Sudden draining of glacial lake terentiev BY glacier Retreat

The Columbia Glacier of south-central Alaska is a tidewater glacier surging at nearly 80 feet (24 m) per day out of the Chugach Mountains into Prince William Sound, releasing two cubic miles (8.3 km3) of ice every year into the Gulf of Alaska. It is one of the world's fastest-shrinking glaciers. The glacier presently covers an area about the size of Los Angeles but has lost nearly six square miles of area (15 km2) since the 1980s. With current rates of ice loss, it will lose another six square miles (15 km2) in 15-20 years and then rise off its base to above sea level.

The retreat of the Columbia Glacier has been associated with an unusual phenomenon, where a deep lake created when the glacier dammed a valley suddenly drained as the valley became open as the glacier retreated. Terentiev Lake, located along the western margin of Columbia Glacier, covers about four square miles (10 km2) and lost about 400 feet (120 m) of water in 1990 as the glacier moved away from the side valley. Geologic evidence suggests that Columbia Glacier has acted as a "cork in a bottle," blocking the valley at several times in the past several thousand years, and has advanced and retreated during this time, alternately draining and filling the lake each time. Observations of the lake have confirmed rapid draining of the lake in successive stages of 115 feet (35 m) sometime between August and October of 1982, 155 feet (47 m) between June and September of 1986, and 226 feet (69 m) between June 1989 and May of 1990. Each of the outbursts for each stage is reported to have occurred in less than one day.

Carbon dating of trees shows that after older similar events, the glacial ice dam lake has refilled, over periods of hundreds of years, between 1650 to 850 years before the present. The cycle repeated again, with observations indicating that the lake drained again on several occasions similar to the successive draining in stages between 1982 and 1990. Along the margins of the lake, streams are cutting through a 40-foot (12-m) thick depositional sequence of glacial gravels and alluvium that includes two buried forest horizons. The alluvium consists of sand, gravel, and clay that decrease in thickness toward the headwaters of the streams. The buried forests contain a basal horizon of black peat, ranging from one to eight inches (3-20 cm) thick, that the tree trunks extend upward from, still in growth positions. The trees are up to 13 feet (4 m) tall, at which point the trunks are truncated by another soil horizon, out of which grows a second, higher layer of paleo-forest. Most trees' trunks are 2-4 inches (4-8 cm) in diameter and still contain bark, suggesting that they were buried by the sand and gravel quickly. The trees in the lower horizon were buried by poorly sorted glacially-derived gravels, sands, and muds, and those in the upper horizon were deposited by more stratified sands and gravels.

The episodic draining of Lake Terentiev by retreat of the ice dam made by Columbia Glacier is preserved in the record of buried forests, gravels, and the present-day bathtub-like rings around the lake. The sequence indicates that the lake has repeatedly filled slowly, then drained catastrophically during a surge of water that gushes out in a crack between the glacier and the bedrock in less than a day. The glacial sediments including the forest and peat layer represent sequential filling of the lake as

Columbia Glacier advanced, probably over a period of tens to hundreds of years (based on the tree rings and thickness of the peat horizons). The forests would grow in sediment deposited along the lake shore, then gradually be submerged beneath the cold meltwaters of glacial Lake Terentiev. Burial of the forests by the gravels was sudden as indicated by the trees still in growth position, with bark preserved. The gravels and sands along the margin of the lake are interpreted to have formed during high velocity water flow associated with the sudden draining of the lake. As the water rushed toward \_/

i i the small opening between the glacier and mountain to gush into Prince William Sound, it carried the accumulated sand and gravel from around the lake, depositing ridges of gravel that buried the juvenile forests that had been submerged as the lake was gradually filling.

Photograph of Lake Terentiev, Alaska, showing bathtub like rings indicating the position of the shoreline at different stages of the successive draining of the lake (T. Kusky)

hazard to ocean traffic and shipping lanes and has sunk numerous vessels, including the famous sinking of the Titanic in 1912, killing 1,503 people.

