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The variation in atmospheric radiocarbon concentration during the last 1800 years was measured in a giant tree grown on Yaku Island in the southern part of Japan. The observed gradual decrease of radiocarbon concentration in the past agrees with the computed variation based on the paleomagnetic data, which show a dominant effect of variation of geomagnetic field strength on the variation of atmospheric radiocarbon concentration.
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Kigoshi et al. (1966) studied this question.
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