Geochemical insights into the relationship of rock varnish and adjacent mineral dust fractions

Archaeometric studies on the petroglyphs and rock varnish at Kilwa and Sakaka, northern Saudi Arabia

“…For comparison, Figure 2 also shows statistics of the results of our previous measurements on type I varnish. The enrichment patterns in CS and Kilwa samples are in good agreement with published values for the elements typically enriched in varnish, for example, Mn, Pb, Ba, Co and Ce (Andreae et al, 2020; Dorn, 2007; Macholdt et al, 2017b; Otter et al, 2020; Potter & Rossman, 1977; Xu et al, 2019).…”

“…The relatively low Mn concentrations and Mn/Fe ratios are consistent with varnish development under predominantly arid conditions (Liu & Broecker, 2008;Macholdt et al, 2017b). Figure 2 shows the elemental composition data in the form of enrichment factors against the average upper continental crust composition (Rudnick & Gao, 2003) values for the elements typically enriched in varnish, for example, Mn, Pb, Ba, Co and Ce (Andreae et al, 2020;Dorn, 2007;Macholdt et al, 2017b;Otter et al, 2020;Potter & Rossman, 1977;Xu et al, 2019).…”

“…While there is an ongoing debate about the details of rock varnish formation, there is now a broad consensus that the Mn and other enriched elements in the varnish matrix, as well as the embedded detrital minerals, are derived from dust deposition and are transformed into the varnish coating by a sequence of dissolution and re‐precipitation events (e.g., Dorn & Krinsley, 2011; Goldsmith et al, 2014; Perry et al, 2005; Thiagarajan & Lee, 2004; Xu et al, 2019). For further details and references on this issue, see Andreae et al (2020), Macholdt et al (2017b, 2019) and Otter et al (2020).…”

“…While there is an ongoing debate about the details of rock varnish formation, there is now a broad consensus that the Mn and other enriched elements in the varnish matrix, as well as the embedded detrital minerals, are derived from dust deposition and are transformed into the varnish coating by a sequence of dissolution and re-precipitation events (e.g., Dorn & Krinsley, 2011;Goldsmith et al, 2014;Perry et al, 2005;Thiagarajan & Lee, 2004;Xu et al, 2019). For further details and references on this issue, see Andreae et al (2020), Macholdt et al (2017bMacholdt et al ( , 2019 and Otter et al (2020). In spite of the widespread occurrence of rock varnish and the large numbers of publications discussing various aspects of its geochemistry, there is still very little quantitative information on its rate of formation in different geological and climatic settings.…”

Analysis of Rock Varnish from the Mojave Desert by Handheld Laser-Induced Breakdown Spectroscopy

“…For the latter LIBS system, which had a laser spot size of ~400 μm, the LIBS spectra acquired over limited ranges of the ultraviolet (240-342 nm), violet (382-469 nm) and near-infrared (470-907 nm) spectral regions documented that the thin (<1 mm) dark varnish layer had an overall composition different from its basalt substrate, containing H, Li, Na, Mg, Al, Si, K, Ca, Ti, Mn, Fe It is important to note that relative emission line intensities in Figure 1 are not indicative of absolute elemental abundances, but instead reflect a combination of element emissivity and concentration. Thus, the strong intensity for the Li, Na, Ca and Ba spectral lines is the direct consequence of the low ionization potential of these elements, despite the modest concentrations expected to be present within the varnish [10,15,31,33,36]. The intensity of the Mn and Ba lines, which are most pronounced in the surface layer formed under present-day climate conditions, progressively decreased in intensity relative to the Si peak in subsequent laser shots.…”