4.3 Carbon Cycling

what is the carbon cycle

• what are the four spheres

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

• hydrosphere (water/ocean)

• biosphere (living things)

• lithosphere (ground)

• atmosphere (air)

variety of forms that carbon is exchanged

• 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

what do autotrophs do to carbon dioxide

convert inorganic carbon dioxide into organic compounds via photosynthesis

• include carbohydrates, lipids and proteins

CO2 concentration gradient in autotrophs

• should be low as use it up

• CO2 higher concentration in atmosphere

• creates a concentration gradient so CO2 passively diffuses into autotroph

CO2 diffusion in organisms

• aquatic producers usually diffuse directly into the autotroph

• terrestrial plants diffusion occurs at stomata

how do organisms produce CO2

how does it leave the organism

• respiration; breakdown of sugars

• diffuse into the ocean or atmosphere due to build up of it in tissue creating a conc gradient

what is the compensation point

the uptake of CO₂ by photosynthesis may at times be balanced by the production of CO₂ by respiration

• the net carbon dioxide assimilation is zero (intake = output)

what determines the amount of CO2 in the atmosphere

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

• If there is more net respiration than overall photosynthesis occuring, atmospheric carbon dioxide levels should increase

Carbon dioxide in water

• dissolves in water

• some of it will remain as a dissolved gas,

• remainder will combine with water to form carbonic acid  (CO₂ + H₂O  ⇄  H₂CO₃)

what does carbonic acid do (remainder of CO2 in ocean)

dissociate to form hydrogen carbonate ions  

(H₂CO₃  ⇄  HCO₃^– + H⁺)

what do autotrophs absorb/ use them form

dissolved carbon dioxide and hydrogen carbonate ions

• use them to produce organic compounds

which animals are made of carbon

what form is this carbon in/ what can happen

example; reef building coral, mollusca

• calcium carbonate, can be fossilised in limestone

how is calcium carbonate formed

what is this the subsequent development of

when hydrogen ions come into contact with rocks and sediment on the ocean floor and acquire metal ions

• limestone

how else can calcium carbonate be formed

• what does this calcium carbonate form

living animals combine hydrogen carbonate ions with calcium

• hard exoskeleton of coral and the main part of a mollusca shell

what happens when the animals made of calcium carbonate die and settle on the seabed

hard components may become fossilised in the limestone

how is methane produced

name of process

from organic matter in anaerobic conditions by methanogenic archaeans

methanogenesis

what are methanogens

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

Anaerobic conditions where methanogens may be found include:

• 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)

main ways methane is produced from

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

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

where does methane go

can accumulate underground or go into atmosphere

how does methane accumulate underground

When organic matter is buried in anoxic conditions (e.g. sea beds), deposits of methane may form underground

how does methane accumulate in the atmosphere

Rising global numbers of domesticated cattle may be increasing the levels of methane being released into the atmosphere

lifespan of methane once it reaches the atmosphere

why

10-12 years

methane is oxidised in the atmosphere

why are methane levels not really high/relatively constant

what highly reactive molecule causes this

Methane will be naturally oxidised to form carbon dioxide and water  (CH₄ + 2 O₂  →  CO₂ + 2 H₂O)

• hydroxyl radicals react with the methane to form this

how is peat formed

when organic matter is not fully decomposed because of acidic and / or anaerobic conditions in waterlogged soils

what microorganisms in most soils decompose dead organisms

what does this do

what does this require, why

saprotrophic bacteria and fungi

return nutrients to the soil for cycling

aerobic conditions, cell respiration is required to fuel digestive reactions

why does partial decomposition happen

• Waterlogged regions may lack oxygenated air spaces within the soil; anaerobic

• Anaerobic respiration produces organic acids (e.g. acetate), resulting in acidic conditions 

• Saprotrophic bacteria and fungi cannot function effectively in anaerobic/acidic conditions, preventing decomposition

• forms peat

how is coal formed

• When peat is compressed under sediments, heat and pressure force out impurities and remove moisture

• remaining material has a high carbon concentration

• undergoes a chemical transformation to produce coal

oil/natural gas formation

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 organic matter, creating anoxic conditions; prevent decomposition

• As a result of the burial and compaction, the organic material becomes heated & 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)

• takes place over millions of years

what organic compounds undergo combustion reaction and how

what type of reaction is this

(rich in) hydrocarbons which are heated

exergonic reaction (produces energy)

by products of combustion reactions

where does this carbon dioxide go

water and carbon dioxide

atmosphere

combustion sources

fossil fuels (coal, oil and natural gas)

biomass

hydrocarbons from biomass

• 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

what do carbon fluxes describe

the rate of exchange of carbon between the various carbon sinks / reservoirs

what are the four main carbon sinks

• lithosphere (earth crust)

• hydrosphere (oceans)

• atmosphere (air)

• biosphere (organisms)

conversion processes involved in carbon fluxes (6)

• Photosynthesis – removes CO2 from the atmosphere, fixes it in producers as organic compounds 

• Respiration – releases CO2 into the atmosphere when organic compounds are digested in living organisms

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

• Gaseous dissolution – 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 are burned as a fuel source

what are carbon fluxes measured in

issue with measuring

measured in gigatonnes

• 1 gigatonne of carbon = 1 billion metric tonnes

carbon fluxes are large and based on measurements from many different sources so estimates have large uncertainties 

main causes for flux change

climate conditions, natural events and human activity 

how do climate conditions influence flux change

• Rates of photosynthesis will likely by higher in summer seasons - direct sun/long 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

how do natural events influence flux change

• 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

how does human activity influence flux change

• Deforestation for agricultural purposes 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

where are atmospheric CO₂ concentrations measured, since when by who

at the Mauna Loa Observatory (in Hawaii) since 1958 by Charles Keeling

what is the carbon flux pattern that can be seen from the continuous and regular measurements of atmospheric CO₂ concentrations

• CO₂ levels fluctuate annually (lower in the summer months; photosynthesis)

• 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