This is the sound of a splitting iceberg.
Like ice cubes cracking in a glass of warm soda, except louder.
Disintegrating icebergs generate significant noise pollution at the world's poles, affecting narwhals, whales and other aquatic life, according to a study published June 18 in Oceanography. And with temperatures rising at the poles due to climate change, the oceans will only get louder.
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The splitting of an iceberg over 20 minutes in the Antarctic's Southern Ocean produces enough sound energy to equal that of 214 supertankers, according to the study.
"Sound from ice breakup in the Southern Ocean can be significantly greater than anthropogenic noise sources and thus are a major contributor to the overall ocean noise budget," the authors state.
It had been assumed so far by oceanographers that the loudest noises in the oceans of the Arctic and Antarctic are generated by direct human activity, such as the sonic booms emitted by vessels exploring for oil, by shipping traffic, and sonar.
Scientists have previously listened in on icebergs running aground and scraping against the sea floor. But this is the first time that an iceberg's final moments before disintegration have been documented in sound.
Between April and June 2007, the A53a iceberg was drifting in the Weddell Sea of Antarctica when it hit a shoal. Robert Dziak of Oregon State University and his colleagues had installed an array of hydropones in the area, and they were able to listen in to the sharp bursts of sound called harmonic tremors the iceberg made as it spinned.
By July, the iceberg got free of the shoal, and floated north into the warmer Scotia Sea where it started melting. Melt ponds appeared on the surface, a sure sign it was disintegrating. By September, it was no more. Researchers were most interested in these final moments of A53a.
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They recovered the data from the hydropone array and listened to the sounds, called icequakes. It was entirely different from harmonic tremors of an iceberg running aground.
Since sound is a form of energy, Dziak was able to calculate the energy flows through the ocean over a 20 minute period, which equaled the sound energy generated by 214 ships.
A number of aquatic mammals are affected by noise pollution. At the poles especially, the oceans are usually so quiet that the animals are not habituated to anthropogenic noise.
For example, when there is a blast of sound in the Arctic, narwhals go out of their way to avoid the source during their annual migrations. Sometimes, the route change takes them into waters that are not quite out of winter, leading to flash freezing of the animals in ice. A number of such ice entrapment events have been recorded in recent years.
Photo: a melting iceberg. Bob Dziak, Oregon State University