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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.

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the carbon cycle

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. 

why study the carbon cycle
Why study the carbon cycle?
  • to learn why atmospheric carbon dioxide has tended to decrease over the lifetime of the earth and why it underwent large swings between glacial and interglacial periods of the ice ages
  • to learn why atmospheric carbon dioxide is increasing at only about half the rate that one would expect, given the current rate of burning fossil fuels 
  • to predict future concentrations of atmospheric carbon dioxide 
  • to assess the potential of carbon 'sequestration' (planting new trees) as a strategy for slowing the buildup of atmospheric carbon dioxide
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



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


Atmosphere 725

(Annual increase ~3)








Surface water

Dissolved inorg. 700

Dissolved org. 25

(Annual increase ~ 0,3)

Short-lived biota



Long-lived biota ~450

(Annual decrease ~1)



Detritus decomposition





Respiration &







Surface biota


2 - 5




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)




Fossil fuels

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.)

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)

The 'mg/l CaCO3' unit reports the concentration of CaCO3 in pure water that would provide the same buffering capacity as the water sample in question. This does not mean the sample contains that much CaCO3.   In fact, it tells you nothing about how much of the buffering is due to carbonates, it is only a measure of equivalency.
  • 1 meq/l = 2.8 dKH = 50 mg/l CaCO3 
what controls the pco 213
What controls the pCO2 ?

Sensitivity of pCO2 to changes in DIC, Buffer factor, Revelle factor

ca. 10




m eq mol/kg

Buffer factor

m mol/kg

dic mmol kg to pg
DIC: mmol/kg to Pg
  • DIC= 2.35 mmol/kg seawater
  • Volume Oceans= 1370 e15 m^3
  • Rho_water= 1000 kg/m^3
  • Mass_water= 1370 e18 kg
  • DIC for Oceans: 3220 e15 mol
  • DIC carbon:

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)

time rate of changes
Time rate of changes

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 (short term) organic carbon cycle

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

Calvin-Benson cycle


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. 


Deeper layers

  • Fecal pellets and dying marine organisms decay as they settle into the deeper layers of the ocean, consuming dissolved oxygen and giving off (dissolved) carbon dioxide. 
  • Hence, these layers have much higher carbon dioxide concentrations and lower oxygen concentrations than the waters just below the surface. 

The Marine Carbonate System

  • Carbon dioxide: enters the ocean from atmosphere; steady exchange at surface
  • Solubility: in water is low! But CO2 reacts with water to form ions: carbonate (CO32-) and bicarbonate (HCO3-)
  • Most of marine CO2 is stored as bicarbonate. Therefore, CO2 is never limiting photosynthesis in seawater, but it is in freshwater (low pH prevents bicarbonate)
  • Respiration: CO2 is added, reacts with water:H2O + CO2 = HCO3- + H+; [H+] rises, which means pH sinks!
  • Primary production: CO2 is removed, HCO3- converts to free CO2 to keep the chemical balance; [H+] falls, which means pH rises!
  • Soft tissue (weiches Gewebe)

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
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".

present future co2 increase
Present/future CO2 increase

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.


Equilibrium relationships between these species:

pCO2:Partial pressure atm.

[ ]:Concentrations/activities

pH= - log10 [H+]









the long term organic carbon cycle
The long term organic carbon cycle
  • Only a tiny fraction of the organic material that is generated by photosynthesis each year escapes the decay process by being buried and ultimately incorporated into fossil fuel deposits or sediments containing more dilute fragments of organic material. 
  • Through this slow process, carbon from both terrestrial and marine biosphere reservoirs enters into the long term organic carbon cycle.  The rate is so slow as to be virtually unmeasurable. 
  • Weathering releases carbon back into the other reservoirs.

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