Hydrous ringwoodite synthesized from olivine in Steve Jacobson’s laboratory.(Photo: Steve Jacobsen)
Evidence of water beneath Earth's surface could alter the understanding of the composition of the planet and how its oceans formed.
Researchers have discovered evidence of water — enough to fill oceans — embedded in minerals deep beneath the surface of the United States that could alter the current understanding of the Earth's composition and how it was formed. Were this water in liquid form, which it's not, the discovery could be considered the planet's largest underground water reservoir, researchers said.
The team of researchers, led by Northwestern University geophysicist Steve Jacobsen and University of New Mexico seismologist Brandon Schmandt, has found pockets of magma about 400 miles beneath the Earth's surface — clear signs of the presence of water, though not in its familiar liquid form.
The components of water (your classic hydrogen and oxygen) are trapped inside the molecular structure of minerals in mantle rock, embodying a fourth form of water that is neither liquid, ice nor vapor.
Water is typically cycled down beneath the surface through plate tectonics, or movements of the Earth's crust. The pressure and high temperatures of the plates moving down causes the water to be squeezed up and the rocks to partially melt. However, most melting occurs in a shallower part of the mantle, in the upper 50 miles, Schmandt said, rather than at the 400 mile marker they found in their research. The area where they found the water is in a layer known as the transition zone, which rests between the upper and lower mantles.
"We knew about the water cycle, but we didn't know how deep it extended," Jacobsen told USA TODAY. "It looks like the same process occurring in the very shallow mantle is occurring at a deeper layer."
Earthquakes and volcanoes are an expression of what is going on inside the Earth, Jacobsen said. The melting of rocks that occur because of plate movement is the source of magma for volcanoes, he said. Jacobsen also said that his and Schmandt's research may help explain how the oceans were formed at the beginning of the Earth's formation.
"It may be why we have stable oceans on the surface," Jacobsen said.
Scientists have been looking for this missing water for decades, Jacobsen said. Joseph Smyth, a geologist at the University of Colorado Boulder and a colleague of Jacobsen's, first theorized that there was water in the mantle in 1987. Previously, it had been thought that the pressures in the mantle were too high to sustain water in the minerals.
"It's a very remarkable discovery that suggests that the region is in fact hydrous and is effectively storing water or hydrogen in the interior," Smyth said to USA TODAY.
The water in the mantle rock, which could equal the amount of water in the world's oceans, may be an integral part of sustaining water on the surface, Smyth said. It also implies that the Earth's oceans likely formed from water stored deep beneath the Earth, rather than from a comet or asteroid, as other scientists have theorized, Smyth said.
"Geophysicists have debated where…Earth's water came from," Jacobsen said. "From inside the Earth out of rocks as the planet formed, or did Earth's water come from things like comets much later on? I think this finding is supporting the idea that oceans and the atmosphere came from (inside the Earth)."
Because the water is so deep beneath the surface and not in a liquid form, Jacobsen said it could probably not be extracted for practical use for people on the surface.
"The depth of this water is around 400 miles, or 700 kilometers, and the deepest hole ever drilled in the Earth is about 10 kilometers," Jacobsen said. "The story has more to do with understanding the composition of the Earth."
Collaboration of two laboratories
The research was a convergence of both Jacobsen's research in the lab, where he studied mantle rock under simulated high pressures, and Schmandt's research using seismic waves from earthquakes to get a clear picture of the structure of the deep crust and mantle.
Jacobsen's lab examined ringwoodite, a unique mineral that can hold water at incredible depths. His findings built on the discovery of ringwoodite in a diamond brought up from the depth of 400 miles by a Brazilian volcano in March.
"The (discovery of the ringwoodite diamond) showed us at least one place there is water, but it didn't tell us how expansive the water would be," Jacobsen said. "Schmandt found with seismic work that…water is everywhere beneath North America."
Schmandt used the USArray, part of a National Science Foundation program that deploys thousands of seismic instruments across the continent to detect the evidence of partial melt, or magma, beneath the surface. Melt slows the speed of the seismic waves, giving Schmandt's team a clear picture of Earth's internal structure.
"The array of seismometers gave us a real ability to look at the geography down there," Schmandt said. "The main one that stuck out the most was…the decreases in seismic quality are consistent with the pockets of melt."
It took Schmandt's team about eight years to measure the whole of the United States, but they want to expand their findings beyond the North American continent. Unfortunately, other countries don't have the seismic equipment needed to measure the partial melt.
"In the future as more seismometers are placed, we will be able to see beneath more places," Jacobsen said. "Looking for this signature of water globally is the next step."
The report appears in the new edition of the journal Science.
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