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".