UV and solar radiation effects on natural and anthropogenic carbonates studied by Electron Spin Resonance

Abstract

This is the author-submitted, pre-peer-review version of a manuscript submitted to Geochronometria for the MACh 2026 conference proceedings. The manuscript is currently under peer review. Light exposure in trapped-charge dating is commonly considered primarily as a bleaching process. However, previous Electron Spin Resonance (ESR) studies on calcite and historical lime mortars have shown that UV irradiation can also generate radiation-induced paramagnetic centres analogous to those produced by ionising radiation, indicating that optical exposure may involve both defect removal and formation. Here, ESR spectroscopy was used to investigate the effects of UV, simulated solar, and gamma irradiation on two natural calcarenitic limestones and on experimental lime binders produced directly from the same parent rocks. UV irradiation generated a clear orthorhombic CO2- centre in both natural and anthropogenic carbonates. In the limestones, gamma and simulated solar irradiation produced much weaker features compatible with the same centre, although extensive spectral overlap prevented an unambiguous assignment. In contrast, the newly recarbonated binders exhibited a clearly resolved orthorhombic CO2- signal following UV, gamma, and simulated solar irradiation. The optical irradiation-time response curves were described by single saturating exponential functions, with lower saturation intensities under UV and simulated solar irradiation than under gamma irradiation. Importantly, simulated solar exposure produced a measurable and reproducible CO2- response in both anthropogenic carbonates. These results demonstrate that optical exposure of carbonate materials may involve competing processes of bleaching, retrapping, and defect generation. This behaviour has direct implications for interpreting signal resetting and residual doses in carbonate geochronometry. At the same time, the material-dependent UV and solar responses indicate that ESR parameters may provide a basis for developing material-specific indicators of past UV exposure under terrestrial and planetary conditions.