Comparative study of a stalagmite sample by stratigraphy, laser induced fluorescence spectroscopy, EPR spectrometry and reflectance imaging

In the last few years, it has been shown that multi-proxy data are recorded in speleothems and that these secondary deposits can be used to retrieve records of environmental evolution in extra-glacial continental conditions. The goal of many current research is to obtain a better understanding of the processes leading to the growth of these chemical sediments and to relate them to changes in environmental conditions. In the present research, the multi-proxy study of a well-laminated speleothem sample points out the interest of reflectance trend measurement as a water excess indicator. Results from stratigraphy, laser induced fluorescence spectroscopy, EPR spectrometry and reflectance imaging have been combined in order to get a better understanding of the environmental conditions pertaining during speleothem growth. Several parameters have been measured: (i) Mn2+ concentration evolution (shown to be linked to soil processes); (ii) linewidth ΔH± of the low field Mn2+ EPR line (linked to crystalline properties of the speleothem); (iii) intensities of the laser excited fluorescence (linked to organic matter content of calcite) and reflectance (linked to calcite porosity); (iv) wavelength λpeak of the intensity maximum of laser excited fluorescence bands (linked to the size of trapped organic molecules). Other data resulting from statistical treatment of the annual fluorescence cycles have also been used. Significant correlations demonstrate the covariation of λpeak and calcite reflectance with the hydrological regime of the cave. In the well drained soils of a karstic area, Mn2+ and λpeak appear to be accurate proxies for soil moisture evolution, directly linked to the water excess. These results are confirmed by the comparison with historical knowledge of environmental changes of the surrounding plateau.