The biospheric carbon cycle is primarily concerned with the atmospheric gas carbon dioxide, its incorporation into organic matter by photosynthesis, and its subsequent release by the respiration of all the biota. Carbon also occurs in the earth's rock predominantly as calcium and magnesium carbonates. These compounds are largely organic in origin, being the mineralised remains of the skeletons of marine organisms.
By subsequent geological shifting, carbonate rocks also occur in land and add to soil nutrients and plant nutrition by their subsequent weathering. Such transfers are small on a globe scale compared with the exchanges between the atmosphere and the biota.