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 CH 4 over the saturation value in equilibrium with the atmospheric CH 4 in the oxygenated open ocean, using stable carbon isotope as a tracer. We found that subsurface (∼100 m depth) seawater is supersaturated (up to 12%) with 13 C‐enriched CH 4 (up to −33.1‰) relative to surface seawater in equilibrium with the atmosphere (−47‰), suggesting that in situ addition of 13 C‐enriched CH 4 must be responsible for CH 4 enrichment at depth. The δ 13 C of CH 4 emitted from sinking particles (from −36.7 ± 1.2‰ to +5.9 ± 7.5‰) is within the range of that of excess CH 4 in seawater, suggesting that the major source of subsurface excess CH 4 is sinking particles. The unusually 13 C‐enriched δ 13 C composition of CH 4 emitted from sinking particles suggests that active microbial CH 4 oxidation occurs within the oxic/anoxic boundary of these particles. On the basis of the Rayleigh equation, we estimated that at least 62% of CH 4 produced within the anoxic center of sinking particles is oxidized within 100 m of the surface.