Carbon cycle

Atmosphere

Carbon in the atmosphere is mostly in the form of carbon dioxide with some methane and hydrofluorocarbons. The amount of carbon dioxide in the atmosphere is increasing.

Acknowledgement: NASA.

Vegetation

Plants store carbon as carbohydrates made from carbon dioxide from the atmosphere. Land plants take up about a quarter of all carbon dioxide that enters the atmosphere.

Acknowledgement: Public domain.

Soil and organic matter

Soil contains a lot of carbon in the form of dead plant material and in the many bacteria and other small organisms that live there.

Acknowledgement: Public domain.

Coal, oil, gas

Carbon has been locked up in fossil fuels, built up from once-living things, for millions of years.

Acknowledgement: Public domain.

Sediments and sedimentary rock

The carbon cycle overlaps the rock cycle. Ocean sediments and the rocks they turn into contain huge amounts of carbon. This is mostly in calcite and limestone.

Acknowledgement: Bordalier institute.

Ocean surface

Exchange of carbon dioxide between the ocean and the atmosphere takes place at the surface.

Acknowledgement: NASA.

Deep ocean

Most of the carbon entering the ocean ends up in the deep ocean where it can be carried by currents for hundreds of years or be lost in sediments.

Acknowledgement: Public domain.

Burning fossil fuels

In fossil fuels, the carbon is stored in long-chain hydrocarbons, and then through combustion with oxygen in our cars or in factories, the carbon is converted to CO₂, which is released to the atmosphere. And in addition, a number of other byproducts are also produced through inefficiencies in combustion like CO which are atmospheric pollutants.

Acknowledgements: Dr Sara Mikaloff-Fletcher, National Institute of Water and Atmospheric Research (NIWA)

Respiration

Plants and animals release carbon dioxide into the atmosphere through the process of respiration. Respiration releases energy from carbohydrates, with carbon dioxide as a waste product.

Acknowledgement: The University of Waikato.

Burning

All the mass of all the trees that you’re seeing around is mostly coming from atmospheric CO₂. So when we cut down forests and burn forests, then we are releasing CO₂ into the atmosphere.

Acknowledgements: Dr Sara Mikaloff-Fletcher, National Institute of Water and Atmospheric Research (NIWA).
Scrub fire footage, courtesy Scion

Photosynthesis

Plants take in carbon dioxide from the atmosphere during photosynthesis. They use energy from the sun to combine the carbon dioxide and water to form carbohydrates.

Acknowledgement: The University of Waikato.

Decay

In the soil, decomposers (such as microbes and soil animals) break down dead plant material. As well as making nutrients available for living plants, this process also releases carbon dioxide into the atmosphere.

Acknowledgement: Maria Minor and Alastair Robertson (Massey University)

Carbon dioxide exchange

There is a constant vigorous exchange that is going two ways between the ocean and the atmosphere. So once the CO₂ goes into the ocean then that CO₂ forms carbonic acid, bicarbonate and carbonate ion, and it means that the ocean can take up more CO₂, so at present, there is about 60 times as much carbon in the ocean as there is in the atmosphere.

Acknowledgements: Dr Sara Mikaloff-Fletcher, National Institute of Water and Atmospheric Research (NIWA)

Phytoplankton

Plants take up CO₂ – they essentially breathe it in and use it to build their physical structures – and phytoplankton are basically teeny tiny microscopic plants that live in the surface of the ocean, so essentially you have in the surface of the ocean these little tiny microscopic plants are taking carbon from the water and using it to build parts of their bodies.

Now as the phytoplankton mature, there are a couple of different things that can happen. One thing that can happen is they can be recycled back into the biological processes of the surface ocean – so maybe zooplankton eats them, maybe they die and they are returned to their component minerals by natural processes and that carbon is taken up by some other phytoplankton. But the other thing that can happen is that, when they die, they can precipitate down into the deep ocean and when that happens because ocean circulation is so slow that carbon can be stored or sequestered in the ocean for a very long period of time.

Acknowledgements:
Dr Sara Mikaloff-Fletcher, National Institute of Water and Atmospheric Research (NIWA)
Phytoplankton and zooplankton images courtesy of NIWA
Satellite image of phytoplankton bloom off East coast of NZ, by Jeff Schmaltz, courtesy of NASA and MODIS Rapid Response Team

Sinking sediment

When phytoplankton die, many sink and take their carbon (calcium carbonate shells) to form sediments at the bottom of the ocean. This is called a biological pump, removing carbon from the ocean and atmosphere systems for long periods of time.

Acknowledgement: National Oceanic Atmospheric.

Deep circulation

Some carbon from phytoplankton is carried by deep ocean currents that might not return it to the surface for hundreds of years.

Acknowledgement: Public domain.

Rock formation

Sediments containing lots of calcium carbonate from shells can be turned into rock over millions of years. This limestone rock can be pushed up to form land by tectonic Earth movements. Some carbon trapped in the sediments forms gas and oil.

Acknowledgement: Verity Coomer.

Weathering and run-off

Rain slowly dissolves minerals from rocks – a process called weathering. These minerals eventually get washed into the sea, where some minerals, such as calcium carbonate, add to sediments at the bottom of the ocean. A lot of organic matter from the soil also gets carried to sea by rivers.

Acknowledgement: Terry and Janice Dowdeswell.