Yves Perrette

Infrared spectroscopy tracing of sediment sources in a small rural watershed (French Alps).

The present article describes a first attempt to use infrared spectroscopy to trace the origin of suspended river sediments. Fifty samples of the main potential sediment sources within a small catchment area (990 ha) in the French Alps were collected and compared with samples of suspended sediment from the river, collected on various dates during 2006 and 2007 using sediment traps. Two major categories of sediment source were identified: topsoils and river channel sediments. For the qualitative part of the study, each of these two main categories was divided into two sub-categories, that is to say, cultivated and pastureland topsoils, and riverbed and riverbank sediments. Discriminant analysis on the source samples showed that Diffuse Reflectance Infrared Fourier Transform (DRIFT) spectroscopy can be used to differentiate between the four potential source materials. To determine whether or not immersion in the river altered the infrared spectra of these source materials, we measured the infrared spectra of samples that had been immersed in the river, in litter bags, for periods of up to 24 days. Immersion did not cause any major changes in the infrared spectra. The contribution of each type of source material to the suspended sediment in the river was quantified using partial least squares (PLS) analyses of DRIFT spectra to compare actual river sediment samples with an experimental model. This model was produced from the DRIFT spectra of a range of calibration samples produced by mixing source material samples in different ratios. The predictions of the model were valid and fell within the confidence interval calculated for the calibration set. Comparisons between suspended sediment samples and the model indicate that the predominant source of the sediment is riverbank erosion, which, in this case, is probably due to trampling by cattle.

Sedimentary charcoal pattern in a karstic underground lake, Vercors massif, French Alps: implications for palaeo-fire history

Knowledge on processes of charcoal transportation is crucial for fire reconstruction based on sedimentary charcoal. Charcoal is susceptible to long-distance transport by water. A lake basin with a large and long catchment area is likely to accumulate charcoal from many fires, not only those produced by fires nearby the lakeshore. Here we test the potential of charcoal transportation by analysing sedimentary charcoal accumulated in an underground lake within a karstic massif. Fires cannot spread around the lake, nor within the karstic massif. Organic materials, including charred particles, are generated several kilometres from the lake on the karstic plateau above. The pattern of sedimentary charcoal shows that the underground lake records continuously produced charcoal by wild fires or human-made biomass burning (slash-and-burn, charcoal kilns) over centuries and millennia, but also stored charcoal from eroded soils. Although the charcoal series shows a certain high variability signal, fire frequency reconstruction cannot be performed owing to chronological uncertainties. The charcoal accumulation corresponds to a more or less regular background input. Such background input is empirically well described in palaeo-fire reconstruction, but was never experimentally displayed. This study provides evidence that the pattern (surface, length, slope, etc.) of catchment areas is crucial for interpreting sedimentary charcoal series. Large catchment areas draining long rivers are not suitable for high-resolution and spatially precise fire reconstructions.

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

Abstract 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) Mn 2+ concentration evolution (shown to be linked to soil processes); (ii) linewidth Δ H ± of the low field Mn 2+ 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 λ peak 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 λ peak and calcite reflectance with the hydrological regime of the cave. In the well drained soils of a karstic area, Mn 2+ and λ peak 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.

Characterisation of rapid infiltration flows and vulnerability in a karst aquifer using a decomposed fluorescence signal of dissolved organic matter

Abstract The fluorescent properties of dissolved organic matter (DOM) have been used as natural tracers in various water systems. However, only few studies have focused on groundwater systems, specifically, on karst systems. The aim of this study was to develop the use of the DOM fluorescence signal as a natural tracer, considering the multiple compartments of a karst system. This method was applied to the Lez hydrosystem, which supplies the city of Montpellier with drinking water. The hydrodynamics and hydrochemistry of the spring were monitored beginning March 2006. The DOM fluorescence was measured by the excitation–emission matrix spectroscopy technique. The analysis of the total fluorescence signal confirms the efficiency of this tool to trace rapid infiltration flows. Moreover, the decomposition of the signal into different fluorophores complements the information provided by the total signal. Indeed, the fluorescence emitted by the humic compounds seems to be the ideal tool for identifying rapid infiltration flows. Nevertheless, the fluorescence of protein-like compounds is better correlated with the inflow of faecal bacteria at the outlet. This decomposition of the fluorescence signal is an interesting way to provide information on both the rapid infiltration flow as well as the vulnerability of the karst aquifers.

Investigation of Organic Matter Entrapped in Synthetic Carbonates—A Multimethod Approach

Organic matter (OM) entrapped in calcite is regularly used for environmental studies; however, insertion mechanisms and types of interaction remain poorly understood. The present study used a new methodology to investigate interactions between OM and the calcite matrix during crystallization processes with humic acid (HA) entrapment. A multimethod approach confirmed that HA is both adsorbed onto the calcite surface and incorporated into the calcite lattice during crystallization. Our results also confirm the log-linear correlation between fluorescence intensity and calcite matrix HA concentration. Fourier transform infrared spectroscopy showed that HA in colloidal conformation is adsorbed onto the calcite surface as a result of the structure of the OH stretching band. We also developed a new method based on synchrotron analysis that uses sulfur as a tracer element for entrapped HA and that localizes the OM electrostatically adsorbed onto the calcite surface. Changes in the sulfur environment, determined using X-ray absorption near-edge structure spectroscopy, indicated more complex insertion mechanisms than simple adsorption of HA during calcite crystallization. Desorption experiments revealed the stability of the OM atomic structure and its layered nature. These results allowed us to draw up a general model of OM insertion in calcite.

Spatial distribution of soda straws growth rates of the Coufin Cave (Vercors, France).

2area of the roof of the Coufin Cave entrance chamber. Because of the very slow and sometimes inexistent water feeding of those stalactites, hydrochemistry analysis were not achieved and drop rate effect on growth were neglected; this study is based on a geomorphological and geostatistical work. By measuring a large number of soda straws in a very small area for which most of the parameters affecting stalactite growth could be considered uniform, and because flow rates are very slow (frequencies are always superior to 1 drop per half hour), we could ascribe differences in growth rates to variations in the global increase of water flow through the unsaturated matrix. Statistical and geostatistical analyses of the measurements showed that this set of similarly shaped stalactites actually consisted of three Gaussian populations with different mean growth rates: fast growth rate (FGR- mean of 0.92 mm.y -1 ), medium growth rate (MGR- mean of 0.47 mm.y -1 ) and low growth rate (LGR- 0.09 mm.y -1 ). Plotting the lengths and spatial distribution of the 20 longest FGR soda straws revealed that there is a rough pattern to the water flow through the cave roof. Even if no direction is statisticaly different from others, the observed directional pattern is consistent with local and regional tectonic observations. Plots of the spatial distribution of the soda straws show that FGR soda straws follow lines of regional geological stress, whereas MGR and LGR soda straws are more dispersed.

Flowstone growth in Gournier River (Vercors, France): a diachronic landscape analysis by 3D modelling and photo draping

*kim.genuite@univ-smb.fr Citation: