Asphalt in carbon-14-dated archaeological samples from Terqa, Syria

Abstract

The presence of fossil fuel compounds in archaeological samples giving ages much higher than expected has long been suspected but never proved by appropriate chemical analyses. An excessively high conventional ¹⁴C age was found in an archaeological charcoal sample from Terqa, Syria¹ (S313, Table 1). Its¹⁴ C age of 28,700 yr BP was at variance with the archaeological context, and with the ¹⁴C age of several other samples¹ (such as S283, Table 1) obtained from the same area and expected to date from ∼3000 BC or younger. The inconsistency can be explained by assuming contamination with geologically old material—either industrial petroleum products or ancient asphalt. We report here organic geochemical investigations of the sample S313 and of sample S267, which gave a ¹⁴C age ∼2,000 yr too old. A third charcoal sample (S283) from the same area, whose ¹⁴C age was consistent with its known historical age, and an asphalt sample found stuck to the bottom of a goblet from another site in the area dated at 1700 BC on typological grounds, were also analysed. Similar studies of used crankcase oils from motor vehicles, weathered 0–2 yr, and of hydraulic transmission fluid show that modern motor oil cannot be the contaminant at this excavation site. Our data clearly indicate that ancient asphalt must be the source of contamination. Caution should be exercised, therefore, in interpreting ¹⁴C dates of archaeological samples from areas containing asphalt or other fossil fuel deposits.