The minerals which the human economy depends on are part of a global cycle. How do these work, and how much are humans interrupting the natural cycles?
Elements of the Life Cycle
The availability of elements for the creation and maintenance of life depends on a relatively small number of elements cycling through the troposphere and upper crust.
Carbon (C) is passing continuously between the inorganic and organic forms. Inorganic carbon is in an oxidised form, which is reduced through photosynthesis when it enters the lifecycle through plant biomass growth. Hence CO2 is reduced to glucose (C6H12O6). As a more complex chain of carbon atoms, the glucose contains much more energy than CO2. It is this energy storage which fuels living organisms. Through respiration, carbon is oxidised through a series of steps to return to its lowest energy level, inorganic CO2.
Oxygen (O) has atomic number 8, is one of the most abundant elements on and in Earth. It is a primary constituent of silicon dioxide (SiO2), or quartz, and as O2 is just under 21% of the atmosphere by volume.
By mass, it is 89% of water (H2O). In the Stratosphere, oxygen takes the form ozone (O3), creating an essential shield against UV radiation from the Sun.
Oxygen O2 takes the central role in cellular respiration, being breathed in by animals, and exhaled as CO2. The counterpart to animal respiration is plant transpiration, whereby during photosynthesis chloroplasts in leaves 'breathe in' CO2 and 'exhale' O2.
Nitrogen (N) is involved in both biotic and abiotic processes: fixation, ammonification, nitrification and denitrification.
78% of the atmosphere is molecular nitrogen (N2). However, different to oxygen, nitrogen cannot be used by organisms directly. Plants are dependent on nitrobacter for their nitrogen. This strain of bacteria 'fix' nitrogen from the air, and provide the nitrogen in nodules at the roots of plants.
Nitrates (NO3-) are manufactured from the air by the Haber-Bosch Process, and the release of large quantities into agriculture can cause accumulation in waterbodies, resulting in eutrophication.
Sulphur (S) (or Am. sulfur) is an essential element for life, and moves to and from the soil minerals through biological and physical processes.
The forms of sulphur in nature include H2S (hydrogen sulphide, or 'rotten egg gas'), sulphide minerals (e.g. pyrite (fool's gold) FeS2), and elemental sulphur. Sulphur is expelled during volcanic activity.
As an impurity in coal and oil, sulphur dioxide SO2 is a major pollutant during fossil fuel combustion, and sulphur oxides (SOx = SO, S2O, SO2) are responsible for the formation of acidic rain.
Phosphorus (P) is an element which is essential to life, obtained by mining. The balance of phosphate in the environment, particularly in water bodies, is a major concern in environmental management.
Living cells also use phosphate to transport cellular energy in the form of adenosine triphosphate (ATP). Phosphate (PO4-3) is available naturally in any ecosystem as a fertiliser, but is now artificially manufactured in large quantities. Organophosphorus compounds are a commercially important group of phosphorous compounds.
The over-abundance of phosphate in a water catchment area can lead to eutrophication of lakes and rivers. Eutrophication is an oversupply of nutrient, and encourages surface species, such as algae, to dominate, at the expense of sub-surface communities. The loss of photosynthetic plants in the photic zone and benthic community results in depletion of oxygen levels, and consequently anaerobic conditions.
Approximately 1010 moles of phosphorous are deposited by sediments in the oceans each year. Sediment carrying P are of three types:
- Phosphorous associated with calcium carbonate
- Organic phosphorous (associated with organic carbon)
Apatite is a phosphate mineral with formula Ca5(PO4)3(F,Cl,OH). Apatite is one of the few minerals which can be produced and used by biological systems.
Joseph Priestley and Antoine Lavoisier are credited as being the discoverers of the carbon cycle, in the 1770s-80s.
Carbon in the Earth's Carbon Cycle has both organic and inorganic forms. Most inorganic carbon is carbon dioxide (CO2), carbonate (CO32-) and hydrogen carbonate (HCO3-, aka bicarbonate). Organic carbon is carbon in living and dead organisms, fossil fuels, and other organic deposits dispersed in rock, water and the atmosphere.
Respiration of living organisms liberates solar energy stored in glucose during photosynthesis.
Carbon is passing continuously between the inorganic and organic forms. Inorganic carbon is in an oxidised form, which is reduced through photosynthesis when it enters the lifecycle through plant biomass growth. Hence CO2 is reduced to glucose (C6H12O6). As a more complex chain of carbon atoms, the glucose contains much more energy than CO2. It is this energy storage which fuels living organisms. Through respiration, carbon is oxidised through a series of steps to return to its lowest energy level, inorganic CO2.
Carbon in the crust is of biological origin, and has a residence time of 2.7 x 105 years. This carbon is mainly provided by the deposits of living organisms which used calcium carbonate (CaCO3) in their shells, and forms sedimentary carbonate rock, such as limestone, or subsequent rocks through metamorphism. Carbonate rock is essentially a permanent reservoir of carbon, however a small amount of carbon is released back into the atmosphere through volcanoes.
In anaerobic conditions, such as in swamps, there is insufficient oxygen available to oxidise the carbon in detritus (leaves and dead organisms), so carbon is removed from the surface environment in what is known as a carbon sink. It is this carbon which is compressed and heated underground to form the fossil fuels: coal, gas, and oil. Thanks to the anaerobic land and ocean carbon sinks, the Earth has retained a balance between oxygen and carbon dioxide through the mechanism of life.
Human activities are currently the greatest source of imbalance to the finely-tuned carbon cycle
Carbon as CO2 in the atmosphere has a short residence time. On average, a CO2 molecule released during respiration or decomposition will circulate in the air for just 3.2 years, before being returned to the life cycle through photosynthesis. The atmosphere mixes due to wind systems on about the same timescale. This means that local variations in CO2 concentrations can be created. There are also natural variations in the influx and outfluxes of carbon, due to variations in climate, weather, and volcanic activity. Human activities are currently the major cause of carbon imbalance in the planet's carbon cycle, releasing over 50 Gt of carbon dioxide annually from fossil fuel burning.
Estimates of carbon reservoirs
/Gt (billion tonnes carbon)
- Atmosphere: 800
- Biomass: 550
- Soil: 2300
- Reactive sediments: 6000
- Ocean surface: 1000
- Deep ocean: 37,000
- Fossil fuels: 10,000
Estimates of natural carbon flows
/Gt (billion tonnes carbon) annual
- Plant respiration: 60
- Photosynthesis: 120
- Microbial decomposition and respiration: 60
- Air-sea gas exchange: 90 released / 92 uptake
Estimates of carbon accumulation/sinks
/Gt (billion tonnes carbon) annual
- Anthropogenic (primarily burning fossil fuel) contributions to atmosphere: 9
- Total natural atmosphere exchange: 5
- Net terrestrial uptake: 3
- Net ocean uptake: 2
Conclusion: the atmosphere has a net 4Gt per year accumulation of carbon due to the burning of fossil fuel and other human activities.