Carbon Cycle Transfers (AQA A Level Geography)

• Carbon stores can be thought of as a natural reservoir of carbon before the transfer
• Every carbon store can be a carbon source and a carbon sink
• Earth's carbon stores change constantly (they are dynamic) over time depending on location, time and scale
• Carbon transfer occurs at a plant, or sere level such as the lithosere

Plant scale carbon cycling shown through a tree

• A tree's wood acts as a carbon store
• Wood is approximately 50% carbon
• Carbon is transferred between the atmosphere, the biosphere and pedosphere through the processes of:
  • Photosynthesis by the tree which removes CO₂ directly from the atmosphere 
  • Respiration by the tree and microbes in the soil, returns carbon to the atmosphere as CO₂
  • Decomposition of leaf litter or death of the tree, also returns carbon to the atmosphere or soil
  • Combustion due to wildfires releasing large amounts of stored carbon in the tree

• The lithosere is one example of terrestrial carbon cycling
• A ‘sere’ is a stage in the succession of vegetation in an ecosystem
  • A lithosere is vegetation succession that occurs on bare rock
  • A hydrosere occurs in freshwater e.g. a pond
  • A halosere occurs in salt-rich conditions e.g. salt marshes
  • A psammosere occurs in sandy areas e.g. sand dunes
• When environmental equilibrium or balance is reached further succession stops - the final stage of a sere is reached 
• The end nature of the vegetation found is mostly due to the climatic conditions and is known as the 'climatic climax community'
•  In the UK, the usual climatic climax community for a lithosere is a deciduous wood
• The carbon cycle at a ‘sere’ scale is much more complex involving numerous different stores, and many transfers which all vary over space and time

Carbon cycling at a 'sere' (lithosere) level, note the complexity of the interactions

Flows and transfers of carbon at a continental scale

• The carbon cycle at a continental scale involves all of the fast and slow carbon cycles 
• The connections between the stores is very complex
• The rate of transfers varies over time due to changing conditions on the planet
• Human activity has added another dimension at this scale, particularly with increased additions of carbon dioxide to the atmosphere

Halosere carbon transfer - oceans

• There are three key processes that occur in oceans which allow them to be a carbon sink, store and transfer: A physical, solubility and biological cycle
• Oceans can absorb more carbon than it emits (a net gain of 0.6 Gtc yr-1)
• Oceans are important in regulating the composition of the atmosphere and is a two-way relationship with carbon being moved both downwards and upwards through the oceans

Physical pump / thermohaline circulation

• Considered the most important transfer 
• Carbon dioxide (CO₂) is absorbed by the ocean's surface through diffusion
• Dissolved CO₂ is then taken from the surface down to the intermediate and deep ocean stores through downwelling currents (96 GtC per year)
• The thermohaline circulation then distributes the carbon around the planet
• Cold water absorbs more CO₂, therefore, as the equatorial waters move toward the poles, more CO₂ is absorbed
• Salinity increases at the same time, making the water denser, therefore, the water sinks (downwelling) taking CO₂ from the ocean's surface to the deep ocean stores
• Allowing more diffusion to occur at the surface and helping to regulate the carbon stored in the atmosphere
• However, there is also the upwelling of carbon from intermediate and deep oceans to the surface oceans (105.6 GtC yr-1) 
• Through upwelling currents and turbulence created by surface winds, previously stored carbon in the intermediate and deep ocean stores, return to the ocean’s surface and then back into the atmosphere

Image showing the three carbon cycles of oceans (a halosere) and the link with the thermohaline circulation

Biological cycle/sequestration of carbon through photosynthesis

• The biological cycle sequesters carbon in the ocean through photosynthesis by phytoplankton and other marine animals which converts CO₂ into organic matter (10GtC per year)
• This acts as a biological pump transporting carbon from the oceans' surface to the intermediate and deep ocean stores (10 GtC per year)
• As the biological organisms die, their dead cells, shells and other parts sink into the mid and deep water
• Also, the decay of these organisms releases carbon dioxide into the intermediate and deep water stores 
• Oceans regulate the composition of the atmosphere by moving carbon from the ocean’s surface (where it may vent back into the atmosphere) and storing it in the mid and deep ocean store, along with the dissolved carbon store, which regulates the carbon cycle

