Large ¹⁴ C excursion in 5480 BC indicates an abnormal sun in the mid-Holocene

Significance Carbon-14 contents in tree rings tell us information of the past cosmic ray intensities because cosmic rays produce 14 C in the atmosphere. We found a signature of a quite large increase of incoming cosmic ray intensity in the mid-Holocene (the 5480 BC event) from the measurement of 14 C content in North American tree rings. The cause of this event is supposed to be an extremely weak sun, or a combination of successive strong solar bursts and variation of a solar magnetic activity. In any case, 14 C variation of the 5480 BC event is extraordinary in the Holocene, and this event indicates the abnormal solar activity compared with other periods.