The Earth's surface is famously a pretty wet place, but a new study suggests that the mantle is home to much more water than was previously believed. Observations of seismic activity around the Mariana Trench have revealed that subducting tectonic plates are dragging more water deeper into the Earth, which could change our understanding of the global water cycle.
The Mariana Trench is best known for being the deepest part of the ocean, plunging all the way down almost 11,000 m (36,000 ft) at its lowest known point. That's because the trench is the meeting place of two tectonic plates – the tiny Mariana Plate and the huge Pacific Plate. Being denser and older, the latter is actually sliding under the former, which lets vast amounts of seawater pour down through the cracks into the Earth's crust and the upper mantle.
But it's not just as a liquid. Under the higher temperature and pressure at that depth, the water can be locked into the lower plate's rocks as hydrous minerals, which then sink deeper into the mantle as the plate does. So just how much water ends up down there?
"Previous estimates vary widely in the amount of water that is subducted deeper than 60 miles (97 km)," says Douglas A. Wiens, research adviser for the study. "The main source of uncertainty in these calculations was the initial water content of the subducting uppermost mantle."
To get a better understanding, researchers from Washington University in St. Louis used a network of 19 seismographs deployed on the bottom of the ocean around the Mariana Trench, backed up by seven land-based ones on the nearby Mariana Islands. With those instruments, the team listened to over a year's worth of seismic movements to get a closer look at the structure and speed of the plates.
The team found that the hydrated rock in the area extends almost 32 km (20 mi) below the seafloor. Previous studies imaged underground structures by firing sound waves from air guns from research vessels, but these could only peer a fraction of that distance down.
"Previous conventions were based on active source studies, which can only show the top 3-4 miles into the incoming plate," says Chen Cai, first author of the study. "They could not be very precise about how thick it is, or how hydrated it is. Our study tried to constrain that. If water can penetrate deeper into the plate, it can stay there and be brought down to deeper depths."
The observations suggest that in the Mariana Trench, four times more water is captured and dragged down than previous studies estimate. If this trend applies to other similar regions around the world, then there could be about three times more water in the deep mantle than we thought. That lines up well with the recent discovery of exotic forms of ice trapped in diamonds, which are also evidence of a wetter mantle.
So where does all that water end up? It's commonly believed that most of it is ejected back to the surface through volcanic eruptions, sometimes hundreds of miles away. But, the team says the new findings suggest that far more water is going in than is coming out, which may prompt a closer look at our understanding of the global water cycle.
The research was published in the journal Nature.
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