The carbon cycle

 THE CARBON CYCLE: 

 

The carbon cycle is the biogeochemical movement of carbon between the atmosphere, biosphere, geosphere, hydrosphere, pedosphere, cryosphere and the anthroposphere (You et al., 2017). The carbon cycle contains natural carbon sources and sinks, which describes where carbon is naturally extracted and stored. These spheres all interact to create a natural balance in the amount of carbon present on earth. This exchange of carbon has since become unbalanced due to anthropogenic activity's such as carbon emissions from human actions and fossil fuels. The carbon cycle also describes the long-term and short-term cycling of carbon which can happen over millions of years or hundreds of years. The long-term carbon cycle describes the storage of carbonate rocks (such as limestone), in the earth's crust, which get released by volcanic and tectonic activity. The short-term carbon cycle occurs by processes such as photosynthesis and decomposition, which is released or absorbed by plants and organisms (Hannah, 2015). Due to an increase in the amount of atmospheric carbon, there has been a great increase in temperature, which generates positive feedback and further effects to the natural balances of the carbon cycle. Carbon can come in different isotopic forms which can be measured and used to determine the carbon concentrations from different time periods, implying our current knowledge of CO2 concentrations. 

The hydrosphere is the combined mass of water on earth in the forms of lakes, rivers, and oceans. The main forms of carbon found in the ocean are from dissolved species of organisms, suspended carbon, and from living organisms (Romankevich & Vetrov, 2013). The hydrosphere is one of the biggest carbon reservoirs and contributes to the carbon cycle in two ways, through the biological pump and the physical pump. The biological pump explains the movement of inorganic carbon from the atmosphere into surface waters via photosynthetic organisms. When these organisms die and decompose, they sink, and carbon becomes sequestered at the bottom of the ocean. Because shells and exoskeletons contain calcium carbonate, these shells sink via sedimentation and form limestone over many years. This then is released by processes such as volcanoes and tectonics (Hannah, 2015). The hydrosphere interacts with the atmosphere, biosphere, and geosphere heavily. This is because CO2 from the atmosphere gets absorbed by photosynthetic organisms (phytoplankton), in which these organisms decompose and form limestone rock (Ding et al., 2021). This limestone subsequently gets uplifted and forms mountains, in which carbon becomes cycled back into the atmosphere. The physical pump describes the diffusion of carbon dioxide from the atmosphere into cold deep waters, which form carbonate and bicarbonate ions in the water. This is purely temperature dependent as waters with higher temperatures hold less dissolved gas (Brewin et al., 2021). In the polar regions, there is more dissolved carbon dioxide in the waters as it is cold. The thermohaline circulation then drives this dissolved carbon in cold waters, into the deep ocean which allows for further uptake of carbon at the surface. 

 

This diagram shows the fluxes of carbon in the hydrosphere, via processes such as photosynthesis and diffusion. It shows the biological and physical pump of the ocean. (Brewin et al., 2021) 

The biosphere includes all living organisms on the surface of the earth. It can be organisms such as plants, animals, and micro-organisms. The biosphere cycles carbon as terrestrial plants absorb carbon dioxide to carry out the process of photosynthesis. Organisms such as animals, plants and microorganisms decompose, releasing certain isotopes of CO2 back into the atmosphere (Knorr et al., 2007). Animals also respire CO2 into the atmosphere, which contributes to the cycle. The biosphere strongly interacts with the atmosphere as the absorption and re-emission of carbon dioxide allows for a cyclic natural balance. The increasing amount of CO2 in the atmosphere due to anthropogenic activities may lead to a positive feedback loop of an increase in temperature (Delire et al., 2003). This could lead to terrestrial plant death and release of carbon dioxide back into the atmosphere, making an imbalance.  

 

This image describes the interactions of carbon between the hydrosphere, biosphere, anthroposphere and atmosphere. In this diagram you can see the exchange of carbon to phytoplankton in the hydrosphere, and the exchange back to the atmosphere via respiration/decomposition. In the left diagram we see the exchange of carbon from the atmosphere to the soil and plants. (Hannah, 2015). 

