IB Environmental systems and societies 2.3B Carbon Cycle Notes

Carbon Is Stored in Organic and Inorganic Forms

1. Carbon exists in both organic and inorganic forms across different reservoirs.
2. These carbon stores regulate atmospheric CO₂ and keep ecosystems in balance.

Organic Carbon Stores

1. Organic carbon sits in living organisms and in fossilised remains like crude oil, coal, and natural gas.
2. It can return to the system through decomposition, combustion, or being eaten.

Inorganic Carbon Stores

1. Inorganic carbon exists as dissolved CO₂, carbonates, and bicarbonates in water, air, and soil.
2. It is not part of living tissue.

Carbon Stores Stay in Equilibrium When Inputs Match Outputs

1. A carbon store is in equilibrium when the rate of carbon absorption equals the rate of release.
2. This balance keeps the store's carbon content stable over time.
   1. Absorption moves carbon in, for example photosynthesis and ocean uptake.
   2. Release moves carbon out, for example respiration, decomposition, and combustion.

Residence Time Varies Hugely Between Stores

Different stores hold carbon for vastly different lengths of time, depending on how fast it moves in and out.

Carbon Flows: Transfers Move It, Transformations Change It

1. Carbon moves between organic and inorganic stores through biological and physical processes.
2. These flows are either transfers (carbon changes location, not form) or transformations (carbon changes chemical form).

Transfers

1. Feeding: carbon moves up trophic levels as animals eat plants.
2. Defecation: carbon returns to soil as organic waste.
3. Death and decomposition: carbon passes from organisms to decomposers and soil.

Transformations

1. Photosynthesis: inorganic CO₂ and H₂O become organic glucose using light energy.
2. Cellular respiration: organic molecules release CO₂ and H₂O for energy.
3. Dissolution: atmospheric CO₂ dissolves into ocean water.
4. Combustion: burning biomass or fossil fuels converts organic carbon into CO₂.
5. Fossilisation: partly decomposed organic matter becomes fossil fuels over geological time.

Building a Systems Diagram of the Carbon Cycle

1. A good diagram links the atmosphere, producers, consumers, decomposers, soils, and fossil fuel stores with labelled arrows.
2. Identify the main carbon stores:
   1. Biosphere: plants, animals, and decomposers.
   2. Lithosphere: fossil fuels and sedimentary rocks.
   3. Hydrosphere: dissolved carbon in the oceans.
3. Add the flows:
   1. Photosynthesis: CO₂ → plants.
   2. Feeding: plants → animals.
   3. Respiration: plants and animals → CO₂.
   4. Decomposition: dead matter → soil and atmosphere.
   5. Fossilisation: organic matter → fossil fuels.
   6. Combustion: fossil fuels → CO₂.

Carbon Sequestration Locks Carbon Away