A few decades ago, seismologists identified a thin layer with a lower velocity in Earth’s metallic liquid core. The origin of this layer, known as the E’ layer, has been a mystery. Now, geoscientists have revealed that water from the Earth’s surface can penetrate deep into the planet, altering the composition of the outermost region of the core and creating the E’ layer.
Kim et al. suggest that chemical exchange between the core and mantle over gigayears of deep transport of water may have contributed to the formation of the putative E’ layer. Image credit: Yonsei University.
“Seismic studies have shown that the density of the Earth’s outer core is around 10% lower than that of pure iron-nickel alloy,” said Arizona State University scientist Dan Shim and colleagues.
“In order to explain such a density deficit, substantial amounts of light elements are required in the core.”
“Although the exact make-up of the light elements in the Earth’s core is still debated, silicon has been proposed as an important candidate.”
“However, the density deficit in the Earth’s core cannot be explained by silicon alone.”
According to the new study, over billions of years, surface water has been transported deep into the Earth by descending, or subducted, tectonic plates.
Upon reaching the core-mantle boundary, about 2,900 km (1,800 miles) below the surface, this water triggers a profound chemical interaction, altering the core’s structure.
In high-temperature and -pressure laser-heated diamond-anvil cell experiments, Dr. Shim and co-authors demonstrated that subducted water chemically reacts with core materials.
This reaction forms a hydrogen-rich, silicon-depleted layer, altering the topmost outer core region into a film-like structure.
Additionally, the reaction generates silica crystals that rise and integrate into the mantle.
This modified liquid metallic layer is predicted to be less dense, with reduced seismic velocities, in alignment with anomalous characteristics mapped by seismologists.
At the interface where subducting water meets the core, a chemical exchange occurs to form a hydrogen-rich layer in the topmost outer core and dense silica in the bottom of the mantle.
“For years, it has been believed that material exchange between Earth’s core and mantle is small,” Dr. Shim said.
“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.”
“This discovery, along with our previous observation of diamonds forming from water reacting with carbon in iron liquid under extreme pressure, points to a far more dynamic core-mantle interaction, suggesting substantial material exchange.”
The findings appear in the journal Nature Geoscience.
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T. Kim et al. A hydrogen-enriched layer in the topmost outer core sourced from deeply subducted water. Nat. Geosci, published online November 13, 2023; doi: 10.1038/s41561-023-01324-x
