Английская Википедия:Blue-ice area

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Файл:Miller Range, Antarctica - Meteorite (2).jpg

A blue-ice area in the Miller Range with a meteorite

A blue-ice area is an ice-covered area of Antarctica where wind-driven snow transport and sublimation result in net mass loss from the ice surface in the absence of melting, forming a blue surface that contrasts with the more common white Antarctic surface. Such blue-ice areas typically form when the movement of both air and ice are obstructed by topographic obstacles such as mountains that emerge from the ice sheet, generating particular climatic conditions where the net snow accumulation is exceeded by wind-driven sublimation and snow transports.

Only about 1% of Antarctic ice area can be considered to be blue-ice area, but they have attracted scientific interest due to the large numbers of meteorites that accumulate on them; these meteorites either fall directly on the blue-ice area and remain there, or they fall elsewhere into the ice sheet and are transported to the blue-ice area by ice flow. Additionally, ice up to 2.7 million years old has been obtained from blue-ice areas. Blue-ice areas are sometimes used as runways for aircraft.

Appearance

Файл:Transantarctic pho 2017334 lrg.jpg

A glacier in the Transantarctic Mountains. Blue ice is seen as a lighter blue in the photograph, while the darker blue is formed by re-frozen melt ponds.

Blue-ice areas have a generally smooth and oftenШаблон:Sfn rippled appearance, a blue colourШаблон:Sfn and a sparseness of bubbles in the ice.Шаблон:Sfn This light blue colour is a consequence of the absorption of light by ice and air bubbles encased within it, and is the source of the name "blue-ice area". It contrasts markedly with the white colour of Antarctic plainsШаблон:Sfn and can be seen from space and from aerial images,Шаблон:Sfn while the density of the blue ice makes it appear on radar images as a dark ice form.Шаблон:Sfn Scalloped or rippled surfaces have almost regular surface patterns, although wholly smooth blue-ice areas exist as well,Шаблон:Sfn and the terrain even of rippled surfaces features very low aerodynamic roughness, perhaps among the lowest of all permanent natural surfaces.Шаблон:Sfn This is because most aerodynamic drag is caused by surface anomalies less than a centimetre long, not larger uneven forms.Шаблон:Sfn Wave structures form through sublimation.Шаблон:Sfn

The occurrence of supraglacial moraines at blue-ice areas has been reported;Шаблон:Sfn these form when debris contained within a glacier accumulates at the surface due to melting or sublimation.Шаблон:Sfn Small depressions in the ice known as cryoconite holes are common and are formed where rocks got embedded in the ice,Шаблон:Sfn but are absent on more mountainous blue-ice areas.Шаблон:Sfn

Typical blue-ice areas often feature intense katabatic winds, with average winds reaching Шаблон:Convert and gusts of up to Шаблон:Convert; such winds can remove and take up large amounts of snow.Шаблон:Sfn They are usually warmer than comparable snow-covered areas, sometimes by up to Шаблон:Convert, which makes them identifiable from brightness temperature imaging. This warming is due to the lower albedo of the blue ice compared to snow, which results in them absorbing more sunlight and warming more.Шаблон:Sfn Blue-ice areas also alter the climate above them.Шаблон:Sfn

As commonly defined, blue-ice areas display little or no evidence of melting,Шаблон:Sfn thus excluding glaciers and frozen lakes in the Antarctic Dry Valleys where sublimation-dominated ice also occurs, but which may be more comparable to the ablation areas of regular glaciers.Шаблон:Sfn

Occurrence

Файл:Antarctic blueice hg.jpg

Occurrence of blue-ice areas (dark blue) in Antarctica

Blue-ice areas were first discovered in 1949-1952 by the Norwegian–British–Swedish Antarctic Expedition. They have been identified only in Antarctica,Шаблон:Sfn although similar ice patches on Greenland have been reportedШаблон:Sfn and blue ice is widespread at glaciers worldwide.Шаблон:Sfn Blue-ice areas make up only about 1% of the Antarctic surface ice;Шаблон:Sfn however, they are locally commonШаблон:Sfn and scattered across the continent, especially in coastal or mountainous areas,Шаблон:Sfn but not directly beside the coastline.Шаблон:Sfn

They have been found in Dronning Maud Land, the catchment of the Lambert Glacier, the Transantarctic Mountains and Victoria Land.Шаблон:Sfn Individual locations in Antarctica include areas of the Allan Hills,Шаблон:Sfn the Queen Fabiola Mountains (the Yamato ice field there covers an area of Шаблон:Convert and is the largest such structure),Шаблон:Sfn Scharffenberg-BotnenШаблон:Sfn and the Sør Rondane Mountains.Шаблон:Sfn Their location has been correlated with specific atmospheric pressures, temperaturesШаблон:Sfn and a relative humidity of less than 100%.Шаблон:Sfn

Origin and processes

Blue-ice areas are regions where more snow is removed by sublimation or by wind than accumulates by precipitation or wind-driven transport,Шаблон:Sfn leading to the emergence of (blue) ice. In most of Antarctica, the net tendency is for snow to accumulate except in coastal Antarctica where melting occurs and blue-ice areas where sublimation dominates.Шаблон:Sfn This sublimation occurs at rates of Шаблон:Convert snow water equivalent and is balanced by ice flow, with the sublimation rate decreasing with elevationШаблон:Sfn and increasing with temperature. Summer also increases the sublimation rate, although it still occurs during winter.Шаблон:Sfn Winds remove snow that rest on the surface and could even scour exposed ice away, although the occurrence of scouring is not established without doubtШаблон:Sfn and the role of abrasion is also unclear.Шаблон:Sfn

