Point 4 of 7 — Ecological Balance in Earth Systems, Biogeochemical Cycles and Food WebsQuestion bank: 0 questions (0 with pictures)
04The carbon cycle
In: photosynthesis fixes atmospheric CO₂ into carbohydrate — 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ — and the carbon passes along the food chain.
Out: respiration in all organisms returns CO₂; decomposers release it from dead matter; combustion of wood, coal, oil and gas returns ancient carbon that had been locked out of the cycle for millions of years.
The oceanis the largest active reservoir: CO₂ dissolves and equilibrates with HCO₃⁻ and CO₃²⁻, shell-forming organisms make CaCO₃, and when they sink the carbon becomes limestone or is returned by upwelling.
Because the ocean absorbs CO₂, its pH falls — ocean acidification — which dissolves the shells of corals, molluscs and plankton.
Terrestrial sinks: living biomass and soils hold far more carbon than the atmosphere, so deforestation both releases carbon and removes the sink.
Human disturbance: burning fossil fuels and clearing forests move carbon into the air faster than photosynthesis and dissolution can take it back.
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