CARBON CYCLE

The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, hydrosphere, and atmosphere of the Earth. It is a very important cycle as it allows for the fourth richest element to be cycled and reused.

In the atmosphere, carbonic acid (H₂CO₃) forms by the reaction of carbon dioxide and water. This weakly acidic water reaches the earth as rain; it reacts with minerals at the earth’s surface, slowly dissolving them into their component ions through the process of chemical weathering. These component ions are carried in surface waters like streams and rivers eventually to the ocean, where they precipitate out as minerals like calcite (CaCO₃). Through continued deposition and burial, this calcite sediment forms the rock called limestone. This cycle continues as seafloor spreading pushes the seafloor under continental margins in the process of subduction. As seafloor carbon is pushed deeper into the earth by tectonic forces, it heats up, eventually melts, and can rise back up to the surface, where it is released as CO₂ and returned to the atmosphere. This return to the atmosphere can occur violently through volcanic eruptions or hotsprings.

Biology plays an important role in the movement of carbon between land, ocean, and atmosphere through the processes of photosynthesis and respiration. Plants take in carbon dioxide (CO₂) from the atmosphere during photosynthesis, and release CO₂ back into the atmosphere during respiration. Carbon is released through the decay of animal and plant matter as bacteria and fungi break down the carbon compounds in dead animals and plants and convert the carbon to carbon dioxide; or burning of biomass.

In the oceans, phytoplankton (microscopic marine plants that form the base of the marine food chain) use carbon to make shells of calcium carbonate (CaCO₃). The shells settle to the bottom of the ocean when phytoplankton die and are buried in the sediments. The shells of phytoplankton and other creatures can become compressed over time as they are buried and are often eventually transformed into limestone. Additionally, under certain geological conditions, organic matter can be buried and over time form deposits of the carbon-containing fuels coal and oil. It is the non-calcium containing organic matter that is transformed into fossil fuel. The carbon is emitted through burning fossil fuels and limestone.

BY: DOAN VIET HONG (3)

Bibliography:

John Arthur Harrison, Ph.D. "The Carbon Cycle: What Goes Around Comes Around," Visionlearning Vol. EAS-2 (3), 2003.
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