Methane production, consumption and its carbon isotope ratios in the Southern Ocean during the austral summer

Abstract. The distribution of dissolved CH₄ in the Southern Ocean at 140° E was measured during the austral summer. Surface CH₄ was supersaturated on average, and the calculated mean sea-air flux rate was 0.32 μmol m⁻² d⁻¹. The vertical distributions exhibited a CH₄ maximum at approximately 125 m (ΔCH₄, 2.94 nM) below the chlorophyll-rich layer, suggesting a relationship between CH₄ production and plankton dynamics in this area. CH₄ oxidation and ocean movement characteristics in the deep layer led to the enrichment and fluctuation of δ¹³C_CH4. We estimated the influence of Southern Ocean CH₄, a source of isotopically heavy CH₄ to the atmosphere, on the global CH₄ budget to be approximately 0.19 Gg d⁻¹.