Carbon Cycle


    The carbon cycle is one of the major biogeochemical cycles describing the flow of essential elements from the environent to living organisms and back to the environment again. This process is required for the building of all organic compounds and involves the participation of many of the earth's key forces. The carbon cycle has affected the earth throughout its history; it has contributed to major climatic changes, and it has helped facilitate the evolution of life.

    The complexities of the carbon cycle are depicted in the diagram below:

    The carbon cycle is one of the earth's fastest recycling processes--each atom of carbon has been recycled numerous times.  For this reason, the carbon cycle has no specific beginning or ending point on the diagram. The carbon cycle passes through three main stages: reservoirs, assimilation, and release.

    Much of the earth's carbon is contained in the atmosphere which serves as a reservoir, and that is where we will begin our explanation.  Atmospheric carbon consists mostly of carbon dioxide and has two major sinks: terrestrial ecosystems and marine ecosystems, both of which deal with photosynthesis as a part of assimilation and respiration as a part of release.

The two scenes below represent the two major sinks of the carbon cycle described above:

                                                                     Terrestrial Ecosystem                                                 Marine Ecosystem

    Another carbon sink is the weathering of mountains and other rock formations formed by plate tectonics, mainly silicate weathering.  Carbon dioxide is consumed
from silicate weathering as seen in this equation:    CaSiO3 + 2CO2 + 2H2O => CaCO3 + SiO2 + CO2 + 2H2O

    A major source of atmospheric CO2 is degassing from volcanic activity which acts as a release of carbon dioxide.  Conversely, the process of subduction of crust provides a sink for CO2 .  The effects of the formation of volcanoes and the subduction of continental crust is also discussed on the next page.

    Another important source of carbon in the atmosphere is in the decomposition of organic material.  Carbon dioxide is captured by plants throughout their lives and heterotrophic organisms in turn obtain a part of this carbon. The element is transferred from organism to organism when plants are eaten by herbivores which are in turn eaten by carnivores along the food chain. All these organisms go through respiration, excrete organic waste, and eventually die and decompose which releases carbon into the soil either as carbonates or fossil fuels. The carbon dioxide drawn into marine ecosystems is eventually released through oceanic respiration. Through time and pressure, the organic material buried in soil and sediments may eventually become fossil fuels such as coal and oil, which then become additional sources of carbon. When these are burned, they emmit tremendous amounts of carbon back into the atmosphere. The burning of fossil fuels, however, is occurring at much higher rates than is their production.

    Thus the atmospheric carbon that is sequestered through weathering and the terrestrial and marine ecosystem sinks is eventually recycled through the processes of decomposition, respiration, and tectonic forces, releasing them once more into the atmosphere.

    The following flow chart is another way to view the carbon cycle by concentrating more on the role of organisms and observing how biological functions lead to changes in the carbon cycle.

The carbon cycle also has major effects on global climate:

          -  Carbon Cycle Modelling, ed. Bert Bolin. 1981, John Wiley & Sons
          -  Tectonic Uplift and Climate Change, ed. William F. Ruddiman. 1997, Plenum Press
          -  Natural Sinks of CO2, ed. Dr. Joe Wisniewski & Dr. Ariel E. Lugo. 1992, Kluwer Academic Publishers
          -   GE 70A Reader by Mark Morris, Mark Harrison, and Stephen Mojzsis
          -   The Cosmic Perspective by Jeffrey Bennett, Megan Donahue, Nicholas Schneider, and Mark Voit, 1999 Addison Wesley Longman.