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. Other biogeochemical cycles involve oxygen, nitrogen and sulfur.
Basic concepts • reservoirs - forms in which carbon resides within the earth system- usually expressed in terms of the mass of carbon in Gigatons (Gt) = Petagram (Pg) • transfer mechanisms - processes that move carbon between reservoirs - they usually involve a physical process and a chemical reaction • transfer rate - expressed in terms of Pg per year • residence time for carbon in a reservoir - estimated by dividing the amount of carbon in that reservoir by the transfer rates in and out of it. For example, the residence time for atmospheric carbon dioxide is 760 Pg divided by 60 Pg/year yields ~13 years
S total flux out of the reservoir M content if a substance in the reservoir Turnover Time, renewal time single reservoir with source flux Q, sink flux S, and content M Q S=kM M The equation describing the rate of change of the content of a reservoir can be written as
Atmosphere 725 (Annual increase ~3) Deforestation ~1 ~93 ~90 ~60 ~120 ~1 Surface water Dissolved inorg. 700 Dissolved org. 25 (Annual increase ~ 0,3) Short-lived biota ~110 ~15 Long-lived biota ~450 (Annual decrease ~1) ~15 ~40 Detritus decomposition 54-50 Primary production ~40 Respiration & decomposition ~36 Litter ~60 ~40 ~38 Surface biota 3 2 - 5 ‹1 Detritus ~4 2 - 5 Intermediate and Deep water Dissolved inorg. 36,700 Dissolved org. 975 (Annual increase ~ 2,5) Soil 1300 - 1400 (Annual decrease ~1) Peat (Torf) ~160 ‹1 5 Fossil fuels oil, coal, gas 5,000 - 10,000 Land Sea 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.)
Seawater Carbonate System imgres
Alkalinity is the measure of the pH- buffering capacity of the water • Sum ( neg. charges) = Sum (pos. charges) • Conservative ions do not undergo acid-base reactions: Na+,K+,Ca2+,Cl- • Non-conservative ions: H+,OH-,HCO3-,CO32- • Alk= Sum ( neg. charges for non-conservative ions) - Sum (pos. charges for non-conservative ions)
Global mean seawater properties Approximations:
pH pCO2 m eq mol/kg Buffer factor m mol/kg
Organic pump 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.
Example for Hard parts: 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
Dissolution of mineral calcite (and aragonite): 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.
Biological Pump(s) 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".