The Ice. Stephen J. Pyne

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СКАЧАТЬ and away from the coast, and acquire a collective identity as the Antarctic pack. The expansion of the pack measures the outward march of subfreezing temperatures that accompanies the encroaching polar night.

      The Southern Ocean is never far from freezing. Much of the surface water is perennially supercooled. As less sunlight heats open waters and as cold air streams off the continent, water turns to ice. The fundamental crystal is a hexagon, but it can aggregate in two habits: sometimes hexagon stacks on hexagon to make long filaments and needles of ice, and sometimes hexagons are annexed side-by-side to make plates and disks. Initially, both habits are apparent. But the crystals exist in two states, and two kinds of ice result. Along the sea surface, needles give way preferentially to plates. The heat of fusion released during crystallization locally warms a site, and platy crystals appear between needles. As the platy structure expands, it coats the surface with a filmy grey sheen known as grease ice. Away from the surface, however, no such preference occurs. Ice crystals proliferate into an unstructured slush called frazil ice. The two kinds of ice evolve in different ways and contribute differently to the mass of the pack.

      Which ice predominates apparently depends on the role of snow, the formation of polynyas, and the local hydrometeorology. Congelation ice requires a stable environment, which it progressively sheets over. Frazil ice requires open, turbulent waters. It is assisted by winds that break apart the embryonic ice sheeting, by the convective mixing that results from the liberation of brine during surface-ice freezing, and by the persistence of open water like leads and polynyas. Snowfall assists both ice masses, contributing directly to the surface of congelation ice sheets, and it may be vital to the formation of frazil ice by providing suitable nuclei. Depending on the turbulence of the local seas, ice crystals may coat the surface in the form of congelation ice or they may ride like turbid sediment, frazil ice, within subsurface waters. Congelation and frazil ice frequently combine. Where frazil ice forms a dense sludge, congelation ice may grow along the exposed frozen surface; and where congelation ice evolves a well-developed structure, frazil ice may collect in clumps. Yet the two ices are also competitive. The sealing of the surface by congelation ice, for example, prevents snowfall from furnishing new nuclei for frazil ice.

      While some pack expansion is attributable to a simple process of freezing along the perimeter, most seems to occur by a process of interstitial freezing between floes. Storms, offshore winds, and ocean currents break up the ice veneer, the protofloes rift outward, and interstitial leads between them freeze. Where open water persists, sea ice—frazil ice in particular—can form in abundance. Polynyas thus coincide, not accidentally, with major centers of ice production. Some polynyas are semipermanent features of the Southern Ocean—in part the product of warm water upwelling and persistent winds. The pack expands not by a process of simple accretion along its margin but by a more complex interaction of winds, water, and ice.

      In many places, the pack will consist of about equal portions of congelation ice and frazil ice. In the Ross Sea, however, congelation ice predominates; and particularly in the protected, polynya-free region of McMurdo Sound, where fast ice is abundant, congelation ice is the norm. But in the Weddell Sea, where polynyas persist, between 50 and 90 percent of sea ice consists of frazil ice. Since nearly one-third of the entire Antarctic pack belongs within the Weddell gyre, frazil ice is a significant constituent of the ice field. Frazil ice tends to be more common near the coast (within 30–40 kilometers), where windblown snowfall is more abundant. Whatever the mixture, however, frazil ice will be supplemented by snow and other ices that become incorporated within the general sea ice matrix.

      Congelation ice brings structure to pack ice. Initially, the mingling of ice needles and ice plates creates a porous crystalline scaffolding called skeletal ice. Filamentlike crystals branch outward toward patches of water that are characterized by reduced salinity and higher freezing points. This framework thickens and spreads laterally across the sea surface into a sheen of grease ice. The evolution of congelation ice, if unbroken, interferes with the production of subsurface frazil ice and fundamentally redefines the boundary between air and sea. When this evolution is complete, the exchange of mass and energy between atmosphere and ocean ends. In its place, the pack initiates fluxes of salt, water, and heat between the ice and the ocean. Some of these processes involve positive feedback mechanisms, such that the presence of sea ice encourages the further production of sea ice. Snow insulates the ice floes, the ice floes insulate the sea, the chilled boundary of air and sea promotes fog, which further reduces insolation. Storm tracks and ice edge become interdependent. A dense pack increases albedo and reduces mixing. Ice leads to ice.

