According to a recent paper published by MBARI geologists and their colleagues, methane gas bubbling through seafloor sediments has created hundreds of low hills on the floor of the Arctic Ocean. These enigmatic features, which can grow up to 40 meters (130 feet) tall and several hundred meters across, have puzzled scientists ever since they were first discovered in the 1940s.
Writing in the January issue of Geophysical Research Letters, MBARI geologists Charlie Paull and William Ussler and their coauthors described the results of field work they conducted on the Beaufort Sea Shelf, offshore of the north coast of Canada. In this area of year-round sea ice and permafrost, the team spent over a month mapping the seafloor and collecting sediment cores and gas samples from these underwater hills, which they call "pingo-like features."
"Pingos," small, dome-shaped, ice-cored hills, are found in many Arctic regions. "Pingo-like features" are similar in shape and size to pingos on land, but are found underwater, on the continental shelf in several parts of the Arctic. Previous studies have suggested that pingo-like features are pingos that formed on land but were submerged when sea level rose following the end of the last ice age, over 10,000 years ago.
Based on their geologic fieldwork and subsequent chemical analysis of the gas and sediments from eight pingo-like features, Paull and his coauthors propose an alternative hypothesis: Pingo-like features form when methane hydrate (a frozen mixture of gas and seawater) decomposes beneath the seafloor, releasing gas that squeezes deep sediments up onto the seafloor like toothpaste from a tube.
The geologists based this hypothesis on a number of observations and measurements. First, sound waves bounced through the pingo-like features showed that they were not built up from layers, but consist of a jumbled mixture of sediment and small nodules of fresh-water (rather than salt-water) ice. Carbon-14 dating of organic matter in the sediment at the crests of several hills showed that this sediment was deposited before the last ice age, thousands of years before sediments on the surrounding seafloor. Finally, many of the pingo-like features were surrounded by shallow "moats," where the seafloor within a kilometer of the hill had apparently subsided.
Even with evidence that pingo-like features were made of older, deeper sediment that had been pushed up from beneath the seafloor, the geologists still had to figure out what geologic process could generate enough pressure to lift seafloor sediments. The most obvious source of such pressure was methane gas, which the researchers observed bubbling out of the tops of several pingo-like features.
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