Regional Differences in Carbon-14 Data of the 993 CE Cosmic Ray Event

Cosmogenic nuclides such as C-14 from tree rings and Be-10 and Cl-36 from ice cores are excellent proxies for the past extremely large solar energetic particle (SEP) events, which are dozens of times larger than the largest SEP event in the history of observation. So far, several rapid C-14 increases have been discovered, which are considered to have originated from extreme SEP events (or set of successive SEP events) from verifications using multiple cosmogenic nuclide analyses in natural archives. Although these events are characterized by a rapid increase in cosmogenic nuclide concentrations, C-14 data recorded worldwide do not always show similar variations, especially during the 993 CE event, where a rapid increase was recorded in either 992-993 CE or 993-994 CE in several records. We present new C-14 data of the Japanese cedar sample for the 993 CE event. Although the latest data show no significant increase in 1 year, an overall increase pattern is consistent with the previously reported C-14 data of the Japanese cedar, which supports that a significant C-14 increase occurred from 993 to 994 CE in the Japanese sample. Given the dominant C-14 production in high latitudes by SEPs, the difference in timing of increase may be a transport effect in the atmosphere. Moreover, the difference in the timing of the C-14 increase can cause a 1-year age-determination error using the 993 CE radiocarbon spike. Compared with the C-14 data between tree samples from high latitude and midlatitude, including Japan, high-latitude data can capture C-14 changes originating from SEP events more quickly and clearly and may be more suitable for a SEP event exploration in the past.

Automated summary

Cosmogenic nuclides are important proxies for studying past solar energetic particle events. New C-14 data from a Japanese cedar sample supports a significant C-14 increase from 993 to 994 CE. Comparing C-14 data from tree samples at high latitudes and midlatitudes, high-latitude data show clearer and quicker changes reflecting solar energetic particle events.