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One example of such research is the exploration of Antarctic subglacial lakes, bodies of water at the bed of the ice sheet, which SCAR has acted to facilitate over the past 20 years, and which culminated in field programmes to measure and sample three individual lakes in 2012/13: Lake Vostok, led by a Russian team; Lake Ellsworth, led by the British; and Lake Whillans, led by the USA.
The experiment is simple: a small explosion sets off a sound wave, which travels down to the ice base, where it is reflected and subsequently recorded by the receiver. The two-way travel time is noted and converted into distance as the speed of sound in ice is known reasonably well. Thus, a measure of ice thickness is possible using a simple seismic reflection test, adapted for harsh polar field conditions by Robin. While the process of data acquisition is time consuming (two boreholes for each data point, meaning that a single datum would take at least a day to record), by aligning measurements along a survey line a profile of ice-sheet thickness, and therefore bed topography, could be derived. In this way, the first two-dimensional cross-section measurements of the Antarctic ice sheet and its subglacial landscape were obtained.
The data collected by these surveys transformed our knowledge of the continent, proving it to be a single landmass, showing the ice to be several kilometres thick (at Vostok Station, for example, it was measured as c. 3.7 km) and, in large parts of West Antarctica, revealing the bed to be over a kilometre below sea-level.
Radio-echo sounding (RES), as it was known (essentially ice-penetrating radar), was able to chart ice thickness, therefore, in an analogous way to seismic sounding. The major advantage of RES over seismic sounding was that it did not require the drilling of boreholes and could be deployed on a moving platform to obtain cross-section information during transit. The most significant innovation by Evans and Robin was to consider how RES could be mounted and used effectively on aircraft. In the late 1960s their Cambridge team, supported by funding and logistics from the US Antarctic Research Program, demonstrated the use of airborne RES with instant and revolutionary success.
For example, the first subglacial lake was discovered from data collected on one of the first long-range survey flights in 1969. An unusually flat subglacial radio-echo surface beneath the Russian base at Sovetskaya Station in central East Antarctica was received and attributed to a ‘thick layer of water beneath the ice’ (Robin et al. 1970). Shortly afterwards, the first inventory of 17 subglacial lakes was documented from East Antarctic RES data (Oswald & Robin 1973; Fig. 1). Lake Vostok, the gigantic subglacial lake in East Antarctica, was detected by RES in December 1974 (Robin et al. 1977), although its true extent was not established at this time. These early discoveries showed the bed of the Antarctic Ice Sheet to be wet in many places, that water could be stored in ‘lakes’ and that the distribution of subglacial lakes was widespread across the surveyed regions of the continent.
Although subglacial lakes were undeniably discovered using RES by Robin and his team, the first mention of ‘lakes’ in Antarctica was made by a Russian pilot (Robinson 1960) who, as part of an experiment to determine ice-surface landmarks to aid flight orientation, identified ‘oval depressions with gentle shores’ on the ice surface. Although these features were referred to as ‘lakes’ by pilots who observed them, such as Robinson, there was no connection made between these features and water beneath the surface.
Ridley et al. (1993) analysed ERS-1 data from East Antarctica, and noticed a remarkably flat surface at and to the north of Vostok Station. This surface, which pilots may have observed in the 1950s, coincides remarkably well with RES evidence of a large subglacial lake established by Robin et al. (1977), and delineates the outline of the lake beneath (the ice surface above large subglacial lakes is flat owing to the frictionless contact between ice and water at the ice-sheet base). The combination of satellite and RES data confirmed that this lake, Lake Vostok, was over 240 km long and more than 50 km wide. These data also showed the existence of the lake directly beneath Vostok Station, where Kapitsa and others had collected seismic ice thickness measurements around 30 years previously.
Unlike previous publications on subglacial lakes during the 1970s, Kapitsa et al. (1996), which featured on the front cover of Nature magazine, was met by worldwide media attention and considerable new scientific interest, particularly from the microbiological community, which instantly regarded the lake as an extreme yet viable habitat for life, cut off from the rest of the planet for sufficient time to allow novel adaptations to have developed (Ellis-Evans & Wynn-Williams 1996).