The pedosphere is the crust of the earth where soil and rock processes occur. This is a large carbon sink as decaying plant matter becomes part of the soil, and microorganisms in the soil release carbon in their excretions. They are also interacting with the terrestrial biosphere as plants and roots decompose, releasing carbon into the soil. The deeper crust contains copious amounts of carbonate rock, which get released via the process of karstification (Jiang et al., 2012). This process describes the uplift of carbonate outcrops to the surface, which is via tectonic activity. There are many other natural processes that affect or expose carbon in the pedosphere, such as erosion, rainfall, management of the land, temperature, and other abiotic factors, like pH (Ross et al., 2020). The pedosphere has an influence on the atmosphere and biosphere, as carbon gets exchanged between plants and serves as a sink, and a source for the atmosphere.  

The geosphere is the mantle and crust of the earth and includes all rocks and minerals. It can be distinguished from the pedosphere, as it is the deeper layers of rock found below the pedosphere surface. The geosphere is the largest carbon sink, as it contains 1015 gigatons of carbon (Kvenvolden, 1988). The sources of carbon in the geosphere include limestone and methane. The geosphere consists largely of limestone, due to the sedimentation of calcium carbonate in marine skeletons and shells over a prolonged period, which get stored in the crust. When rocks in the geosphere, such as silica, is weathered, it releases calcium and bicarbonate.  It is considered the long-term sink of carbon in the carbon cycle. Methane is the reduced form of carbon and is an isotope that gets released in the shallow geosphere. Volcanoes and hydrothermal vents release this carbon from the shallow geosphere into the atmosphere and hydrosphere (Sano et al., 2017).  

The atmosphere contains substantial amounts of carbon dioxide, as it is a greenhouse gas that contributes to the earth's warming. Due to anthroposphere activities, atmospheric carbon concentrations have increased, causing a global temperature increase (Harde, 2017). This happens due to short wave solar radiation striking the earth, which gets rebounded off earth's surface in the form of long wave radiation, infrared. CO2 absorbs this solar radiation and emits infrared radiation in the form of heat, causing warming to the atmosphere. CO2 is cycled from the biosphere and the hydrosphere, as CO2 gets drawn out by plants and by the ocean's surface. CO2 and other forms of carbon also get emitted during volcanic eruptions, which also contributes greatly to the global temperature increase. If there is a further increase in carbon emissions, the uptake of carbon via plants and the ocean will be further reduced, as warmer liquids hold less gas, so less will be diffused into the oceans. Plants will also be unable to carry out photosynthesis, as the temperature will be too hot for them to survive. 

The cryosphere relates to all regions on the earth that have water in the form of a solid, such as ice, snow, and frost. The cryosphere is typically associated with cold regions such as the arctic and Antarctica. The cryosphere contributes to the carbon cycle, as organic matter from the soil is frozen over, trapping methane in the permafrost. Because of the increase in fuel emissions, the global temperature will increase and create a positive feedback of further warming, as all the methane trapped in the permafrost will be released (Parmentier et al., 2017). The ice on the earth also has a high albedo, meaning that solar radiation is reflected more at the poles. So, when the sea ice melts, the albedo is reduced causing further warming to the earth. This will affect the biosphere and hydrosphere as warmer waters will have reduced diffusion uptake of carbon into the waters, as well as reduced photosynthetic activity. The cryosphere plays a significant role in the thermohaline circulation which distributes carbon in several ways, so a decrease in temperature will stop the drivers in this circulation, causing a strong positive feedback loop of further warming. 

The anthroposphere composes of all human activities occurring on earth. Human activities such as fossil fuel use and carbon emissions from factories, have affected the natural balance of the carbon cycle (Delire et al., 2003). The emissions have caused global atmospheric CO2 concentrations to increase, as carbon that has been stored for many years gets released from the respective carbon stores. Other human activities that have thrown the carbon cycle out of balance is deforestation. There will be an increased amount of decomposition and respiration of decaying matter, as well as less vegetation to absorb this carbon. This has detrimental impacts on all “spheres” of the carbon cycle, as the increase in carbon causes feedback loops which reduce the uptake or release carbon from their long-term storage.  

All the spheres interact and have a natural balance that keeps the carbon concentration at an equilibrium. Because there has been an increase in carbon, the global temperature has increased which has affected the climate of the earth. There have been more extreme weather events such as droughts, typhoons, and heavy rainfall.  

