Explain what is meant by a ‘biogeochemical cycle’, and use examples to illustrate the main processes that occur within them, particularly in relation to their importance for global environmental change. The Blue Marble (NASA 2002) Introduction In this essay the Earth’s biogeochemical cycle will be presented as a closed system that exists with finite resources. The parts of the biogeochemical cycle to be described are the atmosphere, lithosphere, hydrosphere and biosphere; and their dynamic series of complex exchanges. Natural processes inside the biogeochemical cycle shall be identified.
Global environmental change will be examined as the ongoing transformation over time of the worlds land surfaces, water, the atmosphere, (in terms of its chemical makeup, temperature fluctuations and climate change), and the responding flora and fauna adaptations. The links between changes in the biogeochemical cycle and global environmental change will be explored, citing increases in atmospheric carbon dioxide levels and subsequent warming, and ozone depletion. Finally, reference will be made to the impact of human activities, such as industrialisation and deforestation, on the biogeochemical cycles along with global environment change.
The biogeochemical cycle The ‘biogeochemical cycle’ is the relationship between the processes that exist to maintain life on Earth as we know it. For (Marshak and Prothero 2001) a biogeochemical cycle “involves the passage of a chemical amongst non-living and living reservoirs in the Earth system, mostly near the surface. Nonliving reservoirs include the atmosphere, the crust, and the ocean, while living reservoirs include plants, animals and microbes” (2001 712). For Kemp a biogeochemical cycle is one in which an element “moves between sources and sinks along well established pathways.
To facilitate progress through the cycle, it may change state – from a liquid to a gas for example – often in combination with other chemicals and involve both organic and inorganic phases” (Kemp 2004: 64). In an ‘open system’ resources are supplied constantly and the process ceases when the inputs are exhausted. Resources which may be consumed in an open system could include coal, oil or natural gas, but the obvious example would be the sun, which supplies energy to the ‘earth/atmosphere system’ (Kemp 1994: 6). The Earth is a closed system.
It is one in which “the total amount of matter in the system is fixed” (Kemp 1994: 6) and existing resources are finite, although the elements that change in state become available for use in other reactions. The key feature of the biogeochemical cycle is the everchanging, ongoing, dynamic series of complex interreactions which makes this rock habitable, as procedures that allow an element to be used and reused an infinite number of times are “essential to the successful operation of a closed material flow system such as the earth/ atmosphere system” (Kemp 2004: 64).
Biogeochemical cycles involve the circulation of chemical elements through ecosystems such as water (hydrosphere), land (lithosphere), the air (atmosphere) and living creatures (biosphere). These processes vary: carbon reservoirs include the ocean, the atmosphere, the biosphere, and fossil fuel deposits; while the chief nitrogen reservoirs are the atmosphere, living organisms and inorganic compounds in the lithosphere.
The time that chemical elements take to circulate varies: carbon can be stored in the oceans perhaps for years, while fossil fuels may lock carbon up for millions of years (Kemp 2004: 64). Fig 1: Example of a biogeochemical cycle: the carbon cycle. The blue numbers indicate how much carbon moves between reservoirs each year. (Wikipedia contributors 2006a) The lithosphere The lithosphere consists of “the earth’s crust and the underlying rigid section of the mantle. It is thinnest beneath the ocean basins, but thickens under the continental blocks to as much as 300 km in places” (Kemp 2004: 25).
Examples of the lithosphere contributing to the biogeochemical cycles would include continental drift and vulcanism. Continental drift can be seen in: divergence; such as sea floor spreading (where the continental plates are spreading and new mantle is being built) or rifting (as seen on land margins); subduction (where the heavier oceanic plate is sinking beneath the lighter lithospheric plate); and transformation (where two plates move in opposite directions past each other.)
Divergence and subduction are significant for the biogeochemical cycle as they make and destroy mantle thus changing the materials accessible for other processes. The most spectacular instance of the lithosphere contributing to the biogeochemical cycle would be vulcanism. Volcanoes differ in their size, impact and composition. They can eject massive amounts of ash, gas and vapour into the atmosphere.
The gases in and of themselves can be deadly, there can be localised rain events and even acid rain, lahars or jokulhlaups, as illustrated in Figure 2, and if the volcanic plume carries significant amounts of material into the stratosphere the jets stream can carry material around the globe (Kemp 1994: 105-106), reflecting the incoming solar radiation, resulting in ‘global dimming’, (Sturman and Tapper 2006: 474, 512) where the diminished levels of sunlight are such as to cause noticeable cooling at the ground level (Kemp 1994: 108).
Fig. 2: Volcanic Impact on the Global Environment (Wikipedia contributors 2006c) As one example, Mt Pinatubo erupted in June 1991, killing hundreds and causing US$260 million worth of damage locally (Smith 2001: 114). Globally, the volcanic plume went more than 30 km into the atmosphere (Smith 2001: 158) and contributed to the very cool summer in New Zealand in 1992-1993 (Sturman and Tapper 2006: 415). The atmosphere The atmosphere is the mixture of gases, called air, enveloping our planet (Marshak and Prothero 2001: 611).
The chemical components in air are “78% nitrogen (N2) and 21% oxygen (O2), with minor amounts (1% total) of argon, carbon dioxide (CO2), neon, methane, ozone, carbon monoxide, and sulphur dixode” (Marshak and Prothero 2001: 37). It provides a breathable atmosphere for the biosphere; plants, both aquatic and terrestrial, breath carbon dioxide and expel oxygen which is breathed by fauna who exhale carbon dioxide. Wind can move soils and erode rocks and form tornadoes.
In conjunction with solar radiation and water, the atmosphere creates clouds, precipitation such as rain, snow and hail, and storm events such as cyclones or hurricanes. A significant atmospheric process is the ozone cycle. Ozone (O3) is “a gas that absorbs harmful (short-wave length) radiation from the sun” (Marshak and Prothero 2001: 612) and is “essential component of the earth/atmosphere system because of its ability to protect the biosphere” (Kemp 2004: 361). Most ozone (80%) is found at an altitude of around 25km (Sturman and Tapper 2006: 465), as illustrated in Figure 3.