The Ice. Stephen J. Pyne

       [The ship] came to the limits of the world, to the deep flowing

       Oceanus … shrouded in mist and cloud.…

      —Homer, The Odyssey, Book XI

      I will not say it was impossible anywhere to get in among this Ice, but I will assert that the bare attempting of it would be a very dangerous enterprise and what I believe no man in my situation would have thought of. I whose ambition leads me not only farther than any other man has been before me, but as far as I think it possible for man to go, was not sorry at meeting with this interruption, as it in some measure relieved us from the dangers and hardships, inseparable with the Navigation of the Southern Polar regions.

      —Capt. James Cook, Journals, Voyage of the Resolution and Adventure (1774)

      PROGRADATION

      They raft across a wine-dark sea, sometimes isolated, sometimes strung together, circling the continent like gears within a vast orrery of floating ice.

      A veil of low cloud and grey fog, a stygian current of black sea, a mobile breakwater of white ice: these define the terrane of the pack and the oscillating perimeter of The Ice. The boundary is multiple. The cold core of the global atmosphere and the tangled vortex of the world ocean roughly coincide with the ice field of Greater Antarctica, creating a complex zone of mixing. The Ice is surrounded by a circumference of swirling storms, where Antarctic and subantarctic air masses mingle; by braided currents and fronts that mix Antarctic and subantarctic waters; by a shadow line that separates the varied calendar of temperate time from the two seasons of the polar day. That all of these zones approximately overlap accounts for the intensity of Antarctica’s isolation. Of these processes, sea ice is both a product and a producer. As the zones wax and wane with the seasons, the ice field grows and decays on a grand scale, and the pack becomes the effective boundary of the continent.

      While the iceberg is the most interesting ice mass in Antarctica, the pack ice is the most interesting ice terrane. Its rapid life cycle, its explosive winter growth and catastrophic summer collapse, the infinite movements of its numberless floes, with their constant rupturing and resuturing, the volatility of its position as a solid-phase boundary between two fluid regimes, air and sea—all give the pack a collective dynamism and variety unparalleled among Antarctic ices. This is the most complex and active of Antarctic systems. In part, the pack reflects this vigor and reduces it. The floes form an ice membrane between air and sea, atmosphere and hydrosphere; between land and the air-sea matrix, Antarctica and the fluids that bind it to the Earth; between biosphere and cryosphere, life and an inorganic lithosphere. In this twilight zone between Earth and Ice are mixed sea and sky, sea and ice, sky and ice, sea ice and land ice, life and lithosphere. Even in their colors and geometry the black sea and white ice recapitulate the two polar seasons: the summer day and the winter night.

      This intermingling, so characteristic of the pack and so different from what goes on in the rest of the ice field, leads not to more complexity but to less. As the pack matures, it reduces dramatically the interactions among its component systems, and the reductionism and solipsism of The Ice are boldly extended outward. The mingling of sea, sky, and ice makes this region among the cloudiest on Earth: a perennial fog hangs cloyingly over the pack; the scene is shrouded in a stormy grey twilight. The ragged front of the pack creates a geographic warp, an icescape where space expands and dissolves, where time slows and distorts. Around the ice field the floes orbit, waxing and waning with the seasonal tides: at times loose and free-floating in the circumpolar current, like ice fragments trapped within the rings of Saturn, and at times frozen more or less solid, slowly creaking around, their gyres like Aristotelian spheres of quintessential crystal. Only the power of the outside world reverses the trend to greater and greater simplicity and uniformity. For millennia, the pack was the commanding barrier not only to human travel but to human mind. One could reach The Ice only by passing through the pack.

      Glaciology of the Pack

      The progradation of the pack begins with the coming of the austral twilight. The sun circles low on the horizon, a cold white globe. As ambient temperatures fall below the freezing point of seawater, sea ice begins to form. Ice forms first in protected embayments along the coast, most rapidly in the protected seas that ring the continent. Some takes the form of congelation ice, which organizes surface crystals into a scaffolding; some takes the form of frazil ice, slushy clumps of ice crystals suspended within supercooled seawater. From their structured mixing the floe evolves, by a series of stages. More ice and snow are added. Metamorphism restructures the ice breccias that comprise the floe. What happens on the microscopic level is then repeated on a macroscopic level. Individual floes interact, form a matrix, accept more ices, move around 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,