Sources and sinks of methane in sea ice: Insights from stable isotopes

We report on methane (CH4) stable isotope (d13C and d2H) measurements from landfast sea ice collected near Barrow (Utqiagvik, Alaska) and Cape Evans (Antarctica) over the winter-to-spring transition. These measurements provide novel insights into pathways of CH4 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 situ CH4 oxidation consumed a substantial fraction of the CH4 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 was also supersaturated in CH4 but with surprisingly high d13C 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.