 

References: 

Romankevich, E. A., & Vetrov, A. A. (2013). Masses of carbon in the Earth’s hydrosphere. Geochemistry International, 51(6), 431–455. https://doi.org/10.1134/s0016702913060062 

Ding, W., Nie, T., Peng, Y., Sun, Y., Xue, J., & Shen, B. (2021). Validating the deep time carbonate carbon isotope records: Effect of benthic flux on seafloor carbonate. Acta Geochimica, 40(3), 271–286. https://doi.org/10.1007/s11631-021-00467-1 

Brewin, R. J. W., Sathyendranath, S., Platt, T., Bouman, H., Ciavatta, S., Dall'Olmo, G., Dingle, J., Groom, S., Jönsson, B., Kostadinov, T. S., Kulk, G., Laine, M., Martínez-Vicente, V., Psarra, S., Raitsos, D. E., Richardson, K., Rio, M.-H., Rousseaux, C. S., Salisbury, J., Walker, P. (2021). Sensing the Ocean Biological Carbon Pump from space: A review of capabilities, concepts, research gaps and future developments. Earth-Science Reviews, 217, 103604. https://doi.org/10.1016/j.earscirev.2021.103604 

Knorr, W., Gobron, N., Scholze, M., Kaminski, T., Schnur, R., & Pinty, B. (2007). Impact of terrestrial biosphere carbon exchanges on the anomalous CO2increase in 2002-2003. Geophysical Research Letters, 34(9). https://doi.org/10.1029/2006gl029019 

Jiang, Z., Lian, Y., & Qin, X. (2012). Carbon cycle in the EPIKARST systems and its ecological effects in South China. Environmental Earth Sciences, 68(1), 151–158. https://doi.org/10.1007/s12665-012-1724-x 

Ross, C. W., Grunwald, S., Vogel, J. G., Markewitz, D., Jokela, E. J., Martin, T. A., Bracho, R., Bacon, A. R., Brungard, C. W., & Xiong, X. (2020). Accounting for two-billion tons of stabilized soil carbon. Science of The Total Environment, 703, 134615. https://doi.org/10.1016/j.scitotenv.2019.134615 

Sano, Y., Kinoshita, N., Kagoshima, T., Takahata, N., Sakata, S., Toki, T., Kawagucci, S., Waseda, A., Lan, T., Wen, H., Chen, A.-T., Lee, H., Yang, T. F., Zheng, G., Tomonaga, Y., Roulleau, E., & Pinti, D. L. (2017). Origin of methane-rich natural gas at the West Pacific convergent plate boundary. Scientific Reports, 7(1). https://doi.org/10.1038/s41598-017-15959-5 

Kvenvolden, K. A. (1988). Methane hydrate — a major reservoir of carbon in the shallow geosphere? Chemical Geology, 71(1-3), 41–51. https://doi.org/10.1016/0009-2541(88)90104-0 

Harde, H. (2017). Scrutinizing the carbon cycle and CO2 residence time in the atmosphere. Global and Planetary Change, 152, 19–26. https://doi.org/10.1016/j.gloplacha.2017.02.009 

Parmentier, F.-J. W., Christensen, T. R., Rysgaard, S., Bendtsen, J., Glud, R. N., Else, B., van Huissteden, J., Sachs, T., Vonk, J. E., & Sejr, M. K. (2017). A synthesis of the Arctic terrestrial and marine carbon cycles under pressure from a dwindling cryosphere. Ambio, 46(S1), 53–69. https://doi.org/10.1007/s13280-016-0872-8 

You, H. S., Marshall, J. A., & Delgado, C. (2017). Assessing students' disciplinary and interdisciplinary understanding of Global Carbon Cycling. Journal of Research in Science Teaching, 55(3), 377–398. https://doi.org/10.1002/tea.21423 

Delire, C., Foley, J. A., & Thompson, S. (2003). Evaluating the carbon cycle of a coupled atmosphere-biosphere model. Global Biogeochemical Cycles, 17(1). https://doi.org/10.1029/2002gb001870 

Hannah, L. (2015). Carbon Sinks and sources. Climate Change Biology, 403–422. https://doi.org/10.1016/b978-0-12-420218-4.00019-6