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

Abstract

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.

References

  1. Poupinet, G., Pillet, R. & Souriau, A. Possible heterogeneity of the Earth’s core deduced from PKIKP travel times. Nature 305, 204–206 (1983)

    ADS  Article  Google Scholar 

  2. Tanaka, S. & Hamaguchi, H. Degree one heterogeneity and hemispherical variation of anisotropy in the inner core from PKP(BC)-PKP(DF) times. J. Geophys. Res. 102, 2925–2938 (1997)

    ADS  Article  Google Scholar 

  3. Creager, K. C. Large-scale variations in inner core anisotropy. J. Geophys. Res. 104, 309–314 (1999)

    Article  Google Scholar 

  4. Garcia, R. & Souriau, A. Inner core anisotropy and heterogeneity level. Geophys. Res. Lett. 27, 3121–3124 (2000)

    ADS  Article  Google Scholar 

  5. Niu, F. & Wen, L. Hemispherical variations in seismic velocity at the top of the Earth’s inner core. Nature 410, 1081–1084 (2001)

    ADS  CAS  Article  Google Scholar 

  6. Yu, W.-c., Wen, L. & Niu, F. Seismic velocity structure in the earth's outer core. J. Geophys. Res. 110, B02302, 10.1029/2003JB002928 (2005)

    ADS  Article  Google Scholar 

  7. Souriau, A. & Poupinet, G. The velocity profile at the base of the liquid core from PKP(BC+Cdiff) data: an argument in favor of radial inhomogeneity. Geophys. Res. Lett. 18, 2023–2026 (1991)

    ADS  Article  Google Scholar 

  8. Kennett, B. L. N. & Engdahl, E. R. Traveltimes for global earthquake location and phase identification. Geophys. J. Int. 105, 429–465 (1991)

    ADS  Article  Google Scholar 

  9. Souriau, A. & Roudil, P. Attenuation in the uppermost inner core from broad-band GEOSCOPE PKP data. Geophys. J. Int. 123, 572–587 (1995)

    ADS  Article  Google Scholar 

  10. Kennett, B. L. N., Engdahl, E. R. & Buland, R. Constraints on seismic velocities in the earth from traveltimes. Geophys. J. Int. 122, 108–124 (1995)

    ADS  Article  Google Scholar 

  11. Song, X. & Helmberger, D. V. A. P wave velocity model of Earth's core. J. Geophys. Res. 100, 9817–9830 (1995)

    ADS  Article  Google Scholar 

  12. Zou, Z., Koper, K. D. & Cormier, V. F. The structure of the base of the outer core inferred from seismic waves diffracted around the inner core. J. Geophys. Res. 113, B05314, 10.1029/2007JB005316 (2008)

    ADS  Article  Google Scholar 

  13. Gubbins, D., Masters, G. & Nimmo, F. A thermochemical boundary layer at the base of Earth's outer core and independent estimate of core heat flux. Geophys. J. Int. 174, 1007–1018 (2008)

    ADS  Article  Google Scholar 

  14. Dziewonski, A. M. & Anderson, D. L. Preliminary reference Earth model. Phys. Earth Planet. Inter. 25, 297–356 (1981)

    ADS  Article  Google Scholar 

  15. Calvet, M., Chevrot, S. & Souriau, A. P-wave propagation in transversely isotropic media: II. Application to inner core anisotropy: effect of data averaging, parametrization and a priori information. Phys. Earth Planet. Inter. 156, 21–40 (2006)

    ADS  Article  Google Scholar 

  16. Loper, D. & Roberts, P. A study of conditions at the inner core boundary of the Earth. Phys. Earth Planet. Inter. 24, 302–307 (1981)

    ADS  Article  Google Scholar 

  17. Dziewonski, A. M. & Anderson, D. L. Preliminary reference Earth model. Phys. Earth Planet. Inter. 25, 297–356 (1981)

    ADS  Article  Google Scholar 

  18. Dalziel, S. B., Hughes, G. O. & Sutherland, B. R. Whole-field density measurements by ‘synthetic schlieren’. Exp. Fluids 28, 322–335 (2000)

