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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 liquid water can exist above the lower melting point of ice under high pressure. 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 Fresh water, 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 isolated from Earth's atmosphere, subglacial lakes are influenced by interactions between ice, water, , and . They contain active biological communities of Extremophile Microorganism that are Adaptation to Psychrophile, Oligotroph 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.
Not all lakes with permanent ice cover can be called subglacial, as some are covered by regular lake ice. Some examples of perennially ice-covered lakes include Lake Bonney and Lake Hoare in Antarctica's McMurdo Dry Valleys as well as Hodgson Lake, a former subglacial lake.
A Hydrostatics seal is created when the ice is so much higher around the lake that the equipotential surface dips down into impermeable ground. Water from underneath this ice rim is then pressed back into the lake by the hydrostatic seal. The ice rim in Lake Vostok has been estimated to a mere 7 meters, while the floating level is about 3 kilometers above the lake ceiling. If the hydrostatic seal is penetrated when the floating level is high, the water will start flowing out in a jökulhlaup. Due to melting of the channel the discharge increases exponentially, unless other processes allow the discharge to increase even faster. Due to the high hydraulic head that can be achieved in some subglacial lakes, jökulhlaups may reach very high rates of discharge. Catastrophic drainage from subglacial lakes is a known hazard in Iceland, as volcanic activity can create enough meltwater to overwhelm ice dams and lake seals and cause glacial outburst flooding.
In 1959 and 1964, during two of his four Soviet Antarctic Expeditions, Russian geographer and explorer Andrey Kapitsa used seismic sounding to prepare a Stratigraphy 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. The technique of using RES is as follows: 50-meter deep holes are drilled to increase the signal-to-noise ratio in the ice. A small explosion sets off a Sound, which travels through the ice. This sound wave is reflected and then recorded by the instrument. The time it takes for the wave to travel down and back is noted and converted to a distance using the known speed of sound in ice. RES records can identify subglacial lakes via three specific characteristics: 1) an especially strong reflection from the ice-sheet base, stronger than adjacent ice-bedrock reflections; 2) echoes of constant strength occurring along the track, which indicate that the surface is very smooth; and 3) a very flat and horizontal character with slopes less than 1%. 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 and revealed that these lakes are interconnected.
Systematic profiling, using RES, of the Antarctic Ice Sheet took place again between 1971–1979. During this time, a US-UK-Danish collaboration was able to survey about 40% of East Antarctica and 80% of West Antarctica – further defining the subglacial landscape and the behavior of ice flow over the 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 Hydrology “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. In 2009, it was revealed that Lake Cook is the most hydrologically active subglacial lake on the Antarctic continent. Other satellite imagery has been used to monitor and investigate this lake, including ICESat, CryoSat-2, the Advanced Spaceborne Thermal Emission and Reflection Radiometer, and SPOT5.
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 February 2012, Russian ice-core drilling at Lake Vostok accessed the subglacial lake for the first time. 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. The team collected water samples and bottom sediment samples down to 6 meters deep.
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. Notably, subglacial lake Grímsvötn's hydraulic seal remained intact until 1996, when significant meltwater production from the Gjálp eruption resulted in uplift of Grímsvötn's ice dam.
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. Hydrothermal activity beneath the Mýrdalsjökull ice cap is thought to have created at least 12 small depressions within an area constrained by three major subglacial . Many of these depressions are known to contain subglacial lakes that are subject to massive, catastrophic drainage events from volcanic eruptions, creating a significant hazard for nearby human populations.
and weathering processes drive a diverse set of chemical reactions that can drive a unique food-web and thus Nutrient cycle and energy through subglacial lake ecosystems. No photosynthesis can occur in the darkness of subglacial lakes, so their are instead driven by chemosynthesis and the consumption of ancient organic carbon deposited before glaciation. Nutrients can enter subglacial lakes through the glacier ice-lake water interface, from Hydrology connections, and from the physical, chemical, and biological weathering of subglacial .
Melting of the layer of glacial ice above the subglacial lake also supplies underlying waters with iron, nitrogen, and phosphorus-containing , in addition to some dissolved organic carbon and bacterial cells.
Concentrations of solutes in subglacial lakes, including major and like sodium, sulfate, and , are low compared to typical surface lakes. These solutes enter the water column from glacial ice melting and from sediment weathering. Despite their low solute concentrations, the large volume of subglacial waters make them important contributors of solutes, particularly iron, to their surrounding oceans. 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 subglacial water column is influenced by the exchange of water between lakes and streams under ice sheets through the subglacial drainage system; this behavior likely plays an important role in biogeochemical processes, leading to changes in microbial habitat, particularly regarding oxygen and nutrient concentrations. Hydrology connectivity of subglacial lakes also alters water residence times, or amount of time that water stays within the subglacial lake reservoir. Longer residence times, such as those found beneath the interior Antarctic Ice Sheet, would lead to greater contact time between the water and solute sources, allowing for greater accumulation of solutes than in lakes with shorter residence times. Estimated residence times of currently studied subglacial lakes range from about 13,000 years in Lake Vostok to just decades in Lake Whillans.
