Ruprecht-Karls-Universität Heidelberg

Carbon Cycle Group IUP Radiocarbon

Radiocarbon in Atmospheric CO2

Radiocarbon (14C) is the natural radioactive carbon isotope which is produced in the atmosphere by cosmic ray induced reactions with atmospheric nitrogen. The radioactive half life of 14C is 5700 years. In an undisturbed steady state situation the atmospheric 14CO2 activity corresponds to an equilibrium between production in the atmosphere and decay of 14C in all carbon reservoirs exchanging CO2 with the atmosphere. The natural equilibrium level of atmospheric 14CO2 has been disturbed through man's activities in the last century, (1) by the ongoing input of fossil fuel CO2 into the atmosphere known as Suess effect, and (2) through nuclear detonations in the atmosphere in the 1950s and early 1960s (14C bomb effect).

CO2 from the burning of fossil fuels, due to its age of several hundred million years is free of 14C. Adding fossil fuel CO2 to the atmosphere therefore leads not only to an increase of its mole fraction but also to a dilution of the 14C/C ratio in CO2. While the Suess effect caused a reduction of the 14C/12C ratio in atmospheric CO2 by only several percent in the 1940s, the 14C bomb effect resulted in a global increase of 14CO2 by more than 80 %. Equilibration of this bomb 14C spike with the other carbon reservoirs, ocean and terrestrial biosphere lead to a decrease of the atmospheric 14C/C ratio after the atmospheric test ban treaty in 1963 with today's activity ratios approaching again natural levels (Δ14C ≈ 0‰) (Levin et al., 2012; 2013). Today, global fossil fuel emissions are responsible for the ongoing decrease of Δ14CO2 observed globally. In polluted areas e.g. on the European continent, a surplus of CO2 from the burning of fossil fuels is clearly detectable by significant 14C/C depletions in regional atmospheric CO2 relative to measurements made in background air i.e. over the Atlantic ocean or at a high mountain site as for example Jungfraujoch. Regional 14CO2 observations can thus be used to validate potential emissions reductions in the catchment area of the measurements sites (e.g. Levin and Rödenbeck, 2008).

For further information on the global distribution of 14C in atmospheric CO2 see Levin and Hesshaimer (2000) and Levin et al. (2010).
Development of 14C in atmospheric CO2 in the northern hemisphere in the last 50 years. Data before 1959 have been derived from tree rings (Stuiver and Quay, EPSL 53, 349-362, 1981). From 1959 to 1986 measurements were performed at the Alpine site Vermunt subsequent data from 1986 onwards are from the Swiss High Alpine Research Station Jungfraujoch (Levin et al., 2010).
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