A seismic survey reveals a huge amount of water driven into the Earth's interior

Researchers at the University of Washington, Saint-Louis, in the arts and sciences, have had significant consequences on the global water cycle.

"People knew that the subduction zones could bring down the water, but they did not know how much water," said Chen Cai, who has just completed his doctoral studies at the University of Hawaii. University of Washington. Cai is the first author of the study published in the November 15 issue of the journal Nature.

"This research shows that subduction zones move far more water than previously thought in the depths of the Earth," said Candace Major, Program Director at the Division of Marine Science. the National Science Foundation, which funded the study. "The results highlight the important role of subduction zones in the Earth's water cycle."

"Previous estimates vary considerably with respect to the amount of water sub-engineered at more than 60 km," said Doug Wiens, Distinguished Professor of Earth and Planetary Sciences at Robert S. Brookings, and Research Advisor from Cai for the study. "The main source of uncertainty in these calculations was the initial water content of the highest sub-tubular mantle."

To conduct this study, researchers listened to the rumblings of the Earth for more than a year (from ambient noise to real earthquakes) with the aid of a network of 19 passive seismographs at the bottom of the Ocean deployed in the Marianas Trench, as well as seven seismographs based on islands. The plaque is where the plate of the Western Pacific Ocean slides under the Mariana Plate and sinks deep into the Earth's mantle when the plates converge slowly.

The new seismic observations paint a more nuanced picture of the Pacific plate that bends in the trench, which resolves its three-dimensional structure and tracks the relative velocities of rock types with different water retention capabilities.

The rock can hang in the water and hold it in different ways.

The seawater at the top of the plate descends into the earth's crust and the upper mantle along the fault lines that link the area where the plates meet and fold. Then he is trapped. Under certain conditions of temperature and pressure, chemical reactions force water in a non-liquid form in the form of hydrated minerals – wet rocks – by blocking the water in the rock of the geological plate. All the while, the plaque continues to sink deeper and deeper into the Earth's mantle, drawing water with it.

Previous studies conducted in subduction zones such as the Mariana Pit have shown that the subduction plate may contain water. But they could not determine how much water it contained or how deep it was.

"The previous conventions were based on active source studies, which can only show 3 to 4 miles high in the incoming plate," Cai said.

It was referring to a type of seismic study that uses sound waves created by firing an airgun onboard an oceanographic research vessel to create an image of the underground rock structure .

"They could not be very precise about its thickness, nor about its hydration," said Cai. "Our study has tried to limit that, if the water can penetrate deeper into the plate, it can stay there and be brought deeper."

The seismic images obtained by Cai and Wiens show that the hydrated rock zone of the Marianas Trench extends for almost 20 miles below the seabed, much deeper than previously thought.

The amount of water that can be retained in this hydrated rock block is considerable.

For the Mariana Trench region alone, four times more water sub-ducts than previously calculated. These features can be extrapolated to predict conditions in other ocean trenches around the world.

"If other old slabs, subjected to a cold subduction, contain layers of hydrated mantle of similar thickness, estimates of the overall flow of water in the mantle at depths greater than 60 miles shall to be multiplied by three, "said Wiens.

And for the water on the Earth, everything that flows must rise. Sea level has remained relatively stable over geological time, ranging from less than 1,000 feet. This means that all the water that goes down into the Earth in the subduction zones has to go up one way or another and not continually accumulate inside the subduction zones. Earth.

Scientists believe that most of the water that falls into the trench returns from the Earth into the atmosphere in the form of water vapor as volcanoes explode for hundreds of kilometers. But with the revised water estimates from the new study, the amount of water entering the earth seems to far exceed the amount of water coming out of it.

"Estimates of water coming out of the volcanic arc are probably very uncertain," said Wiens, who hopes that this study will encourage other researchers to reconsider their models of return of the water. water of the Earth. "This study will likely result in a reassessment."

Beyond the Mariana Trench, Wiens and a team of scientists recently deployed a similar seismic network off the coast of Alaska to examine how water is entering the Earth.

"Does the amount of water vary considerably from one subduction zone to another, depending on the type of defect you have when the plate bends?" Wiens asked. "There have been suggestions on this in Alaska and Central America, but no one has yet examined the deepest structure as we have done in the Mariana Trench."

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More information:
Chen Cai et al. Water intake in the Mariana subduction zone estimated from ocean bottom seismic data, Nature (2018). DOI: 10.1038 / s41586-018-0655-4