Methane field discovered off Gisborne coast

A "huge network" of frozen methane and methane gas has been discovered in ocean sediments 20 kilometres to 50km off Gisborne.

A joint New Zealand-German research team found 99 gas flares in a 50-square kilometre area, venting in columns up to 250 metres high, the National Institute of Water and Atmospheric Research (Niwa) said.

Methane was also found building up beneath a large landslide and being released along the landslide margin, and there were indications of large areas of methane hydrates - ice-like frozen methane - below the sea floor.

The discoveries were made by a 16-member team using state-of-the-art seismic and echosounder technology on board the Niwa research vessel Tangaroa.

The concentration of sea floor gas vents was the densest known off the New Zealand coast, and the vents were in much shallower water than usual.

Venting usually happened around a depth of 800m on large ridges in the middle of the continental slope, Niwa marine geologist and voyage leader Dr Joshu Mountjoy said.

In this case venting was going on along the edge of the shelf in as little as 200m of water.

The work is part of a larger project focused on the interaction between gas hydrates and and slow-moving active landslides. The area surveyed was known to have large active landslides, up to 15km long and 100m thick.

Researchers were also hoping to understand whether some methane was reaching the atmosphere, rather than being mixed up in the water column and consumed by biological processes as normally happened, Mountjoy said.

"Methane is a very effective greenhouse gas and seabed methane release has the potential to dramatically alter the earth's climate," he said.

"As ocean temperatures change the methane hydrate system has the potential to become unstable."

It would be interesting to find out whether global warming was changing the ocean system off Gisborne and causing more methane expulsion than previously. Higher ocean temperatures could change conditions so ice could turn back into a gas.

It remained to be seen whether the area off Gisborne was sensitive to climate change, Mountjoy said.

"We may be entering into a situation where global climate change is influencing the methane hydrate system."

The researchers were also trying to understand what caused the large, slow landslides in the area.

In a recently submitted scientific paper they proposed the landslides might be the sea floor equivalent of glaciers, with frozen methane rather than water ice. Alternatively pressurized gas could be causing landslides to move down slope.

One implication if large landslides were found to be moving slowly, rather than catastrophically, was that slow landslides were unlikely to cause tsunami.

Work still needed to be done to work out how much methane was coming out of the sea floor, Mountjoy said.

"The concentration of vents we saw is really something unique and special," he said.

"We think it's going to become a major site of interest. It's very unusual to see such a high concentration in one area."

Research to understand the interaction between gas hydrates and slow-moving landslides is to carry on during the next decade, and will include drilling into landslides in 2016.

This first part of the project is a collaboration between Niwa, GNS Science and the University of Auckland from New Zealand, GEOMAR and the University of Kiel from Germany, Oregon State University from the USA, and the University of Malta.

Mountjoy said work was being done around the world looking at methane hydrates as an energy source, but that was not the focus of the project.