The carbon cycle preserves a record of many processes on the Earth throughout the planet's history and includes many geologic, biologic, ocean, and atmospheric systems. Many volatile substances including water and carbon dioxide were degassed from the deep interior of the Earth during the early Archean, and some has been added by cometary and meteorite impact. The early atmosphere of the Earth was rich in carbon dioxide (C02), and since the Archean this C02 has been progressively removed by the precipitation of limestones (with a composition close to CaC03), and by photosynthesis that converts the C02 (along with nitrogen, phosphorus, and sulfur) into organic matter, releasing free oxygen in the process. The development of life on the Earth enhanced the formation of limestones and other carbonates, since many organisms secrete calcium carbonate for their shells and tissues. Inorganic processes since the Archean formed other limestones.
over long geologic times carbon dioxide returns to the atmosphere by decomposition of limestones subducted to the Earth's deep interior, releasing carbon dioxide through gases dissolved in magmas that rise to the surface. Plate tectonics and the supercontinent cycle also play a large role in cycling carbon between the atmosphere and rock sphere. When many continents collide to form a supercontinent, the passive margins on these continents that contain thick limestone sequences are uplifted above sea level. The tectonic uplifting of carbonate rocks causes them to be exposed to the atmosphere during continental collisions. The calcium carbonate (CaC03) then combines with atmospheric Co2, depositing it in the oceans. Thus continental collisions and times of supercontinent formation are associated with drawdown and reduction of Co2 from the atmosphere, global cooling, and sea-level changes.
The mass of carbon stored in the limestone and organic matter reservoirs on Earth is huge, about 2,000 times greater than all the carbon presently in the atmosphere and oceans combined. Living plants contain about the same amount of carbon as that in the atmosphere, so human activities such as deforestation that change the vegetation balance on the planet may significantly change the balance between atmospheric and living organic reservoirs for the carbon, putting more Co2 in the atmosphere and altering global climate.
Living plants take Co2 out of the atmosphere and release one molecule of oxygen for every molecule of carbon dioxide used to make organic matter. When these plants die, much of the organic matter is oxidized and returned into C02, but some escapes this process and becomes buried in organic sediments in another reservoir to store carbon. There is a delicate balance between the carbon cycle and the oxygen cycle, as the amount of oxygen released indicated by the mass of the present-day mass of the organic carbon reservoir is 30 times the present atmospheric level, so there is recycling of both carbon and oxygen on geological timescales. Similar relationships exist among biological, geological, and atmospheric processes for the geochemical cycles of nitrogen, phosphorus, and sulfur. Plants absorb these elements in fixed proportions from different environments, store them in organic soils, where groundwater can leach these elements into the hydrological system, bringing them to the ocean, where they form building blocks for new life.
See also asthenosphere; atmosphere; biosphere; carbon cycle; climate change; convection and the Earth's mantle; energy in the Earth system; Gaia hypothesis; greenhouse effect; hydrosphere; lithosphere; magma; passive margin; photosynthesis; Precambrian; subduction, subduction zone; supercontinent cycles; weathering.
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