1. Earth Systems Science Chapter 8
THE CARBON CYCLE
The circulations of the atmosphere, hydrosphere, and lithosphere were studied in previous chapters. Here, we learn how nutrients are recycled in the earth system. We focus on carbon in particular due to its importance for biological activity and for global climate.
Nutrients: substances normally in the diet that are essential to organisms.

2. Earth Systems Science Chapter 8
THE CARBON CYCLE
carbon cycle: dynamics
The short term terrestrial organic carbon cycle
The short term marine organic carbon cycle
The long term organic carbon cycle
The short term inorganic carbon cycle; interaction with the biological pump
The long term inorganic carbon cycle: the carbonate-silicate geochemical cycle

3. THE CARBON CYCLE: DYNAMICS

4. THE CARBON CYCLE: DYNAMICS
Reservoirs
Locations, or types of regions, where the substance you are tracking is stored.
Value of reservoir depends on the net flux
STELLA diagram of global C cycle used in our lab, adapted Chameides and Perdue (1997)

5. THE CARBON CYCLE: DYNAMICS
The atmosphere
A variety of processes are related to flux into and out of the atmosphere.
These may vary seasonally, resulting in a seasonal cycle in atmospheric carbon concentration.
Steady state: same as dynamic equilibrium

6. THE CARBON CYCLE: DYNAMICS
Residence time, or response time, or e-folding time
Average amount of time that a substance (e.g. atom of C) remains in a reservoir under steady state conditions
Residence time = T = (reservoir size) / outflow rate or (reservoir size) / inflow rate
T(atm) = 760 (Gt-C) / 60 (Gt-C/yr) = 12.7 yr
T = time in which a perturbed system will return to 1/e, or ~38%, of original value
rate = 1/T = 1/12.7 (1/yr) = (1/yr) = yr-1

7. THE CARBON CYCLE: DYNAMICS
Residence time T is calculated at equilibrium using total inflow or total outflow
T = (reservoir size) / (total outflow) = (reservoir size) / (total inflow) = (reservoir size) / (flux_out_1 + flux_out_2) = (reservoir size) / (flux_in_1 + flux_in_2)

8. THE CARBON CYCLE: DYNAMICS
Rate constant r is calculated using the individual flow
r_in_1 = flux_in_1 / reservoir r_in_2 = flux_in_2 / reservoir r_out_1 = flux_out_1 / reservoir r_out_2 = flux_out_2 / reservoir

9. THE CARBON CYCLE: DYNAMICS
Oxidized C that is combined with oxygen
examples: CO2, CaCO3
Reduced C that is not combined with oxygen, usually combined with other carbon atoms (C-C), hydrogen (C-H), or nitrogen (C-N)
example: organic carbon in carbohydrates
reduced substances tend to be unstable in the presence of oxygen: organic matter decomposes, metals rust

10. THE SHORT-TERM TERRESTRIAL ORGANIC CARBON CYCLE
Organic carbon: associated with living organisms; contains C-C or C-H bonds
Photosynthesis: C is removed from the atmosphere and incorporated into carbohydrate molecule; becomes organic.
Primary productivity: amount of organic matter produced by photosynthesis (per year, per area)
Primary producers (producers, autotrophs): organisms that store solar energy in chemical bonds (carbohydrates) for other organisms to consume
Respiration: C is returned to the atmosphere; becomes inorganic
Net primary productivity (NPP): primary productivity - respiration
Image Name: North America NDVI Image Date: March 1990-November 1990 Image Source: AVHRR Mosaic

11. THE SHORT-TERM TERRESTRIAL ORGANIC CARBON CYCLE
Photosynthesis: CO H20  CH (solar energy)
Respiration: CO H20  CH (release energy)
Consumers (heterotrophs): organisms that can not use solar energy directly, get their energy by consuming primary producers
Image Name: Global Greenness Image Date: June 1992 Image Source: AVHRR NDVI

