Fate of Dissolved Methane from Ocean Floor Seeps

Methane is an important greenhouse gas, with a global warming potential that is far higher than that of CO₂. Methane from seafloor seeps, whether naturally occurring or in relation to petroleum infrastructure, has been suggested to be a significant contribution to greenhouse gas releases. Here, we consider the fate of methane from seeps on the Norwegian continental shelf by means of models for dissolution of methane from rising bubbles, mixing and biodegradation of dissolved methane, and mass transfer to the atmosphere. Laboratory experiments with tritium-labeled methane have been conducted to help determine the biodegradation rate of methane in natural seawater, and the results, together with literature data, have been used to guide the modeling. From the modeling study, we present results as a function of biodegradation half-life, treating this as a free parameter to reflect the considerable span in values reported in the literature. Considering three different locations on the Norwegian continental shelf, we find that if the biodegradation half-life of methane is in the range of a 9 to 16 days, as suggested by our experiments, then about 57-68% of the released methane will biodegrade in the water column from a seep at 65 m depth. For deeper locations of 106 and 303 m, we find respectively 75-83%, and more than 99% biodegradation.