Carbon Cycle (Edexcel A Level Geography)

Carbon Stores and Fluxes

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

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  • 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

• 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

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

• 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

Chemical Weathering & Release of Carbon

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:

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