Suess effect

The Suess effect is dilution of the atmospheric concentrations of heavy isotopes of carbon (¹³C and ¹⁴C) by the admixture of large amounts of fossil-fuel derived CO₂, which is depleted in ¹³CO₂ and does not contain any ¹⁴CO₂.^[1] It is named for the Austrian chemist Hans Suess,^[2] who noted the influence of this effect on the accuracy of radiocarbon dating. More recently, the Suess effect has been used in studies of climate change. The term originally referred only to dilution of atmospheric ¹⁴CO₂. The concept was later extended to dilution of ¹³CO₂ and to other reservoirs of carbon such as the oceans and soils.^[3]

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Carbon isotopes

Carbon has three naturally occurring isotopes. About 99% of carbon on Earth is carbon-12 (¹²C), about 1% is carbon-13 (¹³C), and a trace amount is carbon-14 (¹⁴C). The ¹²C and ¹³C isotopes are stable, while ¹⁴C decays radioactively to nitrogen-14 (¹⁴N) with a half life of 5730 years. ¹⁴C on Earth is produced nearly exclusively by the interaction of cosmic radiation with the upper atmosphere. A ¹⁴C atom is created when a thermal neutron displaces a proton in ¹⁴N. Minuscule amounts of ¹⁴C are produced by other radioactive processes, and a significant amount was released into the atmosphere during nuclear testing before the Limited Test Ban Treaty. Natural ¹⁴C production and hence atmospheric concentration varies only slightly over time.

Plants take up ¹⁴C by fixing atmospheric carbon through photosynthesis. Animals then take ¹⁴C into their bodies when they consume plants (or consume other animals that consume plants). Thus, living plants and animals have the same ratio of ¹⁴C to ¹²C as the atmospheric CO₂. Once organisms die they stop exchanging carbon with the atmosphere, and thus no longer take up new ¹⁴C. Radioactive decay then gradually depletes the ¹⁴C in the organism. This effect is the basis of radiocarbon dating.

Photosynthetically fixed carbon in terrestrial plants is depleted in ¹³C compared to atmospheric CO₂. This depletion is slight in C4 plants but much greater in C3 plants which form the bulk of terrestrial biomass worldwide. Depletion in CAM plants vary between the values observed for C3 and C4 plants. In addition, most fossil fuels originate from C3 biological material produced tens to hundreds of millions of years ago; C4 plants did not become common until about 6 to 8 million years ago, and although CAM photosynthesis is present in modern relatives of the Lepidodendrales of the Carboniferous lowland forests, even if these plants also had CAM photosynthesis, they were not a major component of the total biomass.

Fossil fuels such as coal and oil are made primarily of plant material that was deposited millions of years ago. This period of time equates to thousands of half-lives of ¹⁴C, so essentially all of the ¹⁴C in fossil fuels has decayed. Fossil fuels also are depleted in ¹³C relative to the atmosphere, because they were originally formed from living organisms. Therefore, the carbon from fossil fuels that is returned to the atmosphere through combustion is depleted in both ¹³C and ¹⁴C compared to atmospheric carbon dioxide.

See also

• See articles about environmental isotopes.

and original studies of the Suess effect

• in the Northern Hemisphere: Suess, H. E. Science 122, 415 (1955).

• in the Southern Hemisphere: Lerman, J. C., Mook, W. G. & Vogel, J. C. in Radiocarbon Variations and Absolute Chronology (ed. Olsson, I. U.) 275−301 (Wiley, New York, 1970).

References