There are about 44,000 gigatonnes of carbon in the atmosphere and oceans. A gigatonne is one billion metric tonnes, equivalent to the mass of water in over 400,000 Olympic-size swimming pools. Large as this quantity is, it only amounts to a small fraction of one percent of Earth's carbon. Over 90% may reside in the core, most of the rest being in the crust and mantle.
In the photosphere of the Sun, carbon is the fourth most abundant element. The Earth likely started with a similar ratio but lost a lot of it to evaporation as it accreted. Even accounting for evaporation, however, the silicates making up the crust and mantle of the Earth have a carbon concentration that is five to ten times less than in CI chondrites, a form of meteor that is believed to represent the composition of the solar nebula before the planets formed. Some of this carbon may have ended up in the core. Depending on the model, carbon is predicted to contribute between 0.2 and 1 percent by weight in the core. Even at the lower concentration, this would account for half Earth's carbon.
Estimates of the carbon content in the upper mantle come from measurements of the chemistry of mid-ocean ridge basalts (MORBs). These must be corrected for degassing of carbon and other elements. Since the Earth formed, the upper mantle has lost 40–90% of its carbon by evaporation and transport to the core in iron compounds. The most rigorous estimate gives a carbon content of 30 parts per million (ppm). The lower mantle is expected to be much less depleted – about 350 ppm.