= Solution
In seawater, freezing produces nearly salt-free solid and leaves salt in the interstitial liquid. The growing cover is therefore a <mushy layer>, with <sea-ice crystal fabric> determined by both crystal growth and the mechanical history. Crystallographic c-axes are perpendicular to the plates' basal planes; grain elongation and c-axis orientation must not be confused.
For <first-year sea ice> growing by congelation, heat loss first brings the surface water to its salinity-dependent freezing point. Small <frazil ice> crystals may form and collect into a thin continuous sheet, or a quiet surface can freeze directly; the resulting <nilas> provides the initial cover. Once the water surface is protected from strong agitation, heat transported by <heat conduction> upward through the sheet permits freezing onto its underside. Competitive growth favors grains with fast basal-plane growth directed downward, so grains with approximately horizontal c-axes crowd out less favorably oriented grains. The mature <congelation ice> consists predominantly of long vertical columns beneath a finer initial surface layer. Within and between these grains, the advancing cellular interface traps brine-bearing spaces. https://www.pmel.noaa.gov/arctic-zone/essay_wadhams.html[The growth sequence and crystallographic selection] explain why the resulting texture is columnar rather than a random aggregate of floating crystals.
For <pancake ice>, wind and <surface gravity waves> prevent an uninterrupted ice sheet. Supercooled surface water produces suspended <frazil ice>; crystals accumulate into grease ice and then small coherent cakes. Wave-induced convergence, collisions and repeated accumulation of frazil increase cake size and thickness. Collisions and overwash deposit slush around the margins, building the characteristic raised rims. The crystal aggregate is predominantly granular, with much less systematic orientation than in <congelation ice>. A calmer interval can add a columnar basal layer, so a pancake need not be purely granular throughout. https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2002JC001373[Observed pancake structures] include frazil layers, basal congelation growth and rafted cakes.
In Antarctic <consolidated pancake ice>, the active edge initially follows this frazil-pancake route. As concentration rises and waves are attenuated, pancakes jam, raft and become frozen together, with interstitial frazil filling the gaps. The consolidated sheet retains overlapping cake outlines, stacked granular layers and an irregular underside, rather than acquiring the simple fabric of a single quiet-water-grown sheet. Later basal freezing can add <congelation ice>, but the granular fraction often remains large. Snow loading or overwash can flood the top; freezing this slush adds granular <snow ice>, a separate contribution from seawater-derived frazil. These mechanisms produce the heterogeneous <sea-ice crystal fabric> documented in https://tc.copernicus.org/articles/16/2899/2022/index.html[cores from the Antarctic marginal ice zone]. \b[Quiet basal growth produces columnar ice; wave-driven aggregation and consolidation produce predominantly granular, layered ice.]
The main efficient net desalination mechanisms require transport through the ice boundary. In <gravity drainage of sea-ice brine>, cold saline liquid is denser than the water below, so a connected permeable pore network permits downward flow and compensating entry of less saline seawater. Dense brine can collect into channels and convective drainage pathways. For a layer of depth $H$, a <Darcy law> estimate of drainage speed is $u_D\sim K\Delta\rho g/\mu$, giving a drainage time $H/u_D$ and showing directly why <permeability of a porous medium> and <buoyancy> matter. Warm, brine-rich ice drains much more readily than cold ice whose pores have become poorly connected. This is important during growth and can intensify as warming reconnects channels; an isolated sealed brine pocket cannot drain merely because its liquid is dense. The connectivity constraint is supported by https://www.math.utah.edu/~golden/resources/mypapers/GOLDEN_etal_SCIENCE_1998.pdf[sea-ice permeability measurements].
In <meltwater flushing of sea ice>, water supplied by surface snow melt or melt ponds has a hydraulic head above sea level and percolates down connected pores and cracks, displacing brine. It can remove salt rapidly in warm summer ice. Its limits are the supply and head of meltwater and pore connectivity; lateral runoff may reduce the amount available for vertical flushing. Gravity-driven drainage can act without surface melting, whereas flushing cannot.
<Brine expulsion> occurs when additional freezing and solid expansion pressurize an inclusion and force liquid into nearby passages. This is often internal redistribution: expelling brine from one inclusion does not desalinate the entire floe unless that liquid reaches the ocean. <Migration of brine inclusions> involves melting on a warmer wall and refreezing on the colder wall; molecular salt diffusion through liquid connections is also possible. These slow processes are far less effective at exporting bulk salt than connected advective drainage. Diffusion through the ice lattice itself is extremely slow. Likewise, excluding salt from each growing crystal is not by itself proof of salt loss from a whole ice-brine mixture. https://pure.mpg.de/rest/items/item_993947/component/file_3444911/content[Desalination experiments and transport calculations] distinguish net salt export from internal redistribution. \b[Gravity drainage dominates desalination of permeable growing ice; meltwater flushing can dominate summer desalination; expulsion, inclusion migration and diffusion are comparatively ineffective without a boundary-crossing flow.]
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