Tracing ancient solar cycles with tree rings and radiocarbon in the first millennium BCE

The Sun drives Earth's energy systems, influencing weather, ocean currents, and agricultural productivity. Understanding solar variability is critical, but direct observations are limited to 400 years of sunspot records. To extend this timeline, cosmic ray-produced radionuclides like ¹⁴C in tree-rings provide invaluable insights. However, few records have the resolution or temporal span required to thoroughly investigate important short-term solar phenomena, such as the 11-year solar cycle, or ¹⁴C production spikes most likely linked to solar energetic particle (SEP) events. Here we present a continuous, annually resolved atmospheric ¹⁴C record from tree-rings spanning the first millennium BCE, confirming no new SEP's and clearly defining the 11-year solar cycle, with a mean period of 10.5 years, and amplitude of approximately 0.4‰ in ¹⁴C concentration. This dataset offers unprecedented detail on solar behavior over long timescales, providing insights for climatic research and solar hazard mitigation, while also offering enhanced radiocarbon calibration and dating accuracy.