12. THE SHORT-TERM TERRESTRIAL ORGANIC CARBON CYCLE
On land, Net Primary Productivity = 0.5 Primary Productivity
Steady state: flux in = flux out

13. THE SHORT-TERM TERRESTRIAL ORGANIC CARBON CYCLE
Where is the atmosphere in this model?
exogenous to this model
STELLA diagram of terrestrial forest C cycle (adapted from Huggett, 1993)

14. THE SHORT-TERM TERRESTRIAL ORGANIC CARBON CYCLE
aerobic: biological process that uses oxygen for metabolism
aerobe: an aerobic organism; organism whose metabolism is aerobic
metabolism:  The chemical processes occurring within a living cell or organism that are necessary for the maintenance of life. In metabolism some substances are broken down to yield energy for vital processes while other substances, necessary for life, are synthesized. (dictionary.com)

15. THE SHORT-TERM TERRESTRIAL ORGANIC CARBON CYCLE
anaerobic: biological process whose metabolism uses no oxygen
anaerobe: an anaerobic organism; organism whose metabolism is anaerobic
Methanogenesis: an anaerobic form of metabolism
Photosynthesis: CO H20  CH (solar energy)
Respiration: CO H20  CH (release energy)
Methanogenesis: CO CH4  2CH (release energy)

16. Plankton: organisms floating in water
Diatom (SiO2, ~50 mm)
coccolithophorid (CaCO3, ~10 mm)
THE SHORT-TERM MARINE
ORGANIC CARBON CYCLE
Plankton: organisms floating in water
photic zone: ~mixed layer, upper 100m

17. Plankton: organisms floating in water
foraminifer (CaCO3, ~600 mm)
radiolarian (SiO2, ~50 mm)
THE SHORT-TERM MARINE
ORGANIC CARBON CYCLE
Plankton: organisms floating in water
photic zone: ~mixed layer, upper 100m

18. THE SHORT-TERM MARINE ORGANIC CARBON CYCLE
The Biological Pump
Thermohaline
Circulation

19. THE SHORT-TERM MARINE ORGANIC CARBON CYCLE
The Biological Pump
Nutrient Limitation
Organisms (i.e. plankton) require a variety of nutrients to grow. These nutrients are obtained from the ambient water. Nutrients are required in certain ratios: Redfield Ratios
Typically, the organism stops multiplying when one of the required nutrients is depleted.
The depleted nutrient is called the limiting nutrient. If more of the nutrient were present, there would be additional growth.

20. THE SHORT-TERM MARINE ORGANIC CARBON CYCLE
SEAWIFS Mean Chlorophyl September 97 - August 2000
Center of gyres – downwelling – few sources of nutrients – little biological activity
Areas with nutrient input from rivers – or from upwelling – more biological activity
High latitudes generally more productive than low latitudes

21. THE LONG-TERM ORGANIC CARBON CYCLE
On long time scales the processes that are part of the short term cycle are approximately in equilibrium. However, the slower processes associated with geological processes become important.
Reservoir value flux T (Gt-C) (Gt-C/y) (y)
atmosphere soil/sed sed. rock 1e e08

22. THE LONG-TERM ORGANIC CARBON CYCLE
Terrestrial as well as marine organic sediments fill the ocean basins, get buried and lithify, remain in sedimentary rocks until uplift and weathering, or subduction.
This is sometimes referred to as a “leak” from the short term organic C cycle because removal of CO2 leaves one oxygen molecule (O2 ) in the atmosphere:
CO2 + H20  CH

23. THE LONG-TERM ORGANIC CARBON CYCLE
Short circuit the flux from sedimentary rocks to the atmosphere
Fossil fuels are formed from the organic carbon in sedimentary rocks.
How does the burning of fossil fuels affect this system diagram?
How does the deforestation affect this system diagram?
What about reforestation?

