Carbon Cycle (Edexcel A Level Geography)

Carbon Stores & Fluxes

• Carbon is considered to the the ‘building block of life’ as it can be found in all of the earth’s spheres
• It plays a major role in regulating global climate, particularly temperature and the acidity of rain, rivers and oceans
•  
• Atmosphere - as carbon dioxide and compounds such as methane
• Hydrosphere - as dissolved carbon dioxide
• Lithosphere - as carbonates in limestone and fossil fuels (e.g. coal, oil and gas)
• Biosphere - in living and dead organisms
• Cryosphere - biological carbon is stored in permafrost, which prevents bacterial decay

The carbon cycle

• Carbon moves between these spheres as part of the biogeochemical carbon cycle
• The carbon cycle is a closed system so the amount of carbon is constant and finite and it has three components:
• Stores - where carbon is held
• Fluxes (transfers) - the flows which move carbon between stores (from one sphere to another) measured in petagrams or gigatonnes of carbon per year
• Processes - the physical mechanisms which drive the fluxes between stores e.g. photosynthesis and diffusion

• Carbon stores operate as sources (adding carbon to the atmosphere) and sinks (removing carbon from the atmosphere)
• The carbon cycle is balanced (or in equilibrium) when the sources equal the sinks
• When plants and animals die, the carbon they were storing is released back into the atmosphere and the cycle continues
• An undisturbed carbon cycle maintains carbon dioxide levels in the atmosphere and keeps global temperatures steady so Earth can sustain life
• However, when huge amounts of carbon dioxide are released into the atmosphere in a short period of time, the whole cycle can become unbalanced

Geological Carbon Cycle

• This slow part of the cycle is focused on the huge carbon stores in rocks and sediments with reservoir turnover rates of at least 100,000 years
• Organic matter buried deep in sediments are protected from decay which means it takes millions of years to turn into fossil fuels
• Carbon flows through volcanic eruptions, chemical weathering, erosion and sediment formation on the ocean floor

Bio-geochemical Carbon Cycle

• This fast part of the carbon cycle has large fluxes and rapid reservoir turnovers of a few years up to a thousand years
• Carbon is sequestered in and flows between the atmosphere, oceans, ocean sediments, vegetation, soils and freshwater

Geological Stores of Carbon

• The majority of the earth’s carbon is geological, originating from:
  • The formation of sedimentary carbonate rocks (limestone) in the ocean
  • Biologically derived carbon in shale, coal and other rocks
• Carbon is released into the atmosphere by slow geological processes

Geological Processes in the Carbon Cycle

Processes	Results
1. Weathering of rocks	Mechanical, chemical and biological weathering results in the breakdown of rocks
2. Decomposition	Plant and animal particles from decomposition after death store carbon
3. Transportation	Rivers can carry particles to the ocean, where they will be deposited
4. Sedimentation	Over time, sediments build up, burying older sediments below e.g. shale and limestone
5. Metamorphosis	Pressure builds over time in the layers of sediment which eventually leads to deeper sediment changing to rock e.g. limestone becomes marble, shale becomes slate

• Carbon can take between 100 and 200 million years to move between rocks, the soil, the ocean and the atmosphere

Carbon in Limestone & Shale

• 80% of carbon containing rocks in the ocean is from shell-building organisms (corals) and plankton
• When corals and plankton fall to the ocean floor, they form layers and cement together eventually turning into limestone (lithified)
• The remaining 20% of rocks contain organic carbon originating from organisms that have been embedded in layers of mud
• Heat and pressure compress the mud and carbon over millions of years to form sedimentary rock e.g. shale

Carbon in Fossil Fuels

• Coal, oil and natural gas are known as fossil fuels because they have been formed from the remains of organic material over 300 million years ago from the remain
• When organic matter builds up faster than it can decay, the layers of organic carbon develop into coal, oil or natural gas instead of shale

The Formation of Fossil Fuels

Chemical Weathering

• The geological part of the carbon cycle interacts with the rock cycle in a series of constant processes which can be broken down into five stages:

Geological interactions with the Rock Cycle 

Volcanic Outgassing

• The Earth’s crust contains pockets of carbon dioxide which can be disturbed by volcanic eruptions or seismic activity 
• This release of gas that has been dissolved, trapped, frozen or absorbed in rock is called outgassing
• Outgassing happens at:
• Volcanic zones associated with plate boundaries (including subduction zones and spreading ridges)
• Areas with no current volcanic activity, e.g., the geysers in Yellowstone National Park, USA
• Direct emissions from fractures in the Earth’s crust
• The gas released by volcanic eruptions is relatively insignificant in comparison to human activity
• Volcanoes currently emit 0-15 - 0.26 Gt carbon dioxide annually
• Fossil Fuel use emits about 35 Gt