Ambiguities in Direct Dating of Rock Surfaces Using Radiocarbon Measurements

An attempt was made to date rock surfaces with accelerator mass spectrometry (AMS) radiocarbon measurements of rock varnishes or rock weathering rinds. In two case studies, samples pretreated in the laboratory of Dr. Ronald Dorn prior to AMS analysis have been found to contain significant quantities of carbon-rich materials of two distinct classes. Type I material resembles bituminous coal, whereas type II material resembles pyrolized wood charcoal fragments. In samples where these type I and type II materials were separated and AMS-radiocarbon dated, they were found to have widely differing radiocarbon ages. In these cases, the measurement of the radiocarbon age of the entire sample would yield results that are, at best, ambiguous. Neither type I nor type II materials were found in comparable samples that were independently prepared.