There are four main categories of sea ice. The first comes from ice that formed on polar seas in the Arctic Ocean and around Antarctica. The ice that forms in these regions is typically about 10-15 feet (3-4 m) thick. Antarctica becomes completely surrounded by this sea ice every winter, and the Arctic Ocean is typically about 70 percent covered in the winter. During summer many passages open up in this sea ice, but during the winter they re-close, forming pressure ridges of ice that may be up to tens of meters high. Recent observations suggest that the sea ice in the Arctic Ocean is thinning dramatically and rapidly, and may soon disappear altogether. The icecap over the Arctic Ocean rotates clockwise, in response to the spinning of Earth. This spinning is analogous to putting an ice cube in a glass, and slowly turning the glass. The ice cube will rotate more slowly than the glass, because it is decoupled from the edge of the glass. About one-third of the ice is removed every year by the East Greenland current. This ice then moves south and becomes a hazard to shipping in the North Atlantic, and it melts and contributes cold fresh water to the thermohaline circulation.

Icebergs from sea ice float on the surface, but between 81 and 89 percent of the ice will be submerged. The exact level that sea ice floats in the water depends on the exact density of the ice, as determined by the total amount of air bubbles trapped in the ice and how much salt got trapped in the ice during freezing.

A second kind of sea ice forms as pack ice in the Gulf of St. Lawrence, along the southeast coast of Canada, in the Bering, Beaufort, and Baltic Seas, in the Seas of Japan and Okhotsk, and around Antarctica. Pack ice builds up especially along the western sides of ocean basins, where cold currents are more common. Occasionally, during cold summers, pack ice may persist throughout the summer.

Several scenarios suggest that new ice ages may begin with pack ice that persists through many summers, gradually growing and extending to lower latitudes. Other models and data show that pack ice varies dramatically with a four- or five-year cycle, perhaps related to sunspot activity and the El Nino-Southern Oscillation (ENSO).

Pack ice presents hazards when it gets so extensive that it effectively blocks shipping lanes, or when leads (channels) into the ice open and close, forming pressure ridges that become too thick to penetrate with ice breakers. Ships attempting to navigate through pack ice have become crushed when leads close, and the ships are trapped. Pack ice has terminated or resulted in disaster for many expeditions to polar seas, most notably Franklin's expedition in the Canadian arctic and Scott's expedition to Antarctica. Pack ice also breaks up, forming many small icebergs, but because these are not as thick as icebergs of other origins they do not present as significant a hazard to shipping.

Pack ice also presents hazards when it drifts into shore, usually during spring break up. With significant winds pack ice can pile up on flat shorelines and accumulate in stacks up to 50 feet (15 m) high. The force of the ice is tremendous and is enough to crush shoreline wharves, docks, buildings, and boats. Pack ice that has blown ashore also commonly pushes up high piles of gravel and boulders that may be 35 feet (11 m) high in places. These ridges are common around many of the Canadian Arctic islands and mainland. Ice that forms initially attached to the shore presents another type of hazard. If it breaks free and moves away from shore, it may carry with it significant quantities of shore sediment, causing rapid erosion of beaches and shore environments.

Pack ice also forms on many high-latitude lakes, and the freeze-thaw cycle causes cracking of the lake ice. When lake water rises to fill the cracks, the ice cover on the lake expands, and pushes over the shoreline, resulting in damage to any structures built along the shore. This is a common problem on many lakes in northern climates and leads to widespread damage to docks and other lakeside structures.

An unusual pack-ice disaster has been occurring in northern Quebec, Canada, along the Ungava Peninsula on the east side of Hudson Bay. A series of dams has been built in Canada along rivers that flow into Hudson Bay, and these dams are used to generate clean hydroelectric energy. The problem that has arisen is that these dammed rivers have annual spring floods, which before the dams were built would flush the pack ice out of Hudson Bay. Since the dams have been built, the annual spring floods are diminished, resulting in the pack ice remaining on Hudson Bay through the short summer. This has drastically changed the summer season on the Ungava Peninsula; as the warm summer winds blow across the ice they pick up cool moist air, and cold fogs now blow across the Ungava all summer. This has drastically changed the local climate and has hindered growth and development of the region.

Icebergs present the greatest danger to shipping. In the Northern Hemisphere most icebergs calve off glaciers in Greenland or Baffin Island, then move south through the Davis Strait into shipping lanes in the North Atlantic off Newfoundland. Some icebergs calve off glaciers adjacent to the Barents Sea, and others come from glaciers in Alaska and British Columbia. In the Southern Hemisphere, most icebergs come from Antarctica, though some come from Patagonia.

