The Ice. Stephen J. Pyne

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СКАЧАТЬ that reconstitute their inherited ices, perhaps purging them of trapped air. Ice shelves pick up saline ice, which is frozen into bottom crevasses. The freezing of this saline ice can, in turn, capture organic material for the general ice matrix. Brine infiltrates firn and snow—blown as sea spray, insinuating along the permeability boundaries that segregate firn from glacial ice, and rising through vertical fissures in the glacial ice. Blue ice and white ice may be splotched with black (dirty) ice from morainal debris, or with green ice embedded within the ice fabric. The origin of the jade-green ice is uncertain. Green ice includes a mixture of contaminants, especially particulate protein-nitrogen, but its peculiar appearance seems to derive from a pure, bubble-free ice fabric that apparently originates in the vigorous shear zones found in mountain glaciers. This odd ice—like bottles wrapped in snow—may represent the optical effect of light on a highly oriented, clear ice.

      The formation of the berg profoundly alters the composition of the ice. Liberated, the berg adds new materials, loses old ones, and rearranges its inherited constituents. More sea spray is absorbed and more brine infiltrates along the firn boundary. Additional saline ice may be acquired when the berg pauses in its journey, frozen amidst shore and fast ice. While the berg is at sea additional snow is added to the top, although more is also melted. This surface melting—the result of greater sunlight and higher ambient temperatures away from the ice field—percolates into the substratum and, in some cases, collects into snow swamps that drain off the sides of the berg. Meltwater percolation profoundly alters the internal ice structure, and seepage passes through the porous firn until it reaches a low enough temperature to refreeze. This change of state releases heat to the surrounding ice, with the result that the internal temperature of the iceberg rises to 6 degrees C. or higher. The recrystallized ice rearranges the stratigraphy of the inherited ice and adds new ice inclusions. This process of heat transfer by percolating meltwater—much more effective than heat conduction through ice—leads to rapid decay, a thermal rot. The disintegration of glacial ice releases the nitrates bound within the ice. The resulting nutrient bloom around the berg attracts algae and plankton. Other organisms are drawn to these primary producers, and a small marine biosphere encircles the berg, just as, for similar reasons, a marine biosphere encircles Antarctica.

      Icebergs are fundamental to the energy budget and hydrologic cycle of the ice field—and of the Earth. Only a minuscule portion of Antarctic snow melts, and that is restricted to intermittent streams of glacial meltwater confined to rocky oases. A much greater proportion of snow sublimates, vaporized by katabatic winds warming as they pour down from the polar plateau. Some snow is blown to sea by powerful offshore winds. But virtually all the ice on the continent ablates in the form of icebergs, calved from vast ice shelves and isolated outlet glaciers—the Amazons and Mississippis of Antarctica. The discharge of nutrients, water, and eroded earth from Antarctica is thus dissipated around the Southern Ocean by wandering icebergs. In places the deposition of debris by icebergs, “drift,” is considerable. Off the coast of South Africa more than a meter of glacial till has accumulated, evidently the product of iceberg rafting from Antarctica during the past glaciation. Similarly, icebergs affect the distribution of fresh waters around the Southern Ocean. Instead of entering the sea in relatively concentrated streams, there is a slow leaching of fresh water from a wandering mosaic of thaw-points. Since major melting does not occur until the berg leaves the near-shore environment, and usually not until the pack ice is breached, discharge by icebergs is diffuse and far-flung. This, in turn, affects the salinity, density profile, and thermodynamics of the outer Southern Ocean. It requires almost as much energy to melt ice as to raise the temperature of the resulting water to its boiling point. This energy must come from the sea.

      Similarly, the amount of water and heat drained into the Antarctic sink has global consequences. The Ice is the cold core of the planet. The amount of precipitation that falls on Antarctica is small by tropical standards, and only in selected areas does it approach the rates typical of temperate zones. The interior is a cold desert, the most total desert on Earth. But the continent is so huge and its storage capacity so enormous that the quantity of fresh water it contains dominates the global water budget. Over 60 percent of the world’s fresh water resides in the ice of Antarctica, an amount equivalent to sixty years of global precipitation or forty-six thousand years of flow by the Mississippi River. Its fresh water is the largest and most accessible of Antarctica’s mineral resources, and it is all discharged in quantum bits, as bergs.

