A multinational team of researchers led by marine geophysicist Dr Bryan Davy from GNS Science has found what may be the world’s biggest pockmarks on the seafloor about 310 miles east of Christchurch, New Zealand.

Three giant pockmarks – crater-like structures on the seabed – found by the team are possibly twice the size of the largest pockmarks ever recorded.

Scientists believe they are the ancient remnants of vigorous degassing from under the seafloor into the ocean. The structures (the largest being 6.8 miles by 3.7 miles in diameter and 328 feet deep) are at water depths of about 0.6 miles and there is currently no sign of gas being emitted from them.

The team investigated the larger seafloor structures on the German research ship Sonne. Their aim was to determine the geological origin of the structures, which were first noted in 2007.

“Some of the pockmarks are huge compared to similar structures observed elsewhere in the world,” Dr Davy said. “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.”

The geological processes that led to the formation of the larger structures were still unclear.

“There were clear indications in seismic reflection records of gas pockets and fluid flow structures in the deeper sediments underneath the pockmarks,” explained Dr Joerg Bialas from GEOMAR Helmholtz Center for Ocean Research in Kiel, Germany.

“The pockmark features are covered by complex layers of more recent sediment. 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,” he said.

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 328 feet during Ice Ages and subsequent warming of sea temperatures.

“The apparent absence of methane in the shallow sediment and water column at the giant pockmark area was a surprise,” said Dr Ingo Klaucke from GEOMAR and Dr Richard Coffin from the US Naval Research Laboratory.

“While there was no sign of active gas systems in the larger pockmarks, the smaller ones in shallower water appeared to have been sporadically active,” said gas hydrate scientist Dr Ingo Pecher from the University of Auckland. “Gas escape could be occurring from the smaller pockmarks during glacial intervals every 20,000 or 100,000 years.”

“Methane is a potent greenhouse gas and the escape of big volumes would have significant implications for climate change and ocean acidification,” he concluded.

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Bibliographic information: Davy B et al. 2010. Gas Escape Features off New Zealand – Evidence for a Massive Release of Methane from Hydrates? Geophys. Res. Lett., 37, L21309; doi: 10.1029/2010GL045184