When seawater changes the composition of the Earth’s core!

Scientists may have finally solved the mystery of the E’ layer that seismologists observe on the surface of Earth’s outer core. The water brought to the base of the mantle by subductions could explain the nature of this layer and illustrate the strong connections that exist between the surface and the depth.

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The core remains the most mysterious earthly shell. The knowledge we have about its structure, composition and dynamics only comes from indirect data, from seismological studies or modelling. However, the Earth’s core plays an important role in the planet’s overall dynamics, obviously by generating the precious magnetic field, but also by participating in the engine of plate tectonics.

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

The processes that control surface dynamics and the processes that influence the deepest levels of the world have long been seen as completely separate and independent, and indeed appear much more closely linked than we thought. The interface between the liquid outer core and the mantle, located at a depth of 2,891 kilometers, could also play an important role in terrestrial dynamics, especially through intense exchanges of heat and chemical elements. However, the processes taking place at this interface are still largely fragmentary, as is their exact nature.

A layered outer core

Since 1936, we have known, thanks to seismology, that the Earth’s core is divided into two major parts: a solid inner core and a liquid outer core. As the accuracy of the measurements increased over time, seismologists subsequently identified the existence of finer stratification within the outer core. Two fine levels were thus defined, named E’ and F’. The E’ layer occupies the outer part of the liquid core and marks the interface with the mantle, while the F’ layer occupies the base of the outer core and marks the interface with the solid inner core. There is still much debate about the nature of these interfaces.

New results, published in the journal Natural Geosciences, however, could shed light on the nature and origin of the E’ layer. The water supplied by the subduction zones also appears to play an important role.

Seawater is transported to the base of the mantle

Subduction zones represent places on the globe where oceanic plates are gradually swallowed up into the mantle. The crust in question, which is rich in water, will therefore “sink” into the depths of the Earth until it possibly reaches the core-mantle interface. Water initially coming from the oceans will therefore hydrate the lower part of the mantle. But what effect could this hydration have on the rocks of the outer core?

The surface of the outer core has been changed by water

The scientists therefore conducted laboratory experiments in which they reproduced the very high pressures and temperatures prevailing at the interface between the core and the mantle, using a diamond anvil. Their results indicate that the water brought in by the subductions reacts with the silicon present in the outer core, generating silica crystals that will join the composition of the overlying mantle.

The result is that the outer core is covered with a fine liquid level whose composition has changed: it is rich in hydrogen and depleted in silicon. The reduced density compared to the rest of the outer core and the fact that seismic waves propagate more slowly there mean that this change surface conforms to the seismic characteristics defined for layer E’.

This discovery therefore makes it possible to better limit the global water cycle within the Earth system. It also illustrates the powerful connection that exists between the surface and the very deep levels of the globe.

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