The flow of ice is temperature-dependent, but direct measurements of englacial temperature are sparse. The dielectric attenuation of radio waves through ice is also temperature-dependent, and radar sounding of ice sheets is sensitive to this attenuation. Here we estimate depth-averaged radar-attenuation rates within the Greenland Ice Sheet from airborne radar-sounding data and its associated radiostratigraphy. Using existing empirical relationships between temperature, chemistry, and radar attenuation, we then infer the depth-averaged englacial temperature. The dated radiostratigraphy permits a correction for the confounding effect of spatially varying ice chemistry. Where radar transects intersect boreholes, radar-inferred temperature is consistently higher than that measured directly. We attribute this discrepancy to the poorly recognized frequency dependence of the radar-attenuation rate and correct for this effect empirically, resulting in a robust relationship between radar-inferred and borehole-measured depth-averaged temperature. Radar-inferred englacial temperature is often lower than modern surface temperature and that of a steady state ice-sheet model, particularly in southern Greenland. This pattern suggests that past changes in surface boundary conditions (temperature and accumulation rate) affect the ice sheet's present temperature structure over a much larger area than previously recognized. This radar-inferred temperature structure provides a new constraint for thermomechanical models of the Greenland Ice Sheet.

The low-frequency electrical properties of mixtures of silicates and saline H(2)O were measured over broad ranges of temperature and frequency to assess the subfreezing interactions between these materials synoptically, particularly the effects of adsorbed, unfrozen water. Adsorbed water content was determined using nuclear magnetic resonance. Materials were chosen to control effects of grain size and mineralogical complexity, and the initial salt content was also specified. The temperature-dependent DC conductivity of a sand-salt-H(2)O mixture was found to be described well by Archie's law, with either brine or salt hydrate (above and below the eutectic, respectively) as the conductive and partially saturating phase. For materials with pore sizes less than a few micrometers, the brine/hydrate channels become disconnected, and the DC conductivity is controlled by the surrounding ice. High DC conductivity in a montmorillonite-H(2)O mixture is attributed to proton mobility in interlayer adsorbed water. The ice content of the sand mixture was recovered from the static dielectric permittivity using a power-law mixing model. Ice relaxation frequencies were higher than those observed in defect-saturated saline ice, indicating that additional defects are able to form in proximity to silicate surfaces. Five dielectric relaxations related to H(2)O were identified: two orientation polarizations (ice and adsorbed water), two Maxwell-Wagner interfacial polarizations (because of the conductivity differences between hydrate and silicate and adsorbed water and ice, respectively), and a low-frequency dispersion, probably caused by charge hopping. Thicknesses of a few H(2)O monolayers and the preference of hydronium for surface sites, making adsorbed water slightly acidic, favor protons as the mobile charges responsible for these adsorbed-water interfacial polarizations.

We measured the 1 mHz-1 MHz electrical properties of ice-hydrate binary systems formed from solutions of NaCl, CaCl(2), and MgSO(4), with supplementary measurements of HCl. Below the eutectic temperature, electrical parameters are well described by mixing models in which hydrate is always the connected phase. Above the eutectic temperature, a salt concentration threshold of approximately 3 mM in the initial solution is required for the unfrozen brine fraction to form interconnected, electrically conductive networks. The dielectric relaxation frequency for saline ice increases with increasing impurity content until Cl(-) reaches saturation. Because there is insufficient H(3)O(+) for charge balance, salt cations must be accommodated interstitially in the ice. Dielectric relaxations near the ice signature were identified for CaCl(2).6H(2)O and MgSO(4).11H(2)O but not for NaCl.2H(2)O. Ionic and L-defect concentrations in salt hydrates up to approximately 10(-4) and 10(-3) per H(2)O molecule, respectively, follow from the electrical properties, Jaccard theory, and the assumption that protonic-defect mobilities are similar to ice. These high defect concentrations-up to a few orders of magnitude greater than saturation values in ice-indicate that intrinsic disruption of hydrogen bonding in salt hydrates is common.