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Movement of Carbon in the Carbon Cycle

  • A carbon sink is a store of carbon that absorbs more carbon than it releases.
  • Carbon moves between stores in a continuous cycle, via transfers/fluxes. If more carbon is entering a store than leaving it, this is considered a net carbon sink.
  • If more carbon leaves than enters a store, it is a net carbon source.
  • The carbon cycle interacts with the rock cycle in the processes of weathering, burial, subduction and volcanic eruptions. Carbon dioxide is removed from the atmosphere as it dissolves in water which forms carbonic acid: CO2 + H2O → H2CO
  • Carbonic acid falls as rain, and reacts with minerals in the Earth’s surface, dissolving them through chemical weathering.
  • Component ions are carried in surface water to the oceans, where the ions settle as minerals like calcite, which is a form of calcium carbonate.
  • In seawater, marine organisms like coccoliths and molluscs also precipitate calcium carbonate from calcium and bicarbonate ions in the water.
  • When these marine organisms die, their skeletons sink to the bed of the ocean and collect as sediment – burial and compaction eventually results in forming sedimentary limestone.
  • Coral extracts calcium carbonate from seawater when coral dies and is buried by other coral at the bottom, the carbon ends up stored in limestone.
  • Oceanic limestone is exposed by tectonic uplift such as the Himalayas where the peaks were once accumulations of material at the bottom of the ocean.
  • During the process of subduction, tectonic forces cause plate movement, which pushes sea floor under continental margins.
  • Carbonaceous sea floor deposits are pushed under the surface of the Earth it is heated, melted and then rises up through the surface in volcanic eruptions or in CO2 rich hot springs. Through these ways, carbon dioxide is returned to the atmosphere.

 

Photosynthesis

  • In the euphotic zone of water which is the surface, tiny marine plants called phytoplankton turn carbon into organic matter using the sunlight hitting the water, by the process of photosynthesis.
  • Terrestrial plants, photosynthetic algae and bacteria also photosynthesise.
  • Sunlight provides the organisms with energy to combine carbon dioxide from the atmosphere with water, forming carbohydrates, which store energy. Oxygen is a by-product of the process.
  • Carbon dioxide + Water (+sunlight) → Carbohydrate + Oxygen.
  • CO2 + H2O (+sunlight) → CH2O + O

 

Respiration

  • After photosynthesis not for humans though, as we do not photosynthesise, organisms use some stored energy in the carbohydrates to carry out life functions through the process of respiration.
  • Some carbohydrates, however, remain as biomass, like the bulk of the plant. Consumers, like animals and bacteria, get their energy from excess biomass. In respiration, atmospheric oxygen is combined with carbohydrates to liberate the stored energy, with water and carbon dioxide being by-products.
  • Oxygen + Carbohydrate → Energy + Water + Carbon dioxide
  • O2 + CH2O → Energy + H2O + CO2
  • Note: Although respiration and photosynthesis are basically opposites of each other, the processes are not balanced, as in respiration, not all the organic matter is oxidized, as some is buried in sedimentary rock.

 

Decomposition

  • This includes physical, chemical and biological mechanisms which transform organic matter into increasingly stable forms.
  • It includes the break-up of organic material by wet-dry, shrink-swell, hot-cold and other types of cycles. This sort of fragmentation is caused by animals, wind and sometimes plants.
  • Physical mechanisms include leaching and transport in water.
  • Chemical mechanisms/transformations include oxidation and condensation
  • Biological mechanisms include feeding and digestion, aided by catalysts known as enzymes.
  • Decomposers are organisms which break down cells and tissues in dead organisms into large biomolecules, before breaking down the biomolecules into smaller molecules, then individual atoms.
  • The decomposition process ensures that the important elements of life are continually recycled into the soil and are made available for life. For example, plants need supplies of nitrogen, phosphorus and sulphur atoms from the soil to make its DNA, as well as needing carbon, hydrogen and oxygen for photosynthesis.
  • Oceanic carbon pumps as carbon dioxide dissolves in water, and there is a negative correlation between water temperature and amount of CO2 that can be dissolved with an increase in water temperature, less CO2 can dissolve.
  • This results in vertical deep mixing, a term which describes the important movement of CO2 in the oceans.
  • Warm water in surface currents is carried from warm tropics to cold polar regions. When it is here, the water cools down and therefore becomes denser, so it sinks below the surface layer, potentially all the way down to the ocean bed.
  • As cold water returns to the surface, it warms up again, and loses carbon dioxide to the atmosphere. This example of vertical circulation acts as a massive carbon pump, which gives the ocean much more carbon than it would have if it was not constantly replenished.
  • Biological pumps in the ocean also move carbon around. Carbon can be part of marine organisms as organic matter or structural calcium carbonate.
  • When the organisms die, dead parts sink to be decayed, a process that releases carbon dioxide.

 

Combustion

  • This is the reacting/burning of organic material with vegetation and fossil fuel in the presence of oxygen. The process produces carbon dioxide, water and energy.
  • For these products to be made, the organic material tends to be hydrocarbon, mainly made up of hydrogen atoms and carbon atoms.
  • Biomass combustion involves burning living and dead vegetation, either human-induced burning or natural fires.
  • It tends to happen in boreal the northern forests of Alaska, Canada, Russia and Scandinavia, savannah grasslands in Africa, tropical forests in Brazil, Indonesia, Thailand, Laos, Philippines and Peru, temperate forests in US and Western Europe, and agricultural waste after harvests in US and Western Europe.
  • After a fire, the trees die, but the decomposition sets the forest land up for new growth.
  • New trees grow, and these are storing carbon, old trees decompose, and this emits carbon, and organic layers of soil accumulate, storing carbon.
  • This means the balance between production and decomposition determines whether the forest will be a net source or a net sink.

 

Carbon Sequestration

  • This process involves capturing carbon dioxide from the atmosphere and putting it into long-term storage. The two main types of carbon sequestration are geologic and terrestrial or biologic.
  • In geologic sequestration, carbon dioxide is captured from the source, which could be a power plant or an industrial process, and is injected into underground stores in liquid form, such as depleted oil/gas reserves, coal seams, deep salt formations or the deep ocean.
  • Storing carbon in the ocean is ideal over terrestrial systems because the ocean is so large. Ocean carbon sequestration is good as the carbon is literally sunk within weeks/months of it being captured from air/water. Once it is in the ocean, it becomes part of the circulation system.
  • Terrestrial/biologic sequestration uses plants to capture carbon dioxide from the atmosphere and have it stored as carbon in stems, roots and soil. This tries to maximise the amount of carbon that remains stored in the soil and plant material long term.

This is good because it enriches plant ecosystems and enriches wildlife. However, terrestrial sequestration is bad in some ways. If a forest is planted as part of the method, all its stored carbon may be lost in a forest fire or if there is a disease that reaches the forest.