Open AccessCarbon isotopic characterization for the origin of excess methane in subsurface seawater
Author(s) -
Sasakawa Motoki,
Tsunogai Urumu,
Kameyama Sohiko,
Nakagawa Fumiko,
Nojiri Yukihiro,
Tsuda Atsushi
Publication year - 2008
Publication title -
journal of geophysical research: oceans
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.67
H-Index - 298
eISSN - 2156-2202
pISSN - 0148-0227
DOI - 10.1029/2007jc004217
Subject(s) - seawater , anoxic waters , supersaturation , methane , isotopes of carbon , environmental chemistry , saturation (graph theory) , oceanography , carbon fibers , stable isotope ratio , geology , mineralogy , total organic carbon , chemistry , materials science , mathematics , organic chemistry , combinatorics , composite number , composite material , physics , quantum mechanics
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.