We report on methane (CH₄) stable isotope (d¹³C and d ²H) measurements from landfast sea ice collected near Barrow (Utqiagvik, Alaska) and Cape Evans (Antarctica) over the winter-to-springtransition. These measurements provide novel insights into pathways of CH ₄ production and consumption in sea ice. We found substantial differences between the two sites. Sea ice overlying the shallow shelf of Barrow was supersaturated in CH₄ with a clear microbial origin, most likely from methanogenesis in the sediments. We estimated that in situCH₄ oxidation consumed a substantial fraction of the CH ₄ being supplied to the sea ice, partly explaining the large range of isotopic values observed (d¹³C between -68.5 and -48.5 ‰ and d²H between -246 and -104 ‰). Sea ice at Cape Evans wasalso supersaturated in CH₄ but with surprisingly high d ¹³C values (between -46.9 and -13.0 ‰), whereas d²H values (between -313 and -113 ‰) were in the range of those observed at Barrow.These are the first measurements of CH₄ isotopic composition in Antarctic sea ice. Our data set suggests a potential combination of a hydrothermal source, in the vicinity of the Mount Erebus, with aerobic CH₄ formation in sea ice, although the metabolic pathway for the latter still needs to be elucidated. Our observations show that sea ice needs to be considered as an active biogeochemical interface, contributing to CH₄ production and consumption, which disputes the standing paradigm that sea ice is an inert barrier passively accumulating CH₄ at the ocean-atmosphere boundary.