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Melting-induced stratification above the Earth’s inner core due to convective translation


In addition to its global North–South anisotropy1, there are two other enigmatic seismological observations related to the Earth’s inner core: asymmetry between its eastern and western hemispheres2,3,4,5,6 and the presence of a layer of reduced seismic velocity at the base of the outer core6,7,8,9,10,11,12. This 250-km-thick layer has been interpreted as a stably stratified region of reduced composition in light elements13. Here we show that this layer can be generated by simultaneous crystallization and melting at the surface of the inner core, and that a translational mode of thermal convection in the inner core can produce enough melting and crystallization on each hemisphere respectively for the dense layer to develop. The dynamical model we propose introduces a clear asymmetry between a melting and a crystallizing hemisphere which forms a basis for also explaining the East–West asymmetry. The present translation rate is found to be typically 100 million years for the inner core to be entirely renewed, which is one to two orders of magnitude faster than the growth rate of the inner core’s radius. The resulting strong asymmetry of buoyancy flux caused by light elements is anticipated to have an impact on the dynamics of the outer core and on the geodynamo.

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Figure 1: Visualization of the growth of a dense layer in an experimental run.
Figure 2: Evolution of the concentration profile during the growth of a dense layer.
Figure 3: A schematic representation of the translational convective mode.
Figure 4: Thermal departure from the adiabat due to the displacement of the inner core and heat transfer at the ICB.
Figure 5: Growth rate of the radius of the inner core and uniform convective velocity as functions of the inner-core radius.


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This work has benefited from discussions during the CNRS-INSU SEDIT meetings. We thank M. Bergman for discussions regarding inner-core crystallization. The LGIT and the ANR (Agence Nationale de la Recherche) (ANR-08-BLAN-0234-01) have provided financial support for the experiments.

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Authors and Affiliations



M.M., R.D. and T.A. ran and analysed the experiments. T.A. designed the experimental study and built the dynamical model. R.D. and T.A. worked out the thermal conditions on the ICB and assessed the geophysical relevance of the dynamical model. R.D. computed the different scenarios of thermal history. R.D., T.A. and M.M. applied the experimental results to the geophysical context. T.A. and R.D. wrote the paper.

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Correspondence to Thierry Alboussière.

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Alboussière, T., Deguen, R. & Melzani, M. Melting-induced stratification above the Earth’s inner core due to convective translation. Nature 466, 744–747 (2010).

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