Calibration issues of the tree ring records from the bomb period in Croatia

Croatia is situated between 42°23′ and 46°33′ north latitude, and 13°30′ and 19°27′ east longitude. Due to its position within the boundary between NH1 and NH2 zones (Hua et al. 2022), the calibration of radiocarbon results for samples from the bomb peak period raises an issue regarding the choice of the calibration curve. To resolve this issue, radiocarbon analyses of tree rings from Croatian wood were performed. Wood was sampled at two main locations IRB (Zagreb, Croatia’s Capital) (45°49′N, 15°59′E) and Crna Mlaka (45°34′N, 15°46′E), 30 km away from Zagreb. 23 tree-ring samples were radiocarbon analyzed from each location. Additionally, two tree ring samples were analyzed from locations Tijarovićeva street and V. Njegovana street in Zagreb for comparison. Average values of absolute differences between radiocarbon results (Δ 14 C) from both locations and NH1 and NH2 radiocarbon values were calculated. The average of absolute differences between zones and the location Crna Mlaka for zone NH1 is (13 ± 4) ‰, and for zone NH2 is (23 ± 10) ‰. The reduced χ 2 values indicate a preference for NH1 zone for Crna Mlaka site, while the IRB location proved unsuitable for drawing conclusions because the impact of fossil fuels was observed.