| Summary: | The biogeochemical cycle of carbon (C) in the earth system controls fluxes, pools, and transformations associated with life’s most fundamental element. As the most basic building block for all living organisms, organically bound C forms the basis for the overwhelming majority of food chains in ecosystems and global energy flows. The uptake of carbon dioxide (CO2) by autotrophic organisms and photosynthetic transformation of light energy into chemical energy enables conversion of atmospheric CO2 into structural materials for living organisms and ultimately fossil fuels. In the earth system, C exists in different forms and reservoirs, including gaseous, dissolved, and solid forms distributed and continually exchanged among the atmosphere, terrestrial, and aquatic spheres. The most important gaseous forms of C include CO2 and methane (CH4). The liquid phase includes different species of C found in water, including (1) dissolved CO2 and carbonic acid (H2CO3) and its intermediates, (2) dissolved organic compounds (molecules <0.45 µm in size), (3) suspended organic and inorganic colloids/particles (typically >10s nm) containing C, and (4) raw oil. The solid phase comprises C (1) in rocks of organic and inorganic origin, in sedimentary rocks and sediments including coal, (2) on and in soils in the form of carbonates, (3) in dead not dissolved or suspended organic compounds, and (4) in the living biomass of microorganisms, plants, and animals. The C in the atmosphere, terrestrial, and aquatic systems can be characterized according to the amount of C stored in a given reservoir, its mean residence time (i.e., the time needed to exchange each C atom of the considered system or subsystem at least once), and the physical or chemical state of C in a given reservoir or as it exchanges among reservoirs. Different systems can be subdivided into active and inactive drivers of C dynamics based on the C mean residence times. Under natural conditions, almost the entire C stored in sediments or sedimentary rocks, for example, is considered to be inactive with mean residence times longer than 1,000 years. Contrastingly, the C stored in the atmosphere, surface oceans, plant biomass, and soil organic material in the soil is relatively active with residence times ranging from seconds to centuries. However, anthropogenic extractions of fossil fuel C from global sedimentary deposits, for example, have demonstrated how inactive C reservoirs can be rapidly transformed into highly active drivers of global C dynamics.
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