Advancing Paleoclimate Proxies: Insights from a Novel Luminescence Scanner Applied to Stalagmites and Corals

There are several natural climate archives where proxies can be applied to retrieve information about changes in vegetation, soil and water temperature, continental rainfall regimes, as well as variations in sea surface salinity and temperature. Among these records, stalagmites and corals stand out for their high temporal resolution: the former allow the reconstruction of continental precipitation variations, while the latter enable the identification of changes in marine temperature and salinity. Both are predominantly composed of calcium carbonate (CaCO₃), and generally their proxies comprehend  isotopic analyses of carbon and oxygen, as well as magnesium-to-calcium ratios. Given the importance of understanding climate fluctuations in continental and marine environments, the development of new analytical methods to improve the interpretation of these records is essential. In this context, luminescence techniques (Optically Stimulated Luminescence (OSL), Fluorescence, and Phosphorescence) have proven to be promising tools, as they allow the establishment of correlations between luminescent signals and environmental variables such as temperature, precipitation, and salinity. Although the use of OSL is already well established for dating minerals such as quartz and feldspar, its application to carbonate materials as proxies for environmental changes is still recent and under development, while the study of fluorescence and phosphorescence in these materials remains little explored. The development of the first luminescence scanner dedicated to measuring carbonates enabled high-resolution testing of these emissions, specifically in stalagmites and corals. Measurements were performed continuously, at a constant speed of 100 mm/min, along the main growth axis of the stalagmites and from the top to the base of the corals. The experimental protocol was designed to assess temporal variations and consisted of five main steps: (1) X-ray irradiation (40 kV, 300 µA, 100 mm/min); (2) signal reading with LEDs turned off (11x); (3) IRSL signal reading (3x); (4) BOSL signal reading (5x); and (5) signal reading with LEDs turned off (2x). The tests revealed a strong correlation between the blue-light fluorescence signal and oxygen isotopes (ẟ¹⁸O) in the stalagmites, whereas in the coral samples, a greater similarity was observed between the blue-light fluorescence signal and carbon isotopes (ẟ¹³C). Furthermore, the decay tests showed no signal loss over time, suggesting that the stalagmites emit not only optically stimulated luminescence but also fluorescence and phosphorescence. These results demonstrate the potential of the technique not only for detecting quartz and feldspar grains trapped within carbonate matrices but also for investigating intrinsic properties of calcium carbonate itself, opening new perspectives for high-resolution paleoclimate studies. The newly developed equipment enables rapid sequential analyses, thus representing an excellent alternative for material screening. Due to its low cost per analysis, it will be possible to examine a wide range of samples, which constitutes a significant advantage over conventional, well-known methods, typically more expensive and time-consuming.