8.1.3

Biological Carbon

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Carbon in the Food Chain

Land-based plants sequester carbon through photosynthesis. Consumers and other organisms then return this carbon to the atmosphere through respiration.

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Terrestrial primary producers

  • Primary producers (usually plants) are the first organisms in the food chain.
  • Primary producers can make their own energy from sunlight and carbon dioxide through photosynthesis.
    • Photosynthesis is the process where green plants sequester carbon dioxide through chloroplasts in their leaves to produce energy.
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Respiration

  • All living organisms respire and contain carbon.
    • By respiring, organisms release carbon dioxide into the atmosphere.
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Consumers

  • Consumer organisms eat other organisms below them in the food chain.
  • Primary consumers are the first organisms to eat plants.
    • E.g. Bugs, beetles and herbivores such as rabbits.
  • Primary consumers then return the carbon that the primary producer sequestered back into the atmosphere through the process of respiration.
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Decomposers

  • Biological decomposers consume dead organic matter and return the carbon to the atmosphere through respiration.

Sequestration by Phytoplankton

The oceans provide a biological carbon pump. Atmospheric carbon dissolves in the oceans where it can be transformed into terrestrial carbon or biological carbon before returning to the atmosphere.

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Photosynthesis in surface waters

  • Phytoplankton takes carbon dioxide out of the atmosphere through the process of photosynthesis.
  • As these organisms sequester (take up) carbon dioxide, they build their shells from calcium carbonate. This transforms the carbon in the atmosphere into biological carbon.
  • These organisms are consumed by other organisms (e.g. zooplankton). All of these consumer organisms will respire, returning some of the carbon to the atmosphere in the process.
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Carbonate shells sink

  • When phytoplankton and other aquatic organisms die, they sink to the ocean floor.
  • Here, these organisms can accumulate as sediment. Eventually, they can be transformed into sedimentary rock.
  • They can also be decomposed (broken down) by bacteria, which can return carbon into the ocean in the form of dissolved organic carbon.
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Carbonate pump

  • This whole process of carbon entering and moving around the ocean is referred to as the carbonate pump.
  • Without oceans acting as a store for carbon, levels of atmospheric carbon dioxide concentration would be significantly higher.
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The thermohaline circulation

  • The thermohaline circulation refers to the global movement of water.
  • Cold water sinks to the bottom of the ocean while warm water rises to the surface (moving towards cooler water).
  • This process is the reason for the UK’s relative warmth in comparison to other locations at the same latitude.
    • Warm Caribbean water passes the UK as it moves northward towards the poles.
  • Slight changes in temperature can change the operation of both the thermohaline circulation and the carbonate pump.

Dead Organic Matter

Dead organic matter found in the ground or in the soil still contains biological carbon. Different landscapes retain biological carbon in different ways.

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Soils

  • Dead organic matter found in the ground or in the soil still contains biological carbon.
  • The biological carbon can be returned to the atmosphere when decomposed by biological decomposers.
  • The rate at which biological carbon is returned to the atmosphere is dependent on temperature and climate.
  • Human activity such as deforestation and land-use change can speed up this process.
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Mangroves

  • Mangroves are found at tropical coastlines and sequester 1.5 metric tonnes of carbon per hectare every year.
  • Mangrove soils consist of layers of litter, humus and peat, which contain over 10% carbon.
  • Due to being submerged beneath the tidal water twice a day, the soils are anaerobic (without oxygen).
  • Biological decomposers cannot survive without oxygen so the breakdown of the biological material takes a lot longer.
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Tundra soils

  • Tundra regions of the world have permanently low temperatures.
  • The soil contains ancient carbon, which is permanently frozen, stopping the microbe activity that decays the material.
  • The microbe activity only happens once the surface layer thaws.
  • Tundra is a massive carbon store, holding carbon for hundreds of thousands of years.

Jump to other topics

1Tectonic Processes & Hazards

2Option 2A: Glaciated Landscapes & Change

3Option 2B: Coastal Landscapes & Change

4Globalisation

5Option 4A: Regenerating Places

6Option 4B: Diverse Places

7The Water Cycle & Water Insecurity (A2 only)

8The Carbon Cycle & Energy Security (A2 only)

9Superpowers (A2 only)

10Option 8A: Health & Human Rights (A2 only)

11Option 8B: Migration & Identity (A2 only)

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