Icebergs on the move
NIWA Scientist Mike Williams is interested in the daily positions of icebergs, like the big iceberg B15A. Mike wants to learn about ocean circulation and to see what effect icebergs have on ecosystems in Antarctica.
“B15A was particularly interesting,” Mike says. “This was the world’s largest free floating iceberg, stretching over 120 kilometres in length and about 25 km in width”.
Iceberg B15A
Satellite photograph of the B15A iceberg blocking McMurdo Sound, taken on 13th December 2004.
For comparison, the length of B15A was the equivalent of travelling from Auckland to Hamilton. It covered an area of approximately 3,100 km² - about five times the surface area of Lake Taupo. This iceberg broke (calved) off the Ross Ice Shelf in Antarctica in March 2000 in a tabular shape, which is typical for young icebergs. This iceberg started moving and, along its way, got stuck across the entrance to McMurdo Sound.
B15 in McMurdo Sound
Dr Mike Williams from NIWA explains what effect the iceberg B15 had on the penguin colony nearby when it blocked the inlet to McMurdo Sound.
Considering that icebergs only reveal about one tenth of their size above the surface and the majority of the berg is submerged under the waterline, this huge iceberg blocked the sea currents flowing into McMurdo Sound. Mike explains that this had consequences for the ecosystem within the Sound.
Tip of the iceberg
The tip of an iceberg which is visible above the waterline is usually only about 1/7 or 1/8 of the mass of an iceberg. Seawater is slightly denser than ice and this means that the iceberg floats with most of its mass below the surface. In comparison a cork, which has a density of about 20% that of water, floats close to the water surface. Ice has about 90% of the density of water but the density of seawater is also influenced by the temperature and salinity.
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The iceberg sheltered the Sound from wave and current action that would usually break up sea ice. The sea ice in McMurdo Sound, which usually forms to an average thickness of about 2 metres, got as thick as 6 m. With all this sea ice not breaking up, it was difficult for the penguins in McMurdo Sound to access the sea and catch the fish they need for themselves and their chicks.
The significance of B15
Dr Mike Williams from NIWA explains why studying the iceberg B15 was important and what scientists were able to learn from its movement and the impact it had on the environment around it.
The iceberg, which extended to a depth of about 350 to 400 m, also had a direct effect on the sea bed, scouring the seafloor where it made contact. B15A eventually broke into smaller pieces, which have moved further north. Mike explains that initially large icebergs move by action of the ocean currents and later, when they are smaller, they are more influenced by the wind.
Tracking icebergs
Dr Mike Williams from NIWA explains how the icebergs that have calved off the Antarctic ice sheet are being tracked.
“At some stage in the life of an iceberg, the balance between current-driven to wind-driven tips,” says Mike, “but this is hard to predict, as it is very hard to know how and in which shape an iceberg melts. Sometimes when parts of the icebergs have melted away an iceberg can start to ‘ roll’, so parts that have been previously under water may appear and others are moved under the water. This makes working on icebergs extremely dangerous as they can roll without warning."
Measuring icebergs
Dr Mike Williams from NIWA explains how the thickness of icebergs can be measured and why it is hard to do this after an iceberg has calved off the Antarctic ice sheet.
The information that Mike gathers helps other scientists to understand changes in ecosystems, and it also allows scientists to find explanations for changes in paleo-ocean circulation which may have been greatly affected by iceberg movements.
Nature of science
Describing things as accurately as possible is important in science so that scientists can compare their observations with those of others.
