Deep carbon cycle

By returning carbon to the deep Earth, it plays a critical role in maintaining the terrestrial conditions necessary for life to exist.

Nonetheless, several pieces of evidence—many of which come from laboratory simulations of deep Earth conditions—have indicated mechanisms for the element's movement down into the lower mantle, as well as the forms that carbon takes at the extreme temperatures and pressures of this layer.

Not much is known about carbon circulation in the mantle, especially in the deep Earth, but many studies have attempted to augment our understanding of the element's movement and forms within said region.

[7] Thus, the investigation's findings indicate that pieces of basaltic oceanic lithosphere act as the principal transport mechanism for carbon to Earth's deep interior.

These subducted carbonates can interact with lower mantle silicates and metals, eventually forming super-deep diamonds like the one found.

[10] Consequently, scientists have concluded that carbonates undergo reduction as they descend into the mantle before being stabilised at depth by low oxygen fugacity environments.

To illustrate, laboratory simulations and density functional theory calculations suggest that tetrahedrally-coordinated carbonates are most stable at depths approaching the core–mantle boundary.

[18] Considering the core's composition is widely believed to be an alloy of crystalline iron with a small amount of nickel, this seismographic anomaly points to another substance's existence within the region.

[19] Furthermore, another study found that carbon dissolved in iron and formed a stable phase with the same Fe7C3 composition—albeit with a different structure than the one previously mentioned.

[20] Hence, although the amount of carbon potentially stored in the Earth's core is not known, recent research indicates that the presence of iron carbides could be consistent with geophysical observations.