Large Iceberg breaks off Antarctica's Mertz Glacier

The study, undertaken at the Antarctic Climate and Ecosystems Cooperative Research Centre (ACECRC) in Hobart, and in France, was initiated in 2007 during the International Polar Year to study the 'tongue' of the Mertz Glacier and the 'calving' of icebergs from it.

The Mertz Glacier had a large crack in it for two decades. A second crack developed opposite the first in the early part of the 21st century. The collaboration studied whether these two cracks would eventually meet, and the processes that would lead to the calving of an iceberg.

Background

The joint French-Australian team that detected this calving event has been working on a project called "CRACICE" (Cooperative Research into Antarctic Calving and Iceberg Evolution). The iceberg has an area of about 2,550 square kilometers, an overall length of 78 kilometers, width of 33 to 39 km, and represents about half the length of the glacier tongue. Satellite imagery shows the iceberg separation occurred on February 12-13.

The CRACICE team is carrying out a long-term study on how fractures developing in the Mertz Glacier Tongue lead to iceberg calving, and the fate of the icebergs and remaining glacier tongue. Their work involves surveys using satellite data and GPS beacons deployed on the glacier to measure the evolution of the rifts and the calving process. The team has followed the development of two major rifts cutting across the tongue, from opposite sides, over many years. The rifts had almost joined, when a similarly massive iceberg, B-9B, collided with the eastern flank of the tongue leading to the final separation.

A series of images from the ESA ENVISAT satellite with its radar imaging instrument ASAR shows the well developed crevasses from each side of the glacier and the B-9B iceberg approaching from the right side of the glacier flow (East) and pushing the glacier tongue to break at the rift line.

Description of the Calving Event

The Mertz Glacier flows into the ocean with a flux of 10 to 12 Gigatons of ice per year. The floating part of the glacier, which originally extended over 160km from the grounding line to the front, is now only 80km long. The glacier tongue which protruded 100+ km from the coastline is now about 20-25 km long. The new iceberg is 78 km long overall and 33 to 39 km wide with an average thickness of 400 m. The collision of bigger similarly large iceberg, designated B-9B, with the glacier tongue in early February apparently precipitated the calving event. The satellite images available indicate that the event occurred between the 12th and 13th of February, certainly between the 10th and 13th as one can see in the images below. The data being acquired by in situ GPS beacons (to be collected later on this year) will tell much more accurately the sequence of events.

Two large rifts cutting through the southern part of the glacier tongue have been developing over many years. Rifting progressed from the eastern margin of the Mertz Glacier in the 1990s until 2002 when another rift started to develop from the western side. Recently the two rifts had almost joined and the western rift subsequently became very active, leaving the northern part of the glacier tongue attached like a "loose tooth." The final break when B-9B collided with the eastern flank of the glacier tongue. But it did not simply separate along the line of these pre-existing rifts. The break followed most of the western rift and the ice sheared across the section with the eastern rift to produce a clean line which is allowing the southern end of the iceberg to move freely past the remainder of the tongue.

B-9B, itself about 97 km by 20-35 km, is a large part of the B9 iceberg that calved from the Ross Ice Shelf in 1987 and drifted westwards until it ran aground in 1992 on the Ninnis Bank, less than 100 km to the east of the Mertz Glacier Tongue. After remaining in roughly the same location for about 18 years, B-9B recently ungrounded and rotated to collide with the Mertz Glacier Tongue. The Mertz Glacier Tongue originates in a 60km long fjord and had extended a further 100km into the Southern Ocean. It advanced into the ocean at slightly more than 1 km per year. The new iceberg thus represents about 70 years of glacier advance.

The future behavior of the two icebergs is of great interest. Satellite images show that the recently-calved Mertz iceberg is moving into the Adélie Depression, a coastal basin situated between the Mertz Glacier and the French Antarctic station of Dumont D'Urville to the west. This depression one of the major sites of dense water formation which drives the world's deep ocean circulation. The dense water is formed from ocean water that circulates onto the continental shelf and interacts with the glacier tongue, and by high rates of sea ice formation within the Mertz Glacier polynya to the immediate west of the former glacier tongue. The future position of the two giant icebergs will likely affect local ocean circulation, the extent (and timing?) of the polynya, sea ice production, and deep water formation. It also has important implications for the marine biology of this region. A number of on-going field and research activities will follow up this calving event and its impact on the local environment.

Impact on Biodiversity

These polynyas constitute places of high biodiversity and food concentration for birds and marine mammals, in particular emperor penguins the only birds to reproduce during winter in Antarctica. The emperor colony at Pointe Géologie, next to Dumont d’Urville, is closely dependent on the ocean resources. Therefore significant modifications in the marine environment may have large consequences, not only on the local biodiversity but also on this emblematic penguin colony that was brought to prominence in Luc Jacquet's movie "March of the Penguins".