Noble Gas Signatures in Fresh Snow

Noble gas concentrations in water are ideal probes to study the surface and groundwater dynamics by providing indications of flow paths, connectivity between aquifers, and water residence times. In addition, noble gases are ideally suited to study the structure of ice and snow as the modifications of the structural network induced by the incorporation of noble gases are negligible. Additionally, recent noble gas studies have pointed out anomalies in noble gas concentrations derived from groundwater in fractured systems, likely due to the presence of preferential flow paths. It have been suggested that such anomalies originate from conditions at high altitude when rainwater does not have enough time to equilibrate with surface conditions [1]. Potential sources might include snow depending on the region considered.In order to document noble gas concentration patterns in snow we measured the concentration of stable noble gases (He, Ne, Ar, Kr, and Xe) in a series of fresh snow. Here, we outline a methodology to measure noble gases in snow samples.Our results show that snow has elevated He concentrations with depleted concentrations of other noble gases. Similar results have been recorded in sea ice, glacial firn and ground ice, pointing to similar noble gas pattern for bulk ice and snow. In addition, our results show relatively constant (< 14%) He and Ne concentrations while Ar, Kr and Xe show large variability in their concentrations (> 40%). These observations led us to investigate the structure of snow and potential host-sites within the crystal structure, as noble gases are dissolved into those available host sites. Our results show that He and Ne, which are known to have small atomic radii, are likely dissolved into the ice/snow crystal lattice, while heavy noble gases (Ar, Kr and Xe) are likely accommodated into defects or inclusions. That is, smaller variability recorded in light noble gases, are likely due to having He and Ne hosted within the snow crystal lattice structure, whereas heavy noble gas rely on the presence of defects and inclusions, which may randomly appear within the structure during snow formation. These new results can be used to better constrain the origin of ground ice and to investigate the structural transition mechanisms between snow to firn and ice. [1] Warrier, et al., (2013). Geophys. Res. Lett., 40, 3248