When an iceberg runs aground, it can plow a furrow several metres deep in the seabed that may extend for tens of kilometres. Iceberg scour marks have been known from the Labrador Sea and Grand Banks since the early i970s. In the Arctic, many marks are found at depths of more than 400 metres (1,300 feet), whereas the deepest known sill, or submerged ridge, within Greenland fjords is 220 metres (about 725 feet) deep. This unsolved anomaly suggests that icebergs were much deeper in the past or that sedimentation rates within the fjords are so slow that marks dating from periods of reduced sea level have not yet been filled in. It is also possible that an irregular berg can increase its draft by capsizing, though model studies suggest that the maximum gain is only a few percent. Since not all iceberg-producing fjords have been adequately surveyed, another possibility is that Greenland fjords exist with entrances of greater depth. In the Antarctic, the first scours were found in 1976 at latitude 16° W off the coast of Queen Maud Land in the eastern Weddell Sea, and further discoveries were made off Wilkes Land and Cape Hallett at the eastern entrance to the Ross Sea.
In addition, iceberg scour marks have been found on land. On King William Island in the Canadian Arctic, scour marks have been identified in locations where the island rose out of the sea—the result of a postglacial rebound after the weight of the Laurentide Ice Sheet was removed. Furthermore, Canadian geologist Christopher Woodworth-Lynas has found evidence of iceberg scour marks in the satellite imagery of Mars. Scour marks are strong indicators of past water flow
Observations indicate that long furrows like plow marks are made when an iceberg is driven by sea ice, whereas a freely floating berg makes only a short scour mark or a single depression. Apart from simple furrows, "washboard patterns" have been seen. It is thought that these patterns are created when a tabular berg runs aground on a wide front and is then carried forward by tilting and plowing on successive tides. Circular depressions, thought to be made when an irregular iceberg touches bottom with a small "foot" and then swings to and fro in the current, have also been observed. Grounded bergs have a deleterious effect on the ecosystem of the seabed, often scraping it clear of all life.
Both icebergs and pack ice transport sediment in the form of pebbles, cobbles, boulders, finer material, and even plant and animal life thousands of kilometres from their source area. Arctic icebergs often carry a top burden of dirt from the eroded sides of the valley down which the parent glacier ran, whereas both Arctic and Antarctic bergs carry stones and dirt on their underside. Stones are lifted from the glacier bed and later deposited out at sea as the berg melts. The presence of ice-rafted debris (IRD) in seabed-sediment cores is an indicator that icebergs, sea ice, or both have occurred at that location during a known time interval. (The age of the deposit is indicated by the depth in the sediment at which the debris is found.)
Noting the locations of ice-rafted debris is a very useful method of mapping the distribution of icebergs and thus the cold surface water occurring during glacial periods and at other times in the geologic past. IRD mapping surveys have been completed for the North Atlantic, North Pacific, and Southern oceans. The type of rock in the debris can also be used to identify the source region of the transporting iceberg. Caution must be used in such interpretation because, even in the modern era, icebergs can spread far beyond their normal limits under exceptional conditions. For instance, reports of icebergs off the coast of Norway in spring 1881 coincided with the most extreme advance ever recorded of East Greenland sea ice. It is likely that the bergs were carried eastward along with the massive production and outflow of Arctic sea ice.
It is ice-rafted plant life that gives the occasional exotic colour to an iceberg. Bergs are usually white (the colour of snow or bubbly ice) or blue (the colour of glacial ice that is relatively bubble-free). A few deep green icebergs are seen in the Antarctic; it is believed that these are formed when seawater rich in organic matter freezes onto the bottoms of the ice shelves.
Most scientists maintain that adding large numbers of melting icebergs to the oceans may cause an increase in global sea level if the polar regions are not replenished by an equal amount of snowfall. Also, the addition of large amounts of freshwater to the oceans may lower the salinity of the upper layers of the ocean and possibly alter the present convection-current regime.
Apart from local weather effects, such as fog production, icebergs have two main impacts on climate. Iceberg production affects the mass balance of the parent ice sheets, and melting icebergs influence both ocean structure and global sea level.
The Antarctic Ice Sheet has a volume of 28 million cubic km (about 6.7 million cubic miles), which represents 70 percent of the total fresh water (including groundwa-ter) in the world. The mass of the ice sheet is kept in balance by a process of gain and loss—gain from snowfall over the whole ice sheet and ice loss from the melting of ice at the bottom of the ice shelf and from the calving of icebergs from the edges of the ice shelf. The effect of summer runoff and from sublimation off the ice surface is negligible.
Annual snowfall estimates for the Antarctic continent start at 1,000 cubic km (240 cubic miles). If the Antarctic
Ice Sheet is in neutral mass balance, the annual rate of loss from melting and iceberg calving must be close to this value; indeed, estimates of iceberg flux do start at this value, though some run much higher. Such apparently large fluxes are still less than the mean flow rate of the Amazon River, which is 5,700 cubic km (about 1,370 cubic miles) per year. In Antarctica the annual loss amounts to only one ten-thousandth of its mass, so the ice sheet is an enormous passive reservoir. However, if losses from iceberg calving and ice-shelf melting are greater than gains from snowfall, global sea levels will rise.
At present, the size, and even the sign, of the contribution from Antarctica is uncertain. Consequently, Antarctic ice flux has not been included as a term in the sea-level predictions of Climate Change 2007, the fourth assessment report of the Intergovernmental Panel on Climate Change (IPCC). What is more certain is that the retreat of glaciers in the Arctic and mountain regions has contributed about 50 percent to current rates of sea-level rise. (The rest is due to the thermal expansion of water as the ocean warms.) An increasing contribution is coming from a retreat of the Greenland Ice Sheet, and part of this contribution is occurring as an iceberg flux.
