Carbon Cycle

Question 1

Fast/terrestrial carbon cycle

Answer 1

• Refers to the uptake of CO2 by plants through photosynthesis
• CO2 is released back into the atmosphere by plant and animal respiration, whilst CO2 and methane is released through the decomposition of dead, organic matter
• The cycle of carbon between the SOIL, VEGETATION, and the ATMOSPHERE is relatively fast
• PHYTOPLANKTON also absorb CO2 from the atmosphere, as do oceans (sequestration), although the individual molecules of carbon have a 350 year residence time, after which the oceans ventilate the molecule back into the atmosphere

Question 2

Slow carbon cycle

Answer 2

• Refers to the cycling of carbon between rock stores, the atmosphere and oceans through the processes of weathering over millions of years
• Weathering of continental rock creates a net carbon sink in the ocean
• Chemical weathering of rocks by carbonic acid produces carbonate runoff, which is transferred into the oceans, and organisms use it to create their shells
• When these organisms die, the sediment produced eventually forms limestone
• This long term carbon store is later released back into the atmosphere through volcanic activity

Question 3

Atmospheric carbon cycle

Answer 3

• Atmospheric carbon occurs as CO2 and methane
• Methane is a more powerful greenhouse gas but is short lived in the atmosphere
• CO2 is removed from the atmosphere through terrestrial interactions (photosynthesis, water absorption)

Question 4

Oceanic carbon cycle

Answer 4

• Carbon is held in the oceans either in the water in a dissolved form, or in the tissues and shells of oceanic organisms
• Inputs and outputs come from gas exchange with the atmosphere and through an input of organic carbon and carbonate ions from continental run-off
• Small changes in this cycle have global impacts due to the massive size of the cycle
• Ocean sediments are an important long term carbon store

Question 5

Oceanic carbon sequestration

Answer 5

The oceans “take up” carbon (by one of two mechanisms- the physical or biological pumps)

Question 6

Physical (inorganic) pump

Answer 6

• Involves the mixing of surface and deep ocean waters by vertical currents
• This creates a more uniform carbon distribution, both geographically and vertically
• CO2 initially enters the ocean, from the atmosphere, through diffusion
• The dissolved CO2 is transported towards the poles by ocean currents, where it cools and becomes more dense, so sinks (downwelling)
• These carbon molecules can stay at the bottom of the ocean for centuries, before deep ocean currents transport them to areas of upwelling, where they will rise to the surface and diffuse back into the atmosphere

Question 7

Biological (organic) pump

Answer 7

• The actions of marine organisms also contribute to the transfer of carbon between the atmosphere and oceans
• Nearly half of global carbon fixation by photosynthesis happens in the oceans
• Around 50GT of carbon is drawn from the atmosphere by the biological pump every year
• Phytoplankton near the surface of the ocean combine sunlight, dissolved CO2 and water to produce organic material
• Carbon locked in the phytoplankton accumulates in sediments on the ocean floor or is decomposed and released into the ocean as CO2
• Molluscs and crustaceans extract carbonate and calcium ions for their shells or skeletons of calcium carbonate
• Most of this material ends up as ocean sediments and is eventually lithified to form chalk or limestone