Effect of density on electrical conductivity of chemically laden polar ice

[1] Electrical conductivity measurements made using the dielectric profiling technique (DEP) are compared to chemical data from the top 350 m of the Dome C ice core in Antarctica. The chemical data are used to calculate the concentration of the major acidic impurities in the core: sulphuric acid and hydrochloric acid. The conductivity coefficients in solid ice for sulphuric acid (beta(H2SO4)) and hydrochloric acid (beta(HCl)) are found to be 4.9 and 4.5 S m(-1) M-1. These are consistent with previously found values for the acid conductivity coefficient at different sites and suggest that the same conductivity mechanisms are important in all polar ice. A method of rolling regression analysis is used to find the variation of the pure ice conductivity (sigma(infinity) pure) and the conductivity coefficient of sulphuric acid, beta(H2SO4), with depth. Then sigma(infinity) pure and beta(H2SO4) are assessed against changes in core density and hence volume fraction of ice, nu, due to the inclusion of air bubbles in the firn. Looyenga's model for dielectric mixtures applied to conduction in firn broadly predicts the variation observed in sigma(infinity) pure but does not fit well for ice above 110 m. A previous application of the theory of percolation in random lattices is used to model the conductivity coefficient in firn. The coefficient beta(H2SO4) is linked to nu by the power law: beta(H2SO4)(nu) proportional to beta(H2SO4) (1) (nu - nu(c))(t); where nu(c) is a threshold volume fraction below which no conduction can take place and is related to the geometry of the conducting lattice being modeled. The value of the exponent t is also dependent on the structure of the lattice and is here found to be t = 2.5, which is slightly lower than the previously obtained value of t = 2.7 for a structure where each grain has between 14 and 16 nearest neighbors. This model is consistent with the concept of conduction, via liquid H2SO4, taking place at two grain boundaries for firn. The model does not, however, preclude conduction taking place via acid situated at three grain boundaries or in an interconnected vein network at densities above 640 kg m(-3).

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