Topic 4.3 Carbon Cycling

Question 1

Different forms of carbon

Answer 1

• Atmospheric gases – mainly carbon dioxide (CO2), but also methane (CH4)
• Oceanic carbonates – including bicarbonates dissolved in the water and calcium carbonate in corals and shells
• As organic materials – including the carbohydrates, lipids and proteins found in all living things
• As non-living remains – such as detritus and fossil fuels

Question 2

Autotrophs and carbon dioxide

Answer 2

they convert inorganic carbon dioxide into organic compounds via photosynthesis

the level of carbon dioxide within the organism should be low since it is used during photosynthesis, in other words the concentration of carbon dioxide in the atmosphere is always higher

Question 3

The amount of carbon dioxide in the environment will be determined by the level of these two processes:

Answer 3

• If there is more net photosynthesis than cell respiration occurring in the biosphere, atmospheric carbon dioxide levels should drop
• If there is more net respiration than overall photosynthesis occurring, atmospheric carbon dioxide levels should increase

Question 4

Carbonic acid

Answer 4

Carbon dioxide dissolves in water and some of it will remain as a dissolved gas, however the remainder will combine with water to form carbonic acid (CO2 + H2O ⇄ H2CO3)

Question 5

Methanogens

Answer 5

archaean microorganisms that produce methane (CH4) as a metabolic by-product in anaerobic conditions

Question 6

Methanogens may be found in

Answer 6

Wetlands
Marine sediments
Digestive tract of ruminant animals

Question 7

Methane in the long run

Answer 7

it only persists for approx 12 years as it can be naturally oxidized into carbon and water

this is why there isn’t much in the atmosphere despite that quantities being produced

Question 8

By-products of anaerobic digestion

Answer 8

Acetic acid → Methane and Carbon Dioxide (CH3COO– + H+ → CH4 + CO2)

Carbon Dioxide and Hydrogen → Methane and Water (CO2 + 4 H2 → CH4 + 2 H2O)

Question 9

Carbon source: fossil fuels - how is it acquired

Answer 9

Plant/animals –decomposition–> Soil bacteria –fossilation–> Fossils –extraction–> Fossil fuels

Question 10

Partial decomposition

Answer 10

In many soils, saprotrophic bacteria and fungi will decompose dead organisms and return nutrients to the soil for cycling

Question 11

Coal formation

Answer 11

Since the organic matter is not fully decomposed in waterlogged soils, carbon-rich molecules remain in the soil and form peat

• When deposits of peat are compressed under sediments, the heat and pressure force out impurities and remove moisture
• The remaining material has a high carbon concentration and undergoes a chemical transformation to produce coal

Question 12

Formation of oil/natural gas

Answer 12

Oil (i.e. petroleum) and natural gas form as the result of the decay of marine organisms on the ocean floor (takes place over millions of years)

• Sediments (e.g. clay and mud) are deposited on top of the organic matter, creating anoxic conditions that prevent decomposition
• As a result of the burial and compaction, the organic material becomes heated and hydrocarbons are formed
• The hydrocarbons form oil and gas, which are forced out of the source rock and accumulate in porous rocks (e.g. sandstone)

Question 13

Pollution

Answer 13

Fuels -> Combustion -> Atmosphere (CO2)

Question 14

Combustion sources - fossil fuels

Answer 14

• The heat and pressure over time triggers a chemical transformation that results in the compaction of the organic matter
• The resulting products of this process are fossil fuels (coal, oil and natural gas)
• Because this geological process takes millions of years to occur, fossil fuels are a non-renewable energy source

Question 15

Combustion sources - biomass

Answer 15

• Living organisms produce hydrocarbons as part of their total biomass (either for use or as a waste product)
• These hydrocarbons can be extracted and purified to produce an alternative fuel source (e.g. bioethanol and biodiesel)
• Provided new raw materials are provided and waste products are removed, this source of energy is renewable

Question 16

Four main carbon sinks

Answer 16

lithosphere (earth crust)
hydrosphere (oceans)
atmosphere (air)
biosphere (organisms)

Question 17

Carbon conversion processes

Answer 17

Photosynthesis – removes carbon dioxide from the atmosphere and fixes it in producers as organic compounds

Respiration – releases carbon dioxide into the atmosphere when organic compounds are digested in living organisms

Decomposition – releases carbon products into the air or sediment when organic matter is recycled after death of an organism

Gaseous dissolution – the exchange of carbon gases between the ocean and atmosphere
Lithification – the compaction of carbon-containing sediments into fossils and rocks within the Earth’s crust (e.g. limestone)

Combustion – releases carbon gases when organic hydrocarbons (coal, oil and gas) are burned as a fuel source

Question 18

Carbon fluxes

Answer 18

impossible to directly measure so there are estimates measured in gigatonnes

Question 19

Main causes for flux changes

Answer 19

Climate Conditions
- Rates of photosynthesis will likely by higher in summer seasons, as there is more direct sunlight and longer days
- Oceanic temperatures also determine how much carbon is stored as dissolved CO2 or as hydrogen bicarbonate ions
- Climate events like El Nino and La Nina will change the rate of carbon flux between ocean and atmosphere
Melting of polar ice caps will result in the decomposition of frozen detritus

Natural Events

• Forest fires can release high levels of carbon dioxide when plants burn (loss of trees also reduces photosynthetic carbon uptake)
• Volcanic eruptions can release carbon compounds from the Earth’s crust into the atmosphere

Human Activity
- Clearing of trees for agricultural purposes (deforestation) will reduce the removal of atmospheric CO2 via photosynthesis
- Increased numbers of ruminant livestock (e.g. cows) will produce higher levels of methane
The burning of fossil fuels will release carbon dioxide into the atmosphere