Geophysicists have elucidated the genesis of a subtle stratum within Earth’s molten metallic core, characterized by diminished wave propagation velocity. This enigmatic zone, designated the E’ layer, has long baffled researchers. Emerging investigations now posit that H₂O originating from our planet’s surface can infiltrate profound depths, thereby instigating compositional modifications in the core’s uppermost stratum and consequently giving rise to the E’ layer.
Kim et al. propose that geological transformations within the mantle, influenced by extensive H₂O transit over eons, may have played a role in the formation of the theorized E’ layer. Image courtesy of Yonsei University.
“Seismic assessments have indicated that the density of Earth’s outer core is approximately 10% less than that of an unadulterated iron-nickel admixture,” remarked Dan Shim, a scientist affiliated with Arizona State University, alongside his collaborators.
“To account for such a deficit in density, a considerable quantity of lighter elements is posited to be present within the core.”
“While the precise elemental composition of these lighter constituents in Earth’s core remains a subject of ongoing discourse, silicon has been put forth as a significant candidate.”
“Nevertheless, the observed reduction in density within Earth’s core cannot be solely attributed to the presence of silicon.”
According to the latest scientific inquiry, over spans of billions of years, surface H₂O has been conveyed to considerable subterranean depths via descending, or subducted, lithospheric plates.
Upon reaching the boundary between the core and the mantle, situated approximately 2,900 kilometers (1,800 miles) beneath the surface, this H₂O initiates a profound chemical interaction, thereby restructuring the core’s architecture.
Through high-temperature and high-pressure experiments employing a laser-heated diamond-anvil apparatus, Dr. Shim and his colleagues successfully demonstrated that subducted H₂O undergoes a chemical reaction with materials present in the core.
This reaction culminates in the formation of a hydrogen-abundant, silicon-deficient stratum, which subsequently remodels the uppermost region of the outer core into a film-like morphology.
Furthermore, the resultant reaction precipitates silicate crystals, which then ascend and become integrated into the mantle.
This modified liquid metallic layer is projected to exhibit lower density and diminished seismic wave velocities, characteristics that align with anomalous attributes previously delineated by seismologists.
At the juncture where subducting H₂O encounters the core, a chemical exchange transpires, leading to the generation of a hydrogen-rich layer in the topmost outer core and dense silicates within the lower mantle.
“For an extended period, the prevailing belief was that material exchange between Earth’s core and mantle was negligible,” stated Dr. Shim.
“However, our recent experiments conducted under extreme pressures have unveiled a divergent narrative.”
“We ascertained that upon reaching the core-mantle boundary, H₂O reacts with silicon within the core, precipitating silica.”
“This revelation, coupled with our prior observation of diamond formation from the reaction of H₂O with carbon in molten iron under immense pressure, suggests a far more intricate and dynamic interaction between the core and mantle, indicating substantial material transference.”
The discoveries have been published in the esteemed 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
