Discovery of subglacial lakes by radioecho sounding

Nearly 10 years passed after the first publications appeared on the theory of subglacial melting and the existence of lakes in central Antarctica, before a new related development occurred. It started with the use of radio-echo sounding altimetry in aircraft over Antarctica. Pilots of these aircraft reported that the altimeters occasionally showed erroneous altitudes when compared with actual altitudes when their planes flew low above a glacier or an iceberg. This difference was explained as a result of a mixture of a radio-echo reflection from an air/surface boundary and a reflection from the bottom of the glacier or from internal layers within the glacier. This discovery implied that finding the proper frequency and developing appropriate monitoring equipment would result in radio-echo sounding as a new and powerful method for the study of ice sheets.

The first experimental discovery using this method included a series of subglacial lakes below the central part of the Antarctic Ice Sheet by a group of British scientists led by Dr. G. de Q. Robin, at that time the Director of the Scott Polar Research Institute (SPRI) of Cambridge University, and author of the article on the heat regime of the Antarctic and Greenland Ice Sheets (Robin, 1955). The SPRI group worked in close collaboration with the Electromagnetic Laboratory of the Technical University of Denmark (TUD) and the Division of Polar Programs of the U.S. National Science Foundation (NSF). This cooperation was advantageous because it combined the expertise in electronics and antenna design at the TUD, developments of new equipment at the SPRI, and the logistical capabilities of the American LC-130 aircraft for installation of the equipment and long-range flights over large areas of the ice sheet with their highly accurate inertial navigation systems (Robin et al., 1977). These were the days before global positioning systems (GPS or GLONASS) were in use, when flying over featureless terrain was difficult, particularly if you required accurate navigation. As a result, many of the flights, which are shown in Figure 4.1, had to be made within viewing distance of Vostok Station, thus ensuring a reduction in navigation errors.

Figure 4.1. Radio-echo sounding flight lines in Antarctica undertaken during 1967-1975 operations (adapted from Robin et al., 1977). Abbreviations: RIS - Ross Ice Shelf; Antarctic stations: B - Byrd; C - Dome C; D - Dumont d'Urville; H - Halley Bay; M -McMurdo; S - South Pole; V - Vostok.

Most of the results from these flights consisted of lengthy film records, during flight paths, of time versus positions of radio reflections taken by continuous monitoring from within the ice sheet. The film records show the ice/rock interface (the bottom of the ice sheet) as well as numerous near-parallel, internal layer reflections. The bottom of the ice sheet in the majority of the reflections appears irregular with a vertical roughness resolution of more than 500 m and a horizontal length of about 300 km.

In some places the appearance and strength of the reflections were completely different, appearing as horizontal and smooth. An area with especially strong reflections of this kind was recorded near Sovetskaia Station in the central part of East Antarctica, where the ice thickness was about 4,200 m. This was explained by Dr. Robin and his colleagues as an ice-water layer boundary.

Further studies, conducted during the 1971/1973 field seasons, showed that the flat, smooth surfaces (some kilometers in length) with a high reflection coefficient are common elsewhere in East Antarctica (Oswald and Robin, 1973).

Figure 4.2. Radio-echo sounding showing a subglacial lake (G. de Q. Robin, 1993, pers. commun.).

Studies on these peculiarities of bottom reflections were continued in the 1974/ 1975 field season. Analyses of 17 other cases of this kind in East Antarctica showed that all were located in areas with evidence of an ice bed composed of solid rock, where the rate of glacier movement was low (meaning that transport of erosional debris was also low), and the surface above those areas was also nearly horizontal. Dr. Robin and his colleagues interpreted this phenomenon at the bottom of the ice sheet as lakes (Figure 4.2).

Unfortunately, the thickness, or depth, of the lakes was unknown because radio signals do not propagate in water. It was evident only that the thickness is large enough to be larger than the radio-echo signal's wavelength.

Many radio-sounding flights were made during the period 1971/1972 and 1974/ 1975 in the vicinity of Vostok Station, because the majority needed visual contact with the station in order to correct for aircraft navigation. A number of those flights, which registered a large number of "water reflections", led Robin et al. (1977) to suggest that there was a large subglacial lake in the area, with its center located about 190 km to the north-northwest of Vostok Station. So, the discovery of Lake Vostok by radio-echo sounding had been made, but at that time nobody was aware of the ramifications of their findings.

