Abstract
Though tree-ring chronologies are annually resolved, their dating has never been independently validated at the global scale. Moreover, it is unknown if atmospheric radiocarbon enrichment events of cosmogenic origin leave spatiotemporally consistent fingerprints. Here we measure the 14C content in 484 individual tree rings formed in the periods 770–780 and 990–1000 CE. Distinct 14C excursions starting in the boreal summer of 774 and the boreal spring of 993 ensure the precise dating of 44 tree-ring records from five continents. We also identify a meridional decline of 11-year mean atmospheric radiocarbon concentrations across both hemispheres. Corroborated by historical eye-witness accounts of red auroras, our results suggest a global exposure to strong solar proton radiation. To improve understanding of the return frequency and intensity of past cosmic events, which is particularly important for assessing the potential threat of space weather on our society, further annually resolved 14C measurements are needed.
Highlights
If of sufficient amplitude, abrupt changes in the Earth’s atmospheric radiocarbon (14C) abundance[4,5] are recorded in tree rings owing to the short mixing time of the atmosphere[6], including stratosphere-troposphere exchanges
The coherent, rapid increase of 14C concentration in Northern Hemisphere (NH) and Southern Hemisphere (SH) tree rings reveals the global extent of the cosmogenic signature of extreme fluxes of high-energy solar energetic particles (SEP) in 774 (Fig. 2a)
The overall shape of the 14C time-series through the event is similar for both hemispheres, its amplitude is lower in the SH, and, in contrast to the NH signal, the 14C increase in (0.7%) is comparable to the increase in (0.6%) (Fig. 2a)
Summary
Abrupt changes in the Earth’s atmospheric radiocarbon (14C) abundance[4,5] are recorded in tree rings owing to the short mixing time of the atmosphere[6], including stratosphere-troposphere exchanges. Often attributed to extreme fluxes of high-energy solar particles[7,8], distinct 14C anomalies in 774/5 and 992-4 CE4,5,9–13, as well as possibly much earlier in 660 and 3372/1 BCE14,15 have been identified in local proxy archives These so-called Cosmic Events yield anomalies in records of other cosmogenic radionuclides, such as 10Be and 36Cl that are measured in ice cores[7]. A subset of 374 rings in the 770s CE interval originates from 27 records on the Northern Hemisphere (NH) and seven records on the Southern Hemisphere (SH) Another 110 rings that did not reach back into the 8th century CE represent eight NH and two SH records in the 990s CE
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