| Summary: | Variations in atmospheric radiocarbon ( 14 C) concentrations can be attributed either to changes in the carbon cycle 1 —through the rate of radiocarbon removal from the atmosphere—or to variations in the production rate of 14 C due to changes in solar activity or the Earth's magnetic field 2 . The production rates of 10 Be and 14 C vary in the same way, but whereas atmospheric radiocarbon concentrations are additionally affected by the carbon cycle, 10 Be concentrations reflect production rates more directly. A record of the 10 Be production-rate variations can therefore be used to separate the two influences-production rates and the carbon cycle-on radiocarbon concentrations. Here we present such an analysis of the large fluctuations in atmospheric 14 C concentrations, of unclear origin 3 , that occurred during the Younger Dryas cold period 6 . We use the 10 Be record from the GISP2 ice core 5 to model past production rates of radionuclides, and rnd that the largest part of the fluctuations in atmospheric radiocarbon concentrations can be attributed to variations in production rate. The residual difference between measured 14 C concentrations and those modelled using the 10 Be record can be explained with an additional change in the carbon cycle, most probably in the amount of deep-water formation.
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