Carbon Cycle (Edexcel A Level Geography)
Revision Note
Written by: Louise Stone
Reviewed by: Bridgette Barrett
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
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
Examiner Tips and Tricks
Make sure you know the four earth spheres and how they store carbon
Earth’s spheres:
Atmosphere
Hydrosphere
Lithosphere
Biosphere
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
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
Examiner Tips and Tricks
Make sure you read the question carefully and your answer focuses on geological processes such as outgassing and weathering. Do not confuse these with biological processes such as photosynthesis or the carbon pumps that exist in the ocean as you will not gain any marks for these
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
Last updated:
You've read 0 of your 5 free revision notes this week
Sign up now. It’s free!
Did this page help you?