1.3.1 The carbon cycle

Question 1

Carbon cycle

Answer 1

The process by which carbon is stored and transferred.

Question 2

What type of system is the carbon cycle, and why?

Answer 2

The carbon cycle is a closed system – there are inputs and outputs of energy, but the amount of carbon in the system remains the same.

However, some carbon is locked away (sequestered) in long-term stores (e.g. rock and fossil fuels deep underground). If these are released by (e.g. burning fossil fuels) they are effectively inputs.

Question 3

What are the important carbon compounds?

Answer 3

• CO2 – A greenhouse gas
• CH4 – A greenhouse gas
• CaCO3 – Calcium carbonate / limestone
• Hydrocarbons – fossil fuels, e.g. oil, coal, gas

Question 4

Stores of carbon

Answer 4

• Lithosphere
• Atmosphere
• Biosphere
• Cryosphere
• Hydrosphere

Question 5

[Stores of carbon]

Lithosphere

Answer 5

• Over 99.9% of the carbon on Earth is stored in sedimentary rocks such as limestone.
• About 0.004% of the carbon on Earth is stored in fossil fuels, such as coal and oil, in the lithosphere.

Question 6

[Stores of carbon]

Atmosphere

Answer 6

• Carbon is stored as carbon dioxide (CO2) and in smaller quantitative as methane (CH4) in the atmosphere.
• The atmosphere contains about 0.001% of the Earth’s carbon.

Question 7

[Stores of carbon]

Biosphere

Answer 7

• Carbon is stored in the tissues of living organisms. It is transferred to the soil when living organisms die and decay.
• The biosphere contains approximately 0.004% of the Earth’s total carbon.

Question 8

[Stores of carbon]

Cryosphere

Answer 8

• The cryosphere contains less than 0.01% of the Earth’s carbon.
• Most of the carbon in the cryosphere is in the soil in areas of permafrost (permanently frozen ground) where decomposing plants and animals have frozen into the ground.

Question 9

[Stores of carbon]

Hydrosphere

Answer 9

• Carbon dioxide (CO2) is dissolved in rivers, lakes and oceans.
• The oceans are the second-largest carbon store on Earth, containing approximately 0.04% of the Earth’s carbon. The majority of carbon here is found deep in the ocean in the form of dissolved inorganic carbon.
• A small amount is found at the ocean surface where it is exchanged with the atmosphere.

Question 10

Flows of carbon

Answer 10

• Photosynthesis
• Decomposition
• Respiration
• Combustion
• Ocean uptake and loss
• Weathering
• Sequestration

Question 11

[Flows of carbon]

Photosynthesis

Answer 11

• Photosynthesis transfers carbon stored in the atmosphere to biomass.
• Plants and phytoplankton use energy from the Sun to change carbon dioxide and water into glucose and oxygen. This enables plants to grow.
• Carbon is passed through the food chain and released through respiration and decomposition.

Question 12

[Flows of carbon]

Decomposition

Answer 12

• Decomposition transfers carbon from dead biomass to the atmosphere and the soil.
• After death, bacteria and fungi break organisms down. Carbon dioxide and methane are released.
• Some carbon is transferred to the soil in the form of humus.

Question 13

[Flows of carbon]

Respiration

Answer 13

• Respiration transfers carbon from living organisms to the atmosphere.
• Plants and animals break down glucose for energy, releasing carbon dioxide and methane in the process.

Question 14

[Flows of carbon]

Combustion

Answer 14

• Combustion transfers carbon stored in living, dead or decomposed biomass (including peat) to the atmosphere by burning.
• Wildfires cause carbon flow.

Question 15

[Flows of carbon]

Ocean uptake and loss

Answer 15

• Carbon dioxide is directly dissolved from the atmosphere into the ocean. It is also transferred to the oceans when it is taken up by organisms that live in them (e.g. plankton).
• Carbon is also transferred from the ocean to the atmosphere when carbon-rich water from deep in the oceans rises to the surface and releases carbon dioxide.

Question 16

[Flows of carbon]

Weathering

Answer 16

• Chemical weathering transfers carbon from the atmosphere to the hydrosphere and biosphere.
• Atmospheric carbon reacts with water vapour to form acid rain. When this acid rain falls onto rocks, a chemical reaction occurs which dissolves the rock. The molecules resulting from this reaction may be washed into the sea. Here, they react with carbon dioxide dissolved in the water to form calcium carbonate, which is used by sea creatures (e.g. to make shells).

Question 17

[Flows of carbon]

Sequestration

Answer 17

• Carbon from the atmosphere can be sequestered (captured and held) in sedimentary rocks or as fossil fuels. Rocks and fossil fuels form over millions of years when dead animals and plant material in the ocean falls to the floor and is compacted.
• Carbon in fossil fuels is sequestered until it is burned by humans.

Question 18

Human activities causing the movement of carbon

Answer 18

• Hydrocarbon extraction and combustion of fossil fuels. The carbon stored with the fossil fuels are emitted when burned.
• Farming practices – Beef industry. Cows produce methane, which is a greenhouse gas.
• Deforestation – slash and burn techniques causes carbon to be released from the trees. Less CO2 can be absorbed if deforestation happens.
• Urban growth causing an increase in urban growth and imerpeable surfaces.

Question 19

Sequestration

Answer 19

A slow carbon flow. It takes millions of years for carbon to be sequestered in sedimentary rocks.