Rock varnish on petroglyphs from the Hima region, southwestern Saudi Arabia: Chemical composition, growth rates, and tentative ages

Abstract

We investigated rock varnish formed on sandstone and petroglyphs in the Hima area, southwestern Saudi Arabia. To characterize the rock varnish, we made in-situ measurements by portable x-ray fluorescence (pXRF) and analyzed samples by femtosecond laser-ablation inductively coupled–plasma mass spectrometry (fs LA-ICP-MS). Detailed chemical analysis of the rock varnish samples and adjacent soil or aeolian dust yielded information about the varnish’s geochemical context and formation mechanism. Untypically low positive Ce anomalies in the rock varnish samples correlated with negative Ce anomalies in the dust, supporting the hypothesis that the dust is the source of the varnish material. To study the varnish development, we made use of the fact that engraving the petroglyphs exposes a fresh bare sandstone surface without varnish, on which varnish regrows subsequently. We determined by pXRF the areal density of manganese (Mn) and iron (Fe) that had been deposited as rock varnish since the creation of the rock art. The rates of Mn deposition in the newly formed varnish were then estimated by correlating the areal density of Mn in Ancient Arabian and Old Arabic inscriptions with their known age ranges. The observed deposition rates showed substantial variability resulting from differences in exposure conditions of the rock surface, but were in a range comparable with that of our previous measurements in northwestern Arabia. This variability could be reduced significantly by referencing the measurements to the intact varnish adjacent to the individual petroglyphs. This normalization provided a much clearer relationship between varnish deposition and age, and enabled tentative ages to be assigned to rock art motifs without previously known ages. These tentative ages spanned most of the Holocene period and were consistent with the culturally or ecologically derived ages of the animal and human figures depicted in the rock art and the styles of scripts used in different periods.

Geochemical studies on rock varnish and petroglyphs in the Owens and Rose Valleys, California

“…In our studies on Arabian varnishes, we had observed a positive iron intercept in regression analyses of D Fe vs. D Mn , which we suggested to be due to the presence of an Fe oxyhydroxide layer either at the base or the top of the varnish. Such an Fe oxyhydroxide layer has been suggested to act as a catalyst for the formation of the Mn oxyhydroxides [ 17 , 20 ]. In contrast, regression analyses on the data in this study showed no consistent pattern ( Table 3 ).…”

“…Our pXRF measurement technique has been described in detail in previous publications [ 17 , 42 ] and will only be outlined briefly here. Measurements were conducted using a Niton XL3 pXRF (Thermo Fisher Scientific) in the “mining” mode.…”

“…Once the petroglyph has been created, varnish begins to deposit again on the exposed fresh rock surface and, if the rate of accumulation were known, the amount of the regrown varnish could be used to date the rock art. Such a technique would be highly desirable, given the archeological importance of rock art as an expression of ancient cultures and the difficulties encountered in attempts to date petroglyphs by other means [summarized in 17 , and refernces therein].…”

“…The amount of Mn deposited on the rock surface or the Mn/Fe ratios in the varnish have been explored by other authors as potential age indicators [ 34 , 38 , 39 ]. However, all of these approaches have to be viewed with great caution, because the growth rate of varnish is highly variable and depends on a large number of parameters other than age, including the exposure of the rock surface to dust, erosion by wind and water, the orientation and slope of the rock surface, the hardness, roughness, and texture of the rock underneath, and its initial iron content [ 3 , 19 , 22 , 34 , 40 , 41 ], as summarized in [ 17 ] and in S1 Table .…”

“…In two previous studies, we demonstrated the potential of measuring the amount of Mn in rock varnish on petroglyphs and adjacent intact rock surfaces by portable X-ray fluorescence (pXRF) as a tool to determine the growth rate of varnish and to estimate the age of rock art in Saudi Arabia [ 17 , 42 ]. That work benefitted from the existence of distinct time markers in the Arabian rock art in the form of particular types of scripts, which had been used during specific time periods, and of dated paleoclimatic transitions that were reflected in the animal species depicted in the rock art [ 43 – 47 ].…”