Direct Dating of Plaster and Mortar Using AMS Radiocarbon: A Pilot Project from Khirbet Qana, Israel. (Method)

Article excerpt

Introduction

Although radiocarbon dating has been utilised to date important artefacts such as the Dead Sea Scrolls (Jull et al. 1995), archaeologists working on historic-period sites in the Near East generally rely, on techniques such as ceramic typologies, numismatic dating, diagnostic architectural elements, and in situ inscriptions for age determinations of structures and archaeological strata. In certain circumstances, these techniques can assign ages to a feature for less cost than [sup.14]C dating and with greater precision. However, dating by architectural forms and inscriptions is typically dependent on monumental architecture, which is not always present. Ceramics and coins only provide accurate ages when they are sealed within primary contexts and are sufficiently abundant to narrow the age range of ceramic vessel types and coin assemblages. Additionally, many coins were circulated well beyond their age of minting, leading to age assessments that are too old. Radiocarbon dating therefore has a potential value, particularly if it can be applied to primary material in buildings.

Khirbet Qana is a prime example of a site where traditional dating techniques are not adequate. It contains few diagnostic architectural elements and is composed of features and structures cut into or founded on bedrock. There are few intact strata containing abundant coins or ceramics of similar age. Instead there are many thin, heavily disturbed stratigraphic horizons that contain ceramic assemblages of mixed ages. As a result of these limitations, it was unknown whether some structures at Khirbet Qana were Roman (63 BC-330 AD) or Byzantine (363-640 AD) in age, and in other locations it was unclear whether features were late Arab/Crusader (1099-1291 AD) or modern (19th or 20th centuries).

In order to refine the chronology of Khirbet Qana and assess the potential of using accelerator mass spectrometry (AMS) radiocarbon dating at Near Eastern archaeological sites, we radiocarbon-dated fourteen samples of organic matter. Thirteen of these samples were organic fragments sealed within in situ mortar and plaster, and one sample was of charcoal embedded within an uncemented floor. Although there has been some work with [sup.14]C dating of organic material within mortars (Kedar and Mook 1978; Berger 1992), this is the first study to use AMS [sup.14]C to systematically date organic material encased in mortar or plaster to identify multiple phases of construction.

The study area

Khirbet Qana is situated in the lower Galilee region of Israel, within the Bet Netofa Valley (Figure 1). Excavations and an extensive site survey began in 1997 under the direction of Douglas R. Edwards of the University of Puget Sound. Between 1997 and 2001, surface surveys and excavation at Khirbet Qana identified material evidence from the Neolithic through modern periods (6000 BC-19th century A.D). The most extensive occupation of the site, based on architectural and material remains, was during the Roman and Byzantine periods (Table 1).

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Methods

Organic inclusions in plaster and mortar were collected from several areas of the site to address specific research questions. Samples were collected from two of the main excavation areas, a Christian cave shrine, thought to be Crusader in age, and a large public structure, possibly a Byzantine-period synagogue. Samples were also collected from a thick (~ 1m) vaulted wall exposed at the surface to identify its age, from plaster layers within a possible mikvah (Jewish ritual bath) to determine its age and whether it had multiple phases of construction, and from an exposed plastered area (possibly industrial) founded on bedrock.

Organic matter was only collected from in situ plaster and mortar (Figure 2). When there was an abundance of organic material within the plaster/mortar, between 100-500g of plaster was collected. In cases where there were few organics within the plaster/mortar, the material was hand picked in the field to isolate and collect occasional organic fragments. …