Carbon isotopic characterization for the origin of excess methane in subsurface seawater

We collected samples of seawater, zooplankton, and sinking particles in the northwestern North Pacific to determine the source of excess CH4 over the saturation value in equilibrium with the atmospheric CH4 in the oxygenated open ocean, using stable carbon isotope as a tracer. We found that subsurface (similar to 100 m depth) seawater is supersaturated (up to 12%) with C-13-enriched CH4 (up to -33.1%) relative to surface seawater in equilibrium with the atmosphere (-47%), suggesting that in situ addition of C-13-enriched CH4 must be responsible for CH4 enrichment at depth. The delta C-13 of CH4 emitted from sinking particles (from -36.7 +/- 1.2% to +5.9 +/- 7.5%) is within the range of that of excess CH4 in seawater, suggesting that the major source of subsurface excess CH4 is sinking particles. The unusually C-13-enriched delta C-13 composition of CH4 emitted from sinking particles suggests that active microbial CH4 oxidation occurs within the oxic/anoxic boundary of these particles. On the basis of the Rayleigh equation, we estimated that at least 62% of CH4 produced within the anoxic center of sinking particles is oxidized within 100 m of the surface.