Identification of fresh and burnt bat guano and pigeon droppings in Eastern Mediterranean karstic cave sites based on micromorphological and chemical characteristics

Karstic caves in the Eastern Mediterranean region hold significant archaeological importance as they served as an important dwelling place for prehistoric hominins. Yet, such caves were used at the same time as a habitat for birds and bats. As a result, bat guano and bird droppings are not only a major source of cave deposits, but also trigger significant chemical alterations (diagenesis) due to their decomposition that can greatly affect the preservation of archaeological materials as reported from many Palaeolithic cave sites. While the diagenetic processes associated with bat guano degradation are quite well understood, the identification of partially-preserved or burnt guano residues in archaeological cave sites has not received much attention. Here we present a micromorphological, chemical and mineralogical study of modern guano of insectivorous bats, frugivorous bats and pigeons, collected from karstic caves in Israel. Each of these guano types was studied in fresh, charred and ashed forms. Based on the patterns obtained we present guidelines that allow identification of partially-degraded and burnt bat and bird guano remains. These include (1) high phosphorous and calcium associated with high quantities of chitin fragments in insectivorous bat guano; (2) low phosphorous and calcium associated with high quantities of seed coats, fibrous organic matter and red oval organic particles in frugivorous bat and pigeon guano. We found that faecal spherulites are present in bat guano and absent in pigeon droppings. Blackening and rounding of the edges of organic components characterise guano upon charring, while ashed guano is characterised by a micritic calcite groundmass in the case of fruit bat and pigeon guano without any pseudomorphs after calcium oxalate, unlike wood ash. We further show that ashed insectivorous bat guano is characterised by a micritic groundmass that includes phosphatic particles and potassium sulphate. These data indicate that while the combustion of guano presents similarities to other organic-based burnt deposits found in archaeological sites, a careful micromorphological study, aided by certain elemental and mineralogical signatures, can reveal whether or not burnt deposits originate from guano. Last, we studied the micromorphology of in situ degraded frugivorous and insectivorous bat guano in caves, observing that a bedded and laminated structure characterises fresh deposits while, as degradation progresses the proportion of authigenic minerals, appearing as nodules and veins, increases, disrupting the laminated structure. Importantly, chitin fragments in insectivorous bat guano and seed coats and some organic fibers in frugivorous bat guano preserve within degraded bat guano deposits and can serve as indicators for the presence and feeding habits of bats. Applying the guidelines given here to archaeological studies at prehistoric cave sites may produce new insights regarding human behavior, human-bat interactions, as well as for the formation processes at these sites. (C) 2021 Elsevier Ltd. All rights reserved.