The carbon reservoirs

The carbon is engaged in a cycle, but although photosynthesis and respiration make up the biggest part of that cycle, they are not the whole of it. Quantities in the carbon cycle are always given as the amount of carbon, not carbon dioxide, because it is carbon, not oxygen, which moves through the cycle. The diagram illustrates the carbon cycle in a simplified form.

The Earth contains about 1,100 million billion tons (1017 tonnes) of carbon. Almost all of it is contained in carbonate (carbon-containing) rocks such as limestone, and in fossil fuels. Chalk—a very common rock— is made from the calcium carbonate (CaCO3) from the minute shells of marine organisms. Fossil fuels—peat, coal, petroleum, and natural gas— contain about 44 thousand billion tons (4 x 1012 tonnes) of carbon, representing the partially decomposed remains of once-living plants and animals. Methane hydrates contain about double that amount—88 thousand billion tons (8 x 1012 tonnes). This is methane that is held inside the crystal structure of ice. It is found in sedimentary rocks, mainly beneath the sea floor but also in some places on land. Rocks, fossil fuels, and methane hydrates comprise the geological reservoir of carbon.

Although the atmospheric concentration of carbon dioxide is only about 370 parts per million by volume—0.037 percent—and the air contains even less carbon monoxide and methane, the total amount of carbon in the atmosphere amounts to about 803 billion tons (730 billion tonnes). This is the atmospheric reservoir and the figure is an average for the year, because it changes with the seasons. During summer, plants are growing vigorously and photosynthesis draws carbon dioxide out of the air, so the

The carbon cycle. Carbon moves through the air and is absorbed by plants. Plants and animals pass carbon to soil organisms and seabed sediments and their respiration returns it to the air.

animals

plants

water ^

soil organisms volcanoes combustion aquatic organisms j-

seabed sediment

The cycle—sources and sinks

atmospheric reservoir is slightly reduced. In winter, photosynthesis slows or ceases, but respiration continues, so carbon dioxide is being added to the air faster than it is removed and the amount in the reservoir increases.

Much more carbon is held in the ocean reservoir. Carbon dioxide dissolves in water and so the oceans absorb it from the air. It is then called dissolved inorganic carbon (DIC). Aquatic microorganisms, plants, and animals also contain carbon in their tissues. Carbon in their waste products and the remains of dead organisms also dissolves in sea water. This constitutes dissolved organic carbon (DOC). Rivers carry both DIC and DOC to the sea. In all, the ocean reservoir holds about 41,800 billion tons (38,000 billion tonnes) of carbon.

There is also a land reservoir, composed of carbon that is held in the soil—as decomposing organic matter and as air enriched in carbon dioxide that is held in pore spaces between soil particles—and carbon contained in living soil organisms. Soils contain about 1,650 billion tons (1,500 billion tonnes) of carbon, and other living organisms—principally plants—contain 550 billion tons (500 billion tonnes).

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