Prolonged production of C during the ~660 BCE solar proton event from Japanese tree rings.

Annual rings record the intensity of cosmic rays (CRs) that had entered into the Earth's atmosphere. Several rapid C increases in the past, such as the 775 CE and 994CE C spikes, have been reported to originate from extreme solar proton events (SPEs). Another rapid C increase, also known as the ca.

Large 14C excursion in 5480 BC indicates an abnormal sun in the mid-Holocene.

Radiocarbon content in tree rings can be an excellent proxy of the past incoming cosmic ray intensities to Earth. Although such past cosmic ray variations have been studied by measurements of C contents in tree rings with ≥10-y time resolution for the Holocene, there are few annual C data. There is a little understanding about annual C variations in the past, with the exception of a few periods including the AD 774-775 C excursion where annual measurements have been performed.

Another rapid event in the carbon-14 content of tree rings.

Previously, we have observed that the atmospheric (14)C content measured in tree rings showed a strong increase from AD 774 to 775. Although the cause of this event can be explained by a large solar proton event or a short gamma-ray burst, a more detailed discussion of the cause is difficult because the rate of occurrence of such rapid (14)C events remains unknown. Here we report new (14)C measurements from AD 822 to 1020, and the discovery of a second rapid increase of (14)C content from AD 992 to 993.

A signature of cosmic-ray increase in AD 774-775 from tree rings in Japan.

Increases in (14)C concentrations in tree rings could be attributed to cosmic-ray events, as have increases in (10)Be and nitrate in ice cores. The record of the past 3,000 years in the IntCal09 data set, which is a time series at 5-year intervals describing the (14)C content of trees over a period of approximately 10,000 years, shows three periods during which (14)C increased at a rate greater than 3‰ over 10 years. Two of these periods have been measured at high time resolution, but neither showed increases on a timescale of about 1 year (refs 11 and 12).