24. THE INORGANIC CARBON CYCLE
Sources and sinks of atmospheric carbon that do not depend directly on biological activity exist.
source: a reservoir from which the atmosphere gains carbon
sink: a reservoir to which the atmosphere loses carbon
inorganic: not directly related to biological activity
Important reservoirs of inorganic carbon: the atmosphere, the ocean, sedimentary rocks
Sedimentary rock carbon reservoirs consist mostly of: limestone: CaCO3 dolomite: CaMg(CO3)2 (older sedimentary rocks)

25. THE INORGANIC CARBON CYCLE:
rates of diffusion
atm
(CO2)g
(CO2)aq
H2CO3
HCO3-
CO32-
mixed
layer

26. THE INORGANIC CARBON CYCLE:
atm
(CO2)g
(CO2)aq
H2CO3
HCO3-
CO32-
rates of chemical reactions
mixed
layer

27. THE INORGANIC CARBON CYCLE
Atmosphere – Ocean Carbon Exchange
CO2 diffuses between the atmosphere and the ocean
Diffusion: the free or random movement of a substance from a region in which it is highly concentrated into one in which it is less concentrated. In gases and liquids, it happens spontaneously at the molecular level, and continues until the concentration becomes uniform … (Kemp, The Environment Dictionary)
CO2 dissolves in water
dissolve: when two substances go into solution solution: a homogeneous mixture formed when substances in different states … are combined together, and the mixture takes on the state of one of the components (Kemp, The Environment Dictionary)

28. THE INORGANIC CARBON CYCLE
Atmosphere – Ocean Carbon Exchange
CO2 diffuses between the atmosphere and the ocean
The direction and magnitude of diffusion depends on the partial pressure of CO2 in the atmosphere, the amount of CO2 in solution, the solubility of CO2 in water, and on the rate constant of the diffusion process
partial pressure: pressure of one particular gas in the atmosphere solubility: the maximum amount of a substance that will dissolve in a specified liquid (similar to saturation in the atmosphere) rate constant: number representing speed with which diffusion occurs
(CO2)g  (CO2)aq
where g=gas, aq=aqueous = dissolved in water

29. THE INORGANIC CARBON CYCLE
Chemistry of Inorganic Carbon in Water
dissolved CO2 generates carbonic acid
CO H2O  H2CO3
this reaction can go either direction, depending on the relative concentrations of reactants and products. Reaction occurs until chemical equilibrium is reached
reactants: left hand side of equation products: right hand side of equation chemical equilibrium: when relative concentrations of reactants and products reach the point where no net change in concentrations occurs

30. THE INORGANIC CARBON CYCLE
Chemistry of Inorganic Carbon in Water
carbonic acid generates hydrogen ions, bicarbonate ions, carbonate ions
H2CO3  H+ + HCO (bicarbonate ion)
HCO3-  H+ + CO (carbonate ion)
H+ concentration determines the pH of water pH = -log[H+] where [H+] is the concentration of hydrogen ions.
These reactions tend towards chemical equilibrium, depending on the concentrations of bicarbonate and carbonate, the concentration of the H+ ion (pH), and the temperature.

31. THE INORGANIC CARBON CYCLE
Summary
(CO2)g  (CO2)aq diffusion ocean - atm.
CO H2O  H2CO3 CO2 - carbonic acid
H2CO3  H+ + HCO3- carbonic acid - bicarbonate
HCO3-  H+ + CO bicarbonate - carbonate
Interaction with the biological pump
CO H20  CH photosynthesis/decomposition
Ca HCO3-  CaCO3 + H2CO3 calcium carbonate shells
Net Effect: plankton remove CO2 from surface water, drawing more CO2 out of the atmosphere. The organic material, and calcium carbonate shells, eventually sink into the deep ocean.

