Mystery of underwater hill formation solved

Published: Thursday 15 March 2007

scientists have always been puzzled by hills mysteriously rising from the sea floor in the Arctic Ocean. A paper recently published in the journal Geophysical Research Letters (Vol 34 No 1) offers an explanation: methane bubbling through seafloor sediments is the source of the swellings. These hills are called 'pingo-like features' which can rise up to 40 m in height and several hundreds metres across.

The geologists worked on the Beaufort Sea shelf, off the north coast of Canada. They spent over a month mapping the seabed and collecting samples of sediment cores and gas from the region. Pingos are small, dome-shaped, ice-cored hills found in many Arctic regions. They hypothesised the form when a frozen mixture of gas and seawater (methane hydrate) decomposes under the seafloor, releasing carbon dioxide that squeezes the deep sediments onto the seafloor like toothpaste from a tube.

The geologists used sound waves to show the hills' composition of a jumbled mix of sediment and small nodules of fresh, not salt water, ice. They also carbon-14 dated organic matter in the sediment revealing that this sediment was deposited before the last ice age, 10,000 years ago.

Another observation was that the hills were surrounded by shallow 'moats' where the seafloor within a kilometre of the hill was found subsided.

After chemically analysing the gas bubbling out of the hills, the researchers concluded that it originated as methane hydrate--an ice-like mixture of water and methane that forms within sediments under much of the Arctic seafloor and beneath permafrost areas on land. Methane hydrate can remain solid at low temperatures and high pressures, which is not the case on the Arctic Ocean's seafloor. Although the water here is within a few degrees of freezing, the seafloor has been undergoing a gradual warming process since the last ice age.

Over thousands of years, the scientists believe a 'wave' of warming has moved downward through the sediment. When it reaches the frozen methane hydrates hundreds of metres down it can cause methane hydrates to decompose thereby releasing methane into surrounding sediments and causing the seafloor to become a 'pingo' patchwork.

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