SEARCH FOR ANNUAL 14C EXCURSIONS IN THE PAST

Two radiocarbon excursions (AD 774-775 and AD 993-994) occurred due to an increase of incoming cosmic rays on a short timescale. The most plausible cause of these events is considered to be extreme solar proton events (SPE). It is possible that there are other annual C-14 excursions in the past that have yet to be confirmed. In order to detect more of these events, we measured the C-14 contents in bristlecone pine tree-ring samples during the periods when the rate of C-14 increase in the IntCal data is large. We analyzed four periods every other year (2479-2455 BC, 4055-4031 BC, 4465-4441 BC, and 4689-4681 BC), and found no anomalous C-14 excursions during these periods. This study confirms that it is important to do continuous measurements to find annual cosmic-ray events at other locations in the tree-ring record.