The most accurate story of the 664-663 BCE solar energetic particle event told by arctic tree rings

During large solar plasma expulsions into space, extremely elevated fluxes of highly energetic particles arrive at the Earth and catalyze chemical cascades of epic intensity in the atmosphere. These occurrences are seemingly random and rare, but extremely powerful agents that alter the chemistry of Earth’s atmosphere and the carbon cycle. So far, only six super solar storms (also called Miyake events) have been known in the past 14,500 years. The record of these events has been compiled with high-resolution measurements of cosmogenic isotope concentrations in tree rings and ice cores.
Our new study (see Panyushkina et al. 2024) provides two new 14C (radiocarbon) records from arctic and alpine larch tree rings in Eurasia to define the timing and structure of the ca. 660 BCE solar energetic particle event (SEP).  This specific event imprinted a bimodal and prolonged signature of elevated 14C concentrations observed previously in European oak and Japanese cedar.  This unusual 14C signature could be considered as a binary SEP event: two or more successive, closely spaced energetic particle fluxes over 2-6 years, driving a pattern of high production of cosmogenic isotopes in the atmosphere. However, such a binary event has not been previously seen in other SEP 14C records.  Another possibility is a single SEP and resulting binary 14Cspike because of unusual conditions of atmospheric mixing and consequent cosmogenic isotope pathways. Yet another possible explanation might relate to the reduction of a tree’s ability to fractionate against 14C during CO2 exchange fluxes between the ambient atmosphere and the tree, which vary from one geographical location to another. So, are we dealing with an unusual SEP or just a messy SEP signal in 14C tree rings? 
As a dendrochronologist, I recognize that functional traits of trees influence photosynthesis, regulate carbon fractionation, and vary based on tree species and growth conditions. Hypothetically, the SEP 14C signal at ca. 660 BCE might be improved by using tree rings from cold and dry climates, which might perform much better in more faithfully recording the production rate of radiocarbon during the event. For this experiment, we decided to be very selective by choosing high-latitude trees from which to analyze tree rings. Although it is believed that newly formed cosmogenic isotopes mix quickly and evenly by the time they reach the lower troposphere, we hypothesized that the most northern conifer trees close to the North Pole will have stronger and higher fidelity SEP signal, since the Earth geomagnetic field lines are open at the pole so more energetic particles enter the atmosphere from the Arctic stratosphere. Furthermore, trees from the high Arctic conduct photosynthesis in a cold and dry climate, possibly leading to reduced discrimination against 14C during CO2 gas exchange for woody plants.
This experimental protocol was formulated at the University of Arizona. To find the ideal conifer tree rings from the cold and dry climates, we reached out to our Russian collaborators, who archived both a 3000-year-old wood sample from fluvial deposits of the Yamal Peninsula bordering the Arctic Ocean and burial timbers of the Siberian Scythians in the High Altai mountains (see Photo). Radiocarbon measurements from the selected tree rings were made at the AMS facilities of our long-term partners at the Institute of Nuclear Physics in Debrecen, Hungary.   
Analysis of our delta-14C data shows that the SEP flux, regardless of whether it was from a single SEP or multiple, was recorded over two years, 664 BCE and 663 BCE. The prolonged radiocarbon signature from the tree rings manifests a 12‰ rise over two years, which is 3.2-4.8 times higher than the average solar modulation. Carbon-box modeling of the new data suggests that the 664-663 BCE event is comparable to the 774-775 CE solar-proton event. Importantly, we also found that the non-uniform SEP signal in the tree rings is likely driven by the selective ability of stomata to discriminate against 14C in favor of 12C under specific climate conditions regulating low or high rates of gas exchange.  In other words, the photosynthesis of different tree species at different locations may be a primary factor controlling the structure of SEP event signatures.  If transpiration (water loss by evaporation) is low, the stomata may stay wide open and the atmospheric CO2 can freely enter, so that photosynthesis can more readily discriminate against 14C in favor of 12C. Conversely, when stomata are more frequently closed, the pool of CO2 in the leaf is reduced and the plant discriminates less against heavy isotopes (14C and 13C). Ultimately, both the rates of stomatal conductance and photosynthesis determine how effective the trees will be in reducing the amount of 14C (and 13C) used to manufacture photosynthates. Both reduced stomatal conductance and increased rates of photosynthesis could favor the incorporation of more 14C in photosynthates and hence higher 14C content in tree rings, which may make trees from cold and dry climates generally more reliable recorders of the SEP signals. It thus appears that stomatal behavior and its association with tree growth conditions (temperature and moisture) may be an important but overlooked factor in the search for SEP signals in tree-ring radiocarbon.
The arctic tree rings contain the most accurate account of the 664-663 BCE solar energetic particle event. They identified the timing and confirmed the unusual dynamic of this two-year SEP prolonged signal. It seems likely that a lingering effect in the chemical cascade in the atmosphere was induced by a more complex solar emitting incident rather than a single and brief flux of solar energetic particles. The study refined the SEP signal recorded by the high-performing trees, yet the baffling story of 664-663 BCE solar particle emission is waiting to be told, and that would be a story about the Sun’s unusual  behavior.