Melting-refreezing vs melting bags- equilibration method: an intercomparison for gases measurements in sea ice

Observations over recent decades suggest that sea ice plays a significant role in global biogeochemical cycles, providing an active biogeochemical interface at the ocean-atmosphere boundary. However, a pressing need exists to perform methodological intercalibration experiments in sea ice in order to obtain reliable measurements of basic biogeochemical properties. This is the case for two potent greenhouse gases, methane (CH4) and nitrous oxide (N2O). Two methods are reported in the literature: the melting bags-equilibration method, inherited from the widely applied equilibration method in oceanography, and the melting-refreezing method, typically used on ice-core sciences for ancient air extraction. Concentrations of CH4 and N2O were measured at high- resolution (i.e., 5 cm resolution) on four cores collected in spring 2015 in Abatus Bay (Prydz Bay, Antarctica). In agreement with previous unpublished inter-comparison, the melting bags-equilibration method shows constantly higher concentrations for CH4 and N2O than that of the melting-refreezing method. A series of tests have been performed on these two extraction methods to evaluate and understand these differences.