Мұздық астындағы көлдер

Lake under a glacier

A subglacial lake is a lake that is found under a glacier, typically beneath an ice cap or ice sheet. Subglacial lakes form at the boundary between ice and the underlying bedrock, where pressure decreases the pressure melting point of ice. Over time, the overlying ice gradually melts at a rate of a few millimeters per year. Meltwater flows from regions of high to low hydraulic pressure under the ice and pools, creating a body of liquid water that can be isolated from the external environment for millions of years. Since the first discoveries of subglacial lakes under the Antarctic Ice Sheet, more than 400 subglacial lakes have been discovered in Antarctica, beneath the Greenland Ice Sheet, and under Iceland's Vatnajökull ice cap. Subglacial lakes contain a substantial proportion of Earth's liquid freshwater, with the volume of Antarctic subglacial lakes alone estimated to be about 10,000 km3, or about 15% of all liquid freshwater on Earth. As ecosystems isolated from Earth's atmosphere, subglacial lakes are influenced by interactions between ice, water, sediments, and organisms. They contain active biological communities of extremophilic microbes that are adapted to cold, low nutrient conditions and facilitate biogeochemical cycles independent of energy inputs from the sun. Subglacial lakes and their inhabitants are of particular interest in the field of astrobiology and the search for extraterrestrial life.

The water in subglacial lakes remains liquid since geothermal heating balances the heat loss at the ice surface. The pressure from the overlying glacier causes the melting point of water to be below 0 °C. The ceiling of the subglacial lake will be at the level where the pressure melting point of water intersects with the temperature gradient. In Lake Vostok, the largest Antarctic subglacial lake, the ice over the lake is thus much thicker than the ice sheet around it. Hypersaline subglacial lakes remain liquid due to their salt content.

The role of subglacial lakes on ice dynamics is unclear. Certainly on the Greenland Ice Sheet subglacial water acts to enhance basal ice motion in a complex manner. The "Recovery Lakes" beneath Antarctica's Recovery Glacier lie at the head of a major ice stream and may influence the dynamics of the region. A modest (10%) speed up of Byrd Glacier in East Antarctica may have been influenced by a subglacial drainage event. The flow of subglacial water is known in downstream areas where ice streams are known to migrate, accelerate or stagnate on centennial time scales and highlights that subglacial water may be discharged over the ice sheet grounding line.

Russian revolutionary and scientist Peter A. Kropotkin first proposed the idea of liquid freshwater under the Antarctic Ice Sheet at the end of the 19th century. He suggested that due to the geothermal heating at the bottom of the ice sheets, the temperature beneath the ice could reach the ice melt temperature, which would be below zero. The notion of freshwater beneath ice sheets was further advanced by Russian glaciologist Igor A. Zotikov, who demonstrated via theoretical analysis the possibility of a decrease in Antarctic ice because of melting of ice at a lower surface.