Figure 4.3. A map of Dr. Robin's flights over Yostok Station (adapted from Robin et al., 1977). (1) Sketch map of the location of sub-ice lakes detected by radio-echo sounding at the Dome ("B")—Yostok ("Y") region (adapted from Robin et al., 1977). Flight tracks for 1971— 1972 and 1974—1975 seasons are shown. Thickened portions of the lines locate sub-ice water bodies. The contours represent heights in meters above sea level. (2) Required angles between the Sun and low-flying aircraft (Robinson, 1960).

To review the situation around Yostok Station: it was known that most radioecho sounding flights began and ended around Yostok, with each flight identifying the presence of liquid water at the base of the ice sheet. However, a seismic study of the bottom of the ice sheet at Yostok Station was conducted in 1964, with the interpretation that it represented a layer of frozen sediments, not water. This disagreement raised some doubt in the interpretation made by Robin's group, because evidence from seismic studies was traditionally accepted as more accurate than radio-echo sounding.

Despite this disagreement, Dr. Robin insisted on the existence of a large lake, directing special attention to data from flight number 130 along flight line "A—A" in Figures 4.3 and 8.1. He believed that this flight traveled along the long axes of the

Figure 4.3. A map of Dr. Robin's flights over Yostok Station (adapted from Robin et al., 1977). (1) Sketch map of the location of sub-ice lakes detected by radio-echo sounding at the Dome ("B")—Yostok ("Y") region (adapted from Robin et al., 1977). Flight tracks for 1971— 1972 and 1974—1975 seasons are shown. Thickened portions of the lines locate sub-ice water bodies. The contours represent heights in meters above sea level. (2) Required angles between the Sun and low-flying aircraft (Robinson, 1960).

"Lake?

Figure 4.4. Locations of subglacial lakes as identified by radio-echo sounding (adapted from Siegert, 2001). The lakes are concentrated near ice divides.

lake, and crossed the middle of it. The seismic sounding at Vostok Station, however, he believed to be located away from the lake.

It should be noted that the main radio-echo sounding continuous recording camera had jammed film shortly after the aircraft arrived over the lake, and for a major part of the time that the plane flew along the lake it was not working. The scientists on the flight repaired it, and the camera recorded the bottom water reflection again just before the lake ended. The situation was a good example of Murphy's Law. Dr. Robin described the problem: "... one of the radio-echo recording cameras jammed during part of this flight, but the impression formed at the time was that the lake's echoes were present over much more of the discontinuous section A-A [shown in Figure 4.3]. Careful inspection of the 1971-1972 records also revealed lake echoes on flight paths close to this area. We believe that all these echoes originate from a practically continuous body of water, the long dimension of the lake being about 180km with a typical width of 45km'' (Robin et al., 1977). One could finally say that this was the time of the actual discovery of Lake Vostok.

The operational base for the airborne radio-echo sounding undertaken by Dr. Robin was McMurdo Station, and I was fortunate to be there after I had returned from field work on the Ross Ice Shelf Project (RISP). Dr. Robin told me about his discovery at this time, and I mentioned my idea of detecting traces of large subglacial lakes on the surface of the ice sheet, partly from a visual study of the ice sheet surface from low-flying aircraft. I told him that I spent my first winter in Antarctica with a senior navigator of an aviation group of the Soviet Antarctic Expedition (SAE) of 1958, Mr. Robinson - who had told me that when flying from Mirny to Vostok Station he, and other pilots, had seen some relatively large areas on the surface of the plateau that were distinctly different from the rest of it. These areas were always seen in the same places, and pilots used them for navigation, calling them "lakes" (Robinson, 1960).

However, the "lakes" could be seen only when the low-flying plane was traveling away from the "lake" and its angle of view was low. In a short letter to the editor of the SAE Information Bulletin, Robinson mentioned that "... Natural landmarks in the interior of the continent include, in addition to individual mountains and mountain ridges, oval depressions with gentle 'shores' which are visible from an airplane over the plateau. The depth of these depressions usually does not exceed 20-30 m and their length, 10-12 km. These unusual depressions are sometimes called 'lakes' by pilots. These lakes are clearly distinguishable from the air as spots against the white background of the plateau, especially when the angles of the course of the Sun are close to 180°'' (Robinson, 1960). I disagreed with his comment that the ''lakes'' appeared as depressions with depths not exceeding 20-30 m because no one had yet visited these ''lakes'' on the ground. The words ''depressions'' and ''depths'' may have been arbitrarily added to the article, possibly even at the insistance of the editor prior to publishing the letter. From what Robinson told me I got the impression that the differences at the edges of the ''lake'' and the ''lake'' itself were a result of optical properties, hence you could not actually ''see'' the ''lake'' (it depended on a low Sun angle, for one thing).

