A geochemical cycle describes the pathway in which chemical elements transfer within the lithosphere (The Editors of Encyclopaedia Britannica, 2014). Examples discussed in this essay are the carbon and phosphorus cycles which are both essential for the development of life. The groundwater aspect of the water cycle is also included, as it involves the movement of water within the lithosphere. Carbon has many biological functions such as being used in the production of carbohydrates, fats and proteins (Marianne, 2016, pp. 79-92) as well as contributing to the greenhouse effect to keep the Earth at an inhabitable temperature. Phosphorus also has biological functions, as it is an important element in the composition of cell membranes, bones, proteins and molecules such as ATP. Liquid water is a requirement for any life to exist so therefore Earth, as far as we know, is the only planet in our solar system that inhabits life (Genn, 2018, pp. 5-9, 152-157). These cycles can be affected by natural circumstances for example, the density of vegetation, or the presence of oceans and certain types of rocks. These act as large sinks (Middleton, 2019, pp. 1-25) and will have a varying effect on the cycles of ecosystems situated in different locations.
However, anthropogenic activity does have a large role to play in interrupting these natural cycles. There are many of these activities that have an impact but the two largest are related to global climate change and agricultural practices, which will be discussed further in this essay.
The Carbon Cycle
Since the industrial revolution, anthropogenic activities have had a huge impact on the carbon cycle. At the beginning of the industrial period, the concentration of carbon dioxide (CO2) in the atmosphere was approximately 280 parts per million. In 1970 it had increased to 325 parts per million and in January 2020, the atmospheric concentration had peaked at over 413.4 parts per million (“Why CO2 matters for climate change,” 2020). It is believed by scientists that the last time the CO2 concentrations in the atmosphere were this high was approximately 3 million years ago (“Five ways to reduce your carbon footprint,” 2021). Combustion of fossil fuels is the largest contributor to this increase (Marianne, 2016, pp. 79-92) as carbon is released by oxidation (Middleton, 2019, pp. 1-25) through factories, agriculture and methods of transport. It takes millions of years for fossil fuels to be produced from the remains of deceased organisms containing carbon (“Why CO2 matters for climate change,” 2020) and by burning them it speeds up the pathway from the lithospheric store to the atmospheric store, therefore increasing the atmospheric concentration because rates of release exceed rates of absorption. The greenhouse effect is causing global temperatures to rise which in turn, has impacts on the carbon cycle, as shown on the Quinghai-Tibetan Plateau in China. Biomass on the plateau has increased because of enhanced photosynthesis due to global warming. This also links to the melting of permafrosts which contain 1672 Pg of organic carbon. These large carbon sinks then release this carbon that they have contained for centuries into the atmosphere as CO2 (Chen et al., 2013).
Photosynthesis is an important process involved in the carbon cycle, and many anthropogenic activities disrupt this. Firstly, deforestation has decreased vegetation cover by 40% since the industrial revolution. This decreases rates of photosynthesis and results in less CO2 being absorbed from the atmosphere (Marianne, 2016, pp. 79-92). It also decreases the volume of leaf litter as plants are no longer there to shed their leaves, resulting in carbon concentration in the soils decreasing and reducing nutrient recycling (Chen et al., 2013). On the contrary, afforestation increases photosynthesis and decreases atmospheric concentration of CO2 by absorption. However, photosynthesis is not limited to vegetation alone. Phytoplankton play a vital role in marine ecosystems and can be killed by marine pollution caused by careless human behaviour. This results in less carbon being absorbed out of the water and when they die, they sink to the seabed. They decompose and release the carbon they stored into the sediment, returning the carbon to the lithospheric store where it will be recycled in time (Genn, 2018, pp.148-152).
