Abstract
We report on methane (CH4) stable isotope (δ13C and δ2H) 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 (δ13C between –68.5 and –48.5 ‰ and δ2H between –246 and –104 ‰). Sea ice at Cape Evans was also supersaturated in CH4 but with surprisingly high δ13C values (between –46.9 and –13.0 ‰), whereas δ2H 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.
Highlights
The contribution of oceans to the atmospheric methane (CH4) budget is subject to large uncertainties given the small coverage of existing dissolved CH4 measurements and the poor understanding of the processes at play
Observations during the recent decades suggest that sea ice is an active biogeochemical interface at the ocean– atmosphere boundary, contributing up to 60% of the primary production in some parts of the Arctic Ocean (Fernandez-Mendez et al, 2015) and 50% of the CO2 uptake south of 50S (Delille et al, 2014)
We report the CH4 stable isotopic composition in landfast sea ice from the Arctic (Barrow, Utqiagvik, Alaska) and the Southern Ocean (Cape Evans, Ross Sea)
Summary
The contribution of oceans to the atmospheric methane (CH4) budget is subject to large uncertainties given the small coverage of existing dissolved CH4 measurements and the poor understanding of the processes at play. Observations during the recent decades suggest that sea ice is an active biogeochemical interface at the ocean– atmosphere boundary, contributing up to 60% of the primary production in some parts of the Arctic Ocean (Fernandez-Mendez et al, 2015) and 50% of the CO2 uptake south of 50S (Delille et al, 2014). Jacques et al: Sources and sinks of methane in sea ice: Insights from stable isotopes the exchange of CH4 between the ocean and the atmosphere is still largely unknown, as well as the potential for CH4 production and consumption within the sea ice itself. Significant CH4 elevations were measured in the Arctic atmospheric boundary layer, associated with fractional sea-ice cover, the underlying process was not identified (Kort et al, 2012). Due to the lack of measurements and heterogeneity of the system, these fluxes to date are poorly characterized and quantified, so that the role of sea ice as a net sink or source of CH4 remains unclear
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