Since carbon compounds are present in any sphere of the Earth (atmosphere, hydro­sphere, lithosphere, or biosphere), the determination of the carbon isotope ratios obviously plays an important role in the study of the global carbon cycle. In addi­tion, the isotope analysis of other planets provides important information. For exam­ple, the 618O is the same in the rocks of the upper parts of the Earth’s crust and the Moon (618O = 5.5 6 0.2m), proving that the Earth and Moon share the same origin.

An important question in the global carbon cycle is the carbon isotope ratio of the Earth’s mantle. Because of the very high temperature, even isotope composition should be expected; however, there are significant differences in the isotope ratio of different minerals. Diamond and SiC mineral, for example, contain more 12C isotopes than magmatic minerals.

Deviation from the mean carbon isotope ratios refers to the major extinction event. Since the 13C/12C ratio of the biomass is lower than that of the sedimentary carbonate rocks, the sediments forming during the extinction events from the bio­mass show lower 13C/12C ratio than the mean value of the carbonate rocks. The 13C/12C can continue to decrease via the release of methane-hydrate bound to the deep-sea sediments, which is due to bacterial activity that prefers the light carbon isotope. During global warming, the methane-hydrate releases as carbon dioxide, increasing the carbon dioxide content of the atmosphere. The industrial carbon dioxide emission also decreases the 13C/12C ratio because of the burning of fossil fuel. All the above processes are in fact the consequence of biological isotope effects.