The Geomorphology 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. These spatial variations in melting and freezing rates lead to internal convection of water and circulation of solutes, heat, and microbial communities throughout the subglacial lake, which will vary among subglacial lakes of different regions.
The products of sulfide oxidation can enhance the chemical weathering of carbonate and silicate minerals in subglacial sediments, particularly in lakes with long residence times. Weathering of carbonate and silicate minerals from lake sediments also releases other ions including potassium (K+), magnesium (Mg2+), sodium (Na+), and calcium (Ca2+) to lake waters.
Other biogeochemical processes in anoxic subglacial sediments include denitrification, Iron cycle, Sulfur cycle, and methanogenesis (see Reservoirs of organic carbon below).
The microbial inhabitants of subglacial lakes likely play an important role in determining the form and fate of sediment organic carbon. In the anoxic waters sediments of subglacial lake ecosystems, organic carbon can be used by archaea for methanogenesis, potentially creating large pools of methane clathrate in the sediments that could be released during ice sheet collapse or when lake waters drain to ice sheet margins. 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 bacteria in oxygenated upper waters. In subglacial Lake Whillans, scientists found that bacterial oxidation consumed 99% of the available methane. There is also evidence for active methane production and consumption beneath the Greenland Ice Sheet.
Antarctic subglacial waters are also thought to contain substantial amounts of organic carbon in the form of dissolved organic carbon and bacterial biomass. At an estimated 1.03 x 10−2 petagrams, the amount of organic carbon in subglacial lake waters is far smaller than that contained in Antarctic subglacial sediments, but is only one order of magnitude smaller than the amount of organic carbon in all surface freshwaters (5.10 x 10−1 petagrams). This relatively smaller, but potentially more reactive, reservoir of subglacial organic carbon may represent another gap in scientists’ understanding of the global carbon cycle.
Like plants, chemolithoautotrophs Carbon fixation (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 Redox . Common elements used by chemolithoautotrophs in subglacial ecosystems include sulfide, iron, and weathered from sediments.
In addition to mobilizing elements from sediments, chemolithoautotrophs create enough new organic matter to support bacteria in subglacial ecosystems. Heterotrophic bacteria consume the organic material produced by chemolithoautotrophs, as well as consuming organic matter from sediments or from melting glacial ice. Despite the resources available to subglacial lake heterotrophs, these bacteria appear to be exceptionally slow-growing, potentially indicating that they dedicate most of their energy to survival rather than growth. Slow heterotrophic growth rates could also be explained by the cold temperatures in subglacial lakes, which slow down microbial metabolism and reaction rates.
The variable Redox 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, , and chemolithoheterotrophy, in which bacteria consume organic matter while oxidizing inorganic elements.
Some limited evidence for microbial and multicellular in subglacial lakes could expand current ideas of subglacial food webs. If present, these organisms could survive by consuming bacteria and other microbes.
In January 2019, the SALSA team collected sediment and water samples from subglacial Lake Mercer and found diatom shells and well-preserved carcasses from and a tardigrade. Although the animals were dead, the team also found bacterial concentrations of 10,000 cells per milliliter, suggesting the potential for animals to survive in the lake by consuming bacteria. The team will continue analyzing the samples to further investigate the chemistry and biology of the lake.
Lake Vostok is the best-studied Antarctic subglacial lake, but its waters have only been studied through analysis of accretion ice from the bottom of ice cores taken during Russian drilling efforts above the lake. Actively growing bacteria and thousands of unique DNA sequences from bacteria, archaea, and have been found in Lake Vostok's accretion ice. Some DNA appeared to come from multicellular eukaryotes, including species seemingly related to freshwater Daphnia, , and Mollusca. These species may have survived in the lake and slowly Adaptation to the changing conditions since Vostok was last exposed to the atmosphere millions of years ago. However, the samples were likely contaminated by drilling fluid while being collected, so some of the identified organisms probably did not live in the lake. Other subglacial sampling efforts in Antarctica include the subglacial pool of anoxic waters, Hypersaline lake 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 Decomposition organic matter.
During the proposed Snowball Earth period of the late Proterozoic, extensive glaciation could have completely covered Earth's surface in ice for 10 million years. 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 Amphipoda during the Quaternary glacial period.
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.
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