Skip to main content

Thank you for visiting You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

Outer-core compositional stratification from observed core wave speed profiles



Light elements must be present in the nearly pure iron core of the Earth to match the remotely observed properties of the outer and inner cores1,2. Crystallization of the inner core excludes light elements from the solid, concentrating them in liquid near the inner-core boundary that potentially rises and collects at the top of the core3, and this may have a seismically observable signal. Here we present array-based observations of seismic waves sensitive to this part of the core whose wave speeds require there to be radial compositional variation in the topmost 300 km of the outer core. The velocity profile significantly departs from that of compression of a homogeneous liquid. Total light-element enrichment is up to five weight per cent at the top of the core if modelled in the Fe–O–S system. The stratification suggests the existence of a subadiabatic temperature gradient at the top of the outer core.

This is a preview of subscription content, access via your institution

Relevant articles

Open Access articles citing this article.

Access options

Buy article

Get time limited or full article access on ReadCube.


All prices are NET prices.

Figure 1: Experiment geometry and paths taken by the seismic waves through the Earth.
Figure 2: Data, velocity profile in the outer core and self-compression profile using the velocity profile.
Figure 3: Compositional variation in the Fe–O–S liquids that match the observed wave speed profile and core density constraints.


  1. Birch, F. Elasticity and constitution of the Earth’s interior. J. Geophys. Res. 57, 227–286 (1952)

    CAS  Google Scholar 

  2. Stevenson, D. J. Models of the Earth’s core. Science 214, 611–619 (1981)

    CAS  Google Scholar 

  3. Fearn, D. R. & Loper, D. E. Compositional convection and stratification of Earth’s core. Nature 289, 393–394 (1981)

    Google Scholar 

  4. Wood, B. J., Walter, M. J. & Wade, J. Accretion of the Earth and segregation of its core. Nature 441, 825–833 (2006)

    CAS  Google Scholar 

  5. Rubie, D. C., Gessmann, C. K. & Frost, D. J. Partitioning of oxygen during core formation on the Earth and Mars. Nature 429, 58–61 (2004)

    CAS  Google Scholar 

  6. Ozawa, H. et al. Chemical equilibrium between ferropericlase and molten iron to 134 GPa and implications for iron content at the bottom of the mantle. Geophys. Res. Lett. 35, L05308 (2008)

    Google Scholar 

  7. Frost, D. J. et al. Partitioning of oxygen between the Earth’s mantle and core. J. Geophys. Res. 115, B02202 (2010)

    Google Scholar 

  8. Hilty, D. C. & Crafts, W. Liquidus surface of the Fe-S-O system. J. Metals (Trans. AIME) 4, . 1307–1312 (1952)

  9. Lay, T. & Young, C. The stably-stratified outermost core revisited. Geophys. Res. Lett. 17, 2001–2004 (1990)

    Google Scholar 

  10. Garnero, E. J., Helmberger, D. V. & Grand, S. P. Constraining outermost core velocity with SmKS waves. Geophys. Res. Lett. 20, 2463–2466 (1993)

    Google Scholar 

  11. Tanaka, S. Possibility of a low P-wave velocity layer in the outermost core from global SmKS waveforms. Earth Planet. Sci. Lett. 259, 486–499 (2007)

    CAS  Google Scholar 

  12. Alexandrakis, C. & Eaton, D. W. Precise seismic-wave velocity atop Earth’s core: no evidence for outer-core stratification. Phys. Earth Planet. Inter. 180, 59–65 (2010)

    CAS  Google Scholar 

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

    Google Scholar 

  14. Helffrich, G. & Kaneshima, S. Seismological constraints on core composition from Fe-O-S liquid immiscibility. Science 306, 2239–2242 (2004)

    CAS  Google Scholar 

  15. Masters, G. & Gubbins, D. On the resolution of density within the Earth. Phys. Earth Planet. Inter. 140, 159–167 (2003)

    Google Scholar 

  16. Loper, D. E. A model of the dynamical structure of the Earth’s outer core. Phys. Earth Planet. Inter. 117, 179–196 (2000)

    Google Scholar 

  17. Braginsky, S. I. MAC-oscillations of the hidden ocean of the core. J. Geomag. Geoelectr. 45, 1517–1538 (1993)

    Google Scholar 

  18. Gubbins, D., Thomson, C. & Whaler, K. Stable regions in the Earth’s liquid core. Geophys. J. R. Astron. Soc. 68, 241–251 (1982)

    Google Scholar 

  19. Lister, J. R. & Buffett, B. A. Stratification of the outer core at the core-mantle boundary. Phys. Earth Planet. Inter. 105, 5–19 (1998)

    CAS  Google Scholar 

  20. Rose, L. A. & Brenan, J. M. Wetting properties of Fe-Ni-Co-Cu-O-S melts against olivine: implications for sulfide melt mobility. Econ. Geol. 96, 145–157 (2001)

    CAS  Google Scholar 

  21. Usselman, T. M. Experimental approach to the state of the core: part I. The liquidus relations of the Fe-rich portion of the Fe-Ni-S system from 30 to 100 kb. Am. J. Sci. 275, 278–290 (1975)

