The natural carbon cycle is the movement and storage of carbon between the atmosphere,
hydrosphere, lithosphere and biosphere. It’s a closed system so no carbon is lost or gained -
Just transferred.
- The slow part of the carbon cycle is the Geological carbon cycle
- The fast part of the carbon cycle is the biological or physical carbon cycle
The carbon cycle has two components:
1) Stores: Functions as sources (adding carbon to atmosphere) or sinks (removing carbon
from atmosphere. The different stores are:
- Atmosphere: Human activity caused carbon emissions in the atmosphere to increase
rapidly. Eg: Carbon dioxide and methane. The atmosphere stores carbon as
greenhouse gases.
- Hydrosphere (Oceans): It’s a major carbon sink and takes in carbon from the
atmosphere. Carbon is dissolved into the ocean and stored there. Also,
Phytoplankton takes in carbon.
- Biosphere : Plants and trees photosynthesis which takes in carbon dioxide from the
atmosphere to the biosphere. It then releases oxygen into the atmosphere. The
carbon is stored in plants and trees.
- Lithosphere (Earth’s crust - underground): The biggest store of carbon in sediments
and rocks like limestone and chalk. Also, fossil fuels like coal, oil and natural gas
have huge deposits of carbon. Burning fossil fuels and volcanic eruptions releases
carbon into the atmosphere. THIS IS THE SLOW CARBON CYCLE.
2) Fluxes/Flows: Movement of carbon from one store to another.
- Photosynthesis - Takes in carbon from the atmosphere and stores it in the biosphere.
It’s a carbon sink.
- Respiration - Plants and animals use up the oxygen into energy which releases
carbon - It’s a carbon source.
- Decomposition - Living organisms die, and are broken down by decomposers like
bacteria. This releases carbon.
- Diffusion - This is when oceans absorb carbon from atmosphere. This is a carbon
sink.
These are the geological fluxes/processes: OCCURS IN LITHOSPHERE.
- Combustion - When fossil fuels and organic matter such as trees are burnt, which
emit carbon into the atmosphere. They are a carbon source.
- Volcanic outgassing - The earth’s crust has pockets of carbon dioxide. Volcanic
eruptions and earthquakes release pockets of carbon dioxide.
- Weathering - Chemical weathering is where co2 in the air mixes with rainwater to
create carbonic acid. This helps break down and dissolve rocks like Limestone. This
decomposes the rocks which releases carbon dioxide.
,GEOLOGICAL STORES: OCCURS IN LITHOSPHERE.
1) Formation of Carbon rocks - Limestone
- 80% of carbon rocks are formed like this:
- Rain is a weak acid that weathers rocks forming carbon ions, which lead to
rivers carrying carbon ions to the sea and combine with bicarbonate atoms to
create calcium carbonate.
- Calcium carbonate is deposited onto the ocean floor. Shell-building occurs
from organisms like plankton. These organisms form layers on the ocean
floor, which lithify and turn into rocks.
- This leads to limestone to be built, which has been folded up through
mountain building.
- The remaining 20% of carbon rocks contain organic carbon from organisms
embedded in the mud.
- Over millions of years, heat and pressure compress mud and carbon forming
sedimentary rocks like Shale.
2) Formation of Fossil Fuels - Coal
- Formed from the remains of organisms and organic material. These sink to
the bottom of rivers and seas, and were covered by silt and mud. These
started to decay anaerobically and were compressed. The deeper the
deposit, the more heat and pressure is exerted on it.
- This leads to the formation of fossil fuels.
- Oil and gas come from the remains of animals and plants that were buried.
- Coal comes from the remains of trees and plants that were buried.
Compared to the geological cycle, the biological sequestration (Oceans sequestration and
Terrestrial sequestration) operates on a much shorter timescale. It’s the fast carbon cycle.
This is because the carbon exchange between oceans and plants and atmosphere operates
on a shorter timescale than the geological carbon cycle.
- The hydrosphere (oceans) and biosphere (plants) are major carbon stores.
, HYDROSPHERE (OCEANS)
Equatorial oceans are carbon sources, so they produce carbon. This is because of the high
temperatures there so there’s high evaporation rates.
Oceans at higher latitudes are carbon sinks, so they take in carbon. This is because of the
phytoplankton there which photosynthesise, so they take in carbon dioxide and produce
oxygen.
Molecules of carbon dioxide enter the ocean from the atmosphere through diffusion to
surface waters. The carbon dioxide dissolves to create a solution.
The amount of carbon dioxide that dissolves and diffuses into the oceans depends on the
temperature of the water and wind.
Once dissolved in surface water, carbon dioxide can enter the ocean carbon cycle through:
1) Physical pump - Thermohaline circulation.
- Thermohaline circulation is an ocean current that transfers water from the warm
tropical areas to the colder polar areas.
- When seawater is cold, it takes in more carbon dioxide. When the seawater is warm,
it takes in less carbon dioxide.
- Therefore, the warm water in the tropics around the equator is carried to high latitude
near polar areas which is colder. This makes this warm water colder, so it’s denser
and heavier so it sinks below the surface layer and is replaced by colder water from
the tropics which travels to the tropics, where it warms up. The cycle goes on.
- This allows more carbon to be stored in the ocean, due to more colder water around
the ocean, so it takes up more carbon dioxide.
2) Biological carbon pump - Phytoplankton
- The biological carbon pump move carbon dioxide from the ocean surface to
phytoplankton.
- Phytoplankton are microscopic organisms that live in the ocean. They have
chlorophyll to capture sunlight, which enables photosynthesis. This means that they
take in carbon dioxide, and produce oxygen. This is vital for the equilibrium of the
carbon in the atmosphere.
- The carbon is stored in the deep ocean where phytoplankton die. The carbon is also
passed down the food chain by predators of phytoplankton.
- Growth rates of phytoplankton depend on sunlight which influences photosynthesis,
salinity and temperature of water, and amounts of predators.
3) Carbonate pump
- When co2 dissolves into the ocean, it combines with water molecules where
chemical reactions take place to produce bicarbonate and calcium carbonate.
- Shell building organisms like corals and lobsters use calcium carbonate to build
shells.
- Marine organisms like phytoplankton sequester CO2, and use the carbon to build up
shells and skeletons. When they die, their shells dissolve into ocean water, so carbon
becomes part of deep ocean currents.
- Dead organisms and shells that sink to the seafloor become buried and compressed
to form limestone.
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