Evaluating Quercus pollen as a valuable archive of past UV-B levels in the Central Mediterranean: Insights from comparative infrared spectroscopy analyses

The relative abundance of ultraviolet-absorbing compounds (UACs) in pollen and spore exine increases with prolonged and higher exposure to ultraviolet B (UV-B) radiation. This relationship has been extensively studied via transmission Fourier-transform infrared microspectroscopy (transmission micro-FTIR), primarily in Lycopodium spores and airborne Pinus pollen. However, traditional transmission micro-FTIR methods are prone to infrared light scattering and interference, resulting in spectral deformations and reduced reproducibility. Additionally, bisaccate pollen, like Pinus, can travel long distances and may not accurately reflect local UV-B levels. This study compares transmission and attenuated total reflection (ATR) micro-FTIR methods to assess their reproducibility and investigates Quercus pollen as a potential local UV-B proxy in the Central Mediterranean. Samples containing fresh (in situ), trapped (in mosses), and fossilized (from Holocene sediments) pollen grains were subjected to various chemical treatments, after which the macromolecular composition of single and clustered grains was characterized. Results show that ATR micro-FTIR yields significantly more reproducible data than transmission micro-FTIR, demonstrating the suitability of this method for systematic chemo-palynological studies. Quercus ilex pollen consistently display UAC-related absorption bands across modern and fossil samples, and treatment with hydrochloric acid, hydrofluoric acid, and sodium hydroxide does not significantly alter these signatures. A comparison with fresh and trapped Q. cerris and Q. pubescens pollen further supports these findings. These results suggest that relative UAC concentrations in modern and fossil Quercus exine likely reflect accumulated UV-B dosage, pinpointing Quercus pollen as a valuable local UV-B proxy for reconstructing past UV-B levels in the Central Mediterranean.