How carbon dioxide decreased — AQA GCSE Combined Science
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How carbon dioxide decreased explained
Early Earth's atmosphere contained much more carbon dioxide than today.
Read the full explanation
Photosynthesis in algae and plants removed carbon dioxide and stored the carbon in biomass. In photosynthesis, carbon dioxide and water react using light energy to produce glucose and oxygen: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. The glucose is used for growth, so carbon becomes locked in plant and algal tissue. Over millions of years, this process gradually lowered the percentage of carbon dioxide in the atmosphere while raising oxygen. For example, marine algae photosynthesise in surface waters, forming biomass that may sink and become buried. The statement links a biological process to a global atmospheric change, so answers should name photosynthesis, identify algae and plants as the organisms, and explain that carbon is removed from the air and incorporated into biomass.
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. 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.
describe the main changes in the atmosphere over time and some of the likely causes of these changes
You must describe how Earth's atmosphere changed from an early mixture of carbon dioxide, water vapour, methane and ammonia to today's atmosphere of 78% nitrogen and 21% oxygen. Carbon dioxide decreased because it dissolved in oceans, forming carbonates that precipitated as sedimentary rocks. Photosynthesis by algae and plants removed more carbon dioxide and released oxygen, so oxygen gradually increased. The remains of these organisms later formed fossil fuels. Nitrogen increased because it was released by volcanoes, and being unreactive, it accumulated. When describing, give a clear sequence of changes; when explaining causes, link each change to a named process. Use data such as percentage composition to support your answer.
describe and explain the formation of deposits of limestone, coal, crude oil and natural gas.
These deposits are carbon stores formed over millions of years. Limestone is mostly calcium carbonate, CaCO₃, precipitated or compacted from the shells and skeletons of marine organisms such as corals and molluscs, so it forms in warm shallow seas. Coal, crude oil and natural gas are fossil fuels formed from the remains of living organisms. Coal formed from plant material that was buried in swampy conditions where there was little oxygen, so decay was incomplete; heat and pressure over geological time drove off water and gases, raising the carbon content. Crude oil and natural gas formed mainly from marine plankton buried in mud on the seabed. Absence of oxygen prevented full decay, and heat and pressure converted the organic material into liquid and gaseous hydrocarbons. These hydrocarbons then migrated through porous rock and were trapped beneath impermeable caps, forming reservoirs.
Your focus
- State that algae and plants use carbon dioxide during photosynthesis.
- Describe how carbon from carbon dioxide becomes stored in biomass.
- Explain how photosynthesis by algae and plants decreased the percentage of carbon dioxide in the early atmosphere.
Show all 12 objectives
- Identify sedimentary rocks and fossil fuels as carbon-containing stores.
- Describe how carbonate compounds and buried organic remains remove carbon dioxide from the atmosphere.
- Explain how the formation of sedimentary rocks and fossil fuels decreased atmospheric carbon dioxide.
- Describe the main changes in the atmosphere over time, including the decrease in carbon dioxide and increases in oxygen and nitrogen.
- Explain likely causes of these changes, including dissolving in oceans, formation of sedimentary rocks and fossil fuels, photosynthesis, and volcanic activity.
- Use data on atmospheric composition to support descriptions and explanations of change over time.
- Describe the origin of limestone, coal, crude oil and natural gas.
- Explain how burial, limited oxygen, heat and pressure over millions of years form each deposit.
- Distinguish the source organisms of coal from those of crude oil and natural gas.
How carbon dioxide decreased exam tips
Marking Points
- Photosynthesis uses carbon dioxide from the atmosphere as a reactant.
- Algae and plants are photosynthetic organisms that absorb carbon dioxide.
- The carbon from carbon dioxide becomes part of glucose and then biomass.
- Oxygen is released as a by-product, contributing to the rise in atmospheric oxygen.
- Over geological time, repeated photosynthesis by vast numbers of algae and plants reduced the percentage of carbon dioxide in the atmosphere.
- The carbon removed is stored in living tissue and can later be buried, so it is not immediately returned to the air.
