Subglacial lake
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 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.
Physical characteristics
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.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 Lake Hodgson, a former subglacial lake.
Hydrostatic seals
The water in a subglacial lake can have a floating level much above the level of the ground threshold. In fact, theoretically a subglacial lake can even exist on the top of a hill, provided that the ice over it is thin enough to form the required hydrostatic seal. The floating level can be thought of as the water level in a hole drilled through the ice into the lake. It is equivalent to the level at which a piece of ice over it would float if it were a normal ice shelf. The ceiling can therefore be conceived as an ice shelf that is grounded along its entire perimeter, which explains why it has been called a captured ice shelf. As it moves over the lake, it enters the lake at the floating line, and it leaves the lake at the grounding line.A hydrostatic 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.
Influence on glacier movement
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 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.History and expeditions
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. As of 2019, there are over 400 subglacial lakes in Antarctica, and it is suspected that there is a possibility of more. Subglacial lakes have also been discovered in Greenland, Iceland, and northern Canada.Early exploration
Scientific advances in Antarctica can be attributed to several major periods of collaboration and cooperation, such as the four International Polar Years 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 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 to chart ice thickness. Subglacial lakes are identified by 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.
Satellite exploration
In the early 1990s, radar altimeter data from the European Remote-Sensing Satellite provided detailed mapping of Antarctica through 82 degrees south. 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. 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. interpreted ice surface slumping and raising from RADARSAT data as evidence for subglacial lakes filling and emptying - termed "active" lakes. Wingham et al. used radar altimeter 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. who identified 124 such lakes. The realisation that lakes were interconnected created new contamination concerns for plans to drill into lakes.
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., who identified a spectrum of subglacial lake types based on their properties in datasets.
Sampling expeditions
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. Following this meeting, SCAR drafted a code of conduct for ice drilling expeditions and in situ measurements and sampling of subglacial lakes. This code of conduct was ratified at the Antarctic Treaty Consultative Meeting of 2011. By the end of 2011, three separate subglacial lake drilling exploration missions were scheduled to take place.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 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 expedition measured and sampled Lake Whillans in West Antarctica for microbial life. On 28 December 2018, the team announced they had reached Lake Mercer after melting their way through 1,067 m of ice with a high-pressure hot-water drill. The team collected water samples and bottom sediment samples down to 6 meters deep.