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

Radiocarbon content in tree rings can be an excellent proxy of the past incoming cosmic ray intensities to Earth. Although such past cosmic ray variations have been studied by measurements of ¹⁴C contents in tree rings with ≥10-y time resolution for the Holocene, there are few annual ¹⁴C data. There is a little understanding about annual ¹⁴C variations in the past, with the exception of a few periods including the AD 774-775 ¹⁴C excursion where annual measurements have been performed. Here, we report the result of ¹⁴C measurements using the bristlecone pine tree rings for the period from 5490 BC to 5411 BC with 1- to 2-y resolution, and a finding of an extraordinarily large ¹⁴C increase (20‰) from 5481 BC to 5471 BC (the 5480 BC event). The ¹⁴C increase rate of this event is much larger than that of the normal grand solar minima. We propose the possible causes of this event are an unknown phase of grand solar minimum, or a combination of successive solar proton events and a normal grand solar minimum.