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New Zealand : Scientists find mysterious giant pockmarks on chatham rise – 02/04/2013

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Earth Watch Report  -   Discoveries

 

 

Scientists find mysterious giant pockmarks on chatham rise – 02/04/2013

02/04/2013 2:30 pm

New Zealand, German and American scientists have found what may be the world’s biggest pockmarks on the seafloor of the Chatham Rise about 500km east of Christchurch.

Pockmarks are crater-like structures on the seabed caused by fluids and gases erupting through sediments into the ocean.

Three giant pockmarks, the largest being 11km by 6km in diameter and 100m deep, are possibly twice the size of the largest pockmarks recorded in scientific literature. Scientists believe they are the ancient remnants of vigorous degassing from under the seafloor into the ocean.

The structures are at water depths of about 1000m and there is currently no sign of gas being emitted from them.

They are part of a much larger field of many thousands of smaller pockmarks which extend eastward along the Chatham Rise for several hundred kilometres from Banks Peninsula. This vast field covers an estimated 20,000km of seafloor.

Giant pockmarks

An international group of scientists investigated the larger seafloor structures on the German research ship Sonne recently. Their aim was to determine the geological origin of the structures, which were first noted in 2007.

“Some of the pockmarks on the Chatham Rise are huge compared to similar structures observed elsewhere in the world,” said GNS Science marine geophysicist Bryan Davy, one of the scientists who took part in the research voyage.

Methane is a potent greenhouse gas and the escape of big volumes would have significant implications for climate change and ocean acidification

Dr Ingo Pecher

“It’s most unusual for scientists to encounter seafloor structures of this size and complexity. They are big enough to enclose the Wellington city urban area, or lower Manhattan,” Dr Davy said.

The discovery indicated that there was still a lot to learn about the ocean floor off the New Zealand coast.

The geological processes that led to the formation of the larger structures were still unclear, but survey leader on the Sonne, Joerg Bialas from GEOMAR Helmholtz Centre for Ocean Research in Kiel, Germany, said there were clear indications in seismic reflection records of gas pockets and fluid flow structures in the deeper sediments underneath the pockmarks.

“The pockmark features are covered by complex layers of more recent sediment,” Dr Bialas said.Gas release from the larger pockmarks may have been sudden and possibly even violent, with a massive volume being expelled into the ocean and atmosphere within hours or days.

Scientists cannot rule out volcanic activity, directly or indirectly, having generated the release of gas. Another possibility is the release of sub-seafloor hydrocarbon gas through a layer of gas hydrate deposits. This would have coincided with drops in sea level of about 100m during ice ages and subsequent warming of sea temperatures.

 

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