The carbon cycle describes the movement of carbon in its various forms between the different spheres – between the Atmosphere, Hydrosphere, Biosphere and lithosphere. It is described in the diagram above. Carbon can take many pathways through the stores and moves over different timescales. According to NASA’s Earth observatory “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."1
The pathways and processes are described in detail below.
For example, carbon can end up in the atmosphere from many other sources. Carbon can arrive from erupting volcanoes, decaying vegetative matter, respiration from the biosphere and oceans, weathering of rocks and the burning of fossil fuels. However, carbon can leave the atmosphere in many ways too. It can enter the oceans via diffusion and the plants in the biosphere by photosynthesis.
Weathering is hugely important part of the rock cycle and links directly to how carbon is transferred through the lithosphere. Other lithospheric or geologic 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. Carbon in the atmosphere can dissolve in water and form weak carbonic acids. The formula for the formation of carbonic acids is;
CO2 + H2O=H2CO3
This carbonic acid falls as rain and thus the carbon is transferred out of the atmosphere into water stores such as the oceans. If the carbonic acid falls onto land, it can react with minerals in the rocks and soil through the process of carbonation. Here, 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 and bedding planes, 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.1
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 (calcifying) organisms (such as corals) and plankton (like coccolithophores and foraminifera).
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 atmosphere5 and exchanges large amounts of CO2 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 between the atmosphere and oceans. 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.
Source: Derivative work: McSush (talk)CO2_pump_hg.png: Hannes Grobe 21:52, 12 August 2006 (UTC), Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germany [CC BY-SA 2.5 (https://creativecommons.org/licenses/by-sa/2.5)], via Wikimedia Commons
The oceans are able to hold much more carbon than the atmosphere because of 2 pumps;
How the Physical pump of carbon dioxide works;
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 (e.g., 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 CO2 back into the atmosphere.
5. The cycle then repeats.
The Biological Carbon Dioxide Pump
The biological pump is another process that moves CO2 away from the surface ocean. 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, a substantial fraction, perhaps 30 percent, 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. 6
Photosynthesis is the process used by plants, algae and certain bacteria to harness energy from sunlight and turn it into chemical energy. It takes place both on land and within the oceans by tiny marine plants known as phytoplankton. It is important to the carbon cycle as plants take in carbon and convert it to organic matter during the process. The process is as follows;
Basically, plants use sunlight, water and carbon dioxide in the process to produce carbohydrates and oxygen is given off as a biproduct.
Indeed, the process is much more rapid than that mentioned for weathering and the lithosphere. The plants exchange carbon with the atmosphere relatively rapidly through photosynthesis, in which CO2 is absorbed and converted into new plant tissues, and respiration, where some fraction of the previously captured CO2 is released back to the atmosphere as a product of metabolism.
Photosynthesis provides plants with the energy they need to carry out essential life functions. In respiration, Oxygen from the atmosphere is used alongside carbohydrates and this frees up the stored energy. The biproducts are water and carbon dioxide. Plant cells respire, just as animal cells do. The equation for respiration is;
Animals depend on plants for food, energy and oxygen. Once those plants are consumed, carbon dioxide is released into the atmosphere because of cells respiration. In turn, this CO2 produced from respiring cells can be used in photosynthesis again.
Essentially, photosynthesis and respiration are the opposite of one another.
The exchange of carbon dioxide and oxygen thorough photosynthesis or cellular respiration worldwide helps to keep atmospheric oxygen and carbon dioxide at stable levels. This balance can be affected by other factors however 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, Oxygen has become enriched in the atmosphere.
Decomposition is very important in returning carbon from the biosphere to the atmosphere. Some of the carbon from decomposition can also end up in the pedosphere (soils) and lithosphere (rocks). Decomposition includes physical, chemical and biological mechanisms and processes that break down organic matter. Decomposers break down the dead organisms and return the carbon in their bodies to the atmosphere as carbon dioxide by respiration. Indeed, decomposers are organisms that break down dead plants or animals into the substances that plants need for growth.
