2.3 Biogeochemical cyclesIB Environmental Systems and Societies SL: Revision notes

Biogeochemical cycles ensure chemical elements such as carbon continue to be available to living organisms. Every cycle has stores (where an element is held), sinks (stores that take in more than they release) and sources (stores that release more than they take in). The time an element stays in a store is its residence time.

An ecosystem can be a store, a sink or a source of carbon, and may change from one to another over time.

Organic stores of carbon are organisms, crude oil and natural gas. Inorganic stores are in the atmosphere, soils and oceans.

Carbon flows between stores by photosynthesis, feeding, defecation, cellular respiration, death and decomposition. Photosynthesis moves carbon from the atmosphere into producers; feeding moves it along the food chain; respiration and decomposition return it to the atmosphere as carbon dioxide.

Carbon sequestration is the capture of gaseous atmospheric carbon dioxide and its storage in a solid or liquid form, for example in growing trees, peat or in rocks beneath the seabed.

Fossil fuels are stores of carbon with unlimited residence times. They formed when ecosystems acted as sinks in past eras, and they become sources when burned.

Peat bogs, such as the Flow Country in Scotland, are stores because waterlogging slows decomposition. Draining them lets decomposers respire aerobically and turns them into sources. Mature woodland is a sink while photosynthesis exceeds respiration and decomposition.

Agricultural systems can be stores, sources or sinks depending on technique: regular ploughing aerates the soil and releases carbon, while no-till farming, hedgerows, agroforestry and cover crops increase carbon storage.

Carbon dioxide is absorbed into the oceans by dissolving and is released as a gas when it comes out of solution. More dissolved carbon dioxide forms carbonic acid, so the ocean becomes more acidic (ocean acidification). Surface ocean pH has fallen from about 8.2 to about 8.1.

Acidification reduces the carbonate ions available, so animals that build calcium carbonate shells and skeletons, such as oysters, corals and some plankton, grow more slowly or have weaker structures. Oyster hatcheries on the Pacific coast of the United States have reported failures with low-pH seawater.

Human activities, such as burning fossil fuels and clearing forest, have altered the carbon cycle, so measures are required to reduce the effects.

• Reduce fossil fuel use: tackles the cause but can be costly.
• Afforestation and protecting ecosystems: increases sinks, but slow and can be reversed by fire.
• Carbon capture and storage: for example Sleipner in the North Sea, which has injected carbon dioxide into a saline aquifer since 1996. It needs energy and monitoring.
• Carbon pricing: Norway taxed offshore emissions from 1991, giving a financial incentive.

A technocentric view favours technological fixes; an ecocentric view favours protecting ecosystems and reducing consumption.