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2.3.12 The lithosphere contains carbon stores in fossil fuels and in rocks, such as limestone, that contain calcium carbonate.

The residence time for carbon in these stores can be hundreds of millions of years.

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2.3.13 Reef-building corals and molluscs have hard parts that contain calcium carbonate that can become fossilised in limestone.

Limestone is the largest store of carbon in Earth systems. Not all limestone is formed by fossilisation of animal remains; it can also be formed by both biological and non-biological processes. Details of these processes are not required.

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2.3.14 In past geological eras, organic matter from partially decomposed plants became fossilised in coal, and partially decomposed marine organisms became fossilised in oil and natural gas held in porous rocks.

Formation of coal, oil and gas was greatest in specific geological eras when conditions were most suitable, and it took tens of millions of years for significant stores to accumulate.

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2.3.15 Methane is produced from dead organic matter in anaerobic conditions by methanogenic bacteria.

Anaerobic conditions suitable for methanogenesis occur in swamps, rice paddies and the stomachs of cattle and other ruminants.

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2.3.16 Methane has a residence time of about 10 years in the atmosphere and is eventually oxidised to carbon dioxide.

Methane is a potent greenhouse gas.

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The Lithosphere as a Carbon Store

At SL, you learned that fossil fuels are stores of carbon with very long residence times (2.3.7) and that carbon sequestration involves capturing CO₂ and storing it in solid or liquid form (2.3.5). This HL section goes deeper into where carbon is stored in the lithosphere and how it got there.

• The lithosphere is the solid, rocky outer layer of the Earth (crust, outermost mantle and soil)
• Two main carbon stores in the lithosphere: fossil fuels (coal, oil, natural gas) and rocks such as limestone that contain calcium carbonate (CaCO₃)
• Residence time for carbon in these stores can be hundreds of millions of years

Residence time comparisons:

1. Define the lithosphere.

2. Identify the two main forms in which carbon is stored in the lithosphere.

3. State the approximate residence time for carbon in lithospheric stores.

4. Using the table above, calculate how many times longer carbon remains in limestone compared to the atmosphere.

5. Explain why the concept of residence time is important when considering the impact of burning fossil fuels.