Pyrolysis GC–MS and THM-GC–MS studies of a black coating from Little Lost River Cave, Idaho

A black, shiny coating overlies rock paintings in Little Lost River Cave No. 1, located in southeastern Idaho. If this coating is the result of human activity within the cave – a condensate from cooking fires, for example – then a radiocarbon date on the coating would provide a minimum age for the underlying paintings. The work reported here was undertaken to test the assumption that the coating has an anthropogenic origin. Using pyrolysis-gas chromatography–mass spectrometry (py-GC–MS) and thermally assisted hydrolysis-methylation-GC–MS, we have compared samples of the black coating from Idaho to humic acid, an experimental cooking residue, and amberat from packrats. The results show that the coating is not amberat, but bears the most similarity to humic acid, which would indicate a geologic origin for the coating. The age of the coating, then, is not likely related to the age of the paintings.