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Abstract:
Massive methane release due to decomposition of methane hydrate
insea-floor sediments might have caused rapid global warming in the
past(Dickens et al., 1995). However, the degree of global warming has
not beenestimated due to uncertainty over the proportion of methane flux
from the sea-floor to reach the atmosphere. 

According to present observations and model study of methane-bubbling
seep sites, a methane bubble released from the sea-floor does not reach
the atmosphere (Greinart et al., 2006). However, massive methane release
would result in methane-saturated seawater, thus some methane would
reach the atmosphere. Methane in the form of bubbles passing through
undersaturated seawater may potentially reach the atmosphere due to
rapid vertical transport.

In this study, we calculated the methane concentration in the water
column required for methane bubble to reach the atmosphere, using a
one-dimensional numerical model integrated over time predicting 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 minimum amount of methane input from the
sea-floor necessary for methane to reach the atmosphere to the amount of
methane in the sediment. In most cases, our results suggest that the
typical amount of methane in the sediment is significantly lower than the
required minimum amount of methane input. It is, therefore, suggested 
that the massive quantity of methane bubbles released from the sea-floor
would not reach the atmosphere directly but would be dissolved in the seawater.

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