Reassessment of a cosmic ray increase event in 1279/1280 CE using carbon-14 records of tree rings

Extreme solar energetic particle events (ESPEs) rarely occur on the Sun, as known from cosmogenic isotope proxies, the primary being 14C measured in tree rings. Several such events have been identified in tree-ring 14C data as sharp spikes. The most recent spike, around 1279 CE, has been reported in several previous studies, but the results were controversial. Here, we investigated the possible occurrence of an ESPE around 1279 CE by analysing new high-precision Δ14C data from Japanese cedar and Altai larch tree samples for the period 1274-1288. Cross-comparisons of our datasets from different accelerator mass spectrometry facilities demonstrated their high reproducibility. Although our independent measurements do not yield a statistically significant single-year 14C spike, the result supports a small but significant Δ14C enhancement around 1279-1280. The Δ14C data were analysed by using two different carbon-transport models, Ticktack and SOCOL:14C-Ex, showing a robust result. We examined possible causes for the 14C increase, that is, ESPE and variations in galactic cosmic rays and concluded that the two scenarios cannot be distinguished with the existing datasets. Further investigations using ice-core records and 14C data from the southern hemisphere will be crucial to determine the cause of this event. This article is part of the Theo Murphy meeting issue issue 'Radiocarbon and cosmic radiation events'.