Two-thirds of Earth's carbon may be hidden in planet's inner core

A new study has revealed that two-thirds of the Earth's carbon might be hidden in planet's inner core, which means that it is planet's biggest carbon reservoir.

Researchers from University of Michigan and their colleagues suggested that iron carbide, Fe7C3, provides a good match for the density and sound velocities of Earth's inner core under the relevant conditions.

It has been now widely accepted that Earth's inner core consists of crystalline iron alloyed with a small amount of nickel and some lighter elements. However, seismic waves called S waves travel through the inner core at about half the speed expected for most iron-rich alloys under relevant pressures.

Some researchers have attributed the S-wave velocities to the presence of liquid, calling into question the solidity of the inner core. In recent years, the presence of various light elements including sulfur, carbon, silicon, oxygen and hydrogen has been proposed to account for the density deficit of Earth's core.

Iron carbide has recently emerged as a leading candidate component of the inner core. Researchers concluded that the presence of iron carbide could explain the anomalously slow S waves, thus eliminating the need to invoke partial melting.

Jie Li, an associate professor in U-M's Department of Earth and Environmental Sciences said that the model of a carbide inner core was compatible with existing cosmochemical, geochemical and petrological constraints, but this provocative and speculative hypothesis still requires further testing.

The study will be published in the Proceedings of the National Academy of Sciences.