Water is what gives our planet its beautiful blue color and is critical for the existence of life as we know it. Our entire planet is nicknamed after it - the "blue planet", or "pale blue dot". A new study led by geophysicist Steve Jacobsen of Northwestern University and seismologist Brandon Schmandt from the University of New Mexico has yielded evidence that vast oceans worth of water are tied up within Earth’s mantle. The results are published in Science.
Four hundred miles beneath North America, Schmandt and Jacobsen found deep pockets of magma, which indicates the presence of water. However, this isn’t water in any of the three forms we are familiar with. The pressure coupled with the high temperatures forces the water to split into a hydroxyl radical (OH) which is then able to combine with the minerals on a molecular level.
This water, which is bound up in rock, could indicate the largest water reservoir on the planet. It is believed that plate tectonics cycle the water in and out, and the water affects the partial melting of rock in the mantle.
"Geological processes on the Earth's surface, such as earthquakes or erupting volcanoes, are an expression of what is going on inside the Earth, out of our sight," said Jacobsen in a press release. "I think we are finally seeing evidence for a whole-Earth water cycle, which may help explain the vast amount of liquid water on the surface of our habitable planet. Scientists have been looking for this missing deep water for decades.”
To laymen, the Earth has three layers: crust, mantle, and core. It is a bit more complex than that, as the mantle itself has four distinct layers: lithosphere, athenosphere, upper mantle, and lower mantle. Even among those layers, different areas have different features. Many scientists have assumed that the transition zone between the upper and lower mantle (250-410 miles beneath the surface) contained water, though this experiment is the first to provide the necessary direct evidence to support that theory.
”Melting of rock at this depth is remarkable because most melting in the mantle occurs much shallower, in the upper 50 miles," said Schmandt, the paper’s lead author. "If there is a substantial amount of H2O in the transition zone, then some melting should take place in areas where there is flow into the lower mantle, and that is consistent with what we found.”
For this study, the researchers utilized the USArray, which collects information from over 2,000 seismometers in the United States. The observations were supported by computer models that replicated conditions from the transition zone. The key to storing the water, they found, is a mineral called ringwoodite, which is a form of olivine that exists under high pressure and temperature.
"The ringwoodite is like a sponge, soaking up water," Jacobsen said. "There is something very special about the crystal structure of ringwoodite that allows it to attract hydrogen and trap water. This mineral can contain a lot of water under conditions of the deep mantle.”
According to experiments, at depths around 400 miles, the ringwoodite should melt partially. This was done by using diamonds to exert tremendous pressure on the synthesized ringwoodite while subjecting it to high temperatures. The effects were studied with a combination of x-rays, electrons, and light. The researchers found that these experimental conditions supported observations from USArray.
"When a rock with a lot of H2O moves from the transition zone to the lower mantle it needs to get rid of the H2O somehow, so it melts a little bit," Schmandt said. "This is called dehydration melting.” After the rock melts, the researchers say, the water becomes trapped in the transition zone, creating a reservoir.
In March, a paper published in Nature from a different research group used a series of techniques including x-ray diffraction and infrared spectroscopy to confirm that a ringwoodite sample (the first to ever come from within the Earth and not just created in a lab) had a had a water content above one percent. This quantity matches what has been predicted by Schmandt’s experiments. Earth’s mantle is so vast, that if 1% of the material in the transition zone is actually water, it would represent a reservoir three times larger than all of Earth’s oceans combined.
"Whether or not this unique sample is representative of the Earth's interior composition is not known, however," Jacobsen said. "Now we have found evidence for extensive melting beneath North America at the same depths corresponding to the dehydration of ringwoodite, which is exactly what has been happening in my experiments.”
