Methane "burps" create undersea hills

Thawing gas hydrate causes "pseudo-pingos" to grow in the Arctic Sea

A pingo in the Arctic in northern Canada © Charlie Paull
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They are up to 40 meters high, several hundred meters wide and lie deep below the surface of the Arctic Ocean: Strange rounded hills, which were until recently considered as submerged landforms. But now geologists have found that probably rising methane gas caused these "pseudo-pingos" under water.

"Pingos", small domed and icebound hills are a typical phenomenon in many Arctic regions - on land, but some time ago researchers have found very similar structures also on the seabed, in the area of ​​the Arctic continental shelf, as previous studies had postulated that these "pseudo-pingos" may have once been formed as pingos on dry land, but were slowly flooded by the sea level rise 10, 000 years ago after the last ice age.

Field studies in the Arctic Sea

However, researchers at the Monterey Bay Aquarium Research Institute (MBARI) around Charlie Paull and William Ussler have now refuted this theory and described it in the journal "Geophysical Research Letters". They studied for the seabed of the Beaufort shelf, an Arctic marine area off the north coast of Canada. In this region of permafrost and sea ice, they spent a month mapping the ocean floor, collecting gas samples, and drilling sediment samples. Also various other measurements around the Pingo-like hills undertook the scientists.

After completing the laboratory analysis, it was clear that the hills could not have originated on land, but were instead a result of high gas activity in the seafloor. Researchers believe that methane hydrate - a frozen mixture of methane and seawater - decomposes and methane rises through the sediment. It pushes the floor up like toothpaste from a tube. The result is roundish bulges on the seabed - the "pseudo pingos".

Scheme of pseudo-pingo formation © MBARI

Gas bubbles push up sediment

The scientists substantiate their hypothesis with a whole series of data. On the one hand, sonar investigations showed that the hills do not consist of layers like the real pingos, but of a disorderly mixture of sediment and small bubbles of soda rice. C-14 measurements also showed that the organic material on the hummocks is thousands of years older than the surrounding seabed an indication that here originally deeper layers of soil were pushed up here, display

Gas bubbles rise from the top of some pseudo-pingos, which the researchers quickly identified as methane. Chemical analyzes showed that it had to come from gas hydrates, which are usually several hundred meters below the ocean floor in this part of the Arctic Ocean. "We still do not know if this gas has bubbled to the surface within a single year or is slowly moving upwards like a glacier, " Paull said.

Warming started after the Ice Age

In any case, the data suggested that the slow warming of the subsoil since the last ice age may have played a crucial role in this. Because the seawater in this region has heated up so much in the last 10, 000 years that today it is about ten degrees warmer than, for example, the permafrost on land. Due to the sea level rise, former permafrost has been flooded at the same time and was slowly raised. This heat finally reached the gas hydrate deposits several hundred meters deep and began to thaw part of the icy gas mixture, causing the methane to rise.

This process is still going on, new pseudo-pingos continue to emerge. However, since methane is an extremely potent greenhouse gas, the researchers want to investigate in the future, how much methane is released by these processes. "Pingo-like structures are among the places where we see methane rising from the seafloor, " says Paull. "Yet we do not know how crucial this is because we do not know how much gas is released in this way throughout the Arctic or other marine regions." Climate research could these hills will give valuable insights into how methane hydrates behave when heated slowly for example, as climate change progresses.

(MBARI, 07.02.2007 - NPO)