Anchor Ice
Encyclopedia
Anchor Ice is described by the World Meteorological Organization
as "submerged ice attached or anchored to the bottom, irrespective of the nature of its formation." Anchor ice is most commonly observed in fast-flowing rivers during periods of extreme cold, at the mouths of rivers flowing into very cold seawater, in the shallow sub or intertidal during or after storms when the air temperature is below the freezing point
of the water, and in the subtidal in the Antarctic
along ice shelves
or near floating glacier tongues.
due to mixing and contact with the atmosphere. Ice platelets generally form very quickly in the water column and on submerged objects once conditions are optimal for anchor ice formation. Anchor ice in rivers tends to be composed of numerous small crystals adhering to each other in small flocculent masses. Anchor ice in rivers can seriously disrupt hydro-electric power plants by significantly reducing flow or stopping turbines completely.
Another form of anchor ice may be observed at the mouths of Arctic
rivers where fresh water seeps out of the river bed into the ocean up through the sediment. Anchor ice forms if the seawater is below the freezing point
of the river water.
, where submerged ice may be observed to completely cover the substrate to depths of up to 2m, with some anchor ice cover observed at more than 4.5m depth.
anchor ice is perhaps one of the most interesting phenomena of ice formation in the marine environment. The general mechanism of its formation is commonly assumed as the following:
Anchor ice crystals in the Antarctic are generally in the form of thin, circular platelets of 2-10cm in diameter. Large masses of irregularly-oriented crystals form anchor ice formations, which may be as large as 4m in diameter when attached to large immovable objects on the sea floor.
Anchor ice that forms on the underside of sea ice is often referred to as platelet or congelation ice
, and can be hard to distinguish from that formed due to the cooling of the sea ice cover by cold atmospheric conditions.
Anchor ice is thought to be relatively common in the Antarctic, due to large ice shelves
that occupy many areas of the continental coast. Studies and observations of anchor ice formation in McMurdo Sound
, Antarctica have shown that the phenomenon regularly causes the formation of ice on the seafloor to depths of approximately 15m, and rarely to depths of approximately 30m.
, anchor ice has been implicated in drastic zonation of the subtidal fauna
. Many animals are directly affected by the growth of anchor ice, and certain sponge
s have been shown to readily grow anchor ice and to be damaged by it. Anchor ice may also grow on animate or inanimate objects and lift them from the sea floor. In the Antarctic
this will most likely result in the death of an organism, since during much of the year the ocean is covered by annual sea ice, and the organism is likely to become incorporated into this.
Many organisms have actually been found on the surface of ice shelves
in certain places in the Antarctic
, likely due to the anchor icing phenomenon:
World Meteorological Organization
The World Meteorological Organization is an intergovernmental organization with a membership of 189 Member States and Territories. It originated from the International Meteorological Organization , which was founded in 1873...
as "submerged ice attached or anchored to the bottom, irrespective of the nature of its formation." Anchor ice is most commonly observed in fast-flowing rivers during periods of extreme cold, at the mouths of rivers flowing into very cold seawater, in the shallow sub or intertidal during or after storms when the air temperature is below the freezing point
Freezing Point
Freezing Point is a news journal in the People's Republic of China which has been the subject of controversy over its criticism of Communist Party officials and the sympathetic ear it lent to a Chinese historian who had criticized official history textbooks...
of the water, and in the subtidal in the Antarctic
Antarctic
The Antarctic is the region around the Earth's South Pole, opposite the Arctic region around the North Pole. The Antarctic comprises the continent of Antarctica and the ice shelves, waters and island territories in the Southern Ocean situated south of the Antarctic Convergence...
along ice shelves
Ice shelf
An ice shelf is a thick, floating platform of ice that forms where a glacier or ice sheet flows down to a coastline and onto the ocean surface. Ice shelves are only found in Antarctica, Greenland and Canada. The boundary between the floating ice shelf and the grounded ice that feeds it is called...
or near floating glacier tongues.
Anchor ice in rivers
Anchor ice will generally form in fast-flowing rivers during periods of extreme cold. Due to the motion of the water, ice cover may not form consistently, and the water will quickly reach its freezing pointFreezing Point
Freezing Point is a news journal in the People's Republic of China which has been the subject of controversy over its criticism of Communist Party officials and the sympathetic ear it lent to a Chinese historian who had criticized official history textbooks...
due to mixing and contact with the atmosphere. Ice platelets generally form very quickly in the water column and on submerged objects once conditions are optimal for anchor ice formation. Anchor ice in rivers tends to be composed of numerous small crystals adhering to each other in small flocculent masses. Anchor ice in rivers can seriously disrupt hydro-electric power plants by significantly reducing flow or stopping turbines completely.
