Summary
Summary The Carbon Cycle AQA - guide to an A*
- Module
- The Carbon Cycle
- Institution
- AQA
Path to A* compilation of the A-level AQA Geography 'Carbon Cycle' unit.
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1 Changes in the magnitude of major carbon stores The carbon cycle describes the movement of carbon in its various forms between the different spheres –
between the atmosphere, hydrosphere, biosphere and lithosphere.
Through a series of chemical reactions and tectonic activity, carbon takes between 100-200 million years to
move between rocks, soil, ocean, and atmosphere in the slow carbon cycle.
On average, 10–100 million metric tons of carbon move through the slow carbon cycle every year.
Weathering
How carbon is transferred through the lithosphere.
Other lithospheric components include burial and compression of sediments rich in carbon, subduction of
carbon rich rocks at subduction zones along destructive plate margins and volcanic eruptions.
CO2 in the atmosphere can dissolve in water and form weak carbonic acids which falls as rain. (Atm to oceans)
If the carbonic acid falls onto land, it can react with minerals in the rocks and soil through carbonation.
The acid dissolves rocks in a process called chemical weathering and releases calcium, magnesium, potassium,
or sodium ions.
When carbonic acid comes into contact with limestone and passes through joints, it reacts with the rock to form
calcium bicarbonate. The calcium bicarbonate is soluble and is carried away in solution, gradually weathering the
limestone. Rivers carry the ions to the ocean.
Carbon sequestration in oceans and sediments
Once in the oceans, the calcium ions combine with bicarbonate ions to form calcium carbonate.
In the ocean, most of the calcium carbonate is made by shell-building organisms (corals) and plankton
(coccolithophores and foraminifera).
When the organisms die, they sink to the seafloor. Over time, layers of shells and sediment are cemented
together and turn to rock, storing the carbon in stone – limestone.
This limestone can be uplifted by tectonics where it will again be attacked by weathering starting the cycle again.
The rock could also become part of the slow cycle - returning carbon to the atmosphere through volcanoes.
Earth’s land and ocean surfaces sit on several moving crustal plates. When the plates collide, one sinks beneath
the other, and the rock it carries melts under the extreme heat and pressure. The heated rock recombines into
silicate minerals, releasing CO2.
When volcanoes erupt, they vent the gas to the atmosphere and cover the land with fresh silicate rock to begin
the cycle again.
Ocean carbon pumps
The ocean contains 50 times more carbon than the atmosphere and exchanges large amounts of CO 2 with the
atmosphere every year.
, In the past decades, the ocean has slowed down the rate of climate change by absorbing about 30% of human
emissions.
CO2 moves between the atmosphere and the ocean by molecular diffusion when there is a difference between
CO2 gas pressure. For example, when the pressure of atmospheric CO2 is higher than the surface ocean, CO2
diffuses across the air-sea boundary into the sea water.
The oceans are able to hold much more carbon than the atmosphere because of 2 pumps;
The physical pump of CO2
1. Cold Polar ocean waters can dissolve more than twice as much CO2 than in the warm equatorial waters.
2. This means that as major ocean currents (the Gulf Stream) move waters from the tropics to the poles, they are
cooled and can take up more CO2 from the atmosphere.
3. As the waters are cooled as they head to the high latitudes, they become denser and sink into the deep ocean's
taking with them the CO2 accumulated at the surface.
4. The water returns along the ocean bed to the tropics, where it upwells, warms and releases some CO 2 back into
the atmosphere.
5. The cycle then repeats.
The Biological CO2 Pump
The growth of marine plants such as phytoplankton takes CO2 and other chemicals from sea water to make plant
tissue. This happens in the upper layers of the ocean as photosynthesis requires light.
Most of the CO2 taken up by phytoplankton is recycled near the surface, 30% sinks into the deeper waters before
being converted back into CO2 by marine bacteria. Only about 0.1 percent reaches the seafloor to be buried in
the sediments.
Photosynthesis
Plants use sunlight, water and CO2 in the process to produce carbohydrates and O2 is given off as a biproduct.
The process is much more rapid than weathering. Plants exchange carbon with the atmosphere relatively rapidly
through photosynthesis, in which CO2 is absorbed and converted into new plant tissues and provides chemical
energy to carry out life functions.
It occurs both on land and in water – phytoplankton.
Respiration
Plants and animals use O2 and carbohydrates to free up the stored energy. The biproducts are water and CO2.
Animals depend on plants for food, energy and oxygen. Once those plants are consumed, CO 2 is released into
the atmosphere because of cells respiration. In turn, this CO2 produced can be used in photosynthesis again.
Photosynthesis and respiration are the opposite of one another.
The exchange of CO2 and O2 through photosynthesis or cellular respiration worldwide helps to keep atmospheric
O2 and CO2 at stable levels.
This balance can be affected by other factors and not all stored carbon from photosynthesis is released by
respiration as some is stored in biomass and some is stored in soils. As some of the carbon gets stored in rocks
over time, O2 has become enriched in the atmosphere.
Decomposition
Returns CO2 from the biosphere to the atmosphere, some can also end up in the pedosphere (soils) and
lithosphere (rocks).
Decomposers break down dead organisms (organic matter) and return the carbon in their bodies to the
atmosphere as CO2 by respiration. They break down dead organisms into the substances that plants need for
growth.
In some conditions, decomposition can be blocked, if the organic matter is frozen for example, or if it is buried in
anaerobic environments such as in peat bogs.
There are many kinds of decomposer. Each help’s recycle food in its own way.
1. Fungi release chemicals to break down dead plants or animals into simple substances. They absorb some of
these substances for growth, but others enter the soil.
2. Earthworms digest rotting plant and animal matter as they swallow soil. The waste that comes out of their
bodies at the other end contains the important minerals, all ready for plants to take up again.
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