Abstract
Extreme cosmic radiation events occurred in the years 774/5 and 993/4 CE, as revealed by anomalies in the concentration of radiocarbon in known-age tree-rings. Most hypotheses point towards intense solar storms as the cause for these events, although little direct experimental support for this claim has thus far come to light. In this study, we perform very high-precision accelerator mass spectrometry (AMS) measurements on dendrochronological tree-rings spanning the years of the events of interest, as well as the Carrington Event of 1859 CE, which is recognized as an extreme solar storm even though it did not generate an anomalous radiocarbon signature. Our data, comprising 169 new and previously published measurements, appear to delineate the modulation of radiocarbon production due to the Schwabe (11-year) solar cycle. Moreover, they suggest that all three events occurred around the maximum of the solar cycle, adding experimental support for a common solar origin.
Highlights
Extreme cosmic radiation events occurred in the years 774/5 and 993/4 CE, as revealed by anomalies in the concentration of radiocarbon in known-age tree-rings
The new results were obtained at an average precision of 1.71‰ (1 σ) per year, with reported uncertainties encompassing counting statistics, normalization and sample preparation calculated in accordance with standard data reduction procedures[26]
We observed a moderate variation in radiocarbon over the Carrington Event of 1859 CE; our new and previously published data over Event-775 followed a sinusoidal pattern with peak-to-trough amplitude of about 5‰, and a periodicity of about 12 years; likewise, our Event-994 data varied with a 5‰ amplitude and period of around 11 years
Summary
Extreme cosmic radiation events occurred in the years 774/5 and 993/4 CE, as revealed by anomalies in the concentration of radiocarbon in known-age tree-rings. We perform very high-precision accelerator mass spectrometry (AMS) measurements on dendrochronological tree-rings spanning the years of the events of interest, as well as the Carrington Event of 1859 CE, which is recognized as an extreme solar storm even though it did not generate an anomalous radiocarbon signature. Our data, comprising 169 new and previously published measurements, appear to delineate the modulation of radiocarbon production due to the Schwabe (11-year) solar cycle They suggest that all three events occurred around the maximum of the solar cycle, adding experimental support for a common solar origin. This sinusoidal variation has already been measured in tree-ring samples by various groups, and exhibits a peak-to-trough amplitude of around 4–6‰8,10–12 Despite this anticorrelation, the most widely accepted theory for the cause of the anomalous increases in Δ14C is extreme solar storms[3,13]. Establishing whether the Carrington Event, Event-775 and Event-994 all occurred at the same point on the Schwabe cycle may suggest further shared characteristics, such as whether the mechanisms that drove Event-775 and Event-994 can be regarded as extreme versions of those that initiated Carrington
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