Solubility cycle/carbonate pump

• The solubility cycle occurs when CO₂, absorbed by the oceans from the atmosphere, forms carbonic acid which in turn reacts with hydrogen ions to form bicarbonates and then further reactions form carbonates which are stored in the upper ocean
• Some organisms use these carbonates to make their shells or skeletons
• When these organisms die some material sinks to the ocean floor and forms the sea bed sediment store (1750 GtC)
• Over time, through chemical and physical processes, the carbon is transformed into rocks such as limestone
• This process locks up carbon in the long-term carbon cycle and does not allow an easy return to the ocean surface and so prevents possible venting into the atmosphere as the physical pump does

• The main transfers are: 
  • Photosynthesis
  • Respiration
  • Decomposition
  • Combustion
  • Burial and compaction
  • Sequestration 
  • Weathering 
• Weathering:
  • The main process is a form of chemical weathering called carbonation
  • The atmosphere has CO₂ that combines with water vapour to produce a weak acid known as carbonic acid, this makes precipitation slightly acidic
  • Calcium carbonate (calcite) in rocks, reacts with acidic water and forms calcium bicarbonate, which is soluble and is removed in solution by percolating water via streams, rivers and oceans and eventually back into the atmosphere
• Burial and compaction:
  • Oceans also absorb carbon from the atmosphere and this goes into the shells and skeletons of marine creatures as calcium carbonate
  • When the creatures die, they sink to the bottom and build up layers
  • Over time they compact down to create sedimentary rocks such as limestone
  • Under heat and pressure, carbon from organic matter is trapped in the sediment and is converted into hydrocarbons
• Sequestration:
  • Natural sequestration is the process where carbon is removed from the atmosphere and stored in the liquid or solid form e.g. rocks of the lithosphere, plants of the biosphere etc.
  • The formation of hydrocarbons is a good example of natural carbon sequestration as carbon is removed from the atmosphere and stored for a very long time before being released through natural or human processes
  • Carbon capture and storage (CCS) is the technological process of capturing carbon dioxide (CO2) from industrial sources
  • It is then separated, treated and transported to a long-term storage location e.g. from burning fossil fuels or biomass
• Combustion:
  • Tectonic activity over thousands of years moves the sea floor towards destructive plate boundaries, where they are subducted into the mantle
  • The extreme heat and pressure release the carbon in the rock back up to the surface where it returns to the atmosphere through volcanic eruptions by which 200 million tonnes are released per year
  • Burning organic material releases energy, water and CO₂, industrial processes return carbon to the atmosphere that would otherwise have remained stored in rocks for millions of years
  • Wildfires release stored carbon in vegetation back into the atmosphere
• Decomposition: 
  • When plants and animals die, they decompose through animals (worms), bacteria and fungi (collectively called decomposers) breaking down the carbohydrates
  • This releases CO₂ and methane back into the atmosphere, with some of the carbon being transferred into the soil in the form of humus
  • Soil contains millions of tiny micro-organisms which form part of the carbon cycle
  • Decomposition is temperature dependent, with warmer temperatures showing greater microbial activity and faster decomposition
  • Water regulates the rate of decomposition and release of carbon - heavily water-logged areas slow down the rate of decomposition - e.g. peat
• Respiration:
  • Plants, and the animals that feed on them, break down carbohydrates to release the energy that they need to grow and survive
  • As they do this, they release CO₂, as a by-product, through respiration and waste gases, as they digest their food
  • Life on Earth is fuelled by the breakdown of these carbohydrates which releases CO₂ back into the atmosphere
• Photosynthesis:
  • Plants are primary producing organisms (they make their own food), as they use CO₂ from the atmosphere and water in the soil using energy from sunlight to produce carbohydrates
  • Plants ‘fix’ gaseous carbon dioxide into solid form in their living tissues as glucose
  • Oxygen is released as a bi-product
  • In the oceans, microscopic organisms - phytoplankton - also do the same photosynthesising