Destabilisation of the nothern ice front of theWilkins Ice Shelf | ||
| After the colapse of the ice bridge, as expected the northern ice front of the Wilkins Ice Shelf became unstable. On 20 April 2009 the first icebergs detached, as a combination of ENVISAT ASAR and TerraSAR-X images showed. | ||
 | The ice bridge had a size of about 300km2. Until the 27 April about 370km2 were released along the northern ice front. The recent calving is caused by the loss of the stabilising connection to Charcot Island. The new load situation changes the strees state along the ice front. The calving follows failure zones that developed during the last 15 years. We expect that the northern ice front will lose between 570 and 3370km2, after which - hopefully - a new stable ice front will be formed. | |
 | The upper figure shows an ENVISAT ASAR scene from 24 and 27 April 2009. The margins of the former ice bridge are outlines, as well as the aera in which the current calving at the northern ice front takes place. The second image shows a TerraSAR-X stripmap mode scene from 23 April 2009. This high-resolution image reveal numerous details, as for example the capsized icebergs, that appear as dark blocks. | |
Ice bridge on Wilkins Ice Shelf is disrupted
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The narrow ice bridge, that remained after the break-up in May 2008, was deforming since then. One contributor to the deformation was the creep of ice. The shape of the ice bridge, which is at the narrowest location only 900m wide, and the adjacent ice melange provided an ideal surface for storm. Both has contributed to the deformation of the ice bridge and led to the disruption.
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The observation of the deformation of the ice bridge required high-resolution images of the German TerraSAR-X satellite. In these images, the margin of the ice bridge was ascertained in an accuracy of a few meters. The variation of the location of the margins enabled us to analyse the load the ice bridge has experienced during that time. The current events are captured by daily image acquisition of the European ENVISAT satellite. This provides a basis for an analysis of the temporal evolution. The two images of the TerraSAR-X show the state before and after the breach. The image from 1st April shows the early state of disruption, while the one from 6 April 2009, that exhibits the entirely disrupted ice bridge. Between the small bright icebergs, there are capsized icebergs, that appear as dark blocks in the radar image. Currently (9 April) rifts along the northern ice front widen, which hints to imminent retreat of this ice front. This is caused by the loss of the stabilising connection to Charcot Island and supported by the existing rifts and failure zones. |
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Mehr Informationen über die Aufnahmen des ENVISAT ASAR finden Sie unter esa.int
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New rift formation on Wilkins Ice Shelf, Antarktica | |
 | Since 23 November 2008 new rift formation and propagation on the
Wilkins Ice Shelf takes place. The ice shelf did not come to rest
during austral winter as rifts in an eastern area of Wilkins Ice Shelf
were forming. On Aug 10, a new 30km long rift formed parallel to the
northern ice front. End of October another rift formed close to Latady
Island, propagating since 23 November to its current length of 22km. In
the satellite images from ENVISAT ASAR and Terra_SAR-X it is clearly
visible that the rifts in this area are also widening considerably. A
triple junction, formed in July, has been |
| connected with the grounding line of Latady Island on 23 Nov 08 by a new rift. Hence the zone, in which the ice bridge is connected with Latady Island and the central Wilkins Ice Shelf is further weakened. The recent changes are happening on time scales of days and are thus different from the break-up events in Feb and May. This indicates that stresses from the flow of ice are driving the recent changes, which needs however much more investigation. Studies of the team Dr. Angelika Humbert (Polargeophysik, WWU) and Dr. Matthias Braun (ZFL, Uni Bonn) showed, that failure zones in the vicinity of local grounding were developing on the Wilkins Ice Shelf in connection with break-up events since the 90s. The projected scenario following a line from Latady Island via Vere Ice Rise towards the grounding line in the east, might soon be entirely realised. As the new rifts are already southwards of this positive scenario, it becomes more likely that a larger area becomes unstable. Our worst case scenario projects further 2900km2 (5000km2 in sum from Feb on), while the remaining 8000km2 still do not show any sign of instability. | |
 | After two break-up events in February and May 2008, another phase of break-up at Wilkins Ice Shelf has started on 28 June 2008 and last until mid of July. This break-up firstly followed the lines of the failure zones, as we projected. Those zones were strongly extended by the February 2008 break-up event. The break-up along the lines of the failure zones is also the reason, why in this break-up event large icebergs are formed in contrast to the sliver icebergs in February. The lost area is up to date 1220 km2. |
 | The previous break-up event started at 30 May 2008. In this break-up event 160 km2 got lost until 31 May 2008. This was the first documentation of a break-up event in winter. In contrast to the previous break-up event in February, the release of ice started from the interior and spread to the outside, i.e. from the middle of the ice mass towards the ice front. The radar images shown below display the ice surface. The ice appears white as it is cold and there is no moisture or meltwater on the surface. Thus it is proven, that the May 2008 break-up melt water does not play any role. |
 | We have already reported the break-up event of 28 and 29 February 2008, where nearly 425 km2 gotlost. This break-up event started at the western ice front and spread to the interior. The movie on the left shows single radar images, in which the ice appears dark due to surface melting. During this break-up event large icebergs (2 km) as well as small sliver icebergs emerged. Previous to both break-up events, in July 2007 (antarctic winter) a
rift of 52 km length was formed. The rift formation was accomplished
with emergence of many narrow fractures in the now broken-up areas. Dr. Angelika Humbert and Dr. Matthias Braun could show, that the
fracture |
| development is caused by bending stresses induced by buoyancy
forces between unequal thick ice masses. The bending stresses are as
large as 11MPa and thus much larger than the critical stresses
polycrystalline ice can bear. This fracture formation is the actually
important event - the break-up only a consequence. Thus, buoyancy
forces between unequal thick ice masses are, beside melt ponds that
fill crevasses, another reason for the disintegration of ice shelves.
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The current events demonstrare, that there are several reasons for disintegration of ice shelves and the break-up events run dissimilar, i.e. triggered differently. | |
| The central part of the Wilkins Ice Shelf was up to now connected between the two islands by a 14.5-20 km wide, 200-250 m thick mass of ice. This connection acted stabilising on the whole ice plate and was narrowed down to 6 km during the first break-up (narrowest part). The May 2008 break-up has reduced this further therefore only 2.7 km are remaing at the narrowest part. | |
 | Studies of satellite images between 1990 and today revealed, that the Wilkins Ice Shelf is not underlying ordinary, continuous calving, but break-up events with loss of large areas. The break-up event in February had an important effect on the whole ice shelf. In connection with the break-up already existing failure zones extended and coalesced. These zones are a risk for the stability of the ice shelf. Failure zones develop where ice shelves are locally grounded on the sea bed, which happens extraordinarily often in Wilkins Ice Shelf. Usually, these grounded areas act as pinning points and thus positive on the stability of ice shelves. The here observed effect, which is likely to be amplified by the high temperatures and in particular due to the dynamic effect of the break-up events, however, acts destabilising. |
 | The Wilkins Ice Shelf, a 13.000 km2 large area of floating ice, is located along the western Antarctic Peninsula. In the past almost 20 years seven ice shelves along the Antarctic Peninsula have retreated or disintegrated, with the most spectacular break-up of the Larsen B Ice Shelf in 2002. This area has experienced an extraordinary warming in the past 50 years of 2.5°K. The Wilkins Ice Shelf is situated in a temperature zone with mean annual surface temperatures above -9°C, which is supposed to be a line of viability of ice shelves. |
| The rise of temperature along the Antarctic Peninsula and the also rising ocean temperature affect ice shelves in a two fold way: melting processes on the bottom side of ice shelves become more pronounced and could lead to larger ice thickness differences, which caused the current break-up. Furthermore, the complete ice mass gets warmer and will thus have a smaller fracture toughness. | |
| Dr. Angelika Humbert (Polargeophysik, WWU) and Dr. Matthias Braun
(ZFL, Uni Bonn) have been investigating the dynamics of Wilkins Ice
Shelf for months. Our studies are based on time series of satellite
images from the European Space Agency (ESA) and the German Aeropspace Center (DLR). | |
A collection of single satellite images reassembled in a movie shows the temporal development of the break-up:
 February 2008
May 2008
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Further information and diagrams about Wilkins Ice Shelf can be found here. | |
| Publication: M. Braun, A. Humbert and A. Moll 'Changes of Wilkins Ice Shelf over the past 15 years and inferences on its stability' The Cryosphere Discuss., 2, 341-382, 2008 | |
| All satellite images are subjected to the copyright of ESA (2008) and DLR (2008), repectively. They were provided under ESA IPY AO 4032, TERRA-POLAR (DLR AO LAN0013) and the ESA GMES initiative Polar View. Our work on Wilkins Ice Shelf is supported by the German Research Foundation in the priority program 1158 "Antarctic research with comparative studies in Arctiv regions" (e.g. Hu1570/2-1, GlaVoMa project). | |
| Dr. Angelika Humbert und Dr. Matthias Braun, 12.06.2008 | |
