JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. C2, 3051, doi:10.1029/2001JC000887, 2003
[50] This work was designed to provide information needed for a more fundamental understanding of how temperature affects the relationship between the structural and optical properties of sea ice. Analysis of high resolution imagery revealed a large range of brine inclusion sizes, including pockets an order of magnitude smaller and more numerous than previously reported. Vertical thin sections enabled us to distinguish between brine pockets and brine tubes, and to estimate their relative importance to the optical properties of first-year sea ice. This was done by calculating an equivalent cross-sectional area 
for the population of each constituent. Although should be related to the amount of scattering, it does not take into account temperature-dependent changes in the index of refraction or the phase function and, hence, is not a direct measure of the total scattering. Estimates of indicate that, even though tubes contained at least 90% of the brine, pockets are responsible for about 25% of the total equivalent cross-sectional area represented by brine inclusions.
[51] In considering how temperature changes impact , we make a distinction between chemical changes related to the precipitation of salt crystals and structural changes related to the size distributions of brine and gas inclusions. Changes in chemistry are determined strictly by bulk properties (e.g., temperature, salinity, density, brine composition) and should be largely independent of the type and age of the ice. The ice microstructure, on the other hand, is determined not only by the temperature and salinity of the ice, but also by its growth history. Structural properties are least important at temperatures below -23°C where (T) is controlled by hydrohalite precipitation. Between -23° and -8°C, the increasing cross-sectional area of brine and gas inclusions is largely balanced by the decreasing cross-sectional area of mirabilite crystals. Optical measurements reported by Light [1995] indicate that a similar balance was also found in very high salinity (15‰) laboratory-grown ice, suggesting that this may be a fundamental characteristic of first-year sea ice and, possibly, of sea ice in general. Above -8°C, (T) is determined strictly by how the structural elements respond to temperature.
[52] While this study has provided some new insights into sea ice microstructure and how it changes with temperature, important questions remain. For example, most of the precipitated salt crystals appear to be less than 0.01 mm in size but it is unclear what effective size would be appropriate for radiative transfer calculations. Nor is it clear whether salt crystals tend to increase more in size or number as cooling progresses, an issue of concern for predicting how the optical properties change at lower temperatures. In the absense of adequate crystallization nuclei, supersaturation of salts in the brine may even be a possibility. Higher resolution observations and concurrent optical data should help to resolve such questions. Given the importance of scattering by these precipitates at low temperatures, more work should be done to understand the precipitation patterns and optical properties of salt crystals in sea ice. Another interesting question is why new gas bubbles did not normally form within enlarging brine inclusions during warming. This might be related to the presence of microcracks which naturally develop around inclusions as the ice cools and brine is expelled. A film of brine retained in the cracks could keep them from permanently rehealing, allowing them to melt and enlarge somewhat on warming. Such cracks might act as reservoirs for excess brine, or might provide channels through which individual inclusions could maintain hydraulic contact and effectively share a single gas bubble. Alternatively, evidence exists that substantial negative pressures can exist within fluid inclusions. Liquids that become metastably stretched to occupy a larger volume at a lower density than predicted by equilibrium may persist if bubble formation is not energetically favorable [Roedder, 1984]. Since sea ice is a very compliant material, relaxation in the ice matrix could accommodate some of the volumetric strain associated with the apparent lack of bubble formation as well. Answers to some of these questions may involve processes related to the small-scale surface physics which are beyond the scope of this study. It should also be noted that these samples were cooled to solid CO2 temperatures for shipment and it is not known to what degree this cooling might have affected the natural inclusion distributions. It would be worthwhile repeating these measurements with samples in the field to verify the laboratory observations.
Citation: Effects of temperature on the microstructure of first-year Arctic sea ice, J. Geophys. Res., 108(C2), 3051, doi:10.1029/2001JC000887, 2003.