Scientific advances in Antarctica can be attributed to several major periods of collaboration and cooperation, such as the four International Polar Years (IPY) in 1882 1883, 1932 1933, 1957 1958, and 2007 2008. The success of the 1957 1958 IPY led to the establishment of the Scientific Committee on Antarctic Research (SCAR) and the Antarctic Treaty System, paving the way to formulate a better methodology and process to observe subglacial lakes. In 1959 and 1964, during two of his four Soviet Antarctic Expeditions, Russian geographer and explorer Andrey P. Kapitsa used seismic sounding to prepare a profile of the layers of the geology below Vostok Station in Antarctica. The original intent of this work was to conduct a broad survey of the Antarctic Ice Sheet. The data collected on these surveys, however, was used 30 years later and led to the discovery of Lake Vostok as a subglacial lake. Beginning in the late 1950s, English physicists Stan Evans and Gordon Robin began using the radioglaciology technique of radio echo sounding (RES) to chart ice thickness. Subglacial lakes are identified by (RES) data as continuous and specular reflectors which dip against the ice surface at around x10 of the surface slope angle, as this is required for hydrostatic stability. In the late 1960s, they were able to mount RES instruments on aircraft and acquire data for the Antarctic Ice Sheet. Between 1971 and 1979, the Antarctic Ice Sheet was profiled extensively using RES equipment. Using this approach, 17 subglacial lakes were documented by Kapista and his team. RES also led to the discovery of the first subglacial lake in Greenland This imaging revealed a flat surface around the northern border of Lake Vostok, and the data collected from ERS 1 further built the geographical distribution of Antarctic subglacial lakes. In 2005, Laurence Gray and a team of glaciologists began to interpret surface ice slumping and raising from RADARSAT data, which indicated there could be hydrologically “active” subglacial lakes subject to water movement. Between 2003 and 2009, a survey of long track measurements of ice surface elevation using the ICESat satellite as a part of NASA's Earth Observing System produced the first continental scale map of the active subglacial lakes in Antarctica. Gray et al. (2005) interpreted ice surface slumping and raising from RADARSAT data as evidence for subglacial lakes filling and emptying termed "active" lakes. Wingham et al. (2006) used radar altimeter (ERS 1) data to show coincident uplift and subsidence, implying drainage between lakes. NASA's ICESat satellite was key in developing this concept further and subsequent work demonstrated the pervasiveness of this phenomenon. ICESat ceased measurements in 2007 and the detected "active" lakes were compiled by Smith et al. (2009) who identified 124 such lakes. The realisation that lakes were interconnected created new contamination concerns for plans to drill into lakes (see the Sampling expeditions section below). Several lakes were delineated by the famous SPRI NSF TUD surveys undertaken until the mid seventies. Since this original compilation several smaller surveys has discovered many more subglacial lakes throughout Antarctica, notably by Carter et al. (2007), who identified a spectrum of subglacial lake types based on their properties in (RES) datasets.

In March 2010, the sixth international conference on subglacial lakes was held at the American Geophysical Union Chapman Conference in Baltimore. The conference allowed engineers and scientists to discuss the equipment and strategies used in ice drilling projects, such as the design of hot water drills, equipment for water measurement and sampling and sediment recovery, and protocols for experimental cleanliness and environmental stewardship. Lake water flooded the borehole and froze during the winter season, and the sample of re frozen lake water (accretion ice) was recovered in the following summer season of 2013. In December 2012, scientists from the UK attempted to access Lake Ellsworth with a clean access hot water drill; however, the mission was called off because of equipment failure. In January 2013, the US led Whillans Ice Stream Subglacial Access Research Drilling (WISSARD) expedition measured and sampled Lake Whillans in West Antarctica for microbial life. On 28 December 2018, the Subglacial Antarctic Lakes Scientific Access (SALSA) team announced they had reached Lake Mercer after melting their way through 1,067 m (3,501 ft) of ice with a high pressure hot water drill. indicating that the East Antarctic lakes are fed by a subglacial system that transports basal meltwater through subglacial streams. The largest Antarctic subglacial lakes are clustered in the Dome C Vostok area of East Antarctica, possibly due to the thick insulating ice and rugged, tectonically influenced subglacial topography. In West Antarctica, subglacial Lake Ellsworth is situated within the Ellsworth Mountains and is relatively small and shallow. The Siple Coast Ice Streams, also in West Antarctica, overlie numerous small subglacial lakes, including Lakes Whillans, Engelhardt, Mercer, Conway,
accompanied by their lower neighbours called Lower Conway (LSLC) and Lower Mercer (LSLM).

