Ostantarktischer Eisschild stabiler als gedacht: Eis überstand selbst Wärme des Pliozäns vor 4 Millionen Jahren

Während des Pliozäns vor 3-5 Millionen Jahren besaß die Atmosphäre eine ähnlich hohe CO2-Konzentration wie heute. Das Klima war ebenfalls warm, so dass diese geologische Epoche gerne als Analog für die aktuelle Wärmeperiode herangezogen wird. Eine der großen Fragen ist, wie sich die großen Eismassen an den Polkappen damals verhalten haben. Eine Forschergruppe der University of Pennsylvania präsentierte am 15. Dezember 2015 in einer Pressemitteilung neue Ergebnisse zur Ostantarktis, die den Löwenanteil des antarktischen Inlandeises ausmacht. Überraschenderweise fanden die Forscher um Rachel Valletta, dass der ostantarktische Eisschild während der gesamten letzten 14 Millionen Jahre stabil geblieben ist. Sogar das warme Pliozän konnte den Eismassen nichts anhaben. Die Forscher sehen dies als ein gutes Zeichen. Co-Autorin Jane Willenbrink sagt

Dies gibt uns Hoffnung, dass das ostantarktsche Eisschild auch im Zuge der Klimaerwärmung stabil bleibt.”

Im Folgenden die vollständige Pressemitteilung:

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East Antarctic Ice Sheet Has Stayed Frozen for 14 Million Years, Penn Team Reports

Antarctica was once a balmier place, lush with plants and lakes. Figuring out just how long the continent has been a barren, cold desert of ice can give clues as to how Antarctica responded to the effects of past climates and can perhaps also indicate what to expect there as Earth’s atmospheric concentration of carbon dioxide grows.

In a new study in Scientific Reports, University of Pennsylvania researchers use an innovative technique to date one of Antarctica’s ancient lake deposits. They found that the deposits have remained frozen for at least the last 14 million years, suggesting that the surrounding region, the East Antarctic Ice Sheet, or EAIS, has likewise remained intact. The work adds new support for the idea that the EAIS did not experience significant melting even during the Pliocene, a period from 3 to 5 million years ago, when carbon dioxide concentrations rivaled what they are today. “The Pliocene is sometimes thought to be an analog to what Earth will be like if global warming continues,” said Jane K. Willenbring, an assistant professor in the Department of Earth and Environmental Science in Penn’s School of Arts & Sciences. “This gives us some hope that the East Antarctic Ice Sheet could be stable in today’s and future climate conditions.” Willenbring collaborated on the study with lead author and Penn graduate student Rachel D. Valletta, as well as Adam R. Lewis and Allan C. Ashworth from North Dakota State University and Marc Caffee from Purdue University.

Current climate change projections indicate that the marine portion of the West Antarctic Ice Sheet is “a goner,” Willenbring said. Studies from the past few years suggest that sea level will likely rise a few meters as that ice melts. But the East Antarctic Ice Sheet is 20 times more massive. If it melted, the ensuing sea level rise would be even more catastrophic than the western peninsula’s dissolution. To shed light on what could happen in the future to the EAIS, geologists often look to the past. But there is not a scientific consensus about how the EAIS has behaved in different climates throughout history. Some scientists believe the ice sheet experienced significant melting during the relatively warmer conditions of the Pliocene, while others think it has remained almost entirely frozen for the last 14 million years.

Willenbring and colleagues hoped to help clarify the history of the EAIS. They traveled to Antarctica’s Friis Hills in the central Dry Valleys of the eastern portion of the continent. About a foot beneath the surface are sediment deposits from an ancient lake which is known from animal fossils to have been freshwater. Earlier dating established that the volcanic ash deposits at the bottom of the ancient lake are 20 million years old. To see if any melting had occurred in the interim, they analyzed radioactive isotopes of beryllium known as beryllium-10, which form in the atmosphere when cosmic rays collide with oxygen and nitrogen atoms. “Beryllium-10 sticks on to particles quite easily and is associated with lake deposits,” Willenbring said. “We wanted to see if we could use this isotope to figure out how long the sediment was in place and isolated from liquid water.” Beryllium-10 has a known half-life of 1.4 million years. After estimating an initial level of initial concentration of beryllium-10 in their lake samples, the researchers were able to estimate the age of the sediments to be between 14 and 17.5 million years ago.

