Ecology - Carbon Cycling

Carbon fixation: Autotrophs convert atmospheric $CO_2$ into carbohydrates and other carbon compounds through photosynthesis. This reduces the $CO_2$ concentration in the atmosphere.

Carbon in aquatic ecosystems: In water, carbon is present as dissolved $CO_2$ and hydrogen carbonate ions ($HCO_3^-$). $CO_2$ dissolves in water to form carbonic acid ($H_2CO_3$), which dissociates into $H^+$ and $HCO_3^-$, thereby lowering the $pH$.

Methanogenesis: Methane ($CH_4$) is produced from organic matter in anaerobic conditions by methanogenic archaeans. This occurs in environments like wetlands, peat lands, and the digestive tracts of ruminant animals.

Oxidation of Methane: Methane is oxidized to carbon dioxide ($CO_2$) and water ($H_2O$) in the stratosphere. This process ensures that $CH_4$ concentrations remain relatively low, despite being a potent greenhouse gas.

Peat and Fossil Fuel formation: Peat forms when organic matter is not fully decomposed because of acidic and/or anaerobic conditions in waterlogged soils. Over geological time, peat can be compressed and heated to form coal, while silt/mud decomposition in seas leads to oil and natural gas.

Combustion: When biomass or fossilized organic matter is heated in the presence of $O_2$, it undergoes combustion, releasing $CO_2$ and $H_2O$ back into the atmosphere.

Limestone formation: Animals such as reef-building corals and molluscs have hard parts that are composed of calcium carbonate ($CaCO_3$). When these organisms die, their shells and skeletons can settle on the seabed and become fossilized in limestone.

Carbon Fluxes: These represent the rate of exchange of carbon between various reservoirs (sinks) like the atmosphere, oceans, and lithosphere, usually measured in gigatonnes ($Gt$).