A new study published in Science suggests that deep processes in the Earth's mantle gradually elevated East Antarctica and the Gamburtsev Mountains. The elevation allowed snow to survive in the summer and become the nucleus from which the world's largest ice sheet grew.
The Eastern Ice Sheet Antarctica It is the largest ice sheet on Earth. It contains so much frozen water that if it were to melt completely, it would raise global sea levels by about 52 meters. But for decades, scientists have struggled to explain how such a vast ice sheet began to form about 34 million years ago, when the world was about 5 degrees Celsius warmer than today and the seas surrounding Antarctica remained relatively warm. ([University of Southampton][1])
The mystery is more acute in the north. Antarctica was permanently covered in ice at the transition from the Eocene to the Oligocene, but large ice sheets in the Northern Hemisphere did not become established until tens of millions of years later. The decline in atmospheric carbon dioxide contributed to the global cooling, but if it had been the only factor, a similar and more rapid response would have been expected in both polar regions. ([University of Southampton][1])
A new international study suggests that the difference was due not only to climate, but also to the shape of the surface. According to the researchers, the breakup of the supercontinent Gondwana A chain of profound processes began that gradually raised the East AntarcticaThe plateaus and mountains that formed eventually passed an altitude threshold that allowed snow and ice to survive year-round, even when the global climate was still relatively warm.
The study, led by Professor Thomas Grennon and Dr Thea Hinks from the University of Southampton, was published in the journal Science. The team included researchers from the UK, Germany, the Netherlands and Italy, including scientists from Durham University, the GFZ Helmholtz Centre for Earth Sciences and the University of Potsdam. ([University of Southampton][1])
A process that began with the breakup of Gondwana
Credit: Guy Paxman, Durham University, licensed under CC BY-NC 4.0
The story begins more than 160 million years ago, during the Jurassic period, when Africa and Antarctica began to separate from each other. The rifting process did not end with the creation of a new boundary between tectonic plates. According to the model proposed by the researchers, it also caused slow disturbances in the Earth's mantle, known as "Envelope waves".
These are not fast waves like seismic waves from an earthquake, but very slow movements of hot, viscous rock beneath the continents. Such movements can remove dense blocks from the bottom of the continental plate. As some of the heavy material falls to the depths, the plate above it becomes lighter and rises, similar to a floating body rising after a weight is removed. ([Eos][2])
Researchers estimate that mantle waves advanced from the coast into the interior of East Antarctica over tens of millions of years. First, a raised cliff was formed near the coast, and later a vast plateau was raised. The uplift wave continued to move inland, until it reached the area Gamburtsev Mountains – A large mountain range in central East Antarctica, now buried under several kilometers of ice.
To test the idea, the researchers combined geodynamic models, surface evolution models, energy balance models, and ice sheet models. The simulations reconstructed the landscape development of East Antarctica over about 100 million years, producing a coastal cliff, elevated plateau, and interior mountains that resemble structures that exist beneath the ice today. ([University of Southampton][1])
Height that tipped the scales
Height is a crucial element in development glaciersAs you go up in altitude, the air cools, so snow in high mountains may survive the summer instead of melting completely. The annual accumulation of snow allows mountain glaciers to form over time.
According to the simulations, about 50 million years ago, much of the Gamburtsev Mountains was still too low to retain much snow year after year. However, the uplift wave gradually raised large areas above an altitude of about 1.5–2 kilometers – the estimated threshold at which glaciers could establish themselves under the climatic conditions of that time. By about 45 million years ago, a significant part of East Antarctica had already passed this threshold.
The researchers calculated that about 60 million years ago, only about a third of the Gamburtsev Mountains were high enough to retain ice. By 34 million years ago, nearly 90% of the area was above the appropriate elevation threshold. This created mountainous “bridgeheads” where ice could accumulate and spread, until separate glaciers merged into a continental ice sheet.
The ice began to cool itself.
After the first glaciers established themselves, feedback mechanisms came into play that accelerated cooling. Snow and ice reflect more solar radiation back into space than dark rocks and bare ground. As the ice sheet grew, Antarctica absorbed less heat, and the environment became colder.
The team calculated that the ice and albedo feedback could have reduced global temperatures by about 1 degree Celsius. The cooling of the air over the continent also reduced the amount of water vapor it could hold. Since water vapor is a greenhouse gas, drier air weakened the atmospheric insulation effect and allowed for further cooling. Together, these processes helped ice spread from the mountains and plateaus toward the coast.
This may also explain why sea surface temperatures in the Southern Ocean remained relatively warm after the ice sheet began to form. The ice did not form because the entire polar region froze at once, but because the elevated land was cold enough to allow local ice accumulation, which gradually expanded.
Why is the north left without an ice sheet?
The situation in the Arctic was different. The North Pole is located in the middle of an ocean and not on a high continent. Although there were local glaciers in the northern regions, there was no large and high enough land area there at that time on which large ice sheets could establish themselves in the warm climate.
Only after a further decrease in carbon dioxide concentrations and continued global cooling could large ice sheets also form at lower altitudes in Greenland and the continents of the Northern Hemisphere. Thus, the difference between the poles was not due solely to their location, but to a combination of climate, tectonics, and surface elevation.
A promising model, but not the last word
The findings do not negate the central role of carbon dioxide. On the contrary: its decline in concentration provided the broad cooling trend, while the topography of Antarctica determined where and when the cooling could turn into permanent ice. The researchers therefore describe a combination of a geological process lasting more than a hundred million years and relatively rapid climate change.
John Goodge, a geologist at the University of Minnesota-Dolores who was not involved in the study, told Eos that the work highlights the importance of tectonics in the development of ice sheets. However, the conclusions rely heavily on modeling. Direct rock samples from the Gambortsev Mountains, buried deep beneath the ice, could in the future more precisely examine when the ridge rose and how the landscape changed. ([Eos][2])
The study also has implications for the future. It shows that the conditions that allowed the Antarctic ice sheet to form were built up very slowly, over tens and hundreds of millions of years. Ice sheets that disappear due to warming do not necessarily return immediately when temperatures cool down a bit: their recovery may require a rare combination of climate, elevation and topography that is very difficult to recreate.
For the original publication: Opening the original publication
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