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

Abstract

We investigated rock varnish, a thin, manganese- and iron-rich, dark surface crust, on basaltic lava flows and petroglyphs in the Owens and Rose Valleys (California) by portable X-ray fluorescence (pXRF) and femtosecond laser-ablation inductively-coupled-plasma mass spectrometry (fs-LA-ICPMS). The major element composition of the varnish was consistent with a mixture of Mn-Fe oxyhydroxides and clay minerals. As expected, it contained elevated concentrations of elements that are typically enriched in rock varnish, e.g., Mn, Pb, Ba, Ce, and Co, but also showed unusually high enrichments in U, Cu, and Th. The rare earth and yttrium (REY) enrichment pattern revealed a very strong positive cerium (Ce) anomaly and distinct negative europium (Eu) and Y anomalies. The light rare earth elements (REE) were much more strongly enriched than the heavy REY. These enrichment patterns are consistent with a formation mechanism by leaching of Mn and trace elements from aeolian dust, reprecipitation of Mn and Fe as oxyhydroxides, and scavenging of trace elements by these oxyhydroxides. We inferred accumulation rates of Mn and Fe in the varnish from their areal densities measured by pXRF and the known ages of some of the lava flow surfaces. The areal densities of Mn and Fe, as well as their accumulation rates, were comparable to our previous results from the desert of Saudi Arabia. There was a moderate dependence of the Mn areal density on the inclination of the rock surfaces, but no relationship to its cardinal orientation. We attempted to use the degree of varnish regrowth on the rock art surfaces as an estimate of their age. While an absolute dating of the petroglyphs was not possible because of the lack of suitable calibration surfaces and a considerable amount of variability, the measured degree of varnish regrowth on the various petroglyphs was consistent with chronologies based on archeological and other archaeometric techniques. In particular, our results suggest that rock art creation in the study area continued over an extended period of time, possibly starting around the Pleistocene/Holocene transition and extending into the last few centuries.

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

“…The Zissis inscription is particularly questionable since it is on the outside of one of the monastery cells at Kilwa, and may have been altered by later visitors. The other rates on young surfaces are relatively high, consistent with our previous observations that a small amount of Mn varnish forms relatively rapidly (within decades) on freshly cleared surfaces (Andreae et al, 2020;Macholdt et al, 2017aMacholdt et al, , 2018Macholdt et al, , 2019.…”

“…For a quantitative analysis of this variability at different scales, we calculated the coefficients of variation (CV), that is, the ratio of the SD over the mean (expressed as a percentage), of the areal density measurements on the varnished surfaces. Table 1 shows that the variability of D Mn of intact varnish at the site scale (tens of metres to a few kilometres) is in the range of 15-40%, similar to our previous observations (Andreae et al, 2020;Macholdt et al, 2019). There is considerable variability of the varnish density at the microscale on the rock surfaces, ranging from the thickest varnish in microbasins through thin coatings on exposed quartz grains to the total absence of varnish on recently eroded or spallated surfaces (Cremaschi, 1996;Liu & Broecker, 2007;Zerboni, 2008).…”

“…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).…”

“…In several previous studies we developed an improved technique to obtain age estimates from measurements of Mn areal density by portable X-ray fluorescence (pXRF) (Andreae et al, 2020;Macholdt et al, 2018Macholdt et al, , 2019. In this approach we use the ratio between the Mn density on the petroglyph to that of the surrounding intact varnish as the basis for an age estimate, which corrects for the influence of variables other than age that affect varnish deposition.…”

“…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.…”