Mertz Glacier tongue unhinged by giant iceberg
In mid-February 2010 a massive iceberg designated B09B collided with the Mertz Glacier tongue – a section of the glacier that protruded about 100 km from the Antarctic coastline at about 145ºE. The collision precipitated the calving of another massive iceberg, C28, from the tongue, measuring 78 km long and between 33 and 39 km wide. This calving event removed about 80% of the tongue, leaving only a 20 km-long stub. The calving had been anticipated, as rifts cutting across the tongue had been developing over many years, but the timing and collision was not.
The calving event was detected by Australian and French researchers who have been studying the Mertz Glacier as part of the International Polar Year project, Cooperative Research into Antarctic Calving and Iceberg Evolution. The project involves surveys using satellite data and GPS beacons deployed on the glacier, to measure the evolution of rifts and the calving process. The team had followed the development of two major rifts from opposite sides of the Mertz Glacier tongue for some years. The rifts had almost joined when B09B collided with the eastern flank of the tongue, completing the break. B09B originally calved from the Ross Ice Shelf in 1987 and drifted round to the east of Mertz Glacier by 1992. It was grounded for many years and started moving in late 2009.
The region about the Mertz Glacier plays an important role in the global ocean over-turning circulation. Polynyas in the region (areas of open-water or low sea ice concentration) produce about 25% of the Antarctic Bottom Water, which drives the deep over-turning circulation of the global ocean, carrying oxygen and nutrients to the ocean depths in all ocean basins. The effect of strong off-shore winds and heat loss from the ocean, make polynyas very efficient sea ice 'factories'. The salt rejected during the freezing of new sea ice creates the cold, dense water which sinks to the ocean bottom and ultimately forms Antarctic Bottom Water.
The calving of the glacier tongue and the shift of icebergs has changed the geography of the main polynya that was adjacent to the glacier. As C28 drifted west, it initially caused the Mertz polynya to be divided into several smaller areas, which disturbed the ice factory role. At the beginning of April, C28 collided with a submerged peak and split into several massive sections. By the end of April the sections had drifted across the edge of the continental shelf into deep water, about 250–300 km west of the glacier and well clear of the polynya.
Iceberg B09B remains grounded about 50 km north-east of the remaining Mertz Glacier tongue. The behaviour of the Mertz polynya appears to have returned to its previous active ice factory role, after a temporary reduction in sea ice production when C28 was in the area of the polynya.
The changed geography is expected to impact on the ocean circulation in the polynya region and further changes are anticipated when iceberg B09B once again begins to move. This may yet have a longer-term impact on bottom water formation and biological productivity in the region. So there may be flow on effects up the food chain in the region, which includes nearby penguin colonies. A number of on-going field and research activities will follow up this calving event and its impact on the local environment.
NEAL YOUNG1, BENOIT LEGRESY2, RICHARD COLEMAN3, and ROB MASSOM1
1Australian Antarctic Division and Antarctic Climate and Ecosystems Cooperative Research Centre
2 Laboratoire d'Etudes en Géophysique et Océanographie Spatiales
3 Institute of Marine and Antarctic Studies