HOW THE NATURAL 'GLAZES' ON THE WALLS OF KIMBERLEY ROCK SHELTERS HELP REVEAL THE WORLD THE ARTISTS LIVED IN:

Glaze mineral accretion over painted panel, photograph Cecilia Myers, Green et al.

For some time one dream of rock art researchers has been the development of scientific procedures that can directly date rock art. From Dr. Ronald I. Dorn’s pioneering work with cation-ratio dating in the early 1980s to David Whitley and others who worked with radiocarbon dating techniques in the early 1990s a number of direct dating techniques have been tried on rock art with varying degrees of success. And some of these successes have been notable. Nowadays many pictographs can be dated with C¹⁴ dating based upon carbon in their pigments, in the vehicle used to make the paint, or the binder (if present).

Now, seemingly, a team of scientists in Australia has accomplished direct dating from petroglyph panels. “The Kimberley region is host to Australia’s oldest known rock paintings. But people were carving engravings into some of these rocks before they were creating paintings. – The oldest paintings are at least 17,300 years old, and the engravings are thought to be even older – but they have so far proved much harder to date accurately.” (Green and Finch 2021)

Drysdale River, oxcalate rich accretion in fracture post dates painted figure, photograph Cecilia Myers, Green et al.

This team of Australian scientists has now found datable components in the patina formed on the surface of the petroglyphs allowing them to begin to crack this puzzle.

“Some of the rocks themselves are covered with natural, glaze-like mineral coatings that can help reveal key evidence. These dark, shiny deposits on the surface of the rock are less than a centimeter thick. Yet they have detailed internal structures, featuring alternating light and dark layers of different minerals. Our aim was to develop methods to reliably date the formation of these coatings and provide age brackets for any associated engravings. However, during this process, we also discovered it is possible to match layers found in samples collected at rock shelters up to 90 kilometers apart.” (Green and Finch 2021)

Microscope image of alternating layers, photograph Helen Green, Green and Finch.

The team’s research “supports earlier findings that layers within the glaze structure represent alternating environmental conditions in Kimberley rock shelters, that repeated over thousands of years. Our model suggests that during drier conditions, brush fires produce as, which builds up on shelter surfaces. This ash contains a range of minerals, including carbonates and sulfates. We suggest that under the right conditions, these minerals provide nutrients that allowed microbes to live on these shelter surfaces. In the process of digesting these nutrients, the microbes excrete a compound called oxalic acid, which combines with the calcium in the ash deposits to form calcium oxalate.” (Green and Finch 2021)

“These dark calcium oxalate layers also contain carbon that was absorbed from the atmosphere and digested by the microbes that created these deposits. This meant we could use a technique called radiocarbon dating to determine the age of these individual layers.” (Green and Finch 2021) So, not only is carbon a component of the oxalate, but the oxalate layers possess enveloped carbon particles which can be radiocarbon dated.

With this technology, any rock art with this kind of patina or coating should be a candidate for the estimation of its minimum age by testing the coating next to the surface of the stone.

Engraved grooves within dark oxalate surface, Drysdale River, photograph Cecilia Myers, Green et al.

“Globally, oxalate-rich accretions have been found on rock art panels, with the carbon component of the oxalate mineral providing a clear opportunity for radiocarbon dating of associated motifs. Radiocarbon dating of oxalate accretions has provided minimum and, in some cases, bracketing ages for associated rock art, although concerns have been raised about the source of the dated carbon, the possibility of open-system behavior in the accretions, and possibilities of sample contamination by either older or younger organic constituents. Chemical pretreatment procedures have been used with apparent success in dating studies to isolate pure oxalates from other organic materials present, suggesting that some of these risks can be avoided or controlled by improved analytical protocols and a better understanding of their formation mechanism and is essential in relating radiocarbon dates to the time of mineral formation, which may, in turn, be related to associated rock art.” (Green et al. 2021)

The team was also able to cross-reference dates and layers of deposition from other caves in the region suggesting that this phenomenon could be applied to regional studies, not just to specific samples.

“Radiocarbon dating suggests these layers were deposited around the same time showing their formation is not specific to particular rock shelters, but controlled by environmental changes on a regional scale. Dating these deposits can therefore provide reliable age brackets for any associated engravings, while also helping us better understand the climate and environments in which the artists lived.” (Green and Finch 2021) So, not only is this allowing dating of the petroglyphs, it allows understanding of the “climate and environment” at the time it was produced.

Calcium oxalate, known as oxalate of lime in archaic terminology, is a calcium salt of oxalic acid with a chemical formula of CaC₂O₄(H₂O)x. It is also found in 76% of human kidney stones. (Wikipedia) Could this be the reason for our feelings of connection to rock art? While not every petroglyph panel has such deposits on its surface, the ones that do now can be objectively dated, and that is a gift to all rock art researchers (even if kidney stones are not).

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:

Green, Helen et al., 2021, Dating correlated microlayers in oxalate accretions from rock art shelters: New archives of paleoenvironments and human activity, 13 August 2021, Science Advances, Volume 7, Number 33.

Green, Helen, and Damien Finch, 2021, How the natural ‘glazes’ on the walls of Kimberley rock shelters help reveal the world the artists lived in, 16 August 2021, https://phys.org/news/2021-08-natural-glazes-walls-kimberley-reveal.html

Wikipedia, Calcium Oxalate,  https://en.wikipedia.org/wiki/Calcium_oxalate, accessed on 7 October 2021.