Bat guano and preservation of archaeological remains in cave sites

The formation of authigenic minerals in cave sediments can be used to reconstruct the paleochemical conditions that prevailed in the past, and in this way to assess the completeness of the archaeological record. Previous studies indicated that the major driving mechanisms for chemical diagenesis in prehistoric caves that result in the formation of authigenic minerals, are the degradation of bat guano and the local hydrology. We therefore investigated contemporary bat guano deposits in caves and the sediments directly below such deposits. The emerging patterns show that the formation of authigenic minerals occurs under acidic conditions within tens of years. The availability of phosphate, Al, K, and Fe increases with increasing organic matter degradation, while the availability of nitrogen and sulfur decreases. Insectivorous bat guano contains larger amounts of phosphate and is more acidic than fruit bat guano. In the first stages of diagenesis micro-chemical environments form within the degrading bat guano that result in the formation of a wide variety of authigenic minerals. During advanced stages of diagenesis steady state conditions are achieved resulting in authigenic mineral assemblages being dominated by one mineral type at the same locale. This is reminiscent of known distributions of authigenic minerals in prehistoric cave sediments. Under the acidic conditions produced by degrading bat guano deposits, calcareous artifacts and bones are not expected to persist for long periods of time, unless there is a lot of calcite in the sediments that could buffer the sediment water pH. Guano itself is not likely to be preserved. Periods of cave abandonment can be inferred from the presence of authigenic minerals in the sediments as it is during these times that bats mainly occupy the cave. The occurrence of certain authigenic minerals in open-air sites may serve as evidence for areas rich in organic matter containing phosphate (i.e., animal enclosures, latrines, sewers, trash pits) and/or bones. As the degradation process is rapid, the use of authigenic minerals to identify these loci is relevant to almost all archaeological, as well as historical sites.