    CAS  Google Scholar 

  22. Kilburn, M. R. & Wood, B. J. Metal-silicate partitioning and the incompatibility of S and Si during core formation. Earth Planet. Sci. Lett. 152, 139–148 (1997)

    CAS  Google Scholar 

  23. Buffett, B. A. & Seagle, C. T. Stratification of the top of the core due to chemical interactions with the mantle. J. Geophys. Res. 115, B04407 (2010)

    Google Scholar 

  24. Dobson, D. Self-diffusion in liquid Fe at high pressure. Phys. Earth Planet. Inter. 130, 271–284 (2002)

    CAS  Google Scholar 

  25. Garmany, J., Orcutt, J. A. & Parker, R. L. Travel time inversion: a geometrical approach. J. Geophys. Res. 84, 3615–3622 (1979)

    Google Scholar 

  26. Masters, G., Laske, G., Bolton, H. & Dziewonski, A. in Earth’s Deep Interior: Mineral Physics and Tomography from the Atomic to the Global Scale (eds Karato, S.-I., Forte, A. M., Liebermann, R. C., Masters, G. & Stixrude, L.) 63–87 (Geophys. Monogr. 117, American Geophysical Union, 2000)

    Google Scholar 

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

    Google Scholar 

  28. Morelli, A. & Dziewonski, A. M. Body wave traveltimes and a spherically symmetric P- and S-wave velocity model. Geophys. J. Int. 112, 178–194 (1993)

    Google Scholar 

  29. Kennett, B. L. N., Engdahl, E. R. & Buland, R. Constraints on seismic velocities in the Earth from traveltimes. Geophys. J. Int. 126, 108–124 (1995)

    Google Scholar 

  30. Alfé, 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 (2002)

    Google Scholar 

  31. Kind, R. Extensions of the reflectivity method for a buried source. J. Geophys. 45, 373–380 (1979)

    Google Scholar 

  32. Eaton, D. W. & Kendall, J.-M. Improving seismic resolution of outermost core structure by multichannel analysis and deconvolution of broadband SmKS phases. Phys. Earth Planet. Inter. 155, 104–119 (2006)

    Google Scholar 

  33. Boehler, R. Melting of the Fe-FeO and the Fe-FeS systems at high-pressure - constraints on core temperatures. Earth Planet. Sci. Lett. 111, 217–227 (1992)

    CAS  Google Scholar 

  34. Boehler, R. Temperatures in the Earth’s core from melting-point measurements of iron at high static pressures. Nature 363, 534–536 (1993)

    CAS  Google Scholar 

  35. Svendsen, B., Anderson, W. W., Ahrens, T. J. & Bass, J. D. Ideal Fe-FeS, Fe-FeO phase relations and Earth’s core. Phys. Earth Planet. Inter. 55, 154–186 (1989)

    CAS  Google Scholar 

  36. Fei, Y., Saxena, S. K. & Navrotsky, A. Internally consistent thermodynamic data and equilibrium phase relations for compounds in the system MgO-SiO2 at high pressure and high temperature. J. Geophys. Res. 95, 6915–6928 (1990)

    CAS  Google Scholar 

  37. Anderson, O. L., Isaak, D. & Oda, H. High-temperature elastic constant data on minerals relevant to geophysics. Rev. Geophys. 30, 57–90 (1992)

    Google Scholar 

  38. Bina, C. R. & Helffrich, G. Calculation of elastic properties from thermodynamic equation of state principles. Annu. Rev. Earth Planet. Sci. 20, 527–552 (1992)

    Google Scholar 

  39. Morard, G. et al. Structure of eutectic Fe-FeS melts to pressures up to 17 GPa: implications for planetary cores. Earth Planet. Sci. Lett. 263, 128–139 (2007)

    CAS  Google Scholar 

  40. Anderson, O. L., Oda, H., Chopelas, A. & Isaak, D. A thermodynamic theory of the Grüneisen ratio at extreme conditions: MgO as an example. Phys. Chem. Miner. 19, 369–380 (1993)

    CAS  Google Scholar 

Download references


This work was supported by an ERI Visiting Professorship to G.H. We thank A. Jackson for comments and O. Lord for experimental references. Data were provided by the ORFEUS Data Center, de Bilt, the Netherlands, the J-Array data centre, ERI, Tokyo, Japan, and by NIED, Tsukuba, Japan.

Author information

Authors and Affiliations



Both authors contributed equally to the project.

Corresponding author

Correspondence to George Helffrich.

Ethics declarations

Competing interests

The authors declare no competing financial interests.

Supplementary information

Supplementary Information

The file contains Supplementary Text and Data, Supplementary Tables 1- 4, Supplementary Figures 1-6 with legends and additional references. (PDF 3454 kb)

PowerPoint slides

Rights and permissions

Reprints and Permissions

About this article

Cite this article

Helffrich, G., Kaneshima, S. Outer-core compositional stratification from observed core wave speed profiles. Nature 468, 807–810 (2010).

Download citation

  • Received:

  • Accepted:

  • Published:

  • Issue Date:

  • DOI:

This article is cited by


By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.


Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing