Lengths of Schwabe cycles in the seventh and eighth centuries indicated by precise measurement of carbon-14 content in tree rings

Radiocarbon (C-14) is produced in the atmosphere by galactic cosmic rays, which are modulated by solar magnetic activity. Its content in tree rings is retained and provides a record of past cosmic ray intensity and solar activity. We have measured, with 2year resolution, the C-14 content in Japanese cedar tree rings for the period A.D. 600 to 760, which includes a small grand solar minimum in the seventh to eighth centuries. Periodicity analysis of the C-14 data shows that there is a component in the frequency band of the Schwabe cycle, with a period of 12-13years continuing throughout the minimum. This is the fourth case in which an increase in the length of the Schwabe cycle has been observed in a grand solar minimum, after the Maunder Minimum, the Sporer Minimum, and the Fourth Century B.C. Minimum.