Such areas exist even in the coldest parts of Antarctica,Шаблон:Sfn and they are characterized by high mean wind speeds and low precipitation.Шаблон:Sfn Once they have formed, the smooth surface prevents snow from accumulating as it is quickly blown away by the wind, and the blue colour increases the absorption of sunlight and thus sublimation; both these phenomena act to maintain the blue-ice area, and wind-driven transport of warm air can cause the blue-ice area to expand downwind.Шаблон:Sfn

Blue-ice areas are common in mountainous regions. Presumably, irregular surface topography obstructs ice flow and locally creates atmospheric conditions suitable for the development of blue-ice areas. Irregular topography does not need to be exposed to the surface to generate blue-ice areas,Шаблон:Sfn although they must have an effect on the ice surface topography to induce the formation of blue-ice areas. Consequently, many blue-ice areas form when ice thicknesses decrease, which has been postulated to happen during interglacialsШаблон:Sfn although in general the past history of blue-ice areas is poorly known. Such areas may not have existed at all during glacial times when the ice sheet was thicker.Шаблон:Sfn Changes in mean wind speeds cause short-term fluctuations in the land covered by blue-ice areas. Global warming is predicted to decrease wind speeds across Antarctica causing a small decrease in the land surface covered by blue-ice areas.Шаблон:Sfn Thermal contraction of blue-ice can cause icequakes.

Age

Ages of particular blue-ice areas have been inferred from the ages of the meteorites there discovered, although redistribution of meteorites between various areas through ice flow can cause this procedure to yield erroneous age estimates. The oldest blue-ice areas may be up to 2.5 million years oldШаблон:Sfn and the ice in them can be quite old as well, with ages of several hundred thousand years estimated on the basis of ice flow dynamics and radiometric dating and the development of a horizontal stratigraphy. This occurs because ice blocked by obstacles stagnates and moves at a rate commensurate with the ablation rate.Шаблон:Sfn Younger ages have been found as well however, such as 250,000 years old at the Allan Hills, 75,000 years old at the Yamato Mountains,Шаблон:Sfn and 25,000 years old at the Larsen blue-ice area.

Types

Several subtypes have been defined,Шаблон:Sfn which encompass most blue-ice areas.Шаблон:Sfn

• Type I form in the lee of an obstacle and are the most common type of blue-ice areaШаблон:Sfn although they usually cover only a small surface area, compared to the other three types.Шаблон:Sfn They are often 50 – 100 times as long as the obstacle is high, which is often a mountain.Шаблон:Sfn
• Type II form where katabatic winds clear snow from the surfaceШаблон:Sfn until ice appears.Шаблон:Sfn They form on valley glaciers.Шаблон:Sfn
• Type III form where winds blowing on steep slopes – or even over flat terrain – remove snow from the surface.Шаблон:Sfn
• Type IV form by wind removing snow from the lowest part of a glacier basin.Шаблон:Sfn

Meteorites

Файл:Icemvmt.gif

Pathways of meteorites in ice

Blue-ice areas are known primarily for the meteorites that accumulate there. They originally fell on ice elsewhere and were transported by ice flows to the blue-ice area, where they accumulateШаблон:Sfn when the ice they were encased in ablates away; this mechanism has been compared to a conveyor belt that transports meteorites to blue-ice areas.Шаблон:Sfn Additionally, meteorites that fell directly on the blue-ice areas are represented; because of the often great age of the surface a number of meteorites can accumulate even without ice-driven transport.Шаблон:Sfn Over 20,000 meteorites from blue-ice areas were known by 1999, a large share of all known meteorites on Earth.Шаблон:Sfn

Meteorite findings occur only on a minority of all blue-ice areasШаблон:Sfn and are mostly limited to inland blue-ice areas whereas coastal ones tend to be lacking in meteorites.Шаблон:Sfn This might reflect the fact that at low altitude the ice surrounding the meteorites can melt due to solar heating of the meteorite, thus removing it from view.Шаблон:Sfn

Research history

The earliest research in blue-ice areas occurred during the Norwegian–British–Swedish Antarctic Expedition in 1949–1952, and was followed by two decades of mostly geological and geomorphological research. The discovery of meteorites in a blue-ice area of the Yamato Mountains led to an uptick in scientific interest; a number of programs to collect meteorites began. This also led to increased research in the glaciologicalШаблон:Sfn and dynamical properties of blue-ice areas, and later to their meteorological and climatological implications.Шаблон:Sfn

Use

The hard, flat and smooth surfaces of blue-ice areas have been used as aircraft runways (Blue ice runways) in parts of Antarctica.Шаблон:Sfn The very old ice in blue-ice areas has been used to reconstruct past climate, and the temporal resolution may be larger than in deep ice cores.Шаблон:Sfn Blue-ice areas are candidate sites for ice core drilling aimed at recovering 1.5 million year old ice, and 2.7 million year old ice has been recovered from such areas.

References

Sources

• Шаблон:Cite journal
• Шаблон:Cite journal
• Шаблон:Cite journal
• Шаблон:Cite journal
• Шаблон:Cite journal
• Шаблон:Cite journal