The process of the carbon cycle

Define fluxes

The movement of carbon between carbon stores

Explain the process of precipitation

Atmospheric CO2 dissolves in rainwater to form weak acidic rain which has thus increased the acidity in ocean surface water - this can damage marine life

Explain the process of photosynthesis

The flux of carbon from the atmosphere to land plants and phytoplankton averages around 120 gigatonnes a year. Plants absorb CO2 from the atmosphere and return it through respiration.

How much more carbon is exchanged in photosynthesis and respiration than the slow water cycle?

x1000 faster

Explain the process of weathering

• Rainwater is slightly acidic and can slowly dissolve limestone and chalk (carbonation). This slowly releases CO2 into rivers, oceans and the atmosphere.

• This is the most effective beneath a soil cover because the higher concentration of CO2 in the soil makes rainwater highly acidic

• Chemical weathering transfers 0.3 bn tonnes into the atmosphere and the oceans every year

Where has the limestone surface has been lowered by nearly half a metre in the past 13,000 years?

Naber Brow in the Yorkshire Dales

Explain the process of physical and biological weathering

• This leaves rocks exposed to chemical attack and biological weathering contribute to rock breakdown.

• Rainwater mixed with dead and decaying organisms in the soil forms humid acids which attack rock minerals. This process is important in humid tropical environments where decomposition is rapid and forest trees provide litter.

Explain the process of decomposition

This releases CO2 into the atmosphere and mineral nutrients to the soil. Rates of decomposition is dependent on the climate - its fastest rates occur in warm, humid environment such as the tropical rainforest.

Explain the process of combustion

This is when organic material reacts in the presence of oxygen, releasing CO2. This can occur naturally through wildfires or due to human activities such as subsidence farming and global warming.

What is carbon sequestration?

When carbon is absorbed into the oceans

What is the physical/inorganic pump?

The mixing of the surface and deep oceans waters by vertical currents creating a more even distribution of carbon - both geographically and vertically - in the oceans and the atmosphere by diffusion

Explain the physical pump of carbon

• The mixing of the surface and deep oceans waters by vertical currents creating a more even distribution of carbon - both geographically and vertically - in the oceans and the atmosphere by diffusion

• Surface ocean currents then transport the water its dissolved CO2 poleward where it cools, become more dense and sinks. This is downwelling (sinking of dense, salty water in the ocean) - only occurs in certain places

• Downwelling carries the CO2 to the ocean depths where individual carbon molecules may remain for centuries

• Eventually the deep ocean currents transport the carbon to areas of upwelling. There cold, carbon-rich water rises to the surface and CO2 diffuses back into the atmosphere

Where does downwelling occur?

North Atlantic ocean between Greenland and Iceland

Explain the biological pump of carbon

• Carbon is also exchanged between the oceans and atmosphere through the actions of marine organisms

• Globally nearly half of all carbon fixation by photosynthesis takes place in the oceans - around 50GT of carbon is drown from atmosphere by the biological pump every year.

• Marine organisms drive the biological pump - phytoplankton, floating near the ocean surface, combines sunlight, water and CO2 to produce organic material

How does phytoplankton move through organisms?

• Whether consumed by animals in the marine food chain, or through natural death, carbon locked in the phytoplankton either accumulates in sediment and is decomposed and released into the ocean as CO2.

• Other marine organisms such as tiny molluscs extract carbonate and calcium ions from sea water to manufacture plates, shells and skeletons of calcium carbonate

• Most of this carbon-rich material eventually ends up in the ocean sediments and is ultimately lithified (compressed and compacted into solid rock) to from chalk and limestone

Explain the impact of vegetation

Land plants, especially trees in the rainforests and boreal forests, contain huge stores of carbon. Most of this is extracted from atmospheric CO2 through photosynthesis is locked away for decades.