Variations in atmospheric radiocarbon (14C) concentrations can be attributed either to changes in the carbon cycle1—through the rate of radiocarbon removal from the atmosphere—or to variations in the production rate of 14C due to changes in solar activity or the Earth's magnetic field2. The production rates of 10Be and 14C vary in the same way, but whereas atmospheric radiocarbon concentrations are additionally affected by the carbon cycle, 10Be concentrations reflect production rates more directly. A record of the 10Be 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 14C concentrations, of unclear origin3, that occurred during the Younger Dryas cold period6. We use the 10Be record from the GISP2 ice core5 to model past production rates of radionuclides, and find that the largest part of the fluctuations in atmospheric radiocarbon concentrations can be attributed to variations in production rate. The residual difference between measured 14C concentrations and those modelled using the 10Be 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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Data were provided by the National Snow and Ice Data Center, University of Colorado at Boulder, and the WDC-A for Paleoclimatology, National Geophysical Data Center, Boulder, Colorado. We thank K. Hughen for the 14C calibration data and the grey scale record of the Cariaco sediments. This work was supported by the Swiss National Science Foundation and the US Department of Energy.
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