Mysterious 'pocket' of underwater gas discovered in the East China Sea

A mysterious pocket of gas trapped under the seafloor off of the coast of Japan has been detected by geologists who probed the earth with seismic waves.

The composition of the gas is unknown, but it is likely made of either methane or carbon dioxide, produced thanks to magma rising from the Earth’s depths.

The newly-discovered reserve could be an untapped natural resource if it is dominated by methane gas that could be used as fuel, the researchers said.

Alternatively, such trapped gases could represent a ticking time bomb of greenhouse gases ready to escape in o the atmosphere and cause climatic shifts.

Either way, the researchers note that this reservoir is likely not alone — with others expected to be hidden both nearby and in similar settings around the globe. 

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A mysterious pocket of gas trapped under the seafloor off of the coast of Japan has been detected by geologists who probed the earth with seismic waves

Seismologist Andri Hendriyana and colleagues at Japan’s Kyushu University discovered the sizeable reservoir using seismic pressure waves.

These — similar to those generated by earthquakes — can reveal the nature of the rocks beneath the seafloor as they bounce off of them.

‘Seismic pressure waves generally travel more slowly through gases than through solids,’ explains study co-author Andri Hendriyana. 

‘Thus, by estimating the velocity of seismic pressure waves through the ground, we can identify underground gas reservoirs and even get information on how saturated they are.’

The new study used a special computational technique that allowed the researchers to map out cross sections of the material beneath the sea floor in far greater detail than has previously been possible. 

‘In this case, we found low-velocity pockets along the rifting axis near Iheya North Knoll in the middle of the Okinawa Trough, indicating areas filled with gas, ‘ Dr Hendriyana added.

Such rifting axes are where the Earth’s tectonic plates are slowly pulled apart by so-called continental drift, allowing hot magma to seep up from the mantle below.

This magma can both release carbon dioxide and methane — as well as both stimulate the growth of microbes and cause interactions with organic-rich sediments that produce these gases as well.

The team believe some of these gases may be getting trapped under the sea floor — either beneath layers of impenetrable clay or a lid made of a methane-containing ice  — building up to form the kind of reservoir they spotted off of the Japanese coast.

WHAT ARE TECTONIC PLATES?

Tectonic plates are composed of Earth’s crust and the uppermost portion of the mantle. 

Below is the asthenosphere: the warm, viscous conveyor belt of rock on which tectonic plates ride.

The Earth has fifteen tectonic plates (pictured) that together have molded the shape of the landscape we see around us today

Earthquakes typically occur at the boundaries of tectonic plates, where one plate dips below another, thrusts another upward, or where plate edges scrape alongside each other. 

Earthquakes rarely occur in the middle of plates, but they can happen when ancient faults or rifts far below the surface reactivate. 

These areas are relatively weak compared to the surrounding plate, and can easily slip and cause an earthquake.

‘While many people focus on greenhouse gases made by humans, a huge variety of natural sources also exist,’ added paper author Takeshi Tsuji. 

‘Large-scale gas reservoirs along a rifting axis may represent another source of greenhouse gases that we need to keep our eyes on.’

‘Or, they could turn out to be a significant natural resource.’ 

To be the latter, the reservoir would need to be filled mainly with methane, which can be used as a fuel, unlike carbon dioxide. At present, the researchers do not know which of the two gases makes up the largest part of the trapped reservoir.

Either way, however, both are greenhouse gases that could have significant environmental impacts if rapidly released into the atmosphere.

 This body of gas may not be alone, the researchers report.

‘Zones like the one we investigated are not uncommon along rifts,’ said Professor Tsuji.

‘I expect that similar reservoirs may exist elsewhere in the Okinawa Trough as well as other sediment-covered continental back-arc basins around the world.’

The full findings of the study were published in the journal Geophysical Research Letters.

‘We found low-velocity pockets along the rifting axis near Iheya North Knoll in the middle of the Okinawa Trough, indicating areas filled with gas, ‘ Dr Hendriyana said