An onion core: Researchers find hints of a multilayered centre of the Earth

An international research team may have found an explanation for seismic anomalies, i.e., noticeable deviations in the behaviour of earthquake waves, in the Earth’s inner core. A team led by scientists from the University of Münster and comprising team members from Deutsches Elektronen-Synchrotron (DESY), the University of Lille, and the European Synchrotron Radiation Facility (ESRF) simulated high-pressure and temperature conditions of the deep Earth interior. They studied the plastic-elastic behaviour of silicon and carbon alloys of iron using the PETRA III light source at DESY in Hamburg. The findings of these experiments, when extrapolated to conditions of the Earth’s inner core, reveal that onion-like layering induced through the carbon–silicon mixture added to pure iron can explain seismic anomalies detected in the Earth’s core. The researchers published their findings in the journal Nature Communications.

The Earth's core is predominantly composed of iron. However, minor concentrations of lighter elements that form iron alloys, such as silicon, carbon, and oxygen, are also present in the core. While the outer core is liquid, the inner core is solid and thought to consist of these iron alloys. Seismologists have observed in the inner core that compressional sound waves, which are created through events such as earthquakes, travel 3-4% faster parallel to the Earth’s rotation axis compared to those traveling in the equatorial plane. These anomalies in seismic wave velocity, called anisotropy, have different magnitudes when comparing the outer and inner parts of the inner core.

“There have been several hypotheses for the origin of these anisotropies”, states Prof. Carmen Sanchez-Valle from the Institute of Mineralogy at the University of Münster. A possible explanation has been the emergence of a phenomenon called lattice-preferred orientation (LPO), wherein the crystals of the alloys change orientation due to thermal convection patterns or an intrinsic preferential growth. “Unfortunately, there are very little experimental data on how such LPO might look like in the Earth’s iron core, and there are no data on the LPO of iron–silicon–carbon alloy mixtures. Thus, we set out to study the combined effect of silicon and carbon on the deformation behavior of iron,” says Sanchez-Valle.

In order to study the deformation behavior of iron alloys the team around Sanchez-Valle and the team leader Ilya Kupenko synthesized iron-silicon-carbon alloys. At the Extreme Conditions Science beamline P02.2 at PETRA III, the team loaded the alloys into an extreme-states–generation device called a diamond anvil cell, which consists of 2 opposing diamonds with flattened tips that compress the sample to extraordinary pressure and high-temperature conditions. For this experiment, the alloy was initially compressed, then heated to more than 820°C, then further compressed to around a million times atmospheric pressure.

The scientists observed from the X-ray analysis at PETRA III that during the compression of the polycrystalline iron alloy, the sample developed LPO. “We were able to decode the LPO via X-ray diffraction perpendicular to the compression axis,” explains first author Efim Kolesnikov, who was a doctoral student at the University of Münster when the experiment took place. The X-ray method used on the sample, known as radial diffraction, has been extensively developed at P02.2. “The diffraction patterns were analyzed after the experiment to derive plastic properties – specifically, yield strength & viscosity – of the iron–silicon-–carbon alloys, which were further modeled through theory to extrapolate them to inner core conditions.”

From the plasticity properties, the team calculated the difference between compressional sound velocities in the iron–silicon–carbon alloy at the inner core conditions and compared them to that of the pure iron. The result: the differences in anisotropy might be linked to a compositional gradient, since the percentage of iron content increases with increasing core depth. “This matches the different anisotropies of velocities observed in the seismic profiles,” says team leader Kupenko.

The work was funded by the German Research Foundation (DFG) and the European Council ERC grant 'LECOR'.

This is a joint press release from the University of Münster and DESY.