Petrographic, mineralogical and geochemical characteristics of speleothems after cave occupation by bats (Roquette cave, Gard, France)

Speleothems are natural archives that allow the reconstruction of past events, including paleoclimatic changes. Isotopic and mineralogical analyses are key tools for deciphering the environmental record preserved in speleothems. However, cave occupation by bats can modify the morphology and mineralogy of cave systems. This phenomenon, and its potential implications for speleothem-based reconstructions, remains poorly investigated. Speleothems from the Roquette cave (Gard, France) were studied using FTIR, SEM, XRD and LA-ICP-MS to understand how biocorrosion related to bat activity in caves alters their mineralogy. Bands of phosphate minerals, more or less interlayered between calcite crystals, were evidenced and interpreted as mineralizations of bat excreta. Minerals forming one biocorrosion band were identified as crandallite, perhamite, quartz and calcite. Quartz, clays and oxides were systematically associated with the phosphate bands. Phosphate minerals were also present as grains, associated with detrital minerals, whose size and abundance varied between samples. These grains are compared to samples of guano, phosphates, clay pockets and host rock showing the presence of detrital elements and phosphates in all samples. Their preservation in speleothems, together with phosphates, indicates that mixing occurs inside the cave, combining the cave's sediments with phosphate mineralizations from guano and urine. The accumulation of uranium revealed by LA-ICP-MS in the phosphate bands makes these areas unsuitable for dating due to possible mobilization after deposition. This study illustrates that prior to U-Th dating of speleothems, it is crucial to carry out mineralogical investigations particularly with SEM and FTIR, which have proven to be the most effective methods.