Another form of anchor ice may be observed at the mouths of Arctic
Arctic
The Arctic is a region located at the northern-most part of the Earth. The Arctic consists of the Arctic Ocean and parts of Canada, Russia, Greenland, the United States, Norway, Sweden, Finland, and Iceland. The Arctic region consists of a vast, ice-covered ocean, surrounded by treeless permafrost...
rivers where fresh water seeps out of the river bed into the ocean up through the sediment. Anchor ice forms if the seawater is below the freezing point
Freezing Point
Freezing Point is a news journal in the People's Republic of China which has been the subject of controversy over its criticism of Communist Party officials and the sympathetic ear it lent to a Chinese historian who had criticized official history textbooks...
of the river water.
Anchor ice formed during storms
Anchor ice may be formed in the shallow intertidal or subtidal during storms in cold weather, when the uppermost layers of the water column are churned up by strong winds or waves. This type of anchor ice can be found primarily in the ArcticArctic
The Arctic is a region located at the northern-most part of the Earth. The Arctic consists of the Arctic Ocean and parts of Canada, Russia, Greenland, the United States, Norway, Sweden, Finland, and Iceland. The Arctic region consists of a vast, ice-covered ocean, surrounded by treeless permafrost...
, where submerged ice may be observed to completely cover the substrate to depths of up to 2m, with some anchor ice cover observed at more than 4.5m depth.
Anchor ice in the Antarctic
AntarcticAntarctic
The Antarctic is the region around the Earth's South Pole, opposite the Arctic region around the North Pole. The Antarctic comprises the continent of Antarctica and the ice shelves, waters and island territories in the Southern Ocean situated south of the Antarctic Convergence...
anchor ice is perhaps one of the most interesting phenomena of ice formation in the marine environment. The general mechanism of its formation is commonly assumed as the following:
- Antarctic surface waters are forced to flow below a large, thick mass of floating ice (ice shelfIce shelfAn ice shelf is a thick, floating platform of ice that forms where a glacier or ice sheet flows down to a coastline and onto the ocean surface. Ice shelves are only found in Antarctica, Greenland and Canada. The boundary between the floating ice shelf and the grounded ice that feeds it is called...
or glacier tongue) due to tidal motions or normal ocean circulation. - The surface water melts the underside of the mass of ice, causing a slight freshening of the water that brings the temperature of the water into equilibriumThermodynamic equilibriumIn thermodynamics, a thermodynamic system is said to be in thermodynamic equilibrium when it is in thermal equilibrium, mechanical equilibrium, radiative equilibrium, and chemical equilibrium. The word equilibrium means a state of balance...
with the in-situ freezing pointFreezing PointFreezing Point is a news journal in the People's Republic of China which has been the subject of controversy over its criticism of Communist Party officials and the sympathetic ear it lent to a Chinese historian who had criticized official history textbooks...
at depth. - The water, at its freezing pointFreezing PointFreezing Point is a news journal in the People's Republic of China which has been the subject of controversy over its criticism of Communist Party officials and the sympathetic ear it lent to a Chinese historian who had criticized official history textbooks...
at depth (slightly lower than the freezing point at the surface due to the pressure effect on the freezing point), is advectedAdvectionAdvection, in chemistry, engineering and earth sciences, is a transport mechanism of a substance, or a conserved property, by a fluid, due to the fluid's bulk motion in a particular direction. An example of advection is the transport of pollutants or silt in a river. The motion of the water carries...
from under the floating mass of ice and may rise towards the surface due to a variety of factors. - As the water rises the in-situ freezing point increases, leaving the water slightly supercooledSupercoolingSupercooling, also known as undercooling, is the process of lowering the temperature of a liquid or a gas below its freezing point without it becoming a solid....
. SupercoolingSupercoolingSupercooling, also known as undercooling, is the process of lowering the temperature of a liquid or a gas below its freezing point without it becoming a solid....
is relieved by the formation of microscopic ice crystals in the water column. - Ice crystals may coalesce or adhere to submerged objects, including marine organisms, rocks, man-made structures, or other ice formations such as the sea iceSea iceSea ice is largely formed from seawater that freezes. Because the oceans consist of saltwater, this occurs below the freezing point of pure water, at about -1.8 °C ....
, the ice foot, floating glacierGlacierA glacier is a large persistent body of ice that forms where the accumulation of snow exceeds its ablation over many years, often centuries. At least 0.1 km² in area and 50 m thick, but often much larger, a glacier slowly deforms and flows due to stresses induced by its weight...
s or icebergIcebergAn iceberg is a large piece of ice from freshwater that has broken off from a snow-formed glacier or ice shelf and is floating in open water. It may subsequently become frozen into pack ice...
s.
