title: Hidden carbon in Earth's inner core revealed by shear softening in dense Fe7C3.
abstract: Earth's inner core is known to consist of crystalline iron alloyed with a small amount of nickel and lighter elements, but the shear wave (S wave) travels through the inner core at about half the speed expected for most iron-rich alloys under relevant pressures. The anomalously low S-wave velocity (vS) has been attributed to the presence of liquid, hence questioning the solidity of the inner core. Here we report new experimental data up to core pressures on iron carbide Fe7C3, a candidate component of the inner core, showing that its sound velocities dropped significantly near the end of a pressure-induced spin-pairing transition, which took place gradually between 10 GPa and 53 GPa. Following the transition, the sound velocities increased with density at an exceptionally low rate. Extrapolating the data to the inner core pressure and accounting for the temperature effect, we found that low-spin Fe7C3 can reproduce the observed vS of the inner core, thus eliminating the need to invoke partial melting or a postulated large temperature effect. The model of a carbon-rich inner core may be consistent with existing constraints on the Earth's carbon budget and would imply that as much as two thirds of the planet's carbon is hidden in its center sphere.
authors.affiliation: Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, MI 48109; Department of Geology, University of Illinois at Urbana-Champaign, Urbana, IL 61801; Hawaii Institute of Geophysics and Planetology, University of Hawaii at Manoa, Honolulu, HI 96822; binchen@hawaii.edu jackieli@umich.edu.
authors.forename: Bin
authors.identifier: http://orcid.org/0000-0001-8314-9991
authors.initials: B
authors.lastname: Chen
authors.affiliation: Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, MI 48109;
authors.forename: Zeyu
authors.initials: Z
authors.lastname: Li
authors.affiliation: Seismological Laboratory, California Institute of Technology, Pasadena, CA 91125;
authors.forename: Dongzhou
authors.initials: D
authors.lastname: Zhang
authors.affiliation: Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, MI 48109;
authors.forename: Jiachao
authors.initials: J
authors.lastname: Liu
authors.affiliation: Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439; and.
authors.forename: Michael Y
authors.initials: MY
authors.lastname: Hu
authors.affiliation: Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439; and.
authors.forename: Jiyong
authors.initials: J
authors.lastname: Zhao
authors.affiliation: Department of Geology, University of Illinois at Urbana-Champaign, Urbana, IL 61801; Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439; and.
authors.forename: Wenli
authors.initials: W
authors.lastname: Bi
authors.affiliation: Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439; and.
authors.forename: E Ercan
authors.initials: EE
authors.lastname: Alp
authors.affiliation: High Pressure Collaborative Access Team, Geophysical Laboratory, Carnegie Institution of Washington, Argonne, IL 60439.
authors.forename: Yuming
authors.initials: Y
authors.lastname: Xiao
authors.affiliation: High Pressure Collaborative Access Team, Geophysical Laboratory, Carnegie Institution of Washington, Argonne, IL 60439.
authors.forename: Paul
authors.initials: P
authors.lastname: Chow
authors.affiliation: Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, MI 48109; binchen@hawaii.edu jackieli@umich.edu.
authors.forename: Jie
authors.initials: J
authors.lastname: Li
keywords: deep carbon cycle; iron carbide; shear wave velocity; spin transition; spin–phonon coupling