Question

Explain how carbon is cycled between the atmosphere and hydrosphere. Use specific examples.

Answer 1

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Answer 2

Well, begin by taking note of the forms in which carbon is present in both. In the oceans carbon is generally present as the forms that result when carbon dioxide (CO2) is dissolved in water, CO2 itself, H2CO3 (carbonic acid), HCO3- (bicarbonate) and CO3-2 (carbonate). The exact amount of each varies with temperature, pH, pressure of CO2 in the atmosphere above, and so on. In the atmosphere carbon is present mostly as CO2, although there are some other interesting trace gases, such as CH4 (methane).

CO2 goes into the oceans just by dissolving, the way it dissolves in soft drinks to give them a bite (that's the carbonic acid) and a fizz. The amount that dissolves depends on the partial pressure of CO2, which means the amount that's present in the atmosphere at that location (it varies with location, because the atmosphere is not perfectly uniform). The carbonates and CO2 in the water have various possible fates. One is that the CO2 undissolves -- comes out of solution -- and goes back ito the atmosphere. Another is that the carbonate and bicarbonate anions are used by foramineferae (tiny creatures) to build their shells, which are mostly CaCO3 (calcium carbonate). When the creatures die, much of the shell material falls to the bottom of the ocean where, over time, it turns into limestone and is removed into the planetary crust. From there it is occasionally redeposted into the ocean when sedimentary rocks containing carbonates are weathered by rain.

Another important detour for carbon is through the biosphere. Plants -- particularly algae in the ocean -- absorb CO2 and use it to make hydrocarbons that make up their cells, and glucose that they use to store energy. The glucose is "burned" in cellular respiration, the products of which are CO2 and water, so that returns the carbon to the atmosphere pretty quickly. The carbon that is part of the cell material of the plant may end up in the ocean if we're talking an algae, when it dies, or it may end up in the gut of some animal that eats the plant, where it then becomes cycled into the animal's cells, either as cell material or as glucose to store energy. From there it suffers a similar fate, in that if it's part of glucose and burned for energy, it returns to atmospheric CO2 pretty quickly, but if part of the cell material, it only exits the animal when it dies (or gets eaten by something else). Plant and animal remains are mostly consumed by bacteria, but some in shallow seas can in principle become oil, gas and coal deposits, over very long time periods (millions of years).