Abstract
Cosmic rays interact with the Earth's atmosphere to produce (14)C, which can be absorbed by trees. Therefore, rapid increases of (14)C in tree rings can be used to probe previous cosmic-ray events. By this method, three (14)C 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 (14)C by year, the occurrence frequency of such (14)C 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 (14)C measurements using an ancient buried tree during the period between BC 3388 and 3358. We found a rapid increase of about 9‰ in the (14)C content from BC 3372 to BC 3371. We suggest that this event could originate from a large solar proton event.