On the performance of radiocarbon and quartz OSL dating in macrotidal estuarine environments: Four case studies from Western France

The study of estuarine sedimentary archives provides valuable insights into their geomorphological evolution over the past two centuries, enhancing our understanding of estuarine responses to climate change. Establishing a reliable and precise geochronological framework is therefore essential for monitoring these changes. This study evaluates the performance of quartz Single-Aliquot Regenerative (SAR) OSL and AMS 14 C dating in four estuaries along the western coast of France. The results are compared with cartographic data, serving as an independent age control. Of the 14 OSL dated samples, 10 yield depositional ages consistent with cartographic data, whereas the remaining 4 appear to overestimate ages by 20 to 100 years. In contrast, AMS 14 C dating reveals numerous stratigraphic inversions, with at least 12 out of the 16 measured samples overestimating the depositional age in some cases by up to 5,000 years, in total disagreement with cartographic data. The discrepancy between the OSL and radiocarbon ages reflects the constant reworking of allochthonous material, to which is added the further uncertainty associated with the local reservoir age. These factors fundamentally limit the reliability of 14 C dating regardless of the material analysed. By contrast, the OSL signal displays remarkable resilience, with any age overestimation linked to partial bleaching remaining minor (on the order of decades) compared with the errors affecting 14 C ages. This underscores the capacity of OSL dating to resolve short-term environmental changes and positions it as the most reliable tool for constructing high-resolution chronologies of the last centuries in macrotidal estuarine settings.