| Summary: | The atmospheric concentrations of 14C and of 10Be are influenced by variations in isotope production rate due to solar or geomagnetic influence, but also by internal redistribution processes. The various mechanisms are discussed for 14C. While records of 10Be in Greenland ice and of 14C in tree-rings for the past nearly 10,000 years cannot directly be compared because of different geochemical behaviour, a comparison can be made using a carbon cycle model. Short-term variations in both isotopes (with time scale ≅ 100 yr) are highly correlated, which indicates that they are caused by production rate variations, probably due to variations in the sun’s magnetic activity. The long-term trend for the two records is only partly consistent with data on past variations of the geomagnetic field. For 14C, the possibility must be considered that the mean production rate before the start of the tree-ring record was higher than in the past 9000 years and that the observed long-term 14C decrease is thus due to radioactive decay of the “excess” produced in the past.
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