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Hidden carbon in Earth’s inner core revealed by shear softening in dense Fe₇C₃

Chen, Bin and Li, Zeyu and Zhang, Dongzhou and Liu, Jiachao and Hu, Michael Y. and Zhao, Jiyong and Bi, Wenli and Alp, E. Ercan and Xiao, Yuming and Chow, Paul and Li, Jie (2014) Hidden carbon in Earth’s inner core revealed by shear softening in dense Fe₇C₃. Proceedings of the National Academy of Sciences of the United States of America, 111 (50). pp. 17755-17758. ISSN 0027-8424. PMCID PMC4273394. doi:10.1073/pnas.1411154111.

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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 (v_S) 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 Fe_7C_3, 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 Fe_7C_3 can reproduce the observed v_S 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.

Item Type:Article
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URLURL TypeDescription DOIArticle Information CentralArticle
Zhang, Dongzhou0000-0002-6679-892X
Hu, Michael Y.0000-0002-3718-7169
Zhao, Jiyong0000-0002-0777-3626
Alp, E. Ercan0000-0002-4803-8863
Li, Jie0000-0002-3733-4587
Alternate Title:Hidden carbon in Earth’s inner core revealed by shear softening in dense Fe7C3, Hidden carbon in Earth’s inner core revealed by shear softening in dense Fe_7C_3
Additional Information:Copyright © 2014 National Academy of Sciences. Edited by David Walker, Columbia University, Palisades, NY, and approved November 6, 2014 (received for review June 14, 2014). Published online before print December 1, 2014, doi: 10.1073/pnas.1411154111 PNAS December 1, 2014. The authors thank Y. Meng, Y. Wang, Z. Liu, P. Dera, F. Zhang, M. Lang, Y. Shi, and S. Tkachev for their assistance with the synchrotron experiments. We thank Jung-Fu Lin and Dane Morgan for useful discussions. The authors thank the three anonymous reviewers for the constructive reviews of the manuscript. Sector 3 operations and the GeoSoilEnviroCARS gas-loading facility are supported in part by the Consortium for Materials Properties Research in Earth Sciences (COMPRES) [National Science Foundation (NSF) EAR 06-49658]. High Pressure Collaborative Access Team operations are supported by Department of Energy (DOE) NNSA (DE-NA0001974) and DOE BES (DE-FG02-99ER45775), with partial instrumentation funding by NSF. This study was supported in part by GeoSoilEnviroCARS (Sector 13) (NSF EAR-0622171, DEFG02-94ER14466). Use of the Advanced Photon Source is supported by the US DOE, Office of Science (DE-AC02-06CH11357). The authors acknowledge support from Grants NSF EAR-1219891, NSF EAR-1023729, NSF INSPIRE AST-1344133, and Carnegie/DOE Alliance Center CI JL 2009-05246. B.C. acknowledges support from COMPRES and the University of Hawaii. School of Ocean and Earth Science and Technology (SOEST) contribution no. 9228, Hawaii Institute of Geophysics and Planetology (HIGP) contribution no. 2057. Author contributions: B.C. and J. Li designed research; B.C., Z.L., D.Z., J. Liu, and J. Li performed research; M.Y.H., J.Z., W.B., E.E.A., Y.X., P.C., and J. Li contributed new reagents/analytic tools; B.C. and J. Li analyzed data; and B.C. and J. Li wrote the paper. The authors declare no conflict of interest. This article is a PNAS Direct Submission. This article contains supporting information online at
Funding AgencyGrant Number
NSFEAR 06-49658
Department of Energy (DOE)DE-NA0001974
Department of Energy (DOE)DE-FG02-99ER45775
Department of Energy (DOE)DE-FG02-94ER14466
Department of Energy (DOE)DE-AC02-06CH11357
Carnegie/DOE Alliance CenterCI JL 2009-05246
University of HawaiiUNSPECIFIED
Subject Keywords:iron carbide; shear wave velocity; spin–phonon coupling; spin transition; deep carbon cycle
Other Numbering System:
Other Numbering System NameOther Numbering System ID
University of Hawaii, School of Ocean and Earth Science and Technology (SOEST)9228
Hawaii Institute of Geophysics and Planetology (HIGP)2057
Issue or Number:50
PubMed Central ID:PMC4273394
Record Number:CaltechAUTHORS:20141203-132734475
Persistent URL:
Official Citation:Bin Chen, Zeyu Li, Dongzhou Zhang, Jiachao Liu, Michael Y. Hu, Jiyong Zhao, Wenli Bi, E. Ercan Alp, Yuming Xiao, Paul Chow, and Jie Li Hidden carbon in Earth’s inner core revealed by shear softening in dense Fe7C3 PNAS 2014 111 (50) 17755-17758; published ahead of print December 1, 2014, doi:10.1073/pnas.1411154111
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:52339
Deposited By: George Porter
Deposited On:03 Dec 2014 22:00
Last Modified:10 Nov 2021 19:24

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