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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
Turnover Time, renewal time
M content if a substance in the reservoir
S total flux out of the reservoir
MS=kMQ
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
Atmosphere 725(Annual increase ~3)
Surface waterDissolved inorg. 700
Dissolved org. 25(Annual increase ~ 0,3)
Surface biota3
Intermediate andDeep water
Dissolved inorg. 36,700Dissolved org. 975
(Annual increase ~ 2,5)
Short-lived biota~110
Long-lived biota ~450(Annual decrease ~1)
Litter~60
Soil 1300 - 1400(Annual decrease ~1)
Peat (Torf)~160
Fossil fuelsoil, coal, gas
5,000 - 10,000
Respiration &decomposition
~36
Primaryproduction
~40
Detritus~4
Detritus decomposition
54-50
~40 ~38
5
2 - 5
2 - 5
~15~40
~120~60~90~93Deforestation
~1
‹1
‹1
~15~1
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:
What controls the pCO2 ?
pHpCO2
Buffer factor
m mol/kg
m eq mol/kg
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
Organic pump
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.
Examples:
Hard parts
Calcification: calcareous shells or skeletons
Example for Hard parts:
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
Photosynthetic carbon fixation and the flux of organic matter to depth, termed organic carbon pump, generates a CO2 sink in the ocean.
In contrast, calcium carbonate production and its transport to depth, referred to as the carbonate pump, releases CO2 in the surface layer.
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".