The Carbon Cycle. The carbon cycle describes the exchange of carbon atoms between various reservoirs within the earth system. The carbon cycle is a geochemical cycle and since it involves the biosphere it is sometimes referred to as a bio-geochemical cycle.
The carbon cycle describes the exchange of carbon atoms between various reservoirs within the earth system.
The carbon cycle is a geochemical cycle and since it involves the biosphere it is sometimes referred to as a bio-geochemical cycle.
Other biogeochemical cycles involve oxygen, nitrogen and sulfur.
760 Pg divided by 60 Pg/year yields ~13 years
total flux out of the reservoir
content if a substance in the reservoir
Turnover Time, renewal time
single reservoir with source flux Q, sink flux S, and content M
The equation describing the rate of change of the content of a reservoir can be written as
(Annual increase ~3)
Dissolved inorg. 700
Dissolved org. 25
(Annual increase ~ 0,3)
Long-lived biota ~450
(Annual decrease ~1)
2 - 5
2 - 5
Dissolved inorg. 36,700
Dissolved org. 975
(Annual increase ~ 2,5)
Soil 1300 - 1400
(Annual decrease ~1)
oil, coal, gas
5,000 - 10,000
Fig. 4-3 principal reservoirs and fluxes in the carbon cycle. Units are 1015 g(Pg) C (burdens)
and PgC/yr (fluxes). (From Bolin (1986) with permission from John Wiley and Sons.)
Sum ( neg. charges for non-conservative ions) - Sum (pos. charges for non-conservative ions)
Sensitivity of pCO2 to changes in DIC, Buffer factor, Revelle factor
m eq mol/kg
3220 e15 mol x 12 g/mol=38640 Pg
mol = 6.023 molecules (of a molecular substance), or atoms (of an element), or ions (of an ionic substance in solution)
k: Gas exchange coeff.
Zml: mixed layer=40m
= ca. 8 days
When CO2 enters the ocean, approx. 19 out of 20 molecules react with carbonate to form bicarbonate:
The photosynthesis reaction removes carbon atoms from the atmosphere and incorporates them into the living tissue of green plants. It requires energy derived from radiation in the visible part of the electromagnetic spectrum. The chemical reaction:
CO2 + H2O --> CH2O + O2
The respiration (and decay) reaction undoes the work of photosynthesis, thereby returning carbon atoms to the atmosphere:
CH2O + O2 --> CO2 + H2O
Oxygenic photosynthesis, oxygen from sea water
Also: onoxygenic photosynthesis: H2S instead of H2O
The marine biosphere operates like a 'biological pump'. In the sunlit uppermost 100 meters of the ocean, photosynthesis serves as a source of oxygen and a sink for carbon dioxide and nutrients like phosphorous.
DIC and [H+] decrease, net consumption of CO2 in the upper layers, has to be balanced by inorganic carbon by transport
Sink of CO2
The marine biosphere is active only in those limited regions of the ocean where upwelling is bringing up nutrients from below. Once nutrients reach the sunlit upper layer of the ocean they are used up in a matter of days by explosive plankton blooms.
Calcification: calcareous shells or skeletons
Calcification: Some marine organisms combine calciumwith bicarbonate ions to make calcareous shells or skeletons
CO2 balance of calcification: Calcification produces CO2 !Ca2+ + 2 HCO3- = CaCO3 + H2O + CO2
Oceanic blooms of coccolithophorids and production of coral reefs
DO NOT help decreasing the atmospheric increase in CO2
Mineral calcium carbonate shells
Shells sink and eventually dissolve, either in the water column or in the sediments
In contrast, calcium carbonate production and its transport to depth, referred to as the carbonate pump, releases CO2 in the surface layer.
Photosynthetic carbon fixation and the flux of organic matter to depth, termed organic carbon pump, generates a CO2 sink in the ocean.
The ocean plays a major role in the global carbon cycle, exchanging CO2 with the overlying atmosphere.
Uptake of atmospheric CO2 by the oceans is driven by physicochemical processes as well as biological fixation of inorganic carbon species.
The biogenic production of organic material and carbonate minerals in the surface ocean and their subsequent transport to depth are termed the "biological carbon pumps".
Increase in atmospheric CO2 concentrations is observed, which inevitably changes the seawater carbonate chemistry when more CO2 is taken up by the surface ocean.
If CO2 concentrations keep rising at the present rate, it is expected that surface ocean CO2 concentrations will have increased to 3-fold relative to preindustrial values by the end of this century.
This would cause carbonate concentrations and pH to drop by ca. 50 % and 0.35 units, respectively.
pCO2:Partial pressure atm.
pH= - log10 [H+]
Photosynthesis and Respirationare the major pathways totransformforms of C
Calcification: Some marineorganisms combine calciumwith bicarbonate ions to makecalcareous shells or skeletons
CO2 balance of calcification: Calcification produces CO2 !Ca2+ + 2 HCO3- = CaCO3 + H2O + CO2 Oceanic blooms of coccolithophorids and production of coral reefs DO NOT help decreasing the atmospheric increase in CO2