Mr. Robinson and I spent a winter in Antarctica, giving me more time to find out about these ''lakes''. Soon after his short article was published he regrettably lost his life in an airplane crash in the Arctic.

Robinson's ''lakes'' at the surface of the central, and hence thickest, part of the Antarctic Ice Sheet, together with Dr. Robin's discovery of a large subglacial lake near Vostok Station, represented a clear, simple, and important message. A subglacial lake, one to two orders of magnitude larger than the thickness of the ice sheet above it, has to be transparent and exhibit a surface expression of its existence. Conditions of an ice sheet moving along the rough slope of a rocky glacier bed and those floating on the water of a sub-ice lake are vastly different. This difference is revealed at the surface, if the size of the lake is large enough compared with the thickness of the ice sheet above it.

It was strange that this thought did not occur to Mr. Robinson and me when we discussed his ''lakes'' in 1959, because at that time I already knew about permanent melting at the bottom of the ice sheet in central Antarctica.

So, we left Antarctica, and, unfortunately, I never asked him for a look at his flight maps to see where the lakes were located, thinking that we would have sufficient time for that later. Soon after, he lost his life. I later tried to find evidence of these lakes from other pilots, but no one had anything certain to say about them. It looked like Mr. Robinson kept all the information to himself. He was a graduate of the Faculty of Polar Countries of the Geography Branch of the Moscow State University, and hoped to study for a Ph.D. on the subject. In 1975 at McMurdo, in memory of my friend, I tried to find surface evidence of his "lakes" above the huge subglacial lake discovered by Dr. Robin and his team.

Dr. Robin took to the idea enthusiastically, and we soon made a special flight in search of visible evidence. I say "special", because to simulate the conditions of Mr. Robinson's flights the LC-130 aircraft had to fly only a few hundred meters above the surface of the plateau, a practise that was both excessively fuel consuming and dangerous.

Dr. Robin wrote the following about the event:

In the course of an informal discussion of this feature ... I. A. Zotikov ... drew attention to a report by Robinson (I960) ... on the existence of shallow snow surface depressions in the Vostok area ...

A subsequent flight over the large subglacial lake did in fact give visual confirmation of this, with the 'lake's shores' showing up as areas of whiter snow - corresponding well in some cases with the edges of the radio-echo results. We suggest that these 'lakes' are visible because of the difference in appearance of a uniform flat surface (the 'lake') and the gently sloping one (the 'shore'). This could be due either to changes in albedo caused by differences in relative sun angle between the two surfaces, or less directly to changes of texture resulting from different snow accumulation rates.

The extent of the subglacial lake cannot be unambiguously defined, since unfortunately one of the radio-echo recording cameras jammed during part of this flight, but the impression formed at the time was that lake echoes were present over much more of the discontinuous section A-A [Figure 4.3] ... We believe that all these echoes originate from a practically continuous body of water, the long dimension of the lake being about 180 km with a typical width 45 km.

The removal of basal friction over such a large area must be expected to produce some effect on the upper surface of the ice sheet, and in our case of a substantially level basal surface, a level upper surface seems a plausible result. Our measurements are not sufficiently precise, but indicate a mean surface slope of less then 1 in 2,000 compared with the regional value of about 1 in 700.

A large area where traces corresponding to "water" at the bottom are located and where Robinson's "lake" effect has been seen from low-flying aircraft (Figure 4.3(1)) was shaded by Dr. Robin on the map. We know now that this shaded area is where Lake Yostok is located. The actual discovery of what is now called Lake Yostok was published by Robin et al. (1977) in the prestigious journal Philosophical Transactions of the Royal Society of London, and although it was seen by many, at the time it did not produce as much interest in the scientific world as it does now.

Some years later, Dr. Robin's student, Mr. Mclntyre, compiled a map of East Antarctica with an indication of a large subglacial lake, just where Lake Vostok is located (Mclntyre, 1983; Siegert et al., 1996). This lake was marked on Mclntyre's map as Lake Vostok, 11 years before the first mention of the name in any other published literature.

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