32. THE INORGANIC CARBON CYCLE: interaction with the biological pump
atm
(CO2)g
(CO2)aq
H2CO3
HCO3-
CO32-
Net effect: drawdown of atm CO2!
coccolithophorid (CaCO3, ~10 mm)
Diatom (SiO2, ~50 mm)
production
decomposition
mixed
layer
foraminifer (CaCO3, ~600 mm)
radiolarian (SiO2, ~50 mm)
consumption
to the deep ocean
blue = inorganic chemistry red = organic carbon dioxide effect green = organic carbonate effect

33. THE INORGANIC CARBON CYCLE: interaction with the biological pump
atm
(CO2)g
(CO2)aq
H2CO3
HCO3-
CO32-
Net effect: drawdown of atm CO2!
mixed
layer
foraminifer (CaCO3, ~600 mm)
coccolithophorid (CaCO3, ~10 mm)
blue = inorganic chemistry red = organic carbon dioxide effect green = organic carbonate effect

34. THE INORGANIC CARBON CYCLE: interaction with the biological pump
atm
(CO2)g
(CO2)aq
H2CO3
HCO3-
CO32-
H+ ion
H+ ion
mixed
layer
Equilibrium values depend on pH and temperature
pH = -log[H+]
Dissolved CO2 contributes to acidification

35. THE INORGANIC CARBON CYCLE: interaction with the biological pump
From weathering to deposition on the sea floor
Rain drops are slightly acidic to due atm CO2 dissolving in them, resulting in carbonic acid.
Carbonate Weathering: CaCO3 + H2CO3  Ca HCO3- calcium carbonic calcium bicarbonate carbonate acid ion ion
Silicate Weathering: CaSiO H2CO3  Ca HCO SiO2 + H2O wollastonite carbonic calcium bicarbonate silica water acid ion ion

36. THE INORGANIC CARBON CYCLE: interaction with the biological pump
From weathering to deposition on the sea floor
These reactions provide the weathered material that gets washed into the oceans and is available for production of calcium carbonate and silicate shells by plankton in the mixed layer.
As the plankton die, and the shells sink into the deep ocean, they do not dissolve much at first. The shallow and middle depths of the ocean are saturated with respect to CaCO3: there is little acidity to dissolve the shells.
In deeper parts of the ocean they do dissolve more, as these waters often have higher concentrations of dissolved CO2, and therefore carbonic acid, due to the decomposition of organic matter.

37. THE INORGANIC CARBON CYCLE: interaction with the biological pump
From weathering to deposition on the sea floor
carbonate compensation depth (CCD): depth below which the carbonate shells dissolve faster than the rate of shells settling through the water column.
Below the CCD, carbonate shells dissolve, no carbonate is deposited on the ocean floor.

38. THE INORGANIC CARBON CYCLE: interaction with the biological pump
From weathering to deposition on the sea floor
The net result of weathering to deposition is that some carbon is removed from the atmosphere and ends up in calcium carbonate on the ocean floor.
Thus, weathering removes CO2 from the atmosphere and stores it in calcium carbonate sediments. This is another CO2 “leak” from the system. If there were no other source of CO2 into the atmosphere, CO2 concentrations would drop to zero in about a million years.

39. THE INORGANIC CARBON CYCLE: interaction with the biological pump
Summary of the cycle
What process makes up for the CO2 leakage from the atmosphere associated with weathering?
Volcanism, and emission through mid-ocean ridges

40. THE LONG TERM INORGANIC CARBON CYCLE: The Carbonate-Silicate Geochemical Cycle
Net effect: return of CO2 to the atm!
Carbonate metamorphism: CaCO3 + SiO2  CaSiO CO2 calcite silica wollastonite carbon dioxide

41. THE LONG TERM INORGANIC CARBON CYCLE: The Carbonate-Silicate Geochemical Cycle
So, atmospheric CO2 loss by weathering is compensated for by CO2 emissions associated with plate tectonics (volcanic and mid-ocean ridge emissions).
Feedbacks that affect the weathering rate are believed to play a role in regulating atmospheric CO2 levels, and therefore climate, over geologic time scales.