Biological sink

A major biological sink for CO9 that is often overlooked is the calcium carbonate shells of corals, molluscs, and Crustacea. These invertebrate animals deposit CaCOa in the form of protective exoskeletons. In some invertebrates, such as the sderaetinians (hard corals) of tropical seas, photosynthetic dinoflagellates (kingdom Protoctista) known as zooxanthellae live within the ani-... 

Then Arnold (6) found that it was produced by bacterial action on ammonium and nitrate salts. The ocean has been suggested as both a source and a sink [Robinson and Robbins (214)] and there are some indications of a biological sink [Burris (27)]. Bates and Witherspoon (12) suggested the oxidation of molecular nitrogen by ozone as a source, but experiments by Goody and Walshaw (82) showed that it is small. 

Galchenko V. F., Lein A., and Ivanov M. (1989) Biological sinks of methane. In Exchange of Trace Gases between Terrestrial Ecosystems and the Atmosphere (eds. M. O. Andreae and D. S. Schimel). Wiley, New York, pp. 59-71. 

Research has been done on so-called natural biological sinks to determine the amount of carbon sequestration possible by this method. This work has indicated that natural biological sinks such as a pine forest do initially achieve elevated carbon dioxide fixing from the atmosphere but that this effect goes away after approximately 3 to 4 years. This effect, termed down regulation, can be the result of a long-term decrease of nitrogen or other needed nutrients in the forest over time. 

A very significant peculiarity of the atmosphere is that so-called self-cleaning processes take place within the air. Due to these processes even those trace constituents which have no other chemical or biological sinks are relatively quickly removed from the air. Hence these removal mechanisms are of great importance for the control of the atmospheric pathways of some trace gases and aerosol particles (see Chapter 5). 

The major source of Ni is the denitrifying bacteria in soils and oceans, which convert ammonium and nitrate compounds into N2. Major sinks for N2 are nitrogen-fixing bacteria in soils and oceans, lightning, and combustion. These sources and sinks result in an atmospheric lifetime of 17 Myr for N2 on Earth. In the absence of biological sinks, the atmospheric lifetime of N2 would be a minimum of 80 Myr, possibly as long as 1 Gyr in the absence of oxygen produced by photosynthesis. 

The primary sources of selenium are volcanic emanations and metallic sulfides associated with igneous activity. Secondary sources are biological sinks in which it has accumulated. The selenium content of black shales, coal, and petroleum is 10-20 times the crustal abundance (0.05 ppm). Seleniferous black shales are the parent materials of the widespread seleniferous soils of the western plains of the United States. When burned, coal and petroleum containing selenium give rise to a redistribution of particulate Se and SeOg. The average selenium content of U. S. coal is about 3 ppm and of petroleum about 0.2 ppm. Selenium is an essential nutrient for animals and is required at a concentration of about 40 ppb in their diet at concentrations of 4000 ppb and above, however, it becomes toxic to animals. 

As mentioned already, the CO2 cycle has one major problem in the atmosphere -there is no direct chemical sink. In nature, CO2 can only be assimilated by plants (biological sink) through conversion into hydrocarbons (Chapter 2.2.2.3) and stored in calcareous organisms, partly buried in sediments but almost completely turned back into CO2 by respiration hence, CO2 partitions between the biosphere and atmosphere. With respect to time periods being of interest for humankind (from decades to hundreds of years) this natural biogeoehemical recycling can be regarded to be closed or, in other words, the net flux is zero ... 

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