[1] The fluid permeability k of sea ice constrains a broad range of processes, such as the growth and decay of seasonal ice, the evolution of summer ice albedo, and biomass build-up. Such processes are critical to how sea ice and associated ecosystems respond to climate change. However, studies of k and its dependence on brine porosity f and microstructure are sparse. Here we present a multifaceted theory for k(f) which closely captures laboratory and field data. X-ray computed tomography provides an unprecedented look at the brine phase and its connectivity. We find that sea ice displays universal transport properties remarkably similar to crustal rocks, yet over a much narrower temperature range. Our results yield simple parameterizations for fluid transport in terms of temperature and salinity, and permit more realistic representations of sea ice in global climate and biological models. Citation:

We report the growth of GdN thin films and a study of their structure and magnetic and conducting properties. It is demonstrated that they are semiconducting at ambient temperature with nitrogen vacancies the dominant dopant. The films are ferromagnetic below 68 K, and a significant narrowing of the band gap is signaled by more than a doubling of its conductivity. The conductivity in the low-temperature ferromagnetic state remains typical of a doped semiconductor, supporting the view that this material is semiconducting in its ground state and that no metal-insulator transition occurs at the Curie temperature.

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[1] We have imaged sea ice single crystals with X-ray computed tomography, and characterized the thermal evolution of the pore space with percolation theory. Between À18°C and À3°C the porosity ranged from 2 to 12% and we found arrays of near-parallel intracrystalline brine layers whose connectivity and complex morphology varied with temperature. We have computed key porosity-dependent functions of classical percolation theory directly from the thermally driven pore space evolution of an individual sample. This analysis is novel for a natural material and provides the first direct demonstration of a connectivity threshold in the brine microstructure of sea ice. In previous works this critical behavior has been inferred indirectly from bulk property measurements in polycrystalline samples. From a finite-size scaling analysis we find a vertical critical porosity p c,v = 4.6 ± 0.7%. We find lateral anisotropy with p c,pll = 9 ± 2% parallel to the layers and p c,perp = 14 ± 4% perpendicular to them. Lateral connectivity is established at higher brine volumes by the formation of thin necks between the brine layers. We relate these results to measured anisotropy in the bulk dc conductivity and fluid permeability using a dual porosity conceptual model. Our results shed new light on the complex microstructure of sea ice, highlighting single crystal anisotropy and a step toward a realistic transport property model for sea ice based on percolation theory. We present full experimental details of our imaging and segmentation methodology based on a phase relation formulation more widely applicable to ice-solute systems.

[1] We present final results from a program to measure the thermal conductivity of sea ice with in situ thermistor arrays using an amended analysis of new and previously reported ice temperatures. Results from landfast first-year (FY) ice near Barrow, Alaska, and McMurdo Sound, Antarctica, are consistent with predictions from effective-medium models but 10-15% higher than values from the parameterization currently used in most sea ice models. We observe no previously reported anomalous near-surface reduction, which is now understood to have been an artifact, nor a convective enhancement to the heat flow, although our analysis is limited to temperatures below À5°C at which brine percolation is restricted. Results for landfast multiyear (MY) ice in McMurdo Sound are also consistent with effective-medium predictions, and emphasize the density dependence. We compare these and historical measurements with effective-medium predictions and the representation commonly used in sea ice models, developed originally for MY Arctic ice. We propose an alternative expression derived from effective-medium models, appropriate for both MY and FY ice that is consistent with experimental results, k = (r/r i )(2.11 À 0.011 q + 0.09 (S/q) À (r À r i )/1000), where r i and r are the density of pure ice and sea ice (kg m À3 ), and q (°C) and S (ppt) are sea ice temperature and salinity. For the winter and spring conditions studied here, thermal signatures of internal brine motion were observed rarely (22 times in 1957 days), and their maximum contribution to the total heat flow is estimated to be of the order of a few percent.