What lies beneath the Antarctic ice?

“Well, more ice…” may be the general consensus to this oft-pondered question. But beneath the featureless bedrock plain of Antarctica and Greenland lies a fascinating landscape of mountain ranges, cirques, canyons, rift systems, and even lakes, all preserved in time under ice sheets up to 4 kilometres thick [1]. These recently discovered geological systems have held unique records of prehistoric climates and tectonic events, and they still continue to influence the present-day behaviour of our ice sheets and global sea-level rise.

Buried topography

The existence of such dramatic subglacial landscapes was only confirmed in the mid-20th century, when the first airborne radar surveys began to venture into the interior of Antarctica. One of the most striking early discoveries was the identification of the Gamburtsev Subglacial Mountains, a mountain range comparable in height and structure to the Alps in Europe, yet completely entombed under East Antarctica [2]. Interestingly, they exhibit steep peaks, significant reliefs, and radial-shaped drainage patterns, which suggest that they once underwent active fluvial and glacial erosion long before the East Antarctic Ice Sheet formed and engulfed them completely [2].

Just as dramatic is Greenland’s interior landscape. Improvements in radar technology have allowed it to penetrate deep into the ice, revealing astonishing megacanyons of over 750 km in length and, in places, over 800 meters deep (deeper than the Grand Canyon!) [3]. The canyon snakes from central Greenland all the way towards the northwest coast, and is surrounded by fjords and even valleys, which thousands of years ago drained large river systems [4]. These geological features predate glaciation events in the Pleistocene, an era occurring from 2.6 million to 11,700 years ago, and thus provided geologists with more evidence for a much warmer and temperate early Cenozoic climate in the Arctic region [4].

Elsewhere, we can also see subglacial systems on a continental scale. The most prominent example of this is Lake Vostok, a deep, elongated basin that was isolated beneath over 4 kilometres of ice for millions of years, theorized by Russian scientist Andrei Kapitsa in 1957 but only confirmed in 1993 by a joint Russian–British team [5]. More than 400 other smaller lakes and interconnected channels feed into a continental-wide water system driven by geothermal heat and pressure from the ice above [6].

How did we discover all of this with just radar?

While drilling projects can provide us with invaluable physical samples of sediments, meltwater, or ice cores, the majority of what we now know about the subglacial world comes from remote geophysical mapping, using both older techniques and cutting-edge innovations. For instance, Radio-echo sounding (RES) measures ice thickness and how reflective the ice bed is to pulses of radio waves sent by transmitters. Since electromagnetic waves travel at roughly 168 metres per microsecond through glacial ice, radar systems are able to create detailed 3D images of the boundaries of ice, water and bedrock with surprisingly high precision, sometimes capturing features only a few metres across [7].

Gravimetry, on the other hand, measures subtle variations in the Earth’s gravitational field as small as a few milligals. So once radar has mapped the basic topography of an area, scientists use gravimetry to identify underground bodies with lower or higher density than the surrounding ice and thus distinguish what it’s made of. This method has revealed, among other things, that large chunks of East Antarctica sit atop a vast rift system that is no longer active, but still influences ice flows in the present day [8].

Why are subglacial landscapes important?

Understanding this hidden topography is important because it exerts a subtle but significant influence on present-day ice sheet behaviour. Subglacial mountains act as stabilising anchors by pinning ice in place; in other words, they create points of resistance where the ice must flow over or around raised bedrock, which slows its overall retreat [2]. Deep underground troughs have the opposite effect. A trough is a long, deep depression in the bed, and when it slopes downward toward the ocean, it naturally funnels ice toward the coast. This usually increases the speed of outlet glaciers and encourages grounding-line retreat. The grounding line is the place where the ice sheet stops resting on the bed and begins floating on the ocean; so when it moves inland, the glacier becomes more exposed to warm water and loses mass more rapidly [9].

In Greenland, buried valleys from ancient rivers behave like natural channels. Because they are narrow and steep, they guide fast-moving ice streams that feed into marine glaciers. Several of these glaciers have sped up over the last two decades, partly because their underlying valleys provide a smooth pathway for ice to accelerate seaward [4]. Greenland’s subglacial lakes also influence flow above the surface. A subglacial lake is a pocket of water trapped between the ice and the bed. These lakes periodically drain and refill, which changes the pressure at the base of the ice sheet. Even small shifts in this pressure can subtly alter the velocities of ice across large regions, sometimes slowing flow and sometimes increasing it depending on where the water moves [6].

Even the geothermal heat flux beneath these beds matters, because of how uneven it is. In West Antarctica alone, at least 138 subglacial volcanic edifices have been identified, and some regions show heat flow high enough to produce meltwater at the base [10]. Geothermal heat adds water to the basal hydrological system, and even small amounts can reduce friction at the icebed and make the ice more mobile, risking more ice sheet collapse.

In the end, these hidden landscapes prove that ice sheets are shaped from below as much as they are from above. The world under the ice is still changing, quietly but constantly, and understanding it helps us see how our planet is shaped by its own long-forgotten past.

 References

1. Fretwell, P. et al. “Bedmap2: improved ice bed, surface and thickness datasets for Antarctica.” The Cryosphere (2013).

2. Ferraccioli, F. et al. “East Antarctic rifting triggers uplift of the Gamburtsev Mountains.” Nature (2011).

3. Bamber, J. L. et al. “Paleofluvial megacanyon beneath central Greenland.” Nature Geoscience (2013).

4. Cooper, M. et al. “Subglacial landscape evolution of northern Greenland.” The Cryosphere (2023).

5. Kapitsa, A. P. et al. “A large deep freshwater lake beneath the ice of central East Antarctica.” Nature (1996).

6. Livingstone, S. J. et al. “Subglacial lakes in Antarctica and Greenland.” The Cryosphere (2022).

7. Schroeder, D. M. et al. “Radar sounder imaging of subglacial water systems.” Journal of Geophysical Research (2013).

8. Jordan, T. A. et al. “Recent tectonic activity revealed beneath East Antarctica.” Nature Communications (2017).

9. Schoof, C. “Marine ice sheet stability.” Journal of Fluid Mechanics (2007).

10. van Wyk de Vries, M. et al. “Subglacial volcanoes of West Antarctica.” Geophysical Research Letters (2017).