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

The annual (14)C 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 (14)C concentration in tree rings from California, Switzerland, and Finland around 5410 BCE. These (14)C data series show a significant increase of ∼6‰ in 5411–5410 BCE. The signature of (14)C 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 (14)C increase in 5411/5410 BCE rings occurred during a period of high solar activity and 60 years after a grand (14)C excursion during 5481–5471 BCE. The similarity of our (14)C data to previous events suggests that the origin of the 5410 BCE event is an extreme SEP event.