Radiocarbon dating of lime plaster from a Roman period cistern in ancient Gerasa, Jerash in Jordan

Ancient Gerasa (Jerash, since the Islamic period) is a city with a rich archaeological heritage from prehistory onwards, with the periods from Roman times into the Early Islamic period best attested. A Danish-German team has been working in the Northwest Quarter of the city since 2011. Among the findings was a Roman period monumental cistern that was intentionally filled in, at one point in time. The complexity of the archaeology raised chronological questions regarding the construction, destruction and subsequent backfilling of the cistern. This study contributed to answer these questions by radiocarbon dating lime plaster excavated from the cistern, and comparing the results with charcoal radiocarbon dates from other studies. Radiocarbon dating of plaster and mortar in Jerash is challenging because of contamination of geological carbonates from the local limestone geology. Quoting previous mortar dating studies, this study utilized sample characterization by alkalinity screening, petrography, SEM-EDS, and sample pre-treatment and preparation by wet sieving, sedimentation and stepwise injection. The plaster dates argued for the construction of the cistern being in the last half of the 1st century BCE to the middle of the 1st century CE. A few samples had later mortar dates, which argued for the filling event of the cistern taking place sometime in the late 3rd century CE or later.