“We found that the beryllium-10 was almost completely gone, within the resolution of our technique,” Willenbring said. Willenbring said the team was confident that the area had remained frozen since then because if there had been melting, the water would have penetrated the sediments and “reset” the beryllium-10 measurements. “This means that the sediment is definitely older than the time when a lot of people think that Antarctica might have been quite deglaciated,” she said.

By offering support for the idea that the EAIS has been largely stable during the last 14 million years, the research offers some hope that a massive collapse of the ice sheet, and associated sea level rise of tens of meters, may not be imminent. Willenbring, however, cautions that even though carbon dioxide levels in the Pliocene may be analogous to today’s levels, the two situations are not equivalent and thus any conclusions can only be taken so far. “Even though the Pliocene conditions could be an analog for CO2 concentrations today, we’ve probably never experienced such a fast transition to warm temperatures as we’re seeing right now,” she said.

The study was supported by the University of Pennsylvania, North Dakota State University and the National Science Foundation.

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Auch andere Gruppen forschen zur Stabilität des antarktische Eisschildes. Die Université libre de Bruxelles (ULB) entdeckte jetzt Eiserhebungen, die sich im Übergang von Eiszeiten zu Warmzeiten bilden und den Gletscherschwund bremsen. In einer Pressemitteilung gab die Universität am 26. Mai 2015 bekannt:

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Evolution of the Antarctic ice sheet

ULB study sheds a new light on the stability of the Antarctic ice sheet. It shows for the first time that ice rises (pinning points that keep the floating parts of ice sheets in place) are formed during the transition between glacial and interglacial periods, which significantly slows down the response of the ice sheet to climate change.

The Antarctic ice sheet holds as much ice to potentially raise global sea level with 67 meter. Many questions still arise regarding its reaction to climate change, especially for the marine sections of the ice sheet. The contact of the ice sheet with the ocean happens through the formation of ice shelves, which are large sections of floating ice flowing down from the continental ice sheet. Recent observations show that these large shelves are thinning rapidly. Ice shelves can be seen as the cork on a wine bottle lying flat. Removing the cork will lead to the bottle emptying. Therefore, thinning or removing ice shelves make more continental ice to discharge into the ocean, leading to sea level rise. Ice shelves are held in place by contact with embayments and pinning points at the bed. Thinning reduces this contact and reduces the stress that keeps the continental ice normally stable.

For the first time, Dr. Lionel Favier and Prof. Frank Pattyn from the Laboratoire de Glaciologie, Faculty of Sciences of the Université libre de Bruxelles (ULB) in Belgium demonstrate how such pinning points (which form ice rises in the ice shelf, or locally grounded small ice caps within ice shelves) are formed and how they keep ice shelves stable. Ice rises are omnipresent along most of the Antarctic coast and believed to buttress ice shelves to keep them stable. Using a sophisticated ice-sheet model, Favier and Pattyn show that these ice rises actually formed during the deglaciation of the ice sheet, when the grounding line (contact between ice sheet and ice shelf) retreats over the continental shelf. Their major impact during this retreat is that ice rises, once formed, significantly slow down this retreat, which may explain why major changes in ice volume in the past often show a delayed response to climate change.

Favier: “It is amazing to see how a relatively small feature, such as an ice rise, can delay the retreat of a continental ice sheet during a deglaciation by several thousands of years. This understanding is possible thanks to the recent tremendous effort of the glaciological community to improve ice-sheet models”.

Pattyn: “This study sheds a new light on how we think about ice sheet evolution. Ice rises not only influence the stability of ice shelves, but also influence their formation. This insight will help to better constrain future evolution of ice sheets and their contribution to sea level rise”.

The results of this study, funded by the Belgian Science Policy Office (BELSPO), has been published online on 26 May 2015 in Geophysical Research Letters.