Think of all of the dead and decaying leaf matter that falls from deciduous trees in the UK in Autumn or the contents of a compost bin. It is the work of decomposers such as slugs, worms and woodlice that break down that leaf litter or waste organic matter. 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 helps 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.
3. Bacteria are tiny, microscopic organisms. Some kinds live on other living things – for instance, there are millions inside your gut helping you to digest your food. Others live on dead things, and help break them down into the minerals in the soil. 2
Chemical processes can help to decompose animal bodies and plant remains too, via processes such as proteolysis and hydrolysis. The process of decomposition is a part of the nutrient cycle and is essential for recycling the finite matter that occupies physical space in the biosphere. It ensures that the important elements of life, such as nitrogen, phosphorous, oxygen and carbon, are continually cycled.
Combustion is basically the burning of material. It occurs when any organic material is reacted (burned) in the presence of oxygen to give off the products of carbon dioxide and water and energy. The organic material can be any fossil fuel or hydrocarbon such as natural gas (methane), oil, or coal. However, many other types of organic matter are combustible including wood, paper, plastics, and cloth. Organic materials contain at least carbon and hydrogen and may include oxygen. If other elements are present, they also ultimately combine with oxygen to form a variety of pollutant molecules such as sulphur oxides and nitrogen oxides. 3
Combustion occurs because of a range of human and natural factors. People can deliberately set fire to natural vegetation and natural wild fires can occur where leaf litter builds up and is ignited by lightning strikes. Combustion occurs in biomes across the globe including;
1. Tropical forests as found in Indonesia, the Amazon, and central Africa. Burning can be natural but more often than not is caused by forest clearance which is accompanied by burning of vegetation
2. Savannah grasslands in Africa, where seasonal dry periods result in large quantities of dry organic matter available to burn.
3. Mediterranean areas such as southern Europe and parts of California (including the devastating fires of November 2018)
4. Coniferous or boreal forests across Northern Europe and North America
5. Farming areas, as wastes are burnt off at the end of growing seasons.
Forest fires and the carbon cycle
Immediately after a fire, carbon is released in huge quantities into the atmosphere through combustion. Wild fires in forests kill living biomass in forests and reduce carbon gains to near zero.
In the longer term the balance between carbon lost through subsequent decomposition and simultaneous carbon gains through growth of new vegetation is changed. The decomposition of dead biomass that lasts for several decades post-fire can release up to three times as much carbon as that lost in the initial combustion.
Eventually many decades later, as the forest continues to regrow and decomposition tapers off, carbon storage in trees eventually “catches up,” and the carbon balance equalises. 4
1 – NASA Earth Observatory, 2011 , The Slow Carbon Cycle - retrieved 9th December 2018 from https://earthobservatory.nasa.gov/features/CarbonCycle/page2.php
2 – RSPB, 2018, Decomposers – retrieved 10th December 2018 from https://www.rspb.org.uk/birds-and-wildlife/natures-home-magazine/birds-and-wildlife-articles/food-chains/decomposers/
3 – Charles E Ophardt, Virtual Chembook, 2003, Carbon Cycle – retrieved 10th December 2018 from http://chemistry.elmhurst.edu/vchembook/306carbon.html
4 – Christina Frame, Sink or Source? Fire and the forest carbon cycle, Fire Science Brief, Issue 86, January 2010 – retrieved 10th December 2018 from http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1043&context=jfspbriefs
5 – Laurent Bopp, 2002, The Ocean: a Carbon Pump – retrieved 10th December 2018 from http://www.ocean-climate.org/wp-content/uploads/2015/03/ocean-carbon-pump_ScientificItems_BD-2.pdf
6 – Christopher L. Sabine , Water encyclopaedia, Carbon Dioxide in the Ocean and Atmosphere – retrieved 10th December 2018 from http://www.waterencyclopedia.com/Bi-Ca/Carbon-Dioxide-in-the-Ocean-and-Atmosphere.html#ixzz5YqOy6ksY
Written by Rob Gamesby