In considering the effect of iceberg melt upon ocean structure, it is found that the total Antarctic melt is equivalent to the addition of 0.1 metre (0.3 foot) of fresh water per year at the surface. This is like adding 0.1 metre of extra annual rainfall. The dilution that occurs, if averaged over a mixed layer 100-200 metres (330-660 feet) deep, amounts to a decrease of 0.015-0.03 part per thousand (ppt) of salt. Melting icebergs thus make a small but measurable contribution to maintaining the Southern Ocean pycnocline (the density boundary separating low-salinity surface water from higher-salinity deeper water) and to keeping surface salinity in the Southern Ocean to its observed low value of 34 ppt or below
It is interesting to note that the annual production of Antarctic iceberg ice is about one-tenth of the annual production of Antarctic sea ice. Sea ice has a neutral effect on overall ocean salinity, because it returns to liquid during the summer months. Nevertheless, when sea ice forms, it has an important differential effect in that it increases ocean salinity where it forms. This is often near the Antarctic coast. Increased salinity encourages the development of convection currents and the formation of bottom water (masses of cold and dense water). Icebergs, on the other hand, always exert a stabilizing influence on the salinity of the water column. This stabilizing influence manifests itself only when the icebergs melt, and this occurs at lower latitudes.
Individual Arctic icebergs, although similar in numbers to Antarctic bergs (10,000-15,000 emitted per year), are smaller on average, so the ice flux is less. This, however, was not necessarily the case during the last glacial period. It has been postulated that, during the first stage of the retreat of the Laurentide Ice Sheet of North America, a large ice-dammed glacial lake (Lake Agassiz) formed in Canada over much of present-day Manitoba. When the ice dam broke, an armada of icebergs was suddenly released into the North Atlantic. As the icebergs melted, they added so much fresh water at the surface that the normal winter convection processes were turned off in the North Atlantic Ocean. As a result, the Gulf Stream was weakened, and northern Europe was returned to ice-age conditions for another millennium—the so-called Younger Dryas event.
Iceberg Detection, Tracking, and Management
An iceberg is a very large object that can be detected in the open sea both visually and by radar. In principle an iceberg can also be detected by sonar. In the open sea, an iceberg produces squealing, popping, and creaking sounds caused by mechanical stresses and cracking, and these sounds can be detected underwater up to 2 km (more than a mile) away. In summer, bergs can also produce a high-pitched hissing sound called "bergy seltzer," which is due to the release of high-pressure air bubbles from the ice as it melts in the warmer water.
The discovery of an iceberg depends on the alertness of a ship's watchkeepers. A decaying iceberg poses additional hazards because of its trail of growlers and bergy bits. Although small in size, they have masses (up to 120 tons for growlers, up to 5,400 tons for bergy bits) that are capable of damaging or sinking ships. As they drop into the sea, icebergs often roll over and lose their snow layers. In a heavy sea, the bergs' smooth wetted ice surfaces produce a low radar cross section. This makes them difficult to discriminate by eye against foam and whitecaps. Because a ship may steer to avoid a large parent berg, it may be in greater danger from undetected growlers or bergy bits drifting nearby.
The problem of protecting shipping from icebergs is most critical in two regions, the high-latitude Southern Ocean and the northwestern section ofthe North Atlantic. The Southern Ocean threat is increasing because large container ships—those unable or unwilling to use the Panama Canal—can reach high southern latitudes on transit from Australia or New Zealand to Cape Horn. No special measures are currently in place to protect such vessels. In the North Atlantic, the International Ice Patrol was established in 1914 following the loss of the RMS Titanic to an iceberg in April 1912. Its task is to track icebergs as they enter shipping lanes via the Labrador Current and to keep a continuous computer plot of the known or estimated whereabouts of every berg. Reports are transmitted twice a day to ships. In the past, iceberg positions were sited by ships or aircraft. However, it is becoming more common that icebergs are sited by the interpretation of satellite imagery
The most useful type of sensor is synthetic aperture radar (SAR), which combines high resolution with day-and-night weather-independent capability Tools with a pixel size of about 20 metres (65 feet) are capable of resolving most bergs. The new generation of SAR in the early 21st century, such as the Canadian RADARSAT and the European ENVISAT, also surveys wide swaths (up to 400 km [250 miles] wide) in every orbit and thus is capable of surveying the entire danger zone once per day.
During the 1950s and 1960s, attempts were made by the U.S. Coast Guard to find ways of fragmenting icebergs that posed a threat to shipping. All were unsuccessful. Explosive techniques were particularly so, since ice and snow are so effective at absorbing mechanical shock. Often the yield of fragmented ice was no greater than the mass of explosive used. Because of the need to defend offshore drilling and production platforms from icebergs, the viability of explosive techniques has been readdressed more recently. It was found that very cold ice, such as the type found in the lower part of an iceberg, can be fragmented successfully by the use of slow-burning explosives such as Thermit. Thermit can be implanted by drilling. However, implantation is a dangerous process because of the possibility of capsize.
Until these techniques are perfected, icebergs cannot be destroyed. Current protocols call for the location and tracking of threatening icebergs. Iceberg trajectories are then predicted by increasingly sophisticated computer models. If necessary, icebergs are captured and towed out of the way of their targets.
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