Antarctic is known for being earth’s largest single mass of ice. It covers almost 14 million sq km and holds around 61 per cent of earth’s freshwater. However, wide networks of canyons, waterways and lakes that have been discovered buried under the ice sheet have captured the interest of people and are now the focus of novel scientific research. Subglacial lakes are large pockets of water that lie beneath a glacier or the ice sheet and are formed due to geothermal heat that is trapped by the insulating effect of the ice. These lakes may have existed since the continent became glaciated. There are around 400 subglacial lakes scattered across the Antarctic continent (Siegert et al., 2016).These subglacial lakes are proving to be attractive models to explore fundamental themes in limnology as well as have direct global implications such as landscape−lake interactions, the viability and adaptation of organisms to environmental extremes and subglacial aquatic environments as a planetary storehouse of ancient microbes and past climate records (Vincent and Laybourn-Parry, 2008)
Exploration of Antarctic subglacial lakes
The freezing of Antarctica began in the middle Eocene ( about 45.5 million years ago) and intensified during the Eocene−Oligocene about 35 million years ago. Since its formation, the ice sheet has withdrawn a little. However, its eastern and western parts have experienced significant fluctuations in ice volume over time.
The idea of subglacial lakes was proposed as early as the 19th century, when Russian scientist Peter Kropotkin first suggested the existence of freshwater under the Antarctic ice sheets. He theorised that the tremendous pressure exerted by the cumulative mass of thousands of vertical meters of ice could increase the temperature at the lowest portions of the ice sheet to the point where the ice would melt.
During Admiral Byrd’s expedition (1939 to 1941) seismic sounding from the surface of the ice sheet was undertaken to map the subglacial features. The geophysical techniques used during the International Geophysical Year (IGY) 1957-1968, by a number of countries to provide the underlying topography brought many interesting insights of the subglacial structures to light (National Research Council, 2007). The first observation of a subglacial lake was made by Robinson (1964) who identified ‘oval depressions with gentle shores’ on the ice surface (Robinson, 1964). Likewise, the first direct identification of a subglacial lake came as a result of the Antarctic airborne radio-echo sounding (RES) programme from 1968 to 1979 (Robin, 1972). In fact, the first subglacial lake was recognised beneath the Russian base at Sovetskaya Station in central East Antarctica during one of the first RES flights over the Antarctic continent (Robin et al., 1970).
The development of satellites with radar altimeter sensors assisted in locating subglacial lakes on the Antarctic continent. In 1993, altimetry data along with RES from satellite measurements provided evidence of the areal extent of the Vostok Lake, thus confirming it to be the largest known subglacial lake system.Since then, more than 379 lakes have been identified (Siegert et al. 2005). Moreover, recent research has exposed that several lakes are clearly connected to each other (Wingham et al., 2006; Fricker et al. 2007) and form a part of a discontinuous hydrological system rather than isolated entities. The largest lakes clustered in the Dome C-Vostok area of East Antarctic are perhaps hydrologically connected making them an important subject for study of subglacial hydrology, biology and geochemical diversity (Fig. 1).
The Scientific Committee on Antarctic Research (SCAR) constituted a Subglacial Antarctic Lake Exploration Group of Specialists (SALEGOS) in 2000 to understand the formation and evolution of subglacial lake processes and environments determine its origin, evolution, and maintenance of life and understand the limnology and paleoclimate history recorded within it. Recently, some major high-profile scientific projects have emerged. This includes a long standing Russian ice core project’s extension into Lake Vostok, which used the existing 3.5 km deep ice-core borehole. It also had a UK-led mission to explore newly discovered Lake Ellsworth in West Antarctica and a US-led study to access and sample Lake Whillans, an ephemeral subglacial lake at the edge of the West Antarctic ice sheet- using hot-water drilling. Expedition Subglacial Antarctic Lakes Scientific Access (SALSA), funded by the US National Science Foundation was initiated in 2016. This was an integrative study of subglacial geobiology, water column and sedimentary organic carbon and geological processes in Mercer Subglacial Lake beneath the Whillans and Mercer ice streams in West Antarctic.
