AMS Radiocarbon and Cation-Ratio Dating of Rock Art in the Bighorn Basin of Wyoming and Montana

Abstract

Samples of organic matter and rock varnish from seven rock-art sites in the Bighorn Basin of Wyoming and Montana were collected for dating purposes. Petroglyphs sampled include Dinwoody-style figures, shield-bearing warriors, and other well-known Plains rock-art motifs. Accelerator mass spectrometry (AMS) dating of 10 petroglyphs yielded dates from the Early Archaic to the Protohistoric periods. A strong numerical relation between varnish leaching and time was found for petroglyphs older than 1,000 years, permitting the derivation of a cation-leaching curve (CLC) and calibrated cation-ratio (CR) ages for 15 different petroglyphs. No clear numerical relation between varnish leaching and time was found for petroglyphs less than 1,000 years old, possibly due to historical damage or past environmental conditions. As a result, calibrated CR ages could not be derived for six petroglyphs, and they are considered to be only younger than 1,000 years. Although further research is needed to establish whether one CLC can be used for all petroglyphs in the region, these studies constitute the first numerical chronology for rock art in the Bighorn area. Results indicate the occurrence of spatially discrete, but temporally concurrent styles in the Bighorn Basin during the last 800-900 years.

Ambiguities in Direct Dating of Rock Surfaces Using Radiocarbon Measurements

“…In 1986, both vitrinite and CWT were identified and concluded to be "abundant" in the laminar calcrete [(19), p. 77)], a material found in desert soils and in fractures in weathering rinds (20 Beck et al thus fail to cite other pertinent literature; this might seem to imply that the only possible source for the organics is intentional tampering (2-5). The research (9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22) shows that organic heterogeneity is the norm, rather than a cause for suspicion.…”

“…The in situ diagenesis of CWT influences sample age, because CWT is gradually replaced by Mn-Fe oxides (10,12,22). When samples are chemically digested in the lab, and in situ Mn-Fe dissolve, what were originally larger fragments of CWT are reduced greatly in size.…”

“…I was given permission by the Portuguese funding agency to present my findings publicly, and did so at a professional meeting in May 1996 (11,17) (34), fire also comes into direct contact with rock varnish and influences its development (35). In archaeological contexts like petroglyphs, humans may apply charcoal to rock surfaces (10)(11)(12)22).…”

AMS 14C age constraints on geoglyphs in the lower colorado river region, arizona and california

“…There is controversy surrounding the cation-ratio method that was used to "preselect" cobbles for AMS sampling. These issues are reviewed elsewhere (Dorn, 1994a(Dorn, , 1994bFrancis et al, 1993), and many are beyond the scope of this research-because we are not using cation ratios to assign calibrated ages. We are only using the most basic assumption of cation-ratio dating, that cation ratios provide a relative age signal-a finding that has been replicated by several different groups of researchers (Bull, 1991;Dorn, 1983;Glazovskiy, 1985;Pineda et al, 1988;Taylor, 1994;Whitney and Harrington, 1993;Zhang et al, 1990).…”

“…The areas of stone removal are much lighter than the surrounding pavement, due to the exposure of the Av soil horizon and exposure of unvarnished cobbles. The "Nazca lines" of Peru (Clarkson, 1990;Reiche, 1968) are classic examples of geoglyphs, although similar forms have long been recognized in North America (Davis and Winslow, al., 1990), oxalates interbedded with paint (Watchman, 1993a), and organic detritus encapsulated in petroglyphs by rock coatings (Francis et al, 1993;Nobbs and Dorn, 1993).…”