4.3 Carbon cycling

What is carbon cycling?

a biogeochemical cycle whereby carbon is exchanged between the different spheres of the Earth

What are the different spheres between which carbon is exchanged?

The four spheres are the atmosphere (air), lithosphere (ground), hydrosphere (water / oceans) and biosphere (living things)

How is carbon exchanged?

* Atmospheric gases – mainly CO2 and methane (CH4)
* Oceanic carbonates
* Organic matter
* Non-living remains

What happens when CO2 dissolves in water?

Some of it will remain as a dissolved gas, however the remainder will combine with water to form carbonic acid.

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The carbonic acid will then dissociate to form carbonic acid ions.

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This release H+ ions which will increase the acidity

What happens when carbonic acid comes into contact with rocks and sediment?

This commonly results in the formation of calcium carbonate and the subsequent development of limestone

Living organisms and hydrogen carbonate

* Forms calcium carbonate
* This calcium carbonate forms the hardened exoskeleton of coral, as well as forming the main component of mollusca shells
* When the organism dies and settles to the sea floor, these hard components may become fossilised in the limestone

What are methanogens?

Archaean microorganisms that produce methane (CH4) as a metabolic by-product in anaerobic conditions

Conditions where methanogens may be found

* Anaerobic
* Wetlands (e.g. swamps and marshes)
* Marine sediments (e.g. in the mud of lake beds
* Digestive tract of ruminant animals (e.g. cows, sheep, goat)

Reactions by which methanogens produce methane

* Acetic acid → Methane and Carbon Dioxide
* Carbon Dioxide and Hydrogen → Methane and Water

What is causing increased levels of methane?

* When organic matter is buried in anoxic conditions
* Rising global numbers of domesticated cattle

What happens to methane when released into the atmosphere?

Methane will be naturally oxidised to form carbon dioxide and water.

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Therefore there are low amounts of methane in the air even though a lot is being released.

Partial decomposition

When saprotrophic bacteria and fungi decompose dead organisms and return nutrients to the soil for cycling.

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This process requires oxygen

Waterlogged regions

May lack oxygenated air spaces within the soil and thus possess anaerobic conditions

What do waterlogged regions produce and the issue with them?

Anaerobic respiration by organisms produces organic acids resulting in acidic conditions

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Saprotrophic bacteria and fungi cannot function effectively in anaerobic/acidic conditions, preventing decomposition.

Formation of peat

* Since organic matter is not fully decomposed in waterlogged soils, carbon rich molecules remain and form peat.


* When peat is compressed under sediment, heat and pressure will force out impurities and remove moisture.


* Remaining material is high in carbon and undergoes a reaction to form coal.

How are fossil fuels produced?

* Organic compounds become rich in hydrocarbons, when compacted underground for millions of years.

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* Heat and pressure over time triggers a chemical transformation that results in compaction of organic matter.

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* Resulting product is fossil fuel

Production of fuel from biomass

An alternative to relying on fuels produced by geological processes is to manufacture fuels from biological processes.

Carbon fluxes

The rate of exchange of carbon between the various carbon sinks/reservoirs

What are the four main carbon sinks

* Lithosphere
* Hydrosphere
* Atmosphere
* Biosphere

What does the rate of carbon exchange depend on?

* Photosynthesis
* Respiration
* Decomposition
* Gaseous dissolution
* Lithification
* Combustion

How are global carbon fluxes measured?

in gigatonnes

What is the size of a carbon flux dependent on? (three things)

1. Climate conditions
2. Natural events
3. Human activity

Things carbon flux is dependent on: climate conditions

Climate change - higher rate of photosynthesis in summer.

Oceanic temperatures - determines how much carbon is stored as co2 or hydrogen bicarbonate ions

Climate events

Melting of polar ice caps

Things carbon flux is dependent on: Natural events

* Forest fires - high CO2 levels
* Volcanic eruptions can release carbon compounds

Things carbon flux is dependent on: human activity

* clearing of trees for agriculture
* increased number of livestock = high levels of methane
* burning of fossil fuels will release CO2 into the atmosphere

Why do CO2 levels fluctuate anually

* CO2 is lower in summer months because there are longer days with more light = more photosynthesis