Common Pipistrel, European cave bat. Online photograph, public domain.
Another
threat to rock art has recently been scientifically recognized. The effects
that occupation by bats can have on a cave is recognized now to go much farther
than just the accumulation of guano on the cave floor. Occupation by bats of a
cave with parietal art presents a very real threat to that art. This was
illustrated by a team of French researchers who performed in-depth scientific
analysis in a cave that had been prehistorically divided into separate sections
by prehistoric flowstone deposits. This blockage enabled to researchers to
analyze and compare the sections of the cave where bat occupation had been
prevented to the portion of the cave that bats had inhabited.
Grotte de Azé cave, France. Online photograph, public domain.
“The Prehistoric Cave at Azé (France) was divided into two parts by calcite flowstone. This isolated the innermost parts of the cave from the entrance, which remained connected to the surface. Since the closure, bats have not been able to access the cave beyond the flowstone blockage. They have been present only in the cave entrance. The byproducts released by bats had a considerable effect on this entrance part. Gaseous exhalations have changed the composition of the surrounding atmosphere, leading to the development of dome-shaped cupolas and other wall weathering features. Guano deposits built up causing the formation of aerosols and acidic leachates. The resulting corrosion of the limestone has led to the formation of phosphate deposits and biogenic karst. The rate of retreat of the cave walls has been estimated at 5 to 7 mm/ka. Corrosion has also affected archaeological artefacts, as well as any traces left on the walls by humans or animals. Bioglyphs linked to bears as well as anthropic graffiti have completely disappeared from the part of the cave used by bats.” (Barriquand et al. 2021:1)
The bats exhale CO2 which can combine with the moisture in the air to create a weak solution of carbonic acid. Bat droppings and urine also contribute to the damage as described above.
“The presence of apatite in the flowstone at Aze Cave shows that bats were present there before the end of MIS 4 (57 ka), and most probably also during MIS 5, which would have been climatically even more favorable for bats. At least 22 ka ago, calcite flowstone isolated the inner part of the cave. It completely blocked any access and prevented the entry of bats. Since this blockage, the cave walls beyond it have hardly altered, as confirmed by the preservation of bear claw marks on the walls. In contrast, all of the front part of the cave between the entrance and this barrier remained open to the surface and has undergone considerable morphological changes.” (Barriquand et al. 2021:9)
MIS refers to marine isotope stages, cores from ocean sediment are analyzed for variation in isotopic oxygen.
“Marine oxygen-isotope stages or oxygen isotope stages, are alternating warm and cool periods in Earth’s paleoclimate, deduced from oxygen isotope data reflecting changes in temperature.” (Wikipedia) MIS 4 corresponds to an age range from 71 to ca. 84 thousand years ago, and MIS 5 ranges from ca 85 to 139 thousand years BP. (Wikipedia)
With the
knowledge gained from their analysis of the effects that bat populations have
on a cave environment, the team could then look at other caves where cave art
is found.
“Spatial analysis of the position of the art works in the Mas d’Azil Cave, Ariege, France, also shows that these are found only in confined areas not used by bats. In the Gorges d’Amange Cave, Jura, France, the Aurignacian engravings which have been exposed to phosphates are still visible, but have been deeply weathered. In the Chauvet Cave, the art work on the Megaloceros Panel is inscribed into deposits of hydroxylapatite indicating the occupation of this part of Chauvet Cave by bats is older than the artwork. Closure of the cave entrance after the art work was made preserved it from latter biogenic weathering.” (Barriquand et al. 2021:10)
In the example of Mas d’Azil bat residence in portions of the cave is known, but the art is found in locations that the bats do not occupy. In other art caves some damage caused by bats can be seen on existing art as described above. The knowledge gained through this study might someday allow preservation of endangered cave art, this magnificent and important prehistoric treasure.
NOTE: Some images in this posting were retrieved from the internet with a search for public domain photographs. If any of these images are not intended to be public domain, I apologize, and will happily provide the picture credits if the owner will contact me with them. For further information on these reports you should read the original reports at the sites listed below.
REFERENCES:
Barriquand, Lionel, Jean-Yves Bigot, Philippe Audra, Didier Caihol, Christophe Gauchon, Vasile Heresanu, Stephane Jaillet, and Nathalie Vanara, 2021, Caves and bats: Morphological impacts and archaeological implications. The Aze Prehistoric Cave (Saone-et-Loire, France), 21 May 2021, Geomorphology, www.elsevier.com/locate/geomorph
Wikipedia, Marine Isotope Stages (MIS),
https://en.wikipedia.org/wiki/Marine_isotope_stages
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