The existence of subglacial lakes beneath the Greenland Ice Sheet has only become evident within the last decade. Radio echo sounding measurements have revealed two subglacial lakes in the northwest section of the ice sheet. Another potential subglacial lake has been identified near the southwestern margin of the ice sheet, where a circular depression beneath the ice sheet evidences recent drainage of the lake caused by climate warming. Such drainage, coupled with heat transfer to the base of the ice sheet through the storage of supraglacial meltwater, is thought to influence the rate of ice flow and overall behavior of the Greenland Ice Sheet. The majority of Icelandic subglacial lakes are located beneath the Vatnajökull and Mýrdalsjökull ice caps, where melting from hydrothermal activity creates permanent depressions that fill with meltwater. Grímsvötn is perhaps the best known subglacial lake beneath the Vatnajökull ice cap. Other lakes beneath the ice cap lie within the Skatfá, Pálsfjall and Kverkfjöll cauldrons. The Mýrdalsjökull ice cap, another key subglacial lake location, sits on top of an active volcano caldera system in the southernmost part of the Katla volcanic system. These paleo subglacial lakes likely occupied valleys created before the advance of the Laurentide Ice Sheet during the Last Glacial Maximum. However, two subglacial lakes were identified via RES in bedrock troughs under the Devon Ice Cap of Nunavut, Canada. These lakes are thought to be hypersaline as a result of interaction with the underlying salt bearing bedrock, and are much more isolated than the few identified saline subglacial lakes in Antarctica. Professor John Priscu, a prominent scientist studying polar lakes, has called Antarctica's subglacial ecosystems "our planet's largest wetland.”

Microorganisms and weathering processes drive a diverse set of chemical reactions that can drive a unique food web and thus cycle nutrients and energy through subglacial lake ecosystems. No photosynthesis can occur in the darkness of subglacial lakes, so their food webs are instead driven by chemosynthesis and the consumption of ancient organic carbon deposited before glaciation.

Since few subglacial lakes have been directly sampled, much of the existing knowledge about subglacial lake biogeochemistry is based on a small number of samples, mostly from Antarctica. Inferences about solute concentrations, chemical processes, and biological diversity of unsampled subglacial lakes have also been drawn from analyses of accretion ice (re frozen lake water) at the base of the overlying glaciers. These inferences are based on the assumption that accretion ice will have similar chemical signatures as the lake water that formed it. Scientists have thus far discovered diverse chemical conditions in subglacial lakes, ranging from upper lake layers supersaturated in oxygen to bottom layers that are anoxic and sulfur rich. Despite their typically oligotrophic conditions, subglacial lakes and sediments are thought to contain regionally and globally significant amounts of nutrients, particularly carbon.

Air clathrates trapped in glacial ice are the main source of oxygen entering otherwise enclosed subglacial lake systems. As the bottom layer of ice over the lake melts, clathrates are freed from the ice's crystalline structure and gases such as oxygen are made available to microbes for processes like aerobic respiration. In some subglacial lakes, freeze melt cycles at the lake ice interface may enrich the upper lake water with oxygen concentrations that are 50 times higher than in typical surface waters. Melting of the layer of glacial ice above the subglacial lake also supplies underlying waters with iron, nitrogen, and phosphorus containing minerals, in addition to some dissolved organic carbon and bacterial cells. Oxic or slightly suboxic waters often reside near the glacier lake interface, while anoxia dominates in the lake interior and sediments due to respiration by microbes. In some subglacial lakes, microbial respiration may consume all of the oxygen in the lake, creating an entirely anoxic environment until new oxygen rich water flows in from connected subglacial environments. The addition of oxygen from ice melt and the consumption of oxygen by microbes may create redox gradients in the subglacial lake water column, with aerobic microbial mediated processes like nitrification occurring in the upper waters and anaerobic processes occurring in the anoxic bottom waters. Subglacial outflow from the Antarctic Ice Sheet, including outflow from subglacial lakes, is estimated to add a similar amount of solutes to the Southern Ocean as some of the world's largest rivers. The morphology of subglacial lakes has the potential to change their hydrology and circulation patterns. Areas with the thickest overlying ice experience greater rates of melting. The opposite occurs in areas where the ice sheet is thinnest, which allows re freezing of lake water to occur. and may rival the amount of reactive carbon in modern ocean sediments, potentially making subglacial sediments an important but understudied component of the global carbon cycle. Methane has been detected in subglacial Lake Whillans, and experiments have shown that methanogenic archaea can be active in sediments beneath both Antarctic and Arctic glaciers. Most of the methane that escapes storage in subglacial lake sediments appears to be consumed by methanotrophic bacteria in oxygenated upper waters. In subglacial Lake Whillans, scientists found that bacterial oxidation consumed 99% of the available methane. Antarctic subglacial waters are also thought to contain substantial amounts of organic carbon in the form of dissolved organic carbon and bacterial biomass. but over the last thirty years, active microbial life and signs of higher life have been discovered in subglacial lake waters, sediments, and accreted ice. Like plants, chemolithoautotrophs fix carbon dioxide (CO2) into new organic carbon, making them the primary producers at the base of subglacial lake food webs. Rather than using sunlight as an energy source, chemolithoautotrophs get energy from chemical reactions in which inorganic elements from the lithosphere are oxidized or reduced Common elements used by chemolithoautotrophs in subglacial ecosystems include sulfide, iron, and carbonates weathered from sediments. The variable redox conditions and diverse elements available from sediments provide opportunities for many other metabolic strategies in subglacial lakes. Other metabolisms used by subglacial lake microbes include methanogenesis, methanotrophy, and chemolithoheterotrophy, in which bacteria consume organic matter while oxidizing inorganic elements. If present, these organisms could survive by consuming bacteria and other microbes.

