The carbon cycle

The removal of CO₂ from the atmosphere occurs by several natural mechanisms. Fixation of CO₂ in organic form requires an energy source. In biology that energy source is the sun’s radiation, which is coupled to the dark reactions of photosynthesis. While biological capture of carbon is a major regulatory pathway for the short term carbon cycle, there are long term geological processes that have a large impact on the levels of CO₂ in the atmosphere. The carbonate-silicate cycle leads to exchange of Si for C in the earth’s crust,

This chemical reaction, which takes place at depth under the surface of the earth explains why CO₂ is emitted by some volcanos. The limestone, , in the crust originated from why CO₂ in the atmosphere, by way of the oceans. The early Earth had very high levels of CO₂, up to one atmosphere of pressure! Most of that CO₂ was trapped as limestone, which accounts for the vast deposits of limestone at certain locations on earth. The geology of CO₂ uptake by the oceans and deposition as limestone provokes a number of important questions that we should consider today as we ponder the consequences of burning fossil fuels at the current high rate of consumption.

The formation of CaCO₃ in the oceans is thermodynamically favorable. However the process is slow. The calcium content of seawater is about 380 mg/L. We can consider the precipitation of CaCO₃ based on this known concentration and the concentration of CO₃^2-, which is changing with time as the amount of CO₂ in the atmosphere increases. Thus, to fully consider the problem of the uptake of CO₂ we will need to consider the fate of all of the forms of carbonate, including precipitation in the form of CaCO₃.