The establishment of the geographical scale of Lake Vostok in 1996, and speculative ideas on its potential contents, was supplemented in the same year by a second inventory of 77 Antarctic subglacial lakes (Fig. 2), which was collated from a reanalysis of the 1970s RES data collected by Robin's team 20 years earlier (Siegert et al. 1996).
Nine years later, as more RES data were collected by the USA, UK, Russia and Italy, the inventory of subglacial lakes grew to 145 (Siegert et al. 2005; Fig. 3).
Then, another remarkable finding was made. Analysis of ERS-1 altimetric time-series data revealed an area of the central East Antarctic ice sheet to have reduced in elevation by more than 3 m, far in excess of the instrumental error, over 14 months between 1997 and 1999. During the same period, three areas of the ice sheet more than 200 km away rose up by more than a metre. Given that the regions of uplift coincided with the positions of known subglacial lakes at the mouth of a major subglacial valley known as the Adventure Trench, and that the area of subsidence was located at the head of this trench, the measurements were interpreted as evidence of an outburst of subglacial lake water, flowing over 200 km along the axis of the trench into a series of other lakes (Wingham et al. 2006).
Further analysis of satellite interferometry (Gray et al. 2005) and laser altimetry revealed more evidence for basal water flow and subglacial lake outbursts, particularly beneath Whillans Ice Stream (Fricker et al. 2007; 2016), leading to an inventory of 130 so-called ‘active’ subglacial lakes (Smith et al. 2009). Combined with lakes detected from newly acquired RES data, these active lakes pushed the tally within the fourth version of the inventory of Antarctic subglacial lakes to 381 (Wright & Siegert 2011, 2012; Fig. 4), scattered throughout the continent, confirming wet-based conditions over around half the ice sheet. Following subsequent discoveries, by 2016 the tally of known, discrete subglacial lake locations stood at 402 (Siegert et al. 2016a).
Studies of the basal units of the ice core, expecting to take the climate record further back, revealed virtually no gas content, however. Rather than being formed by ice accumulating at the surface, this gas-poor ice was formed instead by lake water freezing to the ice sheet underside, creating over 200 m of ‘accreted ice’ at the ice sheet base. Thus, the ice core had collected a frozen sample of lake water (Jouzel et al. 1999).
One problem with the accreted ice samples was that their extraction from the ice sheet involved their being subjected to the ice-core antifreeze (in this case kerosene). As the samples were not obtained cleanly, and were thus potentially contaminated, the findings of life within the accreted ice were open to challenge.
In January 2013, Lake Whillans became the first subglacial lake to be accessed using a clean hot water drill, and measured and sampled using clean instruments following agreed protocols (Fricker et al. 2011). As a consequence, samples retrieved became the first from a subglacial system in which contamination issues could be assured, adding confidence to the scientific results.
Samples of water and sediment were taken, and returned to the surface for both immediate inspection and transfer to laboratories in the USA. They showed that water within Lake Whillans contained ‘metabolically active’ micro-organisms, and that it was derived primarily from glacial ice melt with a minor component of seawater (Christner et al. 2014; Michaud et al. 2016), making it unique among known subglacial environments within Antarctica.
WISSARD hot water drilling efforts will include a custom water treatment system designed to remove micron and sub-micron sized particles (biotic and abiotic), irradiate the drilling water with germicidal ultraviolet (UV) radiation, and pasteurize the water to reduce the viability of persisting microbial contamination. Our clean access protocols also include methods to reduce microbial contamination on the surfaces of cables/hoses and down-borehole equipment using germicidal UV exposure and chemical disinfection. This paper presents experimental data showing that our protocols will meet expectations established by international agreement between participating Antarctic nations.
The second image shows a map of Antarctica with the relief of the crust thickness below it's ice as indicated by seismic data. Abbreviations: DML, Dronning Maud Land; GSM, Gamburtsev Subglacial Mountains. CREDIT: Baranov, A., Morelli, A., The Moho depth map of the Antarctica region, Tectonophysics (2013).
Something Strange And Hot Is Lurking Beneath Antarctica's Ice