4.3 Carbon Cycling

Construct a Carbon Cycle

The carbon cycle is a biogeochemical cycle whereby carbon is exchanged between the different spheres of the Earth

• The four spheres are the atmosphere (air), lithosphere (ground), hydrosphere (water / oceans) and biosphere (living things)

List forms of Carbon exchange

Carbon is exchanged between a variety of forms, including:

• Atmospheric gases – mainly carbon dioxide, but also methane

• 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

List factors affecting the Carbon Cycle

• Burning Fossil fuels

• Deforestation

• Over-population

List the defining qualities of methanogens

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

• Wetlands (e.g. swamps and marshes)

• Marine sediments (e.g. in the mud of lake beds)

• Digestive tract of ruminant animals (e.g. cows, sheep, goats)

Methanogens produce methane from the by-products of anaerobic digestion, principally acetic acid and carbon dioxide:

• Methane produced by methanogens maybe used as heat ad thus an energy source [biofuel]

Outline the use of methane to produce CO2

Methane will be naturally oxidised to form carbon dioxide and water  (CH4 + 2 O2  →  CO2 + 2 H2O), having interacted with hydroxyl radicals in the upper layers of the atmosphere

• This is why methane levels in the atmosphere are not very large, even though significant quantities are being produced

Outline coal formation

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

Outline natural gas formation

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

• 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)

Outline how organic matter is used for methane production, e.g. manure, cellulose.

• Metabolic by-product of methanogenic archaeans in anaerobic conditions

  • Waterlogged mud, landfill sites, guts of ruminants

• Either released directly into atmosphere or accumulates in soil and gradually reaches surface

• Causes of increased methane production : Cattle farming + landfill disposal of waste food

Define peat and its formation

• A mix of plant material (humus) in wetland under anaerobic/acidic conditions, compacted under its own weight, wherein saprotrophs cannot grow (hence dead leaf matter and organic debris cannot be digested)

New layers of leaf litter and other organic debris fall on top of this older layer of material, further compressing it ⇒ peat

• Formed in water-logged areas [peat bogs]

Outline the processes involved in Ocean Acidification

• Dissolved CO₂ forms carbonic acid H₂CO₃, which may further dissociate to form carbonate CO₃ (2-)

  • This is via the release/dissociation of H protons => Lowers ocean pH

    • Decreases organism’s metabolism and immune response

• Carbonate ions may be absorbed by molluscs and corals to form calcium carbonate

  • CaCO3 used for hard exoskeletons of marine creatures

• Increased dissolution of CO2 :

  • Increase in H+ production ⇒ Lowers ocean pH

  • H+ ions combine with free CO3(2-) ions to form Bicarbonate (competes with molluscs who want to form Calcium carbonate)

    • Reduced availability of CO3(2-) ions for exoskeleton

    • Degradation of shells due to acidity of ocean + deformed shells due to CO3(2-) scarcity

Examples of arguments supporting Global warming

Claim 1:  Climate has changed in the past and current trends merely reflect the Earth’s natural climatic cycle

• Data collected from the Vostok ice core shows several changes in climate over the last 400,000 years (Figure 1)

• At several points in history, global average temperatures have been warmer than those currently observed

Counter Argument:

• Climate changes do occur naturally, but usually not as abruptly as what is seen currently

• When global warming occurred abruptly in the past, it was always highly destructive to life (e.g. Permian mass extinction)

• Atmospheric CO levels positively correlate to average global temperatures and are currently at the highest levels ever recorded

Define lithification

Lithification  – is the process of compressing organic matter over a long period of time until it becomes rock.

Outline why CaCO3 is a source of Carbon

• Found in shells of molluscs and exoskeletons of hard corals and crustaceans

• CaCO3 does not dissolve in alkaline solutions, hence oceans [slightly alkaline] construct the ideal conditions for the formation of sedimentary rock, e.g. limestone with 12% carbon

List the carbon reservoirs on Earth

• Sedimentary rock (limestone deposits)

• Biomass of plants/animals [calcium carbonate shells]

• fossil fuels

• CO2 dissolved in the ocean

Define ruminants

Ruminants  – mammals that have a mutualistic relationship with methanogenic archaeans that help them to digest cellulose from the cell walls in the plants they eat. This creates methane, which is released as gas from the mammal.

List the carbon processes involving carbon exchange

• 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

Define carbon fluxes

Carbon fluxes describe the rate of exchange of carbon between the various carbon sinks / reservoirs

• There are four main carbon sinks – lithosphere (earth crust), hydrosphere (oceans), atmosphere (air), biosphere (organisms)

How does energy flow through a food web

1. Plants (autotrophs) convert solar energy into chemical energy, locked inside the OG matter, through the conversion of inorganic matter from atmosphere [e.g. CO2] into organic matter [1.a) Sugars

2. Consumers feed on the plants, hence the energy is transferred up trophic levels through consumption of the plants’ organic material

• When an organism dies, the chemical energy stored in carbon compounds in its tissues passes to detritivores and saprotrophs

• In a food chain, the arrows represent the transfer of energy, in the form of stored chemical energy in carbon compounds, from one trophic level to the next, by the process of feeding

• Energy procured is used for ATP synthesis, to carry out life-sustaining functions

• Transfer of energy between trophic levels decreases by 90% per trophic level

Explain trends in CO2 levels

• CO2 levels fluctuate annually (lower in the summer months when long days and more light increase photosynthetic rates)

• Global CO2 trends will conform to northern hemisphere patterns as it contains more of the planet’s land mass (i.e. more trees)

• CO2 levels are steadily increasing year on year since the industrial revolution (due to increased burning of fossil fuels)

• Atmospheric CO2 levels are currently at the highest levels recorded since measurements began

How is heat lost to the environment

• As heat, due to respiration

• As waste products, thus heat, after decomposition by sapro/detri

• Indigestible food sorts

• Partial food consumption [excluding bones]

Outline the causes of carbon flux changes and their implications

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

List the consequences of Global Warming

• Dramatic changes in weather patterns

• Increased agricultural productivity for cooler countries

• Rising sea levels destroy coastal habitats

• Loss of biodiversity due to migration of species or extinction (lack of resources/survival advantage)

• Migration of species

• Spread of diseases

• Resource scarcity

• Ocean acidification & coral bleaching