Mineralogical and organic study of bat and chough guano: implications for guano identification in ancient context

摘要

The mineralogical and geochemical evolution of cave guano deposits in France has been investigated in detail. Two testpits were excavated in guano mounds from insectivorous bats and one in a guano mound from omnivorous choughs.Both bats and choughs are thought to be among the main accumulators of guano during the Pleistocene in southwestFrance. Thin section analysis, mineralogical identification and quantification, geochemical analysis, organic mattercharacterization through pyrolysis and thermochemolysis coupled to gas-chromatography, were conducted to betterunderstand the evolution of guano in caves and to identify the underlying factors. Bat guano undergoes mineralizationthrough loss of organic matter and precipitation of phosphate and sulfate minerals. The neoformed minerals includegypsum, ardealite, brushite, francoanellite, hydroxylapatite, monetite, newberyite, and taranakite, and vary according tothe local availability of chemical elements released by the alteration of detrital minerals due to acidic solutions. Choughguano, located at higher altitude in a periglacial environment, does not show similar mineral formation. Organic geochemicalanalysis indicates strong differences between guano. Abundant hydrocarbons derived from insect cuticleswere the dominant feature in bat guano, whereas a mostly vegetal origin typifies chough guano. Geochemical analysispoints to an especially high content of copper and zinc in bat guanos, a few hundreds of μg/g and thousands of μg/g,respectively. Both organic matter analysis and geochemistry may help identification of bat guano in archeological contexts,where phosphate minerals can originate from multiple sources.