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

Abstract

The mineralogical and geochemical evolution of cave guano deposits in France has been investigated in detail. Two test pits 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 southwest France. Thin section analysis, mineralogical identification and quantification, geochemical analysis, organic matter characterization through pyrolysis and thermochemolysis coupled to gas-chromatography, were conducted to better understand the evolution of guano in caves and to identify the underlying factors. Bat guano undergoes mineralization through loss of organic matter and precipitation of phosphate and sulfate minerals. The neoformed minerals include gypsum, ardealite, brushite, francoanellite, hydroxylapatite, monetite, newberyite, and taranakite, and vary according to the local availability of chemical elements released by the alteration of detrital minerals due to acidic solutions. Chough guano, located at higher altitude in a periglacial environment, does not show similar mineral formation. Organic geo-chemical analysis indicates strong differences between guano. Abundant hydrocarbons derived from insect cuticles were the dominant feature in bat guano, whereas a mostly vegetal origin typifies chough guano. Geochemical analysis points 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.