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Arctic Submarine Hills and Methane BubblingMethane sipping in a frozen sea |
By Stefan Anitei, Science Editor
7th of February 2007, 16:27 GMT
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Since the 1940s, scientists were puzzled by hundreds of enigmatic submarine hills, up to 40 meters (130 feet) tall and several hundred meters across, found on the floor of the Arctic Ocean.
A MBARI team of geologists, studying these formations on the Beaufort Sea Shelf, offshore of the north coast of Canada, reached the conclusion they must have been formed by methane gas sipping through seafloor sediments.
The team mapped over a month the seafloor and sampled sediment cores and gas from the "pingo-like" underwater hills.
"Pingo-like" hills are similar in shape and size to real pingos, small, dome-shaped ice-cored hills, found on land all around the Arctic zone.
Scientists believed that these submarine hills were real pingos emerged on land but flooded when sea level got higher at the end of the last ice age, 10,000 years ago.
The new research
is based on the chemical analysis of the gas and sediments from eight pingo-like features that appear when methane hydrate (a frozen mix of gas and seawater) breaks down under the seafloor, spitting gas that pushes deep sediments up onto the seafloor like toothpaste from a tube.
Sound waves technology revealed that the submarine hills were not made up of layers, but of a jumble consisting of sediment and small nodules of fresh-water (not salt-water) ice.
Carbon dating of the crest sediment of several hills pointed the sediment was there before the last ice age, being thousands of years older than those on the neighboring seafloor.
Many hills were surrounded by shallow "moats," where the seafloor within a one kilometer radius looked subsided.
Geologists think the sediments were lifted from the seafloor by pressure exerted by methane gas and observed sipping out of the tops of some pingo-like hills.
Methane hydrate, the source of methane, stays solid only at low temperatures and high pressures, conditions found at hundreds of meters below the seafloor in the Arctic Ocean.
The buried hydrates decomposed gradually, releasing large amounts of methane gas, as this area has been getting warmer in the last 10,000 years.
This seawater is within a few degrees of freezing temperature, but this means at least 10 degrees Centigrade (20 degrees Fahrenheit) more than the permafrost-filled soil.
This "wave" of warming in the last millennia moved downward through the sediment, reaching the frozen hydrates.
The team believes that this methane moved sideways under the seafloor and kept in place by an impermeable frozen soil layer situated between the hydrates and the seafloor, and managed to move toward the surface along faults or in areas with relatively weak sediments.
Areas on the mounds' sides slowly collapsed, forming the "moats".
"We don't know if this gas and sediment was burped up in a single year, or moved slowly like a glacier," said Charlie Paull, one of the geologists.
The study points out that the pingo-like hills are still lifting and dropping methane, a potent greenhouse gas.
"Pingo-like features are one of the places where we see methane coming up through the seafloor. As yet we don't know how important they are, since we don't know how much gas is coming up in the Arctic as a whole or in other seafloor areas," said Paull. "One of the questions we're trying to answer is 'What do buried hydrates do when they are suddenly warmed up?' In this case, we have a field experiment that's been going on for thousands of years."
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