Carbonates at high pressures: Possible carriers for deep carbon reservoirs in the Earth's lower mantle

The high-pressure structural and elastic properties of MgCO₃, CaCO₃, and MgCa (CO₃₎₂ have been investigated with density functional theory. By computing the isotropic elastic constants, we found that the bulk and shear acoustic wave velocities are smaller in carbonates than in their respective silicates for a wide range of pressure. In terms of geophysics, these results provide strong evidence that the low-velocity zones, near the bottom of the Earth's lower mantle, could be associated with the presence of carbon element in the form of carbonate minerals. This is a different model, associated with chemical differentiation, to explain those low-velocity zones, in addition to several other recent ones proposed in the literature.