Scientists have finally solved the mystery of the E’ layer that seismologists see on the surface of Earth’s outer core. Water brought by subduction to the base of the mantle may explain the nature of this layer, which reflects the strong relationship that exists between the surface and depth.

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The core remains the most mysterious terrestrial mantle. What we know about its composition, structure and dynamics comes only from indirect data, seismic studies or modelling. However, the Earth’s core plays a major role in the overall dynamics of the planet, not only by generating valuable magnetic fields but also by participating in the engine of plate tectonics.

The exact nature of the Earth’s core is still poorly defined

Long seen as completely separate and independent, the processes that control surface dynamics and influence the world’s deepest levels actually seem to be much more connected than we thought. The interface between the liquid outer core and the mantle, located at a depth of 2,891 kilometers, may also play a major role in terrestrial dynamics, particularly through intensive exchanges of heat and chemical elements. However, the processes occurring at this interface are still largely fragmented, as is its precise nature.

a stratified outer core

Since 1936, we have known, thanks to seismology, that the Earth’s core is divided into two large parts: a solid inner core and a liquid outer core. As the accuracy of measurements increased over time, seismologists identified the existence of finer stratification within the outer core. Thus two good levels named E’ and F’ were defined. The E’ layer occupies the outermost part of the liquid core, marking the interface with the mantle, while the F’ layer is at the base of the outer core, marking the interface with the solid inner core. However, the nature of these interfaces remains highly debated.

New results, published in the journal nature geologyHowever, E’ may shed light on the nature and origin of the layer. Water brought by subduction zones also appears to play an important role.

Sea water was transported to the base of the mantle.

Subduction zones represent places on the globe where oceanic plates slowly subduct into the mantle. The layer in question, rich in water, would thus “sink” deeper into the Earth, until potentially reaching the core-mantle interface. Therefore initially the water coming from the oceans will hydrate the lower part of the mantle. But what effect might this hydration have on the rocks of the outer core?

The surface of the outer core is covered with water

So scientists conducted laboratory experiments using a diamond anvil to reproduce the very high pressures and temperatures present at the interface between the core and the mantle. Their results indicate that water brought by subduction reacts with silicon present in the outer core, producing silica crystals that will become incorporated into the structure of the upper mantle.

The result is that the outer core finds itself coated by a fine liquid layer whose composition has been modified: it is rich in hydrogen and low in silicon. Its reduced density compared to the rest of the outer core plus the fact that seismic waves propagate more slowly there means that this alteration surface meets the seismic characteristics defined for layer E’ .

This discovery thus makes it possible to better control the global water cycle within the Earth system. It also shows the powerful connection that exists between the world’s surface and much deeper levels.

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