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Cosmic rays interact with the Earth's atmosphere to produce $^{14}$C, whichcan be absorbed by trees. Therefore, rapid increases of $^{14}$C in tree ringscan be used to probe previous cosmic-ray events. By this method, three $^{14}$Crapidly increasing events have been found. Plausible causes of these eventsinclude large solar proton events, supernovae or short gamma-ray bursts.However, due to the lack of measurements of $^{14}$C by year, the occurrencefrequency of such $^{14}$C rapidly increasing events is poorly known. Inaddition, rapid increases may be hidden in the IntCal13 data with five-yearresolution. Here we report the result of $^{14}$C measurements using an ancientburied tree during the period between BC 3388 and 3358. We find a rapidincrease of about 9\textperthousand~ in the $^{14}$C content from BC 3372 to BC3371. We suggest that this event could originate from a large solar protonevent.

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