A Single-Year Cosmic Ray Event at 5410 BCE Registered in ¹⁴C of Tree Rings

The annual ¹⁴C data in tree rings is an outstanding proxy for uncovering extreme solar energetic particle (SEP) events in the past. Signatures of extreme SEP events have been reported in 774/775 CE, 992/993 CE, and ∼660 BCE. Here, we report another rapid increase of ¹⁴C concentration in tree rings from California, Switzerland, and Finland around 5410 BCE. These ¹⁴C data series show a significant increase of ∼6‰ in 5411-5410 BCE. The signature of ¹⁴C variation is very similar to the confirmed three SEP events and points to an extreme short-term flux of cosmic ray radiation into the atmosphere. The rapid ¹⁴C increase in 5411/5410 BCE rings occurred during a period of high solar activity and 60 years after a grand ¹⁴C excursion during 5481-5471 BCE. The similarity of our ¹⁴C data to previous events suggests that the origin of the 5410 BCE event is an extreme SEP event.