Supplementary material to "Significant methane undersaturation during austral summer in the Ross Sea (Southern Ocean)"

Methane (CH4) is a climate-relevant trace gas that is emitted from the open and coastal oceans in considerable amounts. However, its distribution in remote oceanic areas is largely unknown. To fill this knowledge gap, dissolved CH4 was measured at nine stations at 75 degrees S in the Ross Sea during austral summer in January 2020. CH4 undersaturation (mean: 82 +/- 20%) was found throughout the water column. In subsurface waters, the distribution of CH4 mainly resulted from mixing of water masses and in situ consumption, whereas the CH4 concentrations in the surface mixed layer were mainly driven by air-sea exchange and diapycnal diffusion between the surface and subsurface layers, as well as consumption of CH4. With a mean air-sea CH4 flux density of -0.44 +/- 0.34 mu mol m(-2) d(-1), the Ross Sea was a substantial sink for atmospheric CH4 during austral summer, which is in contrast with most oceanic regions, which are known sources.