A rapid cosmic-ray increase in BC 3372-3371 from ancient buried tree rings in China

Cosmic rays interact with the Earth's atmosphere to produce ¹⁴C, which can be absorbed by trees. Therefore, rapid increases of ¹⁴C in tree rings can be used to probe previous cosmic-ray events. By this method, three ¹⁴C rapidly increasing events have been found. Plausible causes of these events include large solar proton events, supernovae, or short gamma-ray bursts. However, due to the lack of measurements of ¹⁴C by year, the occurrence frequency of such ¹⁴C rapidly increasing events is poorly known. In addition, rapid increases may be hidden in the IntCal13 data with five-year resolution. Here we report the result of ¹⁴C measurements using an ancient buried tree during the period between BC 3388 and 3358. We found a rapid increase of about 9‰ in the ¹⁴C content from BC 3372 to BC 3371. We suggest that this event could originate from a large solar proton event.