The Carbon Cycle and Atmospheric Carbon Dioxide Sources

The principal source of carbon dioxide is degassing of the Earth from volcanic activities, mid-ocean ridges, hotspots or subduction-related volcanism. Carbon is converted into different forms through the atmosphere, hydrosphere, and lithosphere. Part of the outgassed carbon dioxide remains in the atmosphere, some of them is dissolved in the oceans, some carbon is preserved as biomass in living or dead organisms, and some is deposited as carbonate rocks such as limestone. 
 

Here, is a table shows Carbon pools in the major reservoirs on earth from Wikipedia:

Pool	Quantity (gigatons)
Atmosphere	720
Oceans (total)	38,400
Total inorganic	37,400
Total organic	1,000
Surface layer	670
Deep layer	36,730
Lithosphere
Sedimentary carbonates	> 60,000,000
Kerogens	15,000,000
Terrestrial biosphere (total)	2,000
Living biomass	600 - 1,000
Dead biomass	1,200
Aquatic biosphere	1 - 2
Fossil fuels (total)	4,130
Coal	3,510
Oil	230
Gas	140
Other (peat)	250

Approximately eighty per cent of atmospheric carbon dioxide is from the Earth’s outgassing(Hibbard. 2000). Meanwhile, respiration and fuel combustion are other two major source of atmospheric carbon dioxide.

Respiration consumes oxygen from the atmosphere and produces carbon dioxide.

Oxygen + carbohydrate > energy + water + carbohydrate

O₂ + CH₂O > energy + H₂O + CO₂

On the other hand, photosynthesis takes carbon dioxide from the atmosphere and produce oxygen.

6CO₂ + 6H₂O (+ light energy) > C₆H₁₂O₆ + 6O₂

The combustion of organic matters will release carbon dioxide to the atmosphere.

C + O₂  > CO₂

Moreover, some of carbon dioxide is produced and released into the atmosphere via metamorphism of limestone （CaCO₃ + SiO₂ > CO₂ + CaSiO₃） and the precipitation of Calcium carbonate from calcium and bicarbonate in the ocean by maritime organisms such as corals.（Ca²⁺ + 2HCO₃^- > CaCO₃ + CO₂ + H₂O）  

Here is a short vedio from NASA briefly describes the carbon cycle of the Earth and the change of atmospheric carbon dioxide. 

From next post, I am going to talk about the impacts of change in atmospheric carbon dioxide, then I will    discuss about the potential methodologies of carbon reduction.

Reference: 

• Carbon pools in the major reservoirs on earth. http://en.wikipedia.org/wiki/Carbon_cycle#cite_note-GlobalCarbonCycle-1
• Falkowski, P.; Scholes, R. J.; Boyle, E.; Canadell, J.; Canfield, D.; Elser, J.; Gruber, N.; Hibbard, K. et al. (2000). "The Global Carbon Cycle: A Test of Our Knowledge of Earth as a System". Science 290 (5490): 291–296.
• The Carbon Cycle and Earth's Climate. http://www.columbia.edu/~vjd1/carbon.htm 
• Chemical precipitation: terrestrial carbon cycle. Encyclopædia Britannica, Inc.