      The salt flux is especially important. When seawater freezes, it liberates salt and releases the latent heat of fusion. The venting of this heat by and large replaces the direct exchange of heat between ocean and atmosphere. The ice crystals themselves hold little salt. Instead salt is extruded into interstitial pores around which further ice crystals form. Its increased salt content lowers the freezing point of the brine, so that the brine pocket does not immediately freeze but becomes mechanically encased by the rapidly emerging ice lattice. As the lattice freezes inward, the brine pocket eventually shrinks. The more rapid the freezing, the more brine is entrapped within the structure and the less homogeneous is the resulting ice lattice. Where frazil ice is abundant, it brings a high proportion of brine to the overall matrix. The more brine, the weaker the ice structure. Meanwhile, the extrusion of brine salts through capillaries and inter-granular discharge channels upsets the density profile of the subsurface waters, and a convective cell develops in the waters beneath the pack.

      As a lattice evolves, the random orientation of the initial ice crystals is replaced by a stronger, more columnar framework. Grease ice and skeletal ice thicken and spread into ice paddies that resemble grey lily pads. As the overall structure grows by the erection of more congelation ice, other ices are captured. Frazil ice attaches in globs to the sides and bottom. Snow falls on the surface. When melted and refrozen, it forms lenses of infiltration ice. Other infiltration ice results from the capture of seawater on the surface by spray and wave. Underwater ice may develop in the form of ice stalactites, growing along brine extrusion channels. Anchor ice—frazil ice that collects on the shallow sea floor—may break loose and rise upward into the ice matrix on the surface. Ice flowers may form on the surface as feathery growths of crystals nucleate on salts excreted along freezing ice columns. Other ices—bergy bits, brash ice, the mechanical debris left by colliding floes—become incorporated into the matrix. The structure may even contain an ice biota, a product of the sudden entrapment of brine impregnated with algae and plankton. From all these sources, out of all these processes, emerges a complex ice fabric, a partially stratified ice breccia that is frequently spongy and malleable.

      This concatenation of ices eventually elaborates the ice paddy into a tabular slab called pancake ice. As pancake ice butts and jostles, its edges curl upward. Snow and seawater collect inside to produce infiltration ice. Further thickening, freezing, and deformation convert spongy slabs of pancake ice into a hardened floe. The floes multiply into an ice terrane, the pack.

      In its shape a floe crudely resembles the platy ice crystals that constitute its microstructure. Floes are roughly equidimensional, 10–100 meters across. Because of constant collisions, their top edges curl up. The thickness of a floe varies with the relative effectiveness of ice production and ice ablation. The floe can ablate from the bottom by melting and from the top by evaporation, sublimation, melting, and wind scour. The resulting equilibrium thickness varies from 2.75 to 3.35 meters. The actual composition and structure of the floe will depend on its unique history.

      This internal history will reflect thermal and mechanical metamorphisms. The collision of floe with floe, driven by wind and wave, can mechanically deform a floe. Ice masses can shear, crumple, and override one another to form pressure ridges and ice massifs. While in the Arctic pack such features are common, in the Antarctic they are restricted to local sites where high stress can accumulate. Some mechanical metamorphism occurs in fast ice, where at least one side of the ice mass is rigidly frozen to the shore, and in the oceanic gyres, like the Weddell, where floes are drawn into a crushing spiral. But more commonly the effect of wind and wave is to shove the pack outward rather than in on itself. СКАЧАТЬ