      Annually, Antarctica produces some five thousand bergs, about 6.5 times the production of the Arctic. The average size of Antarctic bergs is much greater than that of Arctic bergs, each Antarctic berg averaging about one million tons of pure fresh water. Total production equals nearly 690 cubic kilometers of ice. Unlike Greenland bergs, calving off fast-moving glaciers, Antarctic bergs tend to calve from ice shelves or from the tongues of outlet glaciers protruding into the sea. The Greenland bergs, accordingly, resemble small peaks, while the Antarctic bergs resemble great tabular plateaus. On the average, Antarctic bergs are 100–400 meters long, with 12–40 meters of exposed freeboard. The ratio of length to width varies between 1: 1 and 4: 1, with an average of 1.6: 1. Similarly, the ratio of sail to keel, or the exposed freeboard to the submerged stratum, varies widely. For freshly calved tabular bergs, the ratio is 1: 10. The actual value will depend on the source and the character of the ablation process—in particular, the thickness of the firn layer and the vitality of fresh snowfalls, and the relative vigor of bottom melting compared to surface erosion. For domed bergs, the ratio is closer to 1:6; for blocky bergs, 1:4; for drydock bergs, with their much-eroded spindly pinnacles, 1:2.5. Both ratios—of sail to keel and length to width—are influential in determining how responsive the berg is to wind and wave, how it erodes, and how likely it is to overturn.

      Antarctic bergs may reach immense sizes. One sighted in 1927 was reported as 160 kilometers long, with a freeboard height of 35 meters. Others have been measured at 140 × 60 kilometers, 100 × 70 kilometers, and 100 × 43 kilometers. The greatest, tracked in 1965, was 140 kilometers long and featured a surface area of 7,000 square kilometers. One colossal berg, the Trolltunga, began as a severed ice tongue roughly the size of Belgium. The larger bergs cluster near the shore, where they contribute to a chilling of air and sea and to the production of an insulating fog, but once beyond the pack they risk rapid disintegration. In fact, some sort of fragmentation may be necessary to move a berg through the pack. Most bergs will not survive two months, or less than a single summer season, in the open sea.

      The berg is a record of Antarctic ice shapes no less than of ice substances. Its structural cryology chronicles ice deformation, much as its stratigraphy chronicles ice deposition. Berg structure is internal as well as external: it records ice deformations and ice movements experienced while the ice mass existed within the confines of the ice field, and it documents the free-floating behavior of the ice mass in its reincarnation as a berg. Together these two forms of movement—internal flow and external displacement—generate a hierarchy of shapes, extending from the microcosm of ice crystals to the macrocosm of ice sheets. As ice masses respond to new stress fields, their shapes reform. The shape of the berg itself is only a fleeting phase in this history.

      The berg, nonetheless, displays by far the greatest variety of structures. On the microscopic level, there are ice fabrics—mosaics of ice crystals and inclusions like air—that hark back to the origin of the ice mass and that all glacial ices exhibit. As more snow accumulates and as the entire ice matrix flows, the fabrics constantly reform, recrystallize, and reorganize. On a larger scale, other structures become visible—ice fissures and ice faults, crevasses torn by moving ice and icequake; ice strata, the product of the inexorable compression that transforms snow into firn, firn into ice, and glacial ice into metamorphic ice schists and ice gneisses; ice folds, the buckling of ice as it meets earth or more ice; long-wave flexures of ice, undulations of large-scale ice bodies. On the macroscopic level, structures are defined as the ice mass encounters distinct boundaries to its spread—sea, land, other ice. Ice masses become ice terranes. Some of these structures will vanish as new stresses reshape the ice mass. Some will persist for a time, surviving as ghostly sutures or relic fabrics. Some—those that lead to the rupture which finally releases the ice mass from its ice field—will shape the iceberg.

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