The carbon cycle

The biological carbon pump

Phytoplankton = photosynthesise. They take in carbon and turn it into organic matter.

As they are at the bottom of the marine food chain when they get eaten, carbon is passed through the food chain. CO2 is released back into the atmosphere as these organisms respire.

Some organisms like plankton (which does not photosynthesise (different from phytoplankton) sequester CO2, turning the carbon into their hard outer shells and inner skeletons.

Sequestering - to store the CO2 - not release it back into the ocean

When plankton dies their shells dissolve → carbon becomes part of the deep ocean currents. Dead organisms become buried and compressed → limestone (sedimentary rock= fossils)

CO2 from the atmosphere will dissolve into the water → carbonic acid. As concentrations of CO2 increase in the atmosphere, the oceans absorb the CO2 which makes them more acidic. Acidification of the ocean = negative effects. BUT CO2 will also go from the water back into the atmosphere.

Physical pump

Oceanic circulation provides a constant source of new water on the surface whilst transferring surface water into the deep ocean, this is why the ocean stores so much carbon. Water is not stored evenly → in cold water more co2 is absorbed, so the concentration is different around the world.

• CO2 concentration is 10% higher in the deep ocean compared to the surface.
• Polar regions = more carbon compared to tropical regions.
• Warm tropical waters release CO2 into the atmosphere but cold oceans absorb CO2 from the atmosphere.

Thermohaline circulation is the process of the physical pump

Thermohaline circulation is an ocean current that produces both vertical and horizontal circulation of cold and warm water around the world’s oceans. In addition to this, the atmospheric circulation creates large currents in the oceans which transfers water from the warmer tropical areas of the world to the colder polar regions. The rate of circulation is slow; it takes around 1000 years for any cubic metre of water to travel around the entire system. Warm surface waters are depleted of CO2 and nutrients therefore the foundation of the planet’s food chain depends on cool and nutrient-rich water which supports algae to grow.

Water in the North Atlantic is cold and very saline which means it is denser and heavier causing it to sink. When the cold water sinks, warm water is drawn from the ocean surface.

Eventually, cold water is drawn from the bottom of the ocean and then warmed up.

The process in more detail:

1. The main current begins in polar oceans where the water is very cold, and surrounding seawater sinks due to a higher density.
2. The current is recharged as it passes Antarctica by extra cold, salty, dense water.
3. Division of the main current; northward into the Indian Ocean and into the Western Pacific.
4. The two branches warm and rise as they travel northward then loop back southward and westward.
5. The now-warmed surface waters continue circulating around the globe. On their eventual return to the North Atlantic, they cool and the cycle begins again.

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The rate of absorption of CO2 into the ocean depends on ocean temperatures. The colder the water, the more CO2 is absorbed. Therefore, as ocean temperatures increase, the oceans will absorb less CO2 (possibly even emitting some of its stored carbon dioxide). This would accelerate Climate Change and lead to further ocean warming (positive feedback mechanism).

Terrestrial Sequestration

Primary producers sequester carbon through the process of photosynthesis.

• Primary producers take carbon from the atmosphere to photosynthesise and release carbon when they respire.
• Vegetation growth depends on water, nutrients and sunlight.
• When consumers eat plants, carbon from the plants is converted into fats and proteins.
• Microorganisms feed on waste material.
• Animal and plant remains are easier to decompose compared to wood. Decomposition is faster in tropical climates with high rainfall, temperatures and oxygen levels.
• 95% of a tree’s biomass consists of CO2 which is sequestered and converted to cellulose.

CARBON FLUXES VARY

Diurnally - day = positive, night = negative

Seasonally - In the northern hemisphere during winter = plants die leading to high atmospheric CO2 concentrations, spring = plants grow CO2 levels begin to drop

Soil’s capacity to store carbon

Soils store 20-30% of the world’s carbon = the amount sequestered or emitted depends on local conditions.

arid/semi-arid = most important store = any loss of plants will transfer carbon to the soil.

• because there is less vegetation there is less carbon, so any carbon that is there is held onto as a store.

Humus - long term process = 60% of carbon

Factors affecting soil capacity to store carbon:

• Climate - the rate of plant growth + decomposition
• Soil type - clay-rich soils contain more carbon than sandy soils
• Use of soils - land use, cultivation and disturbance can affect how much carbon can be held.

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