Significant methane undersaturation during austral summer in the Ross Sea (Southern Ocean)

Abstract. Dissolved methane (CH₄) was measured at 9 stations along a transect at 75° S in the Ross Sea during austral summer in January 2020. CH₄ undersaturation (mean: 82 ± 20 %) was found in the water column, with a mean air-sea CH₄ flux density of −0.58 ± 0.48 μmol m⁻² day⁻¹, which suggests that the Ross Sea was a net sink for atmospheric CH₄ during the austral summer. Simple box-model calculations revealed that the CH₄ depletion should occur in the surface mixed layer because of CH₄ oxidation and advection of CH₄-poor waters. We propose that freshwater injection caused by sea-ice melting in summer dilutes CH₄ concentrations within the surface layer and thus increases its potential for atmospheric CH₄ uptake in the Ross Sea. Thus, we argue that both CH₄ consumption and sea-ice melting are important drivers of CH₄ undersaturation, which implies that the high-latitude area of the Southern Ocean is a sink for atmospheric CH₄. We estimated that the Southern Ocean (> 65° S) takes up about 0.02 % of the global CH₄ emissions and thus represents a minor sink for atmospheric CH₄.