The planet's largest carbon reservoir is not in permafrost or the Amazon rainforest, it is hidden in the Earth's inner core, according to what the authors of a new study in PNAS call a "provocative and speculative" finding.
As much as two-thirds of Earth's carbon. They suggest that iron carbide, Fe7C3, provides a good match for the density and sound velocities of Earth's inner core under the relevant conditions. The model, if correct, could help resolve observations that have puzzled researchers for decades but they are not claiming it is more than it is.
"The model of a carbide inner core is compatible with existing cosmochemical, geochemical and petrological constraints, but this provocative and speculative hypothesis still requires further testing," says principal investigator Jie Li, an associate professor in the University of Michigan Department of Earth and Environmental Sciences. "Should it hold up to various tests, the model would imply that as much as two-thirds of the planet's carbon is hidden in its center sphere, making it the largest reservoir of carbon on Earth."
It is 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 been touted by some as a leading candidate component of the inner core. In the paper, the researchers hypothesize that the presence of iron carbide could explain the anomalously slow S waves, thus eliminating the need to invoke partial melting.
"This model challenges the conventional view that the Earth is highly depleted in carbon, and therefore bears on our understanding of Earth's accretion and early differentiation," the authors write.
In their study, the researchers used a variety of experimental techniques to obtain sound velocities for iron carbide up to core pressures. In addition, they detected the anomalous effect of spin transition of iron on sound velocities. They used diamond-anvil cell techniques in combination with a suite of advanced synchrotron methods including nuclear resonant inelastic X-ray scattering, synchrotron Mössbauser spectroscopy and X-ray emission spectroscopy.