The flow of carbon through the system
| Carbon is unusual - not only is it very abundant on our planet, but it has one particular feature that makes it different from any other element: it has a strong affinity for itself. This property means that carbon atoms easily bond with other carbon atoms, and can form long, complex chains. As the "backbone" of literally hundreds of thousands of compounds, the ability of carbon to link together into chains is the basis for organic compounds - and the essential component for the building blocks of life. Carbon flows through the environment in a "carbon cycle" by the actions of plants, animals, the ocean, atmosphere and geosphere. The numbers shown on this diagram indicate the mass of carbon exchanged in gigatons (10 to the power of 15 tons) per year. Inorganic carbon dioxide gas in the atmosphere is readily taken up by land-based plants and the ocean (since carbon dioxide readily dissolves in water) where phytoplankton reside, and the action of photosynthesis converts this carbon into organic form. The organic carbon is introduced by primary producers such as land plants and phytoplankton, which supply their respective food webs with carbon, until the animals farther up the food chain exhale (or expire) carbon dioxide gas. As living material dies and decays, microbes and bacteria consume animal tissues and dead plants, and produce methane and carbon dioxide in the process. On land, this gas is either discharged into the atmosphere or retained within the soil. On the dark, cold, ocean bottom, the carbon dioxide produced by decomposition is not available for photosynthesis; instead, it combines with aqueous metal ions (such as calcium and magnesium) to form sediments, although most of it remains dissolved in deep waters as bicarbonate ions. Limestone, a sedimentary rock containing calcium carbonate, is a major "storage area" of Earth's surface carbon. As for all rocks, plate tectonics recycles the limestone at subduction zones, where the rock melts and mixes with the magma in the upper mantle. The carbon in rising magma (that reaches the Earth's land surface through volcanoes and vents) is emitted to the atmosphere as carbon monoxide and carbon dioxide. Another process of atmospheric carbon gain is the burning of fossil fuels by humans. Through metamorphic processes (such as heating and compression), organic material can be converted into carbon-rich "fossil fuels", such as natural gas and oil. The combustion of these compounds, and natural (and man-made) fires also release carbon dioxide and the cycle begins again. |
Credit: GSFC/NASA
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