    Article  Google Scholar 

  19. Gostiaux, L. & Dauxois, T. Laboratory experiments on the generation of internal tidal beams over steep slopes. Phys. Fluids 19, 028102, 10.1063/1.2472511 (2007)

    ADS  CAS  Article  MATH  Google Scholar 

  20. Stacey, F. D. & Davis, P. M. Physics of the Earth Ch. 19 (Cambridge University Press, 2008)

    Book  Google Scholar 

  21. Vočadlo, L. in Treatise on Geophysics (ed. Schubert, G.) Vol. 2, 91–120 (2007)

    Book  Google Scholar 

  22. Alfè, D., Price, G. D. & Gillan, M. J. Iron under Earth's core conditions: liquid-state thermodynamics and high-pressure melting curve from ab initio calculations. Phys. Rev. B 65, 165118, 10.1103/PhysRevB.65.165118 (2002)

    ADS  CAS  Article  Google Scholar 

  23. Poirier, J.-P. Physical properties of the Earth's core. C. R. Acad. Sci. 318, 341–350 (1994)

    CAS  Google Scholar 

  24. Poirier, J.-P. & Shankland, T. J. Dislocation melting of iron and the temperature of the inner core boundary, revisited. Geophys. J. Int. 115, 147–151 (1993)

    ADS  Article  Google Scholar 

  25. Anderson, O. L. & Duba, A. Experimental melting curve of iron revisited. J. Geophys. Res. 102, 22659–22670 (1997)

    ADS  Article  Google Scholar 

  26. Masters, G., Jordan, T. H. & Silver, P. G. & Gilbert, F. Aspherical Earth structure from fundamental spheroidal-mode data. Nature 298, 609–613 (1982)

    ADS  Article  Google Scholar 

  27. Calvet, M. & Margerin, L. Constraints on grain size and stable iron phases in the uppermost inner core from multiple scattering modeling of seismic velocity and attenuation. Earth Planet. Sci. Lett. 267, 200–212 (2008)

    ADS  CAS  Article  Google Scholar 

  28. Buffett, B. A. Onset and orientation of convection in the inner core. Geophys. J. Int. 179, 711–719 (2009)

    ADS  Article  Google Scholar 

  29. Deguen, R. & Cardin, P. Tectonic history of the Earth’s inner core preserved in its seismic structure. Nature Geosci. 2, 419–422 (2009)

    ADS  CAS  Article  Google Scholar 

  30. Labrosse, S. Thermal and magnetic evolution of the earth's core. Phys. Earth Planet. Inter. 140, 127–143 (2003)

    ADS  Article  Google Scholar 

  31. Tritton, D. J. Physical Fluid Dynamics 1–536 (Oxford, Clarendon Press, 1988)

    Google Scholar 

  32. Gubbins, D., Alfè, D., Masters, G., Price, G. D. & Gillan, M. Gross thermodynamics of two-component core convection. Geophys. J. Int. 157, 1407–1414 (2004)

    ADS  CAS  Article  Google Scholar 

  33. Nimmo, F. in Treatise on Geophysics (ed. Schubert, G.) Vol. 2, 31–65, 2007)

    Book  Google Scholar 

  34. Jeanloz, R. & Wenk, H.-R. Convection and anisotropy of the inner core. Geophys. Res. Lett. 15, 72–75 (1988)

    ADS  Article  Google Scholar 

  35. Weber, P. & Machetel, P. Convection within the inner-core and thermal implications. Geophys. Res. Lett. 19, 2107–2110 (1992)

    ADS  Article  Google Scholar 

  36. Wenk, H.-R., Baumgardner, J. R., Lebensohn, R. A. & Tomé, C. N. A convection model to explain anisotropy of the inner core. J. Geophys. Res. 105, 5663–5678 (2000)

    ADS  Article  Google Scholar 

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Acknowledgements

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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Contributions

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.

Corresponding author

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). https://doi.org/10.1038/nature09257

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