The Ice - Stephen J. Pyne Weyerhaueser Cycle of Fire

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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. This tendency accelerates the overall process of pack formation by pushing floes from sites where ice is readily made into more marginal sites and by exposing, under favorable conditions, new seawater for freezing. The ice terrane becomes larger rather than more complex.

      Thermal metamorphism takes several forms. If the insulating snow cover is removed, the exposed surface may contract, leading to thermal cracks. It may also melt, allowing meltwater to percolate into the ice lattice, refreeze as infiltration ice, and release more heat that can induce further local melting. Such melting can occur over and over. The permeation of fresh meltwater through the floe flushes salts out of the interior and strengthens the lattice. In the Antarctic, the presence of brine, algae, and plankton within floes sometimes causes insolation to warm the interior of the floe, not merely the surface. This encourages brine migration and expulsion which, in turn, may lead to the formation of undersea ice—ice stalactites—that protrudes downward from the floe. In ideal systems ice accretes on the bottom of the floe and ablates from the top, and a floe will experience several cycles of metamorphism. Multi-year ice preserves not only more mechanical deformation than single-year ice, but more thermal cycling. There is some multi-year fast ice in protected embayments, and sea ice in the Weddell gyre survives perhaps two seasons. But unlike the Arctic, where sea ice is constantly reworked and converted into fresh ice, the Antarctic experiences an annual renewal of virtually the entire pack. Nearly all sea ice dates from the onset of the austral autumn and expires during the austral summer. The awesomeness of the pack derives from its enormous geographic extent, not its history or internal intricacy.

      Once embedded within the pack, a floe enjoys a collective identity. Floe interacts with floe, and the pack with the sea, the air, and other ice terranes. The pack has a collective geography and a collective history. Geographically, it is organized by two boundaries, one rigid and one dynamic. Near the continent, where it originates, the pack is bounded by land, ice shelves, and fast ice. As the season matures, free-floating floes move north as an ensemble of diverging shards. Later in the season the ice near the continent freezes solid or moves by means of the slow shearing of floe past floe. On its outer margin, the ice is not so rigidly confined. The dynamics of air and sea demarcate this outer fringe, and these processes vary by season and year. The outermost boundary of the pack lies somewhat inside the Antarctic convergence, which defines the perimeter of the Southern Ocean. The actual shape of the pack depends on storms that roughly follow, but do not precisely mimic, the contours of the ocean. Near-shore currents and winds drive coastal floes eastward, most spectacularly around the gigantic gyre that forms in the Weddell Sea. More distant floes spin within the larger, westward drift of the Antarctic circumpolar current.

      The constant pulverization of floes yields ice fragments. Some of these shards are swept under floes, causing them to thicken; some are carried out to sea in clusters and streamers; some are reabsorbed, in the proper season, to make new floes. To this ensemble land ices also contribute. Grounded bergs keep their surrounding near-shore environs cold, divert winds, damp out approaching waves, and ward off warmer water from entering shelves and bays. The presence of land ice thus encourages the formation of sea ice. Some bergs will be frozen in among the pack during the winter. Others, when wind and current urge them, plow through the pack like icebreakers. Their

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