Methane is a potent greenhouse gas that comes from a variety of natural sources, including the shallow seafloor sediment along continental margins—where the thick continental crust rapidly slopes down into the thin oceanic crust. These shallow marine sediment reservoirs include permafrost and methane hydrate, an icy solid that forms when high pressure and low temperatures force water molecules to form a crystalline cage around methane. When temperatures rise or pressure decreases, hydrates melt and release methane into seafloor sediments; from there it can make its way into the air. Since methane traps solar radiation in the atmosphere even more efficiently than carbon dioxide, these concentrated methane-filled snowballs can have a powerful effect on Earth’s climate.
Here Graves et al. assess how methane released at the seafloor makes it into the surface water and its potential role in climate warming. The team measured methane levels in seawater near the gas hydrate stability zone off the shore of the Svalbard Islands in Norway, where gas hydrates form naturally in the seabed. They used sonar to locate the bubble plumes that indicate methane seepage, which occurs at approximately 400 meters water depth, and took samples throughout the water column and surrounding air in this area. The team used these data to calculate the concentrations of dissolved methane in the upper slope and shelf region of the islands.
The researchers found that methane levels were highest close to the seabed but that roughly 60% of the methane leaving the seafloor was oxidized before it had a chance to merge with surface waters. Oxidation carried out by aerobic methanotrophs—bacteria that metabolize methane—is the dominant process (as opposed to dilution, where methane concentrations decrease through water mixing). Because of this, the exchange of methane between the sea surface waters and the air remained relatively low. Using these findings, the team concluded that methane released into the atmosphere comes mainly from the shallower areas of the shelf, where it has a better chance of reaching the surface without being oxidized.
Understanding the interactions between oceanic methane systems and the atmosphere will only become more critical in the future, as rising global temperatures increase the melting rate of methane hydrates. (Journal of Geophysical Research: Oceans, doi:10.1002/2015JC011084, 2015)
—Lily Strelich, Freelance Writer
Citation: Strelich, L. (2015), Rising temperatures release methane locked in the seabed, Eos, 96, doi:10.1029/2015EO039179. Published on 12 November 2015.