Largest Carbon Reservoir: New Study Estimates Deposit in Earth’s Outer Core

A new collaboration from Florida State University (FSU) and Rice University reveals just how much carbon is stored in the Earth's outer core, and it might be the largest carbon reservoir on the planet.

Researchers estimated that the carbon content in the Earth's outer core is somewhere between 0.3% to 2.0% through first-principles molecular dynamics (FPMD) simulations. This estimate could equate to the largest terrestrial carbon reservoir available.

The team behind the new estimate reported the details in the Communications Earth & Environment journal, August 19. The report titled "Earth's Core Could Be the Largest Terrestrial Carbon Reservoir," also presents estimates for iron-carbon alloys in terms of density and compressional wave velocity.

Largest Carbon Reservoir on Earth

While the percentage might appear low, researchers argue that it is still an unbelievably large amount given the volume of Earth's outer core. The United States Geological Survey (USGS) explains that the Earth's outer core is a layer of molten material 2,200 kilometers (1367 miles) thick, covering a solid inner core.

Making the computation means that at 0.3% to 2.0%, the Earth's outer core holds 5.5 to 36.8 x 10^24 grams of carbon.

Mainak Mookherjee, the co-author of the study and a geology professor in FSU's Department of Earth, Ocean and Atmospheric Science, said in a university news article that one of the most difficult challenges in solid-earth science is determining the composition of the Earth's core.

He added that it is already known that the planet's core is mostly iron, but iron has a higher density than the core. The density of the core must be reduced with lighter materials. Carbon is one factor to consider, and that researchers are becoming better at estimating how much there might be.

The new study refines previous estimates on the total amount of carbon present on Earth. Previous estimates place terrestrial carbon content to somewhere between 990 to 6,400 parts per million. Following these estimates would mean that the core, both the outer and inner layers, is the carbon reservoir holding 93%-95% of the Earth's material supply.

Using Simulations To Make Estimates

Considering how the Earth's outer core begins 5,150 kilometers (3,200 miles) beneath the Earth's surface, researchers have to turn to indirect methods to examine its structure and composition. For the new estimate, they used first-principles molecular dynamics simulations.

A 2016 research on the Accounts of Chemical Research explains that first-principles molecular dynamics is a preferred technique for investigating chemical events in solution. The field uses atomic forces computed from electronic structure calculations.

Researchers relied on the known speed of compressional sound waves that travel through the Earth in their study. They used them to build computer models simulating different compositions of iron, carbon, and other elements at the temperature and pressure found at the Earth's outer core.

Lead author and FSU postdoctoral researcher Suraj Bajgain explains that when the velocity of sound waves match the observed rate of sound waves traveling through the planet, that is when they knew that the simulations were starting to be consistent with the chemical composition of the Earth's outer core.