3.9 Radioisotopes in tree rings and ice cores

The radioisotopes 14C and 10Be are produced in the Earth’s stratosphere by the impact of galactic cosmic rays on 14N and 16O. The 14C gets oxidized to form CO2 which is taken up by plants in general and trees in particular where it becomes fixed in annual growth rings. The 10Be gets oxidized and becomes attached to aerosols that can precipitate in snow where it then becomes fixed in annual layers of ice. The solar cycle modulation of the cosmic ray flux can then lead to solar cycle related variations in the atmospheric abundances of 14C (Stuiver and Quay, 1980) and 10Be (Beer et al., 1990). While the production rates of these two radioisotopes in the stratosphere should be anti-correlated with the sunspot cycle, the time scales involved in the transport and ultimate deposition in tree rings and ice tends to reduce and delay the solar cycle variations (cf. Masarik and Beer, 1999). Furthermore, the production rates in the stratospheric are functions of latitude and changes to the Earth’s magnetic dipole moment and the latency in the stratosphere/troposphere is a function of the changing reservoirs for these chemical species. This rather complicated production/transport/storage/deposition process makes direct comparisons between Δ14C (basically the difference between measured 14C abundance and that expected from its 5730 year half-life) and sunspot number difficult.


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