Massive eruption-born craters found
MICHAEL DALY
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Crater-like holes on the sea floor east of the South Island may have been caused by the sudden release of massive amounts of gas, scientists say.
The largest of the so-called pockmarks measures about 11 by 6 kilometres and is 100 metres deep, possibly twice as big as any previously found.
It is in a vast field with thousands of smaller pockmarks, covering an estimated 20,000km of sea floor extending eastward along the Chatham Rise for several hundred kilometres from Banks Peninsula.
The larger pockmarks were studied recently by an international group of scientists, some from New Zealand, on the German research ship Sonne.
The three giant pockmarks found are about about 500km east of Christchurch at a depth of about 1000m.
"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, who took part in the research voyage.
Pockmarks are caused by fluids and gases erupting through sediments into the ocean.
Sonne survey leader Joerg Bialas, from the GEOMAR Helmholtz Centre for Ocean Research in Kiel, Germany, said gas release from the larger pockmarks may have been sudden and possibly violent. A massive amount of gas could have been expelled into the ocean and atmosphere within hours or days.
Scientists could not rule out volcanic activity as the cause of the gas release.
Another possibility was the release of sub-sea floor hydrocarbon gas through a layer of gas hydrate deposits.
That would have coincided with drops in sea level of about 100m during ice ages and subsequent warming of sea temperatures.
Auckland University gas hydrate scientist Ingo Pecher said there was no sign of active gas systems in the larger pockmarks, but the smaller ones in shallower water appeared to have been sporadically active.
"Gas escape could be occurring from the smaller pockmarks during glacial intervals every 20,000 or 100,000 years," he said.
He has a Marsden research grant to investigate gas systems under the sea floor of the Chatham Rise.
"Methane is a potent greenhouse gas and the escape of big volumes would have significant implications for climate change and ocean acidification," he said.
The research has global implications because the episodic and cumulative release of greenhouse gases into the ocean and atmosphere in the geological past would have contributed to episodes of global warming.
Scientists from Germany, the United States and New Zealand would continue to analyse the large amounts of seismic data and sediment samples collected during the voyage to clarify the history of the structures, GNS Science said.
Other scientists in the project are from Otago University, the National Institute of Water and Atmospheric Research and the US Naval Research Laboratory.
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