Deep carbon cycle
The deep carbon cycle (or slow carbon cycle) is geochemical cycle (movement) of carbon through the Earth's mantle and core. It forms part of the carbon cycle and is intimately connected to the movement of carbon in the Earth's surface and atmosphere. By returning carbon to the deep Earth, it plays a critical role in maintaining the terrestrial conditions necessary for life to exist. Without it, carbon would accumulate in the atmosphere, reaching extremely high concentrations over long periods of time.[1]
Because the deep Earth is inaccessible to drilling, not much is conclusively known about the role of carbon in it. 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. Furthermore, techniques like seismology have led to greater understanding of the potential presence of carbon in the Earth's core. Studies of the composition of basaltic magma and the flux of carbon dioxide out of volcanoes reveals that the amount of carbon in the mantle is greater than that on the Earth's surface by a factor of one thousand.[2]
Movement of oceanic plates—which carry carbon compounds—through the mantle
Two models of the carbon content in Earth
Although the presence of carbon in the Earth's core is well-constrained, recent studies suggest large inventories of carbon could be stored in this region. Shear (S) waves moving through the inner core travel at about fifty percent of the velocity expected for most iron-rich alloys.[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. One theory postulates that such a phenomenon is the result of various light elements, including carbon, in the core.[18] In fact, studies have utilised diamond anvil cells to replicate the conditions in the Earth's core, the results of which indicate that iron carbide (Fe7C3) matches the inner core's sound and density velocities considering its temperature and pressure profile. Hence, the iron carbide model could serve as evidence that the core holds as much as 67% of the Earth's carbon.[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.