Massive seafloor craters found in waters off New Zealand

WELLINGTON, April 2 (Xinhua) -- An international team of scientists have found what they believe are the world's biggest " pockmarks" -- craters formed by seafloor eruptions of gas or fluids -- in waters off New Zealand.

The New Zealand, German and U.S. scientists found the pockmarks at a depth of about 1,000 meters on the seafloor of the Chatham Rise, about 500 km east of Christchurch.

The three giant pockmarks, the largest measuring 11 km by 6 km in diameter and 100 meters deep, were possibly twice the size of the largest pockmarks recorded in scientific literature, said a statement from New Zealand's Institute of Geological and Nuclear Sciences (GNS Science).

The craters were part of a much larger field of thousands of smaller pockmarks that extended eastward along the Chatham Rise for several hundred kilometers.

"Some of the pockmarks on the Chatham Rise are huge compared to similar structures observed elsewhere in the world," GNS Science marine geophysicist Bryan Davy said in the statement.

"They are big enough to enclose the Wellington city urban area, or (New York's) lower Manhattan."

Gas release from the larger pockmarks could have been sudden and possibly even violent, with a massive volume expelled into the ocean and atmosphere within hours or days.

Scientists could not rule out volcanic activity having caused the release of gas, but another possibility was the release of sub- seafloor hydrocarbon gas, which would have coincided with drops in the sea level of about 100 meters during ice ages and subsequent warming of sea temperatures.

University of Auckland 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," Pecher said in the statement.

"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 had 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.