| Summary: | In 2012, an unexpected CH4 excess has been reported above open leads in the Arctic Ocean showing that sea ice plays a role in the ocean-atmosphere CH4 dynamics. However, the processes involved there have not yet been identified. We performed CH4 stable isotope (d13C and dD) analyses on sea ice samples, as well as geochemical and physical measurements, to determine the possible pathways involved in CH4 production/removal in or under sea ice. We present results from ice cores collected above the shallow shelf of Barrow (Alaska) from January to June 2009 as well as in the landfast ice of McMurdo Sound (Antarctica) from September to November 2012. We found a clear difference in isotopic signature between the two sites. The McMurdo ice was supersaturated in CH4 and showed isotopic signatures surprisingly enriched in heavy isotopes (d13C between -47 and -12 ‰ and dD between -87 and -350‰). No natural pathways have yet been identified with such isotopic signatures, but we suggest that aerobic CH4 formation in or under the ice might be a candidate. In contrast, the CH4 concentrations were much larger in ice overlying the shallow shelf of Barrow and there the origin of CH4 was clearly biogenic (d13C between -48 and -68 ‰ and dD between -180 and -250‰), thus likely originating from the sediment. In the McMurdo ice, the seasonal evolution shows that CH4 was becoming more enriched in heavy isotopes with time, suggesting the occurrence of aerobic oxidation processes in the ice.
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