Early ¹⁴ C increase in high-latitude trees at 665–664 BC

Abstract The discovery of radiocarbon ( 14 C) peaks in AD 774–775 and AD 993–994 sparked the search for other anomalous events, leading to the identification of one around 660 BC. However, the ∼660 BC event appears to show a more prolonged increase, raising the question whether the event is qualitatively different. To investigate this, we measured high-latitude tree rings from Finnish Lapland, expected to be highly sensitive to energetic particle events. We measured the 14 C content of full rings, as well as their separated earlywood and latewood components. We found that the 14 C concentrations start rising already in the latewood of 665 BC and reach almost its full intensity by 664 BC. This rapid increase is similar to that at another high-latitude site (Yamal, Russia) but contrasts with that of low-latitude sites, which show a later peak. The earlier increase of the 14 C at high-latitude tree rings compared to lower latitudes is consistent with similar observations for the AD 774 and AD 993 Miyake events. Based on carbon-cycle box modeling, the structure of the subsequent amplitude increase can be explained by either single or double initial 14 C pulses. The fast increase coupled with a slower subsequent peak structure suggests similar mechanisms behind the high-latitude observations, i.e., tropospheric 14 C production and/or a fast component of polar air flow across the tropopause combined with the full stratospheric-tropospheric CO 2 exchange. Our results strongly emphasize the need for dynamic carbon cycle models to understand the observed differences between high- and lower-latitude data.