Shim explained that because carbon is an iron-loving element, there is expected to be significant carbon in the core, while the mantle is thought to have relatively low carbon. However, scientists have discovered that there is much more carbon in the mantle than expected. “At the pressures expected for Earth’s core-mantle boundary, hydrogen alloying with liquid iron appears to reduce the solubility of other light elements in the core,” the researcher said. “Therefore, the solubility of carbon, which probably exists in the earth’s core, is locally reduced where hydrogen enters the core from the mantle (via dehydration). The stable form of carbon at the pressure-temperature conditions of the Earth’s core-mantle boundary is diamond. So carbon escaping from the liquid outer core will become diamond when it enters the mantle.” According to Byeongkwan Ko, who led the Geophysical Research Letters paper presenting these findings, the new discovery of a carbon transfer mechanism from the core to the mantle helps shed light on understanding the carbon cycle in the deep interior of the earth. “This is even more exciting given that diamond formation at the core-mantle boundary may have been occurring for billions of years since subduction began on the planet,” he said. Ko’s study shows that carbon leaking from the core into the mantle through this diamond-forming process may provide enough carbon to explain the increased amounts of carbon in the mantle. He and his colleagues also predicted that diamond-rich structures may exist at the core-mantle boundary and that seismic studies might detect them because seismic waves should travel unusually fast through the structures. “The reason that seismic waves must propagate extremely fast through diamond-rich structures at the core-mantle boundary is because diamond is extremely incompressible and less dense than other materials at the core-mantle boundary,” Shim said. Ko, Shim and the rest of the team now plan to continue investigating how the reaction can also change the concentration of other light elements in the core, such as silicon, sulfur and oxygen, and how such changes can affect its mineralogy deep mantle.
