Excursions in the ¹⁴C record at A.D. 774-775 in tree rings from Russia and America

The calibration of radiocarbon dates by means of a master calibration curve has been invaluable to Earth, environmental and archeological sciences, but the fundamental reason for calibration is that atmospheric radiocarbon content varies because of changes in upper atmosphere production and global carbon cycling. Improved instrumentation has contributed to high-resolution (interannual) radiocarbon activity measurements, which have revealed sudden and anomalous activity shifts previously not observed at the common resolution of 5-10years of most of the calibration scale. One such spike has been recently reported from tree rings from Japan and then again in Europe at A.D. 774-775, for which we report here our efforts to both replicate its existence and determine its spatial extent using tree rings from larch at high latitude (northern Siberia) and bristlecone pine from lower latitude (the White Mountains of California). Our results confirm an abrupt ~15‰ ¹⁴C activity increase from A.D. 774 to 776, the size and now the hemispheric extent of which suggest that an extraterrestrial influence on radiocarbon production is most likely responsible. Key Points Excursion in 774-776 A.D. due to a rapid change in 14C production Event must be global and uniform in scale Phenomenon is reproduced in two new locations, making a total of five