The most dominant carbon stores are the oceans and carbonate rocks. Under natural circumstances, carbon is liberated by the processes of seafloor spreading, erosion and emissions from the Earth’s crust by volcanic activity (Middleton, 2019, pp. 1-25). However, agricultural practices, such as spreading nitrate fertilisers, can also have influences on the carbon cycle. Carbonate rocks, for example limestones, in the karst region have enhanced weathering due to the release of nitrate and sulfuric acid from artificial nitrate fertilisers used on crops. The weathering releases CO2 into the atmospheric and oceanic stores, speeding up the carbon cycle’s natural time scale (Li et al., 2020).
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The Phosphorus Cycle
Anthropogenic activities also effect the phosphorus cycle in multiple ways. Firstly, the mining of phosphate rocks. Calcium phosphate is the raw material mined from the earth and taken out of the lithospheric store. It is then converted to ammonium phosphate which can be used for phosphate fertilisers as it is more soluble. This is important for the plants as it makes the phosphate easier for them to uptake and then it can be used in the necessary biological processes. A less destructive method than mining is to extract ammonium phosphates from seabird guano, found in dry areas such as Peru or tropical islands (Genn, 2018, pp.152-157). However, the majority of seabird guano accumulations have already been exploited.
Not only does mining phosphates impact the phosphorus cycle but using the fertilisers themselves also have their effects. Phosphate fertilisers are used because phosphate is a main component of the biological molecule, ATP. ATP is a fundamental part of photosynthesis, therefore without it, plants would suffer stunted growth and would result in a food shortage for people (Izen, 2015, p.88). To avoid this problem, farmers spread artificial phosphate fertiliser so prevent this from happening and increase crop yield. This increases the soil concentration and plant stores, but consequently decreases the lithospheric store. Spreading fertilisers also causes eutrophication as the desired effects it has on plants also effects photosynthetic algae in nearby water sources (Genn, 2018, pp.152-157).
An example of anthropogenic activity influencing the phosphorous cycle is in the mangrove forests of China. Asia and South America run fish and shrimp farms in the mangrove forests because of effective nitrogen abstraction and phosphorus preservation. These features are used to filter the wastes from these farms. This effects the sediment profile, as there was a substantial difference in phosphorus concentration between sediments that are found around the farms and un-touched sediments in other locations (Jiang et al., 2018). This supports the statement that humans do have impacts on the phosphorus cycle.
The Water Cycle
Water is a renewable resource in ample supply which is affected by anthropogenic activities that cause imbalanced distribution and impacts its abundance and condition. Us as humans abstract water for 3 purposes, agricultural, domestic and industrial. 70% of water is used for irrigating crops, 22% is used in industry and 8% for domestic uses. The requirement for water is increasing because of growing global population, as more crops must be grown for food and the demand for domestic purposes increases (Genn, 2018, pp.114-119).
An aquifer is a valuable source of water that is found underground in permeable, porous rocks. It is renewable as it is recharged by groundwater sources. However, aquifer depletion is becoming increasingly common as rates of abstraction exceed rates of recharge. This is the first anthropogenic impact and effects aquifers directly (Genn, 2018, pp.114-119). Global climate change is another that indirectly effects groundwater levels and aquifer recharge. The release of greenhouse gases and aerosol particles alter pathways of water elsewhere in the water cycle, such as precipitation quantities and occurrence (Allan et al,. 2020). Changes in precipitation result in varying levels of groundwater due to altered infiltration, therefore changing aquifer recharge rates. Other activities that can affect infiltration rates are deforestation, afforestation, soil compaction by agricultural practices and urban development. Aquifers can also be recharged artificially, by pumping surplus water from periods of high rainfall back underground, ready to be used in the future. (Genn, 2018, pp.114-129).
Conclusion
Overall, this essay has described that anthropogenic activity has a vast impact on the natural geochemical cycles. Agricultural practices appear to have the widest range of impacts, having affected each cycle individually, particularly the spreading of artificial fertilisers. Global climate change is also having an influence on many aspects and both these impacts have proved to be very destructive in many ways. We as humans must look carefully into preventing these influencers to create a sustainable future for many generations to come.
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