Radiocarbon dating of subfossil trees from the southern French Alps and the extreme solar particle event at ca 14 300 cal yr BP

Subfossil pines from the banks of the Drouzet River in the southern French Alps provide a unique Late Glacial archive for reconstructing past solar activity. Dendrochronological analyses produced three floating chronologies together spanning ≈ 700 calendar years. High-precision 14C measurements obtained at CEREGE with the AixMICADAS accelerator mass spectrometer, and calibrated by Markov Chain Monte Carlo wiggle-matching against the IntCal20 absolute chronology, reveal a large and abrupt Δ14C increase of ≈ 30‰ at 14 300 cal yr BP. Carbon cycle model simulations forced with the GRIP 10Be flux record are compatible with this spike, confirming a sharp enhancement in cosmogenic isotope production linked to an extreme solar particle event (ESPE). Lower geomagnetic field intensity and reduced atmospheric CO₂ during the Late Glacial contributed to amplifying the ∆14C signal. Recent three-dimensional atmospheric 14C modelling indicates that this event represents the most powerful ESPE evidenced so far, much stronger than the largest ground-level enhancement of cosmic rays observed over the last 80 years of instrumental data. These results extend the ESPE record beyond the Holocene and demonstrate that subfossil wood has the potential to offer annually resolved evidence of ancient solar events, providing new constraints for radiocarbon calibration and solar-terrestrial interaction studies. This article is part of the Theo Murphy meeting issue 'Radiocarbon and cosmic radiation events'.