Does Earth's Core Host a Deep Carbon Reservoir?

Two recent studies suggest that some of Earth’s carbon may be hidden in the core, making it the planet’s largest carbon reservoir.

Earth’s formation is one of the most profound events in our history. Meteorites provide information about some of the primordial materials from which Earth formed. However, evidence of volatile elements such as carbon, nitrogen, and hydrogen has long since vanished from these celestial time capsules. Addressing the quantities and origins of volatile elements on Earth is therefore extremely challenging.

Surface reservoirs of carbon are generally well documented, but Earth’s deep interior may contain large hidden inventories. Indeed, two recent studies suggest that some of Earth’s carbon may be hidden in the core, making it the planet’s largest carbon reservoir [1,2].

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.

DCO’s Jackie Li (University of Michigan, USA), Bin Chen (University of Hawaii, USA) and colleagues suggest that iron carbide, Fe7C3, provides a good match for the density and sound velocities of Earth's inner core under relevant conditions [1].

"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," Li said. "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."

In a complementary paper that formed part of his doctoral thesis, Clemens Prescher (Universität Bayreuth, Germany) and colleagues simulated the high pressures and temperatures of Earth’s core in a diamond anvil cell [2]. They showed that at these conditions, carbon dissolves in iron to form a stable phase. Although the iron carbide phase is the same Fe7C3 predicted by Li and colleagues, the structure was found to be different.

They also show that under high temperatures and pressures, Fe7C3 exhibits the elastic, almost “rubbery,” properties predicted from direct seismological observations of Earth’s core.

“If carbon were the only light element in the core, there would be more than one hundred times more carbon in the core than in all of the Earth’s surface regions and rocks,” noted DCO’s Catherine McCammon, one of the senior co-authors.

Li and McCammon both presented their work at the recent DCO International Science Meeting in Munich, 26-28 March 2015.

Image: NASA/JPL

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