AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
How carbon dioxide decreased: Carbon dioxide was also decreased by the formation of sedimentary rocks and fossil fuels that contain carbon.
Photosynthesis was not the only process that lowered atmospheric carbon dioxide.
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Photosynthesis was not the only process that lowered atmospheric carbon dioxide. Carbon dioxide also dissolved in water and reacted to form carbonate compounds. Some of these compounds precipitated or were incorporated into sediments, forming sedimentary rocks such as limestone, which contains calcium carbonate, CaCO₃. Marine organisms built shells and skeletons from carbonates; when they died, these remains could become sedimentary rock, locking carbon away for millions of years. Fossil fuels formed from buried remains of plants and marine organisms. Over time, heat and pressure changed this organic material into coal, crude oil and natural gas, all of which contain carbon. For example, limestone and coal both store carbon that was once in the atmosphere. The statement therefore covers two carbon stores: sedimentary rocks and fossil fuels.
What earns the marks
- Carbon dioxide dissolved in water and reacted to form carbonate compounds.
- Sedimentary rocks such as limestone contain carbon as calcium carbonate, CaCO₃.
- Shells and skeletons of marine organisms were made of carbonates and could become sedimentary rock.
- Fossil fuels formed from buried remains of plants and marine organisms and contain carbon.
- Coal, crude oil and natural gas are fossil fuels that store carbon removed from the early atmosphere.
- The formation of these rocks and fuels removed carbon dioxide from the atmosphere over geological time.
Common mistakes and how to put them right
Stating that sedimentary rocks form directly from carbon dioxide gas.
Put it right by explaining that carbon dioxide first dissolves and forms carbonate compounds, which then form rock.
Claiming that fossil fuels are made of pure carbon.
Put it right by stating that they are carbon-containing fuels formed from buried organic remains.
Saying fossil fuels remove carbon dioxide when they burn.
Put it right by stating that burning releases carbon dioxide; their formation removed carbon.
Exam tips
- Give a named example of a carbon-containing sedimentary rock, such as limestone, and state that it contains calcium carbonate.
- Link fossil fuels to their origin in buried plants and marine organisms rather than describing them as minerals.
- Use the phrase 'over millions of years' to show the geological timescale involved in forming rocks and fuels.
Try the lesson
The start of the How carbon dioxide decreased: Carbon dioxide was also decreased by the formation of sedimentary rocks and fossil fuels that contain carbon. lesson, made from checked teaching notes for this specification point.
Have a go first
Which process contributed to the decrease of carbon dioxide in the Earth's early atmosphere?
Listen
Photosynthesis was not the only process that lowered atmospheric carbon dioxide. Carbon dioxide also dissolved in water and reacted to form carbonate compounds. Some of these compounds precipitated or were incorporated into sediments, forming sedimentary rocks such as limestone, which contains calcium carbonate, CaCO₃. Marine organisms built shells and skeletons from carbonates; when they died, these remains could become sedimentary rock, locking carbon away for millions of years. Fossil fuels formed from buried remains of plants and marine organisms. Over time, heat and pressure changed this organic material into coal, crude oil and natural gas, all of which contain carbon. For example, limestone and coal both store carbon that was once in the atmosphere. The statement therefore covers two carbon stores: sedimentary rocks and fossil fuels. A strong answer earns marks for these points. Carbon dioxide dissolved in water and reacted to form carbonate compounds. Sedimentary rocks such as limestone contain carbon as calcium carbonate, CaCO₃. Shells and skeletons of marine organisms were made of carbonates and could become sedimentary rock. Fossil fuels formed from buried remains of plants and marine organisms and contain carbon. A common slip: Stating that sedimentary rocks form directly from carbon dioxide gas. Instead, explaining that carbon dioxide first dissolves and forms carbonate compounds, which then form rock. Exam tip: Give a named example of a carbon-containing sedimentary rock, such as limestone, and state that it contains calcium carbonate.
Key words
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Sedimentary rocks
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Fossil fuels
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Carbon stores
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See every definition
- Sedimentary rocks
- Rocks such as limestone that contain carbon as calcium carbonate, CaCO₃, formed when carbonate compounds precipitated or were incorporated into sediments.
- Fossil fuels
- Coal, crude oil and natural gas, formed from buried remains of plants and marine organisms, that contain carbon.
- Carbon stores
- Sedimentary rocks and fossil fuels, which lock carbon away for millions of years.
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