Once in the ocean icebergs drift with ocean currents, but because of the Coriolis force are deflected to the right in the Northern Hemisphere, and to the left in the Southern Hemisphere. Most icebergs are about 100 feet to 300 feet (35-92 m) high, and up to about 2,000 feet (610 m) in length. However, in March 2000 a huge iceberg broke off the Ross Ice

Shelf in Antarctica, and this berg was roughly the size of the state of Delaware. It had an area of 4,500 square miles (11,655 km2), and stuck 205 feet (62 m) out of the water. Icebergs in the Northern Hemisphere pose a greater threat to shipping, as those from Antarctica are too remote and rarely enter shipping lanes. Ship collisions with icebergs have resulted in numerous maritime disasters, especially in the North Atlantic on the rich fishing grounds of the Grand Banks off the coast of Newfoundland.

Icebergs are now tracked by satellite, and ships are updated with their positions so they can avoid any collisions that could prove fatal for the ships. Radio transmitters are placed on larger icebergs to more closely monitor their locations, and many ships now carry more sophisticated radar and navigational equipment that helps track the positions of large icebergs and the ship, so that they avoid collision.

Icebergs also pose a serious threat to oil drilling platforms and sea floor pipelines in high-latitude seas. Some precautions have been taken, such as building seawalls around near-shore platforms, but not enough planning has gone into preventing an iceberg's colliding with and damaging an oil platform, or from one being dragged across the sea floor and rupturing a pipeline.

Glaciers as Sensitive indicators of paleoclimate

Glaciers are very sensitive indicators of climate change, as they may melt or advance significantly with relatively small changes in the climate. The planet is presently in an interglacial period, where the large ice caps that covered much of North America, Europe, and Asia have only recently (~10,000 years ago) retreated, and there are still many glaciers left on the planet. Some climate change models suggest that the climate can change suddenly in non-linear manners, rapidly plunging Earth into scorchingly hot conditions or into a dark icy winter that may last hundreds of thousands of years. Many glaciers are presently retreating, many rapidly, in response to warming climate conditions. Glaciers also preserve a yearly record of snowfall, preserved as thin layers in the glacial ice. This ice can be sampled with cores and analyzed in laboratories that are able to extract information about the temperatures and other conditions on the planet at the time each snow layer fell. As such, glaciers are important historical records of past climate that can be used to predict future climate trends.

Glaciologists employ a wide variety of techniques to study glaciers, some being concerned with the movement of glaciers, and they may place stakes in various parts of the glacier and around its edges to measure

Glaciers of Greenland that are showing signs of rapid melting. If the Greenland ice cap melts, global sea levels will rise by an additional 23 feet (7 m). (Shutterstock)

their movement with time and to determine how much the glacier has advanced or retreated. Such measurements have improved in accuracy in recent years with the advent of Global Positioning System (GPS) technologies, where sub-centimeter displacements can now be measured.

Remote sensing technologies are also commonly employed for studies of glaciers. Time-series satellite images can show the position of glaciers at various times, so rates of movement can be calculated. Satellite radar and aircraft radar-altimeter data can be used to determine the thickness of snowfall and compare this information with rates of movement to determine the mass balance of glaciers. This information helps determine whether the glacier is experiencing net loss or gain of volume. Other glaciologists are concerned with the physical conditions of deformation of the ice, temperature of the ice with depth, and how the ice may or may not be coupled to the underlying substratum. This information can be important for determining how fast glaciers are able to move and whether rates of movement may stay the same, increase (surge), or decrease with changing conditions.

Many glacial studies are focused on using the isotopic, pollen, and other records in ice cores to determine the paleoclimate history of the past few tens of millions of years of Earth history. To accomplish this goal, glaciologists must drill and extract ice cores and preserve them at subfreezing temperatures for measurement in the laboratory. The ages of the ice cores must be accurately determined, which in some cases can be done by counting down "annual rings" much like counting tree rings. Once the age of the ice layer is determined, glaciologists may analyze the ice, air bubbles trapped in the ice, or other trapped particles that reveal clues to climate history. Numerous ice cores from Greenland and Antarctica are currently being studied to help decipher the climate history of Earth for the past 100,000 years.

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