Large 14C excursion in 5480 BC indicates an abnormal sun in the mid-Holocene.

Radiocarbon content in tree rings can be an excellent proxy of the past incoming cosmic ray intensities to Earth. Although such past cosmic ray variations have been studied by measurements of 14C contents in tree rings with ≥10-y time resolution for the Holocene, there are few annual 14C data. There is a little understanding about annual 14C variations in the past, with the exception of a few periods including the AD 774-775 14C excursion where annual measurements have been performed. Here, we report the result of 14C measurements using the bristlecone pine tree rings for the period from 5490 BC to 5411 BC with 1- to 2-y resolution, and a finding of an extraordinarily large 14C increase (20‰) from 5481 BC to 5471 BC (the 5480 BC event). The 14C increase rate of this event is much larger than that of the normal grand solar minima. We propose the possible causes of this event are an unknown phase of grand solar minimum, or a combination of successive solar proton events and a normal grand solar minimum.

A signature of cosmic-ray increase in AD 774-775 from tree rings in Japan.

Increases in (14)C concentrations in tree rings could be attributed to cosmic-ray events, as have increases in (10)Be and nitrate in ice cores. The record of the past 3,000 years in the IntCal09 data set, which is a time series at 5-year intervals describing the (14)C content of trees over a period of approximately 10,000 years, shows three periods during which (14)C increased at a rate greater than 3‰ over 10 years. Two of these periods have been measured at high time resolution, but neither showed increases on a timescale of about 1 year (refs 11 and 12). Here we report (14)C measurements in annual rings of Japanese cedar trees from ad 750 to ad 820 (the remaining period), with 1- and 2-year resolution. We find a rapid increase of about 12‰ in the (14)C content from ad 774 to 775, which is about 20 times larger than the change attributed to ordinary solar modulation. When averaged over 10 years, the data are consistent with the decadal IntCal (14)C data from North American and European trees. We argue that neither a solar flare nor a local supernova is likely to have been responsible.

Prolonged production of 14C during the ~660 BCE solar proton event from Japanese tree rings.

Annual rings record the intensity of cosmic rays (CRs) that had entered into the Earth's atmosphere. Several rapid 14C increases in the past, such as the 775 CE and 994CE 14C spikes, have been reported to originate from extreme solar proton events (SPEs). Another rapid 14C increase, also known as the ca. 660 BCE event in German oak tree rings as well as increases of 10Be and 36Cl in ice cores, was presumed similar to the 775 CE event; however, as the 14C increase of approximately 10‰ in 660 BCE had taken a rather longer rise time of 3-4 years as compared to that of the 775 CE event, the occurrence could not be simply associated to an extreme SPE. In this study, to elucidate the rapid increase in 14C concentrations in tree rings around 660 BCE, we have precisely measured the 14C concentrations of earlywoods and latewoods inside the annual rings of Japanese cedar for the period 669-633 BCE. Based on the feature of 14C production rate calculated from the fine measured profile of the 14C concentrations, we found that the 14C rapid increase occurred within 665-663.5 BCE, and that duration of 14C production describing the event is distributed from one month to 41 months. The possibility of occurrence of consecutive SPEs over up to three years is offered.

Tree rings reveal globally coherent signature of cosmogenic radiocarbon events in 774 and 993 CE.

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.

Author Correction: Tree rings reveal globally coherent signature of cosmogenic radiocarbon events in 774 and 993 CE.

The original version of this Article contained an error in the Data Availability section, which incorrectly read 'All data will be freely available via https://www.ams.ethz.ch/research.html .' The correct version states ' http://www.ams.ethz.ch/research/published-data.html ' in place of ' https://www.ams.ethz.ch/research.html '. This has been corrected in both the PDF and HTML versions of the Article.

Atmospheric impacts of the strongest known solar particle storm of 775 AD.

Sporadic solar energetic particle (SEP) events affect the Earth's atmosphere and environment, in particular leading to depletion of the protective ozone layer in the Earth's atmosphere, and pose potential technological and even life hazards. The greatest SEP storm known for the last 11 millennia (the Holocene) occurred in 774-775 AD, serving as a likely worst-case scenario being 40-50 times stronger than any directly observed one. Here we present a systematic analysis of the impact such an extreme event can have on the Earth's atmosphere. Using state-of-the-art cosmic ray cascade and chemistry-climate models, we successfully reproduce the observed variability of cosmogenic isotope 10Be, around 775 AD, in four ice cores from Greenland and Antarctica, thereby validating the models in the assessment of this event. We add to prior conclusions that any nitrate deposition signal from SEP events remains too weak to be detected in ice cores by showing that, even for such an extreme solar storm and sub-annual data resolution, the nitrate deposition signal is indistinguishable from the seasonal cycle. We show that such a severe event is able to perturb the polar stratosphere for at least one year, leading to regional changes in the surface temperature during northern hemisphere winters.

Another rapid event in the carbon-14 content of tree rings.

Previously, we have observed that the atmospheric (14)C content measured in tree rings showed a strong increase from AD 774 to 775. Although the cause of this event can be explained by a large solar proton event or a short gamma-ray burst, a more detailed discussion of the cause is difficult because the rate of occurrence of such rapid (14)C events remains unknown. Here we report new (14)C measurements from AD 822 to 1020, and the discovery of a second rapid increase of (14)C content from AD 992 to 993. The (10)Be flux in the Antarctic ice core shows peaks corresponding to these two (14)C events. The proportions of flux increase ((14)C/(10)Be) of the two events are consistent with each other. Therefore, it is highly possible that these events have the same origin. Considering the occurrence rate of (14)C increase events, solar activity is a plausible cause of the (14)C increase events.