High-pressure and -temperature experiments may explain the reduction of seismic waves velocities at the topmost portion of the Earth’s core. Using the synchrotron facilities PETRA III at DESY and the Advanced Photon Source (APS) at the Argonne National Laboratories (ANL) in the US, an international team of scientists from South Korea, the US and Germany simulated pressure and temperature conditions prevailing at the top of the Earth’s core. They found that the breakdown of hydrous phases formed at the Earth’s surface might cause the formation of a hydrogen-enriched topmost outer core layer. This layer displays a lower density than the rest of the core. Their findings have important implications for the interpretation of seismic wave traveling in this region of the Earth and also provide clues to the fate of surface water, as the researchers, led by Yongjae Lee from Yonsei University (South Korea), report in the journal Nature Geoscience.

The Earth’s core is predominately composed of iron alloys with minor concentrations of lighter elements such as Silicon (Si), Oxygen (O), Sulphur (S), Hydrogen (H), etc. While the inner iron core is solid, the outer core is liquid and in direct contact with the mantle which is made of silicate minerals that might contain deeply transported water. At the interface between the core and mantle, therefore, reactions are likely to take place and are creating regions of significant heterogeneity. “One of our preconceptions is that the boundary between the metallic core and silicate mantle would be chemically separated since their formation”, explains team leader Yongjae Lee. “All of the Earth’s components are, however, intimately interconnected, exchanging materials and energy as an integrated system. The core and mantle boundary should not be an exception.”

“We simulated pressure and temperature conditions to be expected across the core and mantle boundary by using a high-pressure cell, called Diamond Anvil Cell, to create pressure of more than 1 350 000 atm. In addition, infrared lasers that shine through the opposing diamonds on to the compressed sample induced temperatures of 3000-4000 Kelvin,” says the lead author Taehyun Kim from Yonsei University. In the high-pressure cell a piece of iron-silicon alloy was surrounded by H₂O carrying minerals. When these samples were heated up to the target temperatures at pressures, the very small and intense X-ray beam from the extreme condition’s beamline P02.2 at PETRA III and the beamline GSECARS at APS were used to observe an exchange reaction between the iron alloy (FeSi) and the H₂O bearing phases to form iron hydrite (FeH_x) and dense silica (SiO₂).

“Indeed, seismic models of the Earth based on earthquake waves travelling through the planet, support the presence of the so-called E’ layer as a distinguishable thin veneer of a few hundreds of kilometres thickness in the topmost portion of the outer core. The low-velocity layer E′ can now be explained by the enrichment of hydrogen and extraction of silicon”, explains DESY scientist Hanns-Peter Liermann, head of PETRA III beamline P02.2. “Further research will be necessary to understand the complex nature of our planet from the surface to the core. However, our results demonstrate the distinct possibility that active and reactive H₂O cycles could be extended to the deepest region which needs to be considered in understanding the dynamics and evolution of our planet,” explains Lee.

Reference
Kim, T., O’Rourke, J.G., Lee, J. et al. A hydrogen-enriched layer in the topmost outer core sourced from deeply subducted water. Nat. Geosci. (2023). DOI: 10.1038/s41561-023-01324-x