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.

Highlights

  • CH4 undersaturation 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 15 the austral summer

  • 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

  • Lateral transport of water masses by the slope current occurred in the edge of the Ross Sea, where a southward intrusion of saline Modified Circumpolar Deep Water (MCDW) was observed during our cruise (a decrease in MCDW contribution from station R10 (300 m) to R7 (100 m), Figure 2b)

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Summary

Introduction

Methane (CH4) is one of the most important greenhouse gases. The dry mole fractions of CH4 in the atmosphere have 25 increased continuously since the onset of the industrial revolution, contributing more than 20% of the anthropogenic radiative forcing in the lower atmosphere (IPCC, 2021; Saunois et al, 2020).

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