Gamburtsev Aerogeophysical Mapping of Bedrock and Ice Targets (GAMBIT) is an international partnership between the United States, the United Kingdom, Germany, Australia, China and Japan to study the formation of the mountains and the overlying ice sheet that will peel back more than 600 m of ice to explore the last hidden mountain range. Virtually unexplored, the Gamburtsev Mountains represent the largest unstudied area of crustal uplift on earth and is believed to be the starting point for growth of the Antarctic ice sheets. The project will complete a gravity, magnetics and radar study to advance our understanding of the origin and evolution of the polar ice sheets and subglacial lakes. Another major international programme Investigating the Cryospheric Evolution of the Central Antarctic Plate (ICECAP ) is a campaign driven to map the morphology, geology and glaciology of the major marine subglacial basins of East Antarctic. The data is acquired using a long range multi-instrumented aerogeophysical aircraft over the Wilkes and Aurora Subglacial Basins and their margins since 2008. This was a part of both the International Polar Year and Operation Ice Bridge.
Research Prospects in Subglacial Lakes
Antarctic subglacial lakes represent an important component of Antarctic and influence ice dynamics, act as sedimentary traps and house a vast amount of water. Likewise, they also allow determination of thermal conditions within the ice sheet that are related to the geothermal heat flux through the earth’s crust. The lakes, moreover, have the likelihood to be thriving with life and their isolation could mean that they could have unique anomalies or adaptations to this extreme environment. Thus, there are several areas of potential interest that can be explored through examination of Antarctic subglacial lakes and are thus poised to be a major focus of Antarctic science for the next decade or more.
Sustainability of Life in Extreme Conditions
As subglacial environments are extremes with low temperatures, elevated pressures, limited nutrient supply, absence of sunlight and no direct exchange with the atmosphere— questions arise about the existence of life beneath the ice. These environments have been isolated from the surface for millennia and thus, their contents have remained pristine. Microbial metabolism in these systems would rely on metabolic energy and nutrition derived from glacial ice, the bedrock, and/or hydrothermal or geothermal inputs.
In recent years, microbial life, distinct from the flora and fauna, that teems across the Antarctic continent has been found to exist. In 2013, a team of US-based researchers called Whillans Ice Stream Subglacial Access Research Drilling announced that it had successfully extracted liquid water from the Lake Whillans. The researchers confirmed that life not only exists in the subglacial lake, but is highly abundant.
They suggested that waters beneath the Antarctic ice sheet support viable microbial ecosystems and house globally relevant pools of carbon and microbes that can mobilise elements from the lithosphere and influence Southern Ocean geochemical and biological systems (Christner et al., 2014). A Russian team working on a similar project at Antarctic’s subglacial Lake Vostok have found evidences that the lake may contain a complex interdependent set of organisms, zones and habitats that have developed over the tens of millions of years of its existence (Rogers et al., 2013). Further, Antarctic’s subglacial lakes have also attracted astrobiologists who suggest that the lake environments could be analogous to extraterrestrial systems such as icy worlds beneath the Martian ice cap and Jupiter’s moon Europa. Beneath the icy surface of Europa lies a liquid water ocean. Studying ice covered lakes on Earth could provide clues as to how life might adapt to similarly isolated environments on other worlds in the solar system.
Archives of the Past Environments
As the subglacial lakes form a connected hydrological network, they transport a substantial amount of sediment over time that is deposited in the lakes. These sedimentary records would date back to the time at which the lake was formed and could provide climatic information older than five to thirty million years. Thus, the sediment records from subglacial lakes may provide a detailed record of paleoenvironmental information and may help fill the hiatuses in the glacial and paleoclimatic history of Antarctic and the earth.
Subglacial Ice Dynamics and Hydrology
Subglacial lakes are important because of their effect on subglacial hydrology and ice sheet stability. Further, the linkages of subglacial lakes with the onset of rapid ice flow indicate that subglacial lakes and the associated hydrologic systems may be important factors in the drainage and collapse of ice sheets. Thus subglacial lakes crucially affect the velocity and direction of the ice flow as well as the thermal regime of the ice sheet. However, the understanding of these systems is limited and needs to be studied more extensively. An understanding of subglacial hydrology on a continental scale is needed to establish subglacial water distribution and the effects of subglacial water movements on the overlying ice sheet. India, with its sizeable scientific community, has the capability to significantly contribute to unearth the mysteries of subglacial systems.
The pivotal role of water beneath the Antarctic ice sheets has sparked curiosity across various scientific disciplines. The exploration and study of subglacial environments holds great promise for furthering our understanding of Antarctic and the earth as a whole; it’s evolution over geologic time, the existence and adaptability of life and for providing clues to the subsistence of life elsewhere in our universe.