| Summary: | The variations in atmospheric radiocarbon ( 14 C) concentration during the last 50 000 years can be attributed to changes in the 14 C production rate (due to changes in solar activity, the geomagnetic field and/or interstellar galactic cosmic ray flux) and to changes in the global carbon cycle. The relative contributions of these processes is the subject of current debate. Although the discrepancies between the various reconstructions of the past atmospheric radiocarbon concentration increase with age, the relatively good agreement over the last 25 000 years allows a quantitative discussion of the causes of the observed 14 C variations for this period. Using 10Be measurements from Greenland Summit ice cores, we show that, in addition to solar and geomagnetically induced production rate changes, significant changes in the carbon cycle have to be considered to explain the measured 14 C concentrations. There is evidence that these changes are connected to: (1) global deglaciation and (2) climate changes in the North Atlantic region on centennial to millennial time scales related to changes in the ocean circulation. Differences between 10 Be and geomagnetic field records, however, suggest that uncertainties of about 20% still exist in determinations of past changes in the 14 C production rate.
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