Figure 1. Global carbon reservoirs, fluxes,
turnover times. Major
reservoirs are underlined, pool sizes and fluxes are given in Gt (1015 g) C and Gt C yr-1. Turnover times (reservoir divided by largest flux to or from reservoir ) are in parentheses
. To convert Gt C to moles C, multiply by 8.3 x 1010.
Sedimentary carbonates and kerogen are the the largest carbon
reservoirs, followed by marine dissolved inorganic carbon (DIC), soils, surface sediments, and the atmosphere. The living biomass reservoir is somewhat smaller than the atmospheric carbon
reservoir and actively exchanges with the atmospheric reservoir through photosynthesis and respiration.
Global estimates of important fluxes or transfers between
reservoirs are shown. Net primary production (NPP = gross photosynthesis - respiration) is approximately equal in terrestrial and marine environments. NPP @ respiration. New Production =
particulate organic carbon (POC) and dissolved organic carbon (DOC) exported from surface waters. Approximately 20% of the ocean NPP occurs in the coastal ocean; 80% of this is deposited in surface sediments.
Turnover or residence times for the reservoirs range from >>106 yr
for kerogen in the sediment reservoir to 103 -105 yr for peats and soil carbon, to about three years for atmospheric CO2 and less than one yearr for ocean biomass. Because of its small
size and relatively slow equilibration with the ocean reservoir, the atmospheric carbon reservoir is presently out of balance. The difference between atmospheric sources (deforestation and combustion) and sinks (annual atmospheric increment and the
difference between ocean influx and efflux) is the "missing sink" of 1.8 Gt C y-1.
The major long-term sink for carbon is burial in deep sea
sediments. This removal of a small portion (0.1%) of annual NPP is responsible for oxygen in the Earth's atmosphere. Protection of photosynthetically fixed organic carbon from oxidation by photo
synthetic oxygen (respiration), and has permitted accumulation of oxygen in the atmosphere and ocean over geologic time. The carbon cycle is completed by weathering of uplifted marine shales or by combustion of fossil fuels.