Methane oxidation and methane fluxes in the ocean surface layer and deep anoxic waters

Methane is supersaturated in sea water, and is typically at its maximum concentration in near-surface waters, which could support a significant sea-air flux. The magnitude and variability of the flux depends on the mechanisms which produce and consume methane in sea water. Here, we compare measured biological oxidation rates of methane with the diffusional fluxes computed from concentration gradients in the surface layer of the ocean, and show that oxidation of methane in sea water is a mechanism which modulates the flux of methane from marine waters to the atmosphere. Methane fluxes and oxidation rates were investigated in surface waters, at the oxic/anoxic interface and in deep anoxic waters of the Cariaco Basin. Measured oxidation rates were equivalent to 5% of the methane flux into oxygenated waters from the methane-rich deep waters and 10% of the flux into surface waters from the subsurface methane maximum. Thus oxidation was not sufficient to prevent a net sea-air flux. The total methane oxidation rate in the basin amounted to 1.5% of total primary production in the surface layer. Only a small fraction of oceanic primary production would be required to cycle through the methane pool to support the global atmospheric flux from the ocean¹.