Subglacial lake waters are considered to be ultra oligotrophic and contain low concentrations of nutrients, particularly nitrogen and phosphorus. In surface lake ecosystems, phosphorus has traditionally been thought of as the limiting nutrient that constrains growth in the ecosystem, although co limitation by both nitrogen and phosphorus supply seems most common. However, evidence from subglacial Lake Whillans suggests that nitrogen is the limiting nutrient in some subglacial waters, based on measurements showing that the ratio of nitrogen to phosphorus is very low compared to the Redfield ratio. Other subglacial sampling efforts in Antarctica include the subglacial pool of anoxic, hypersaline water under Taylor Glacier, which harbors a microbial community that was sealed off from the atmosphere 1.5 to 2 million years ago. Bacteria under Taylor Glacier appear to have a novel metabolic strategy that uses sulfate and ferric ions to decompose organic matter.

Subglacial lakes under Iceland's Vatnajökull ice cap provide unique habitats for microbial life because they receive heat and chemical inputs from subglacial volcanic activity, influencing the chemistry of lower lake waters and sediments. Active psychrophilic, autotrophic bacteria have been discovered in the lake below the Grímsvötn volcanic caldera. A low diversity microbial community has also been found in the east Skaftárketill and Kverkfjallalón subglacial lakes, where bacteria include Geobacter and Desulfuosporosinus species that can use sulfur and iron for anaerobic respiration. In the western Skaftá lake, the anoxic bottom waters appear to be dominated by acetate producing bacteria rather than methanogens. Life would have survived primarily in glacial and subglacial environments, making modern subglacial lakes an important study system for understanding this period in Earth's history. More recently, subglacial lakes in Iceland may have provided a refuge for subterranean amphipods during the Quaternary glacial period.

Subglacial lakes are an analog environment for extraterrestrial ice covered water bodies, making them an important study system in the field of astrobiology, which is the study of the potential for life to exist outside Earth. Discoveries of living extremophilic microbes in Earth's subglacial lakes could suggest that life may persist in similar environments on extraterrestrial bodies. Jupiter's moon Europa and Saturn’s moon Enceladus are promising targets in the search for extraterrestrial life. Europa contains an extensive ocean covered by an icy crust, and Enceladus is also thought to harbor a subglacial ocean. Satellite analysis of an icy water vapor plume escaping from fissures in Enceladus' surface reveals significant subsurface production of hydrogen, which may point towards the reduction of iron bearing minerals and organic matter. A subglacial lake on Mars was discovered in 2018 using RES on the Mars Express spacecraft. This body of water was found beneath Mars’ South Polar Layered Deposits, and is suggested to have formed as a result of geothermal heating causing melting beneath the ice cap.