Australia: The Land Where Time Began
The Cryosphere - Permafrost
Human and biological systems in regions of permafrost are directly affected by it. Permafrost, perennially frozen ground, is found up to hundreds of metres in thickness and ranges in age between 10s and 100s of millennia, with a geological intransience, though recent changes have shown that is not as permanent a landscape feature as it is often believed. Temperature changes at the surface take a long time to propagate to great depths in the underlying permafrost, as in the case of glaciers, as a result of the timescales of thermal diffusion in permafrost. Permafrost processes have a major effects on infrastructure, hydrology, ecology and carbon cycling in the high latitudes of the Northern Hemisphere as a result of the response of seasonally frozen ground and the upper layers of the permafrost to seasonal and short-term climate change and fluctuations in land-use.
Permafrost has been defined by the International Permafrost Association as ground that maintains a temperature of 0oC or below for at least 2 years. Soils or rocks can meet this definition even in the absence of frozen water, though for practical (engineering) purposes, and in the context of cryospheric science, the ice content is the feature of interest. Slight modifications of the formal definition are required as the primary concern is whether or not pore space water in soils and in rocks will be in the liquid or solid phase. This is dependent on the pressure melting point in the local area and the pore fluid salinity. The freezing point of pure water is depressed by overburden by 0.074oC M/Pa, which results in a freezing point of about -0.2oC beneath 1 km of sediment.
Pressure melting point and the effects of salinity are both relevant in marine environments, with the latter dominating on continental shelves where a freezing point of -1.8oC results from typical ocean salinities. It is necessary for marine sediments with sea water in their pore spaces to drop below this temperature for permafrost to be created. In most of the oceans of the world, in the deep waters temperatures less than -3oC are required by pressure effects to support permafrost. In the modern ocean waters at such low temperature are not found, so according to the author1 permafrost is viable only in cold environments on continental shelves. These conditions are found in continental shelves at high latitudes that at some time in the past were exposed to the atmosphere in times of low sea level during the glaciations of the Pleistocene. Water of low salinity in the sediments is formed as a result of salt rejection as the water is frozen, that has a melting point that is closer to 0oC, which helps in the preservation of frozen ground on continental shelves in the Arctic Basin, as ocean temperature below this are typically seen.
Seasonally frozen ground with an area of 55 x 106 km2, comprises about 58 % of the land mass in the Northern Hemisphere. Over much of this area this seasonally frozen ground thaws in spring and summer, though permafrost, perennially frozen ground covers an area of 23 x 106 km2, about 24 % of the Northern Hemisphere land mass. In the high latitudes of Russia, Canada and Alaska almost half of this is present as continuous permafrost, which is defined as areas with more than 90 % of the landscape with a covering of permafrost. Zones of discontinuous permafrost are present to the south, 50-90 %, and sporadic permafrost, less than 50 %. In most mountain ranges there are pockets of alpine permafrost, and also in areas, that are generally unmapped, of perennially frozen ground beneath glaciers and ice sheets at high latitudes.
Antarctica is the only place in the Southern Hemisphere that is cold enough, and mostly covered by ice, to support the preservation of permafrost, meaning permafrost is rare in the Southern Hemisphere. Much of the 0.3 % of the Antarctic land mass that is not covered by glaciers is in the Dry Valleys, the entire area of which has a covering of permafrost. The area of frozen ground beneath cold-based sectors of the Antarctic Ice Sheet is not known, though it has the potential to be large. The Arctic islands and high altitudes of the Andes also have areas of permafrost.
There is also permafrost in shallow marine shelf environments, though these have not been well mapped. Most of these areas of permafrost formed when the global sea level was more than 100 m lower during the Pleistocene as a result of the expansion of terrestrial ice sheets. At that time large areas of continental shelf were exposed around the world which gave permafrost thousands of years to develop in these exposed shelf environments that fringe the Arctic Ocean, as occurred in the Bering Sea.
During the glacial periods of the Quaternary much of the permafrost of the Earth formed, persisting until the present. In places such as at lower latitudes and mountainous regions where a relatively thin layer of permafrost developed during the glacial periods, that have since thawed, there are relict permafrost features, thus helping demarcate the proglacial zone of the last great ice sheets.
Permafrost on the floor of the sea reaches to more than 100 m in some places. The thickness of permafrost generally ranges from decimetres to more than 1,000 m, the deepest permafrost being found in parts of Siberia, Alaska and northwestern Canada, that eluded glaciers over much of the last glaciation. Near the Lena River in Siberia is the deepest known permafrost. The mean annual temperature was moderated, which limited the growth of permafrost, where ice sheets persisted for extended lengths of time, as they insulated the ground from the cold air temperatures.
The temperature and depth of the surface active layer, the top layer of the ground that is subject to annual cycles of freezing and thawing, varied annually, unlike the deep permafrost. I most environments the active layer is about 1 m deep, depending on the thermal conductivity of the ground, the annual temperature cycle, the type of surface and the depth of snow cover. It is as much as several metres deep in some locations. In locations with a high content of soil water and organic matter the near-surface layer has a high heat capacity which provides a thermal buffer that limits the depth of the active layer. The active layer is exceptionally sensitive to climate change in the short term as atmospheric conditions, such as temperature and snowfall.
The depth of the active layer increases with warmer mean ground temperatures that result from warming of the atmosphere or increased snowfall, leading to degradation of permafrost from above, as atmospheric conditions directly force the active layer. The temperature gradient in the ground is also decreased by warmer surface temperatures, which lead to a reduction of conductive heat fluxes and the decay of permafrost at the base. Permafrost commonly warms throughout when it degrades, melting from above and below. In many places relict permafrost, "burned ice" is present at depth, which the author1 suggests is testament to the sustained periods of colder temperatures that have occurred in the past.
Subglacial permafrost growth is supported by cold-based glaciers, though thinner permafrost than would normally be expected at high latitudes or elevations, is the result of insulation by the glacier cover. The author1 suggests permafrost probably preceded the formation of the ice sheets during the last glaciation, and the ground could warm to the melting point of ice, but no further, after the ice sheet had advanced to cover the ground. Permafrost would be permitted by this to persist, though it would degrade from below. In polar and subpolar environments it is common to find subsurface permafrost in the forefield of glaciers that are retreating. According to the author1 it is possible that free water, such as that generated by strain heating, to be present at the base of a glacier in thermal equilibrium with the ice above it and the permafrost beneath it. In this situation the interface with the permafrost would be impermeable, which would promote high water pressure at the base of the glacier. It is suggested the low-sloping, fast-flowing ice lobes that are present along the Laurentide and Eurasian Ice Sheet margins may have been contributed by the high pressure water at the base of the glacier, via high rates of glacier sliding.
An unusual family of substances, clathrates or gas hydrates, are also found in frozen ground, that are composed of a crystalline ice "shell" or "cage" that holds a gas trapped inside. The pressure of the gas molecules in the shell prevents the ice crystals, that are connected by usual hydrogen bonds, from collapsing into a standard ice crystal lattice.
A specialised range of temperature and pressure, that are sometimes narrow, is required to keep gas hydrates stable, with the most common gas being involved being methane (natural gas) hydrates.
Gas hydrates are particularly common in continental shelf environments that are shallow. Large methane hydrate deposits have been found on the seafloor in the Arctic Basin in sediments on the continental shelf, and in permafrost on land, such deposits being of interest as hydrocarbon energy sources. Among the many gases that are found as gas hydrates is CO2, and the author1 points out that the release of methane or carbon dioxide to the atmosphere as a result of gas hydrate destabilisation caused by warming oceans is a potential "wild card" in the climate system, as the release of large quantities of methane hydrate through destabilisation could potentially trigger abrupt climate change. According to the author1 it is not clear how serious a treat is posed by the release of methane from gas hydrates below the seafloor as oxidation of methane would occur as it passed through the water column, and it is believed the different methane deposits are unlikely to to be destabilised simultaneously.
Permafrost has been used as a thermometer of climate change on decadal- and century-timescales. Boreholes have been drilled in frozen ground to determine the surface temperature history, based on the diffusion of surface temperature signals to depth. These records added to the accumulating body of evidence related to the warming occurring in the 20th century. A recent warming of up to 4oC in Alaska has been indicated by borehole temperatures, though the borehole records from other locations show a negligible change or even cooling. The records from boreholes can be difficult to interpret as the effects of changes of vegetation and snow cover, which cause a decoupling of the signals of air and ground temperature, though these records are an accurate reflection of the history of ground temperatures. It is difficult to invert borehole temperature records for temperature trends on a millennial scale as variability of temperature is smoothed out by thermal diffusion.
According to the author1 the oldest ice on Earth is probably the ice present in deep permafrost. A number of features of the Earth are altered by the presence of permafrost including the water table, the shape of the regional geomorphology, as well as causing significant problems for infrastructure. In ice-rich sediments on the coast in the Arctic, where the rising sea level and reduced ice production have been exposing the coastal margins to increased ocean swell some of the highest rates of coastal erosion have been recorded. Among other aspects of the interaction between permafrost and the climate system are its potentially significant role in the global carbon cycle through the storage, over the long-term, of organic carbon, the melting of which would lead to the destabilisation of methane and carbon dioxide clathrates, thereby releasing these greenhouse gases into the atmosphere
|Author: M.H.Monroe Email: firstname.lastname@example.org Sources & Further reading|