Certain elements involved in the geochemical and biogeochemical cycling are of great concern due to their limited availability both naturally or through industrial manufacture of the compounds involved in the cycles. Some of the processes involved are chemical reactions with other elements or substances. Some of these reactions are irreversible chemical reactions. This implies once the reaction has taken place, the elements are removed from natural cycle and they cannot be replaced again. As such, the more the cases of irreversible reactions, the greater the rate of reduction of these elements from the environment. Ultimately, there could be less or none of these elements existing. On the other hand, certain reactions leads to the release of the elements and compounds. While there are processes that leads to concentration of elements and compounds at a point, other reactions results in the diminishing of the elements and compounds. Some of these elements include phosphorus, nitrogen, iron, manganese, sulfur, carbon, and so forth. Some of the processes involved in the cycle include photosynthesis, sediments deposition in oceans, and weathering of rocks. Certain organisms called prokaryotes are involved in the conversion, of organic and inorganic compounds to produce organic matter through various processes. For example in carbon element, some of the processes are carbon dioxide fixation, methanogenesis, methanotrophy, fermentation, and respiration. For sulfur there is sulfur reduction and sulfur oxidation while for nitrogen, there is nitrogen fixation, nitrification, and dinitrification (Mackenzie, 1999).
Sulfur is usually extracted from the earth by man either with fossil fuels or in sulfur-containing ores or materials meant for the chemical industry. Of the extracted sulfur, part of it ends up in the atmosphere as a product of combustion while a fraction ends up in rivers with either residual waters or sewage. However, some of it is used to manufacture fertilizers applied in agricultural land. In certain instances, volcanic activities has been found to emit traces of sulfur into the atmosphere. Some of the sulfur ends up in ocean through surface runoff and river transport. A combination of sulfur escaping into the atmosphere as a result of fossil fuel combustion (Canfield, 2013), oceanic atmosphere together with its waters, and as fertilizers suggest the future environmental conditions will see an increase in the level of sulfur. The industrial and agricultural activities are expected to continue adding more sulfur into the atmosphere resulting in pollution. An investigation of river waters in the Scandinavian countries and North America suggest one of the river pollutants is sulfuric acid. Sulfur also has a potential to cause more pollution due to its oxides (sulfur oxide) and sulfide (hydrogen sulfide) (Ivan, 1980).
Global sulfur cycle (anthropogenic fluxes)
According to Ivan (1981), biogeochemical cycle of sulfur cycle happens through various ways. For example, some sulfur are emitted into the air from mining activities, some sulfur ends up in the soil from fertilizers applied to the agricultural land, and certain sulfur ends up in the rivers through industrial sewage waters. There is also flux of anthropogenic sulfur that escapes into the atmosphere while a fraction of sulfur flux is due to processes of water erosion. Volcanic and biogenic sulfur flux is another ways of sulfur addition into the environment (Bates et al, 1992). Others sulfur flux include dust emissions, atmospheric precipitation, continents to ocean, and river runoff. Along the ocean, there is biogenic hydrogen sulfide flux present in shallow sediments of coastal areas, sea-spray marine sulfur, and oceanic sulfur rising to the continents, sulfur flux entering the oceans from oceanic atmosphere, sulfur flux emitted from surface of oceans, marine plants containing sulfur, sulfur containing in dead oceanic organisms occurring as mineralized sulfur (Andreae, 1990), some organic sulfur that has been oxidized to form sulfate which is then again returned to the ocean, sediment organic marine sulfur, and reduced sulfur occurring in buried marine sediments. Previous studies suggest almost half of sulfur extracted by man from the earth surface ends up being used to manufacture fertilizers. However, the remaining fraction ends up being used in the chemical industry and eventually ends up in sewage since it is usually discharged as either domestic or industrial sewage which then finds its way to the rivers.
Studies on river waters suggest the presence of sulfur (Aufdenkampe et al, 2011). Sulfur found in rivers has been associated with anthropogenic sulfur from polluted regions and also sulfur from non-polluted areas. Sulfur present in the atmosphere originate from various sources and occur as sulfur oxides and hydrogen sulfide. Some of it being volcanic eruption and other geological features like geysers, fumaroles, lakes, and hot springs. Some of the atmospheric sulfur also originate from the continental surfaces or the oceans. Sulfur cycle in the ocean is contributed by many things. Firstly, there is the sulfur contained in river waters. Oceanic sulfur is also contained in sulfurized oceanic plants and animals which die and get deposited at the bottom of the ocean. Some microorganisms play a key role in actively decaying sulfur in the ocean.
Certain activities leads to the accumulation of certain elements or compounds in some locations while it leads to reduction in other locations. Human extraction of minerals from the earth leads to the reduction of the anthropogenic sulfur. However, human uses of sulfur such as in fertilizers and industrial chemicals leads eventually results in the contamination of rivers and oceans with traces of sulfur (Li et al, 2015). While sulfur is being reduced in the continental surface, it is being accumulated in the ocean in either dissolved form or as sediments in form of phytoplankton and other organisms. Part of the sulfur ends up in the atmosphere causing pollution. It is to be noted that the increased human exploration of sulfur from the earth crust is an irreversible process. Sulfur removed can never be replaced in the mines. It is therefore anticipated that high concentrations of sulfur in oceans, atmosphere, and rivers would cause more harm to the living organisms and man as a result of environmental pollution (Greaver, 2012). The remedy is to either get alternatives or to to find ways of minimizing sulfur leakage.
Andreae, M. O. (1990). Ocean-atmosphere interactions in the global biogeochemical sulfur cycle. Marine Chemistry, 30, 1-29.
Aufdenkampe, A. K., Mayorga, E., Raymond, P. A., Melack, J. M., Doney, S. C., Alin, S. R., & Yoo, K. (2011). Riverine coupling of biogeochemical cycles between land, oceans, and atmosphere. Frontiers in Ecology and the Environment, 9(1), 53-60.
Bates, T. S., Lamb, B. K., Guenther, A., Dignon, J., & Stoiber, R. E. (1992). Sulfur emissions to the atmosphere from natural sources. Journal of Atmospheric Chemistry, 14(1-4), 315-337.
Canfield, D. E. (2013). Sulfur isotopes in coal constrain the evolution of the Phanerozoic sulfur cycle. Proceedings of the National Academy of Sciences, 110(21), 8443-8446.
Greaver, T. L., Sullivan, T. J., Herrick, J. D., Barber, M. C., Baron, J. S., Cosby, B. J., & Novak, K. J. (2012). Ecological effects of nitrogen and sulfur air pollution in the US: what do we know? Frontiers in Ecology and the Environment, 10(7), 365-372.
Ivan, M.V. (1981). The Global Biogeochemical Sulfur Cycle. Institute of Biochemistry and Physiology of Microorganisms. USSR Academy of Sciences. Retrieved from: http://globalecology.stanford.edu/SCOPE/SCOPE_17/SCOPE_17_1.4_Chapter4_61-78.pdf
Li, X., Gan, Y., Zhou, A., & Liu, Y. (2015). Relationship between water discharge and sulfate sources of the Yangtze River inferred from seasonal variations of sulfur and oxygen isotopic compositions. Journal of Geochemical Exploration, 153, 30-39.
Mackenzie, F.T. (1999). Global Biogeochemical Cycles and the Physical Climate System. Boulder: UCAR. Retrieved from: http://www.ucar.edu/communications/gcip/m4bgchem/m4html.html