Intérêt de l'analyse pétrographique des spéléothèmes pour les reconstitutions paléoenvironnementales

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The significance of speleothem petrography in paleoenvironmental reconstruction Speleothem petrography, like oxygen and carbon isotopic ratios, is influenced by various hydrological, chemical, biological and physical factors at the time of growth. While speleothem-based paleoenvironmental studies using stable isotope records are increasingly numerous, interpretations remain problematic. This paper aims to highlight the significance of a petrographical approach to the study of speleothems as a complement to isotopic studies. First, a synthesis of the main calcite crystalline fabrics encountered in speleothems is presented and a terminology suggested. A summary of the current state of knowledge about the relationships between fabrics and (i) the conditions of speleothem precipitation and (ii) speleothem carbon and oxygen isotopic ratios is then provided. It appears that some fabrics are essentially controlled by two main parameters : 1) the chemical efficiency of the solution ; and 2) the drip rate and variability. As a function of the respective importance of these two parameters, fabrics evolve within a continuum from elongated columnar fabric (typically, high intensity and low variability - low chemical efficiency) to dendritic fabric (low intensity and high variability - high chemical efficiency) via two intermediate forms, compact and open columnar fabrics. Other fabrics are associated with specific conditions of precipitation. For example, a high concentration of impurities in the source water can produce a microcrystalline fabric ; precipitation in a tunnel environment can give rise to squat columnar fabric ; and recrystallisation can produce a mosaic fabric. Combined with known conditions of precipitation, fabrics can be used to evaluate the quality of the stable isotope signals recorded by speleothems and can help in the interpretation of paleoenvironmental records.