Embracing icebergs sing eerie duets

They travel from Antarctica to Tahiti, can sound like laughing monkeys, or barking dogs, and some were triggered by the December 2004 tsunami: they are the eerie songs made by some of Antarctica's largest icebergs. The phenomenon occurs when the huge lumps of ice scrape past each other and produce thousands of tiny "icequakes". These are so similar to earthquakes, researchers say, that icebergs could help to better understand and predict tremors. Massive tabular icebergs break off the Antarctic ice shelf about every 50 years. Soon after the last "calving" event in 2000, unusual harmonic tremors were picked up by underwater hydrophones as far as Tahiti. Emile Okal of Northwestern University in Evanston, Illinois, and colleagues soon traced the sounds back to the new icebergs, including what was then the world's largest free-floating berg, B15A. Tidal tune: To find out how the icebergs produce the noises, Doug MacAyeal at the University of Chicago and colleagues constructed a network of seismographs on iceberg C16, which at the time was aground in the Ross Sea and adjoining to B15A. They monitored the tremors – which are inaudible to the human ear, but can be heard if the recordings are speeded up – during the austral summer of 2003. The team found that one particular song was repeated by the giants daily, and the timing matched that of the tides in the Ross Sea. It began vigorously on the first surge then slowly ground to a halt and began again when the tide reversed. The researchers triangulated the source of the tremors and discovered that they came from an area where C16 was rammed by B15A in a collision zone only a few kilometres long.

Pushed by the daily tides, the bergs scraped past each other in many brief, jerky movements. Other theories have been put forward as to the source of the iceberg "songs", but the researchers say this is the first time a mechanism has been demonstrated. Earthquake 'lab': "Each of these movements is only 0.6 millimetres, but they send booming broadcasts into the world oceans," MacAyeal told New Scientist.