Water from Earth’s surface can find its way deep into the planet, and new research explains how it changes the outermost region of the metallic liquid core.
The finding could explain the presence of a thin layer of material inside the planet that has mystified geologists for decades.
Earth’s crust is composed of tectonic plates that grind and slide under each other; over billions of years, these subduction zones have transported water to the lower mantle.
When this water reaches the core-mantle boundary, some 2,900 kilometers (1,800 miles) below the surface, it sets off a powerful chemical interaction. A team from South Korea, the US, and Germany showed this creates a top core layer rich in hydrogen, and sends silica to the lower mantle.
”For years, it has been believed that material exchange between Earth’s core and mantle is small,” says materials scientist Dan Shim from Arizona State University.
“Yet, our recent high-pressure experiments reveal a different story. We found that when water reaches the core-mantle boundary, it reacts with silicon in the core, forming silica.”
The outer core‘s mix of iron and nickel plays a significant role in generating Earth’s magnetic field, which essentially protects life on the planet from solar winds and radiation. So it’s important to understand how Earth’s insides work and have evolved over time.
Earth’s core-mantle boundary changes from silicate to metal quite sharply, and not much is known about the chemical exchanges.
Decades ago, researchers recording seismic waves through Earth’s gooey insides documented a thin layer just over a few hundred kilometers thick, but until now no one knew where this proposed ‘E prime’ layer came from.
“We suggest that such chemical exchange between the core and mantle over gigayears of deep transport of water may have contributed to the formation of the putative E prime layer,” the team writes.
Seismologists mapped out some unusual features that suggest this changed liquid metallic layer will be less dense and have slower seismic speeds. These density differences are considered to involve different concentrations of light elements, like hydrogen or silicon.
But an increase in the concentration of a single light element would make the speed go up while the density goes down, making it hard to reconcile the seismic observation and the dynamic stability of the E prime layer.
Increasing the concentration of one light element while decreasing the concentration of another has been put forward as a possible explanation. However, scientists were not aware of such an exchange process.
The team used laser-heated diamond-anvil cells to mimic pressure-temperature conditions at the core-mantle boundary.
They showed that water that was subducted into Earth’s core could react chemically with the materials there to turn the outer core into a hydrogen-rich film and disperse silica crystals that rise and join the mantle.
The layer of hydrogen-rich, silicon-poor material that forms at the top of the core would have less density and less speed, matching seismic wave observations.
The altered core film might in turn have a significant impact on the deep water cycle, and the team says their results suggest a more complex global water cycle than we thought.
“This discovery, along with our previous observation of diamonds forming from water reacting with carbon in iron liquid under extreme pressure,” Shim says, “points to a far more dynamic core-mantle interaction, suggesting substantial material exchange.”
The study has been published in Nature Geoscience.