Anchor ice crystals in the Antarctic are generally in the form of thin, circular platelets of 2-10cm in diameter. Large masses of irregularly-oriented crystals form anchor ice formations, which may be as large as 4m in diameter when attached to large immovable objects on the sea floor.
Anchor ice that forms on the underside of sea ice is often referred to as platelet or congelation ice
Congelation ice
Congelation ice is ice that forms on the bottom of an established ice cover.- Seawater :On seawater, congelation ice is ice that forms on the bottom of an established sea ice cover, usually in the form of platelets which coalesce to form solid ice....
, and can be hard to distinguish from that formed due to the cooling of the sea ice cover by cold atmospheric conditions.
Anchor ice is thought to be relatively common in the Antarctic, due to large ice shelves
Ice shelf
An ice shelf is a thick, floating platform of ice that forms where a glacier or ice sheet flows down to a coastline and onto the ocean surface. Ice shelves are only found in Antarctica, Greenland and Canada. The boundary between the floating ice shelf and the grounded ice that feeds it is called...
that occupy many areas of the continental coast. Studies and observations of anchor ice formation in McMurdo Sound
McMurdo Sound
The ice-clogged waters of Antarctica's McMurdo Sound extend about 55 km long and wide. The sound opens into the Ross Sea to the north. The Royal Society Range rises from sea level to 13,205 feet on the western shoreline. The nearby McMurdo Ice Shelf scribes McMurdo Sound's southern boundary...
, Antarctica have shown that the phenomenon regularly causes the formation of ice on the seafloor to depths of approximately 15m, and rarely to depths of approximately 30m.
Biological effects
Especially in the AntarcticAntarctic
The Antarctic is the region around the Earth's South Pole, opposite the Arctic region around the North Pole. The Antarctic comprises the continent of Antarctica and the ice shelves, waters and island territories in the Southern Ocean situated south of the Antarctic Convergence...
, anchor ice has been implicated in drastic zonation of the subtidal fauna
Fauna
Fauna or faunæ is all of the animal life of any particular region or time. The corresponding term for plants is flora.Zoologists and paleontologists use fauna to refer to a typical collection of animals found in a specific time or place, e.g. the "Sonoran Desert fauna" or the "Burgess shale fauna"...
. Many animals are directly affected by the growth of anchor ice, and certain sponge
Sea sponge
Sponges are animals of the phylum Porifera . Their bodies consist of jelly-like mesohyl sandwiched between two thin layers of cells. While all animals have unspecialized cells that can transform into specialized cells, sponges are unique in having some specialized cells, but can also have...
s have been shown to readily grow anchor ice and to be damaged by it. Anchor ice may also grow on animate or inanimate objects and lift them from the sea floor. In the Antarctic
Antarctic
The Antarctic is the region around the Earth's South Pole, opposite the Arctic region around the North Pole. The Antarctic comprises the continent of Antarctica and the ice shelves, waters and island territories in the Southern Ocean situated south of the Antarctic Convergence...
this will most likely result in the death of an organism, since during much of the year the ocean is covered by annual sea ice, and the organism is likely to become incorporated into this.
Many organisms have actually been found on the surface of ice shelves
Ice shelf
An ice shelf is a thick, floating platform of ice that forms where a glacier or ice sheet flows down to a coastline and onto the ocean surface. Ice shelves are only found in Antarctica, Greenland and Canada. The boundary between the floating ice shelf and the grounded ice that feeds it is called...
in certain places in the Antarctic
Antarctic
The Antarctic is the region around the Earth's South Pole, opposite the Arctic region around the North Pole. The Antarctic comprises the continent of Antarctica and the ice shelves, waters and island territories in the Southern Ocean situated south of the Antarctic Convergence...
, likely due to the anchor icing phenomenon:
- Organism accumulates anchor ice as it is bathed in supercooled water.
- Organism becomes positively buoyant due to the accumulation of ice, and it lifted from the sea floor.
- Organism floats to the underside of the ice shelfIce shelfAn ice shelf is a thick, floating platform of ice that forms where a glacier or ice sheet flows down to a coastline and onto the ocean surface. Ice shelves are only found in Antarctica, Greenland and Canada. The boundary between the floating ice shelf and the grounded ice that feeds it is called...
or sea iceSea iceSea ice is largely formed from seawater that freezes. Because the oceans consist of saltwater, this occurs below the freezing point of pure water, at about -1.8 °C ....
cover where it freezes in. - AblationAblationAblation is removal of material from the surface of an object by vaporization, chipping, or other erosive processes. This occurs in spaceflight during ascent and atmospheric reentry, glaciology, medicine, and passive fire protection.-Spaceflight:...
of the surface of the ice cover and additional growth of ice on the underside will result in the organism being transported "through" the ice and "deposited" on the surface, largely intact.