- 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.
- Describe the early atmosphere as mainly carbon dioxide with little or no oxygen, plus water vapour, methane and ammonia from volcanic activity.
- Describe the decrease in carbon dioxide as caused by dissolving in oceans to form sedimentary rocks, and photosynthesis by algae and plants.
- Explain that the remains of algae and plants that absorbed carbon dioxide later formed fossil fuels such as coal, oil and natural gas.
- Describe the increase in oxygen as caused by photosynthesis, leading to conditions that allowed more complex life to evolve.
- Describe the increase in nitrogen as caused by volcanic activity, and explain that nitrogen is unreactive so it accumulates.
- Limestone is mainly calcium carbonate, CaCO₃, formed from the shells and skeletons of marine organisms such as corals and molluscs.
- Coal formed from plant material buried in swampy, low-oxygen conditions, so decay was incomplete.
- Heat and pressure acting over millions of years drove off water and gases from buried plant remains, increasing the carbon content to form coal.
- Crude oil and natural gas formed mainly from marine plankton buried in mud on the seabed in the absence of oxygen.
- Heat and pressure over geological time converted the buried organic remains into liquid and gaseous hydrocarbons.
- Hydrocarbons migrated through porous rock and accumulated beneath impermeable traps, forming crude oil and natural gas reservoirs.
- All four deposits are carbon stores formed over millions of years, which is why fossil fuels are described as finite.
Examiner Tips
- 💡Name the process as photosynthesis and state the word equation or balanced symbol equation to show carbon dioxide is used.
- 💡Link the removal of carbon dioxide to the storage of carbon in biomass rather than saying the gas simply disappears.
- 💡Use comparative language such as 'decreased the percentage' rather than 'removed all carbon dioxide', because other processes also affected the atmosphere.
- 💡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.
- 💡Structure your answer as a timeline: early atmosphere, then removal of carbon dioxide, then increase in oxygen, then increase in nitrogen.
- 💡If a graph or table is given, quote at least two figures and link each to a named process to show you are using the data.
- 💡Link each deposit to its source organism and to the conditions of burial, then add the role of heat and pressure over geological time.
- 💡Use the word 'hydrocarbons' for crude oil and natural gas, and 'calcium carbonate' for limestone, to show precise scientific vocabulary.
- 💡When asked to explain, give a cause-and-effect chain such as burial → little oxygen → incomplete decay → heat and pressure → deposit.
Common Mistakes
- Saying plants and algae 'breathe in' carbon dioxide: correct this by stating that carbon dioxide is a reactant used in photosynthesis, not taken in by breathing.
- Writing that photosynthesis produces carbon dioxide: correct this by stating that photosynthesis removes carbon dioxide and produces oxygen.
- Confusing the role of algae with that of animals: correct this by noting that algae are photosynthetic organisms, whereas animals do not photosynthesise.
- Stating that sedimentary rocks form directly from carbon dioxide gas: correct this by explaining that carbon dioxide first dissolves and forms carbonate compounds, which then form rock.
- Claiming that fossil fuels are made of pure carbon: correct this by stating that they are carbon-containing fuels formed from buried organic remains.
- Saying fossil fuels remove carbon dioxide when they burn: correct this by stating that burning releases carbon dioxide; their formation removed carbon.
- Saying carbon dioxide decreased only because plants used it; correction: dissolving in oceans and forming sedimentary rocks were also major causes.
- Claiming oxygen increased before photosynthesis evolved; correction: oxygen was produced by photosynthesis, so it increased after algae and plants appeared.
- Describing nitrogen as the main gas in the early atmosphere; correction: the early atmosphere was mainly carbon dioxide, and nitrogen increased later.
- Saying coal formed from dinosaurs or from marine organisms: coal formed from plant material, while crude oil and natural gas formed mainly from marine plankton.
- Stating that the organisms simply rotted away: decay was incomplete because oxygen was limited, which allowed carbon-rich remains to be preserved.
- Claiming that heat and pressure alone created the deposits without burial or time: burial, low oxygen and millions of years are all needed.