 2002;McGinnis et al 2006;Ostrovsky et al 2008;Yamamoto et al 2009;Del Sontro et al 2010;McGinnis et al 2011). N 2 and O 2 build up in lake waters due to dissolution of air bubbles forced by breaking wind waves, Langmuir currents, and rain drop impacts (Chiba and Baschek 2010;Liang et al 2011), and the concentration (supersaturation) of these gases is positively related to wind speed (Nakayama et al 2002).…”

LAKE 2.0: a model for temperature, methane, carbon dioxide and oxygen dynamics in lakes

“…All bubbles that are released from a given sediment column's surface completely dissolve simultaneously at some depth or evade to the atmosphere. Furthermore, it is known that bubbles with a diameter ≈ 1 cm are unstable and split up (Yamamoto et al, 2009;McGinnis et al, 2006). Hence, in the model it is assumed that a bubble with r b ≥ 0.5 cm splits into two.…”

Geochemical Dynamics of the Natural-Gas Hydrate System in the Sea of Marmara, Offshore Turkey

“…Therefore, applied research on gas hydrates will still be focused on both fields of interest. Beside, the industr i a l i n t e r e s t s , t h e r e i s a d e b a t e o n t h e destabilisation of natural-gas hydrate systems and their contribution to climate change (Henriet and Mienert, 1998;Krey et al, 2009;Maslin et al, 2010;Yamamoto et al, 2009). In fact, if this huge amount of methane bound into the hydrates is released and reach the atmosphere, it would certainly impact on the climate by raising the temperature as methane is a greenhouse gas.…”