Recent studies have highlighted the role of bats in the alteration of cave environments through distinct physical and chemical perturbations. Forming colonies numbering in the hundreds of thousands of individuals, bats release metabolic heat, CO2, and water vapour. Furthermore, the deposition and subsequent degradation of guano, driven by microorganisms, causes the release of gases (CO2, NH3, H2S) and of fluids enriched in phosphoric, sulfuric, and nitric acids. These processes have a dual effect: they induce the corrosion and alteration of carbonate bedrocks and siliciclastic sediments, and concomitantly bring to the formation of diverse mineral assemblages, which are mainly represented by phosphates. The main scope of this thesis was to provide new insights on the pathways which determine the alteration of cave sediments, the consequent formation of authigenic phosphates, and the evolution of the secondary assemblages at varying environmental conditions. A multi-scale approach was developed to elucidate how modifications in the mineralogical assemblages reflect geochemical and textural changes, sometimes leading to the formation of rare and heavily hydrated phases. The integration of micromorphological observations, quantitative mineralogical analysis and bulk geochemistry allowed to describe the multi-stage evolution of a phosphatised sedimentary sequence hosted in Grotte de la Roquette (Gard, France), a cave where intense biocorrosion has significantly reshaped the cave conduits in ancient times, while guano has almost completely disappeared due to degradation. The prolonged release of P-rich acidic fluids from guano decomposition and the progressive leaching of the most mobile elements determined differences in the distribution of secondary minerals along the studied profiles, which developed through impregnation of the sediment matrix and subsequent remobilisation and crystallisation in voids. In this case, the long-lasting evolutionary process that led to the formation of the currently observable associations has reached its conclusion. In other instances, such as the sedimentary deposits of Aven-Grotte de la Baume Saigner (Hérault, France), degrading bat-guano and secondary phosphates coexist, admixed to siliciclastic sediments. Strongly acidic conditions coupled with high Al activity inhibit the formation of Ca-phosphates, and bring to the formation of amorphous Ca-Al-phosphate deposits around limestone fragments contained in the clay-rich sediments. The amorphous material is associated with heavily hydrated aluminophosphates such as taranakite and vashegyite. The latter, a rare and poorly defined mineral, was subject to a detailed mineralogical investigation, which reinforced previous hypotheses regarding the existence of different polytypes. The occurrence of such highly hydrated phases in association with actively decaying guano, representing recent phosphatisation events, as opposed to their absence in more mature contexts, such as the one of Grotte de la Roquette, raises questions regarding their long-term stability. Several authors have proposed that the natural evolutionary pathway for hydrated guano-derived phosphates is their sequential dehydration due to varying environmental conditions. Specifically, exothermic reactions taking place during organic matter degradation are often considered as the primary cause for these transformations. To address the validity of this hypothesis, the taranakite to francoanellite dehydration reaction was taken as a representative example and subjected to a detailed multi-method characterisation, coupling thermodynamic calculations with in situ high-temperature diffraction and spectroscopic techniques. Although the derived equilibrium temperature for the reaction exceeds typical cave conditions, the resulting empirical activation energy suggests that microbially mediated guano degradation might indeed provide sufficient thermal energy to trigger the transformation.
The impact of bat activity on cave evolution: a mineralogical perspective
GALLIANO, YURI
2026-04-16
Abstract
Recent studies have highlighted the role of bats in the alteration of cave environments through distinct physical and chemical perturbations. Forming colonies numbering in the hundreds of thousands of individuals, bats release metabolic heat, CO2, and water vapour. Furthermore, the deposition and subsequent degradation of guano, driven by microorganisms, causes the release of gases (CO2, NH3, H2S) and of fluids enriched in phosphoric, sulfuric, and nitric acids. These processes have a dual effect: they induce the corrosion and alteration of carbonate bedrocks and siliciclastic sediments, and concomitantly bring to the formation of diverse mineral assemblages, which are mainly represented by phosphates. The main scope of this thesis was to provide new insights on the pathways which determine the alteration of cave sediments, the consequent formation of authigenic phosphates, and the evolution of the secondary assemblages at varying environmental conditions. A multi-scale approach was developed to elucidate how modifications in the mineralogical assemblages reflect geochemical and textural changes, sometimes leading to the formation of rare and heavily hydrated phases. The integration of micromorphological observations, quantitative mineralogical analysis and bulk geochemistry allowed to describe the multi-stage evolution of a phosphatised sedimentary sequence hosted in Grotte de la Roquette (Gard, France), a cave where intense biocorrosion has significantly reshaped the cave conduits in ancient times, while guano has almost completely disappeared due to degradation. The prolonged release of P-rich acidic fluids from guano decomposition and the progressive leaching of the most mobile elements determined differences in the distribution of secondary minerals along the studied profiles, which developed through impregnation of the sediment matrix and subsequent remobilisation and crystallisation in voids. In this case, the long-lasting evolutionary process that led to the formation of the currently observable associations has reached its conclusion. In other instances, such as the sedimentary deposits of Aven-Grotte de la Baume Saigner (Hérault, France), degrading bat-guano and secondary phosphates coexist, admixed to siliciclastic sediments. Strongly acidic conditions coupled with high Al activity inhibit the formation of Ca-phosphates, and bring to the formation of amorphous Ca-Al-phosphate deposits around limestone fragments contained in the clay-rich sediments. The amorphous material is associated with heavily hydrated aluminophosphates such as taranakite and vashegyite. The latter, a rare and poorly defined mineral, was subject to a detailed mineralogical investigation, which reinforced previous hypotheses regarding the existence of different polytypes. The occurrence of such highly hydrated phases in association with actively decaying guano, representing recent phosphatisation events, as opposed to their absence in more mature contexts, such as the one of Grotte de la Roquette, raises questions regarding their long-term stability. Several authors have proposed that the natural evolutionary pathway for hydrated guano-derived phosphates is their sequential dehydration due to varying environmental conditions. Specifically, exothermic reactions taking place during organic matter degradation are often considered as the primary cause for these transformations. To address the validity of this hypothesis, the taranakite to francoanellite dehydration reaction was taken as a representative example and subjected to a detailed multi-method characterisation, coupling thermodynamic calculations with in situ high-temperature diffraction and spectroscopic techniques. Although the derived equilibrium temperature for the reaction exceeds typical cave conditions, the resulting empirical activation energy suggests that microbially mediated guano degradation might indeed provide sufficient thermal energy to trigger the transformation.
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