Methane production, consumption, and air‐sea exchange in the open ocean: An Evaluation based on carbon isotopic ratios

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The concentration of methane and its isotopic composition did not appear to vary significantly in the subtropical North Pacific during 1996–1997. Methane enters the atmosphere via the upper mixed layer at a rate of 1.6 μmol m−2d−1 (±0.1) with a δ13C value of approximately −42‰ (±1.5), with no apparent seasonal variation. In comparison, the sea‐air flux of methane in the Sargasso Sea was between 1.6 and 4.4 μmol m−2d−1 with an isotopic composition between −43 and −45‰. Excess methane in surface waters appears to be generated throughout the upper 300 m of the water column by bacterial methanogenesis. The methane concentration maxima occur at the pycnocline, suggesting that the maxima are supported by methanogenesis in suspended particles that accumulate at these depths. Particle incubation experiments show that methane production may occur in these microenvironments. The lack of a diurnal signal implies that methane production and consumption in the ocean is independent of day‐night cycles such as photosynthesis, grazing and vertical migration of zooplankton. The flux to the atmosphere appears to be the main sink for methane in the upper ocean; microbial oxidation and downward eddy diffusion account for only 3 and 6%, respectively, of the total methane loss from the upper 300 m. Below that depth, concentrations decrease and isotopic ratios vary with depth due to bacterial oxidation and mixing of water masses of different ages and different histories of methane input.