Isotope Archaeology: July 8-20, 2007
Radiogenic isotope archaeology at Sagalassos--perspectives from the field situation:
Radiogenic isotopes are of major importance in the Earth Sciences for numerical dating of rocks and the study of various geological processes, such as the origin of magmas, the sources of metals concentrated in ore deposits, the composition of ancient seawater, etc. Radiogenic isotopes can also be principally utilized to study important archaeological aspects, such as provenancing of raw materials of ancient artifacts, archaeometallurgical processes, and the migrations of people. "Isotope Archaeology" is an appropriate, though not yet established, term for this type of quantitative archaeological research. Its analytical techniques required are generally time-consuming and have been developed over decades in the Earth Sciences, where they now rank among the most important methods in geochemical research. They principally involve wet chemical separation of the elements of interest in a clean laboratory and subsequent isotope analysis on a multi-collector sector field mass spectrometer. Any plausible interpretation of radiogenic isotope data in archaeology requires comparative isotope analysis of both artifacts and background geological material. Pb (lead) isotopes and, to a much lesser extent, Sr (strontium), Nd (neodymium), and Os (osmium) isotopes have been applied hitherto to the study of ancient metal production, glass artifacts, building stones and ceramics.
At Sagalassos this type of research has recently been introduced by Dr. Jens Schneider of the Center for Archaeological Sciences (CAS) at K.U.Leuven). The archaeometry group of the CAS adopts a multi-isotope approach to solve archaeological problems, i.e. combined analysis of several isotope tracers per sample, thereby overcoming significant limitations in the data interpretation that result from classical approaches in which only one isotope system is considered. A major part of the research is focused on the development of a methodology. This includes the evaluation of radiogenic isotope tracers rarely applied or completely new in archaeological studies, such as 143Nd, 176Hf (hafnium), 187Os, and 115Sn (tin), as well as the application of Pb and Sr isotopes to archaeological material types beyond those classically used for Pb and Sr isotope archaeometry. Our strongly methodological research philosophy is therefore capable of pointing the way for more applied case study work within the archaeometrical community.
During a geological survey on the territory of Sagalassos conducted between July 8-12 by CAS researchers Prof. Patrick Degryse and Dr. Jens Schneider, along with mineralogist Prof. Jan Elsen, geologist Bert Neyt, and archaeologist Dennis Braekmans (all K.U. Leuven), diverse options for geo-archaeological research utilizing radiogenic isotopes were discussed with respect to the field situation:
1.) The metal and glass artifacts and building stones and wall veneer excavated at Sagalassos reflect a great range of different raw materials and commodities used for their production. We have already started on a general base some research on glass production in antiquity using combined Sr and Nd isotopes. At Sagalassos, one finds a mixture of artifacts produced both from local materials available on the territory and imported raw materials. While the "internal" isotopic characterization of these artifacts is not a problem, there is a clear lack of isotopic background data for comparison. The available rock and metal ore types on the territory of Sagalassos have been thoroughly classified in the past by means of their mineralogy, petrology, and chemical composition, but we only have radiogenic isotope data for a few geological objects. We plan, therefore, to analyze the representative rock and ore types available on the territory for at least Sr, Pb and Nd isotopes to provide a comparative, local database for future archaeometric research. Since the geology on the territory is not too complex and dominated by a few major rock types of very different origin (oceanic basalts, limestone, continental volcanics), this will be useful to define possible end member compositions and point sources in the local production lines, as, on geochemical-petrological grounds, the main rock types will have distinct but characteristic isotopic compositions. Additional "bulk" background information will be provided by isotope analysis of snail shells sampled from different bedrock lithologies.
2.) The majority of metal artifact finds at Sagalassos are either lead or iron objects, the production processes and raw materials of which we have already studied by Pb and Sr isotopes. However, there is also an enormous number of copper alloy artifacts. The origin of the raw materials used for their production is totally unexplored. As with lead, local sources for copper and metals alloyed with it are unknown on the territory and can be excluded. From Pb isotope analysis of representative and well-dated copper alloy artifacts from Sagalassos we therefore expect some clues not only on possible external ore sources, but also on the location of raw metal production centers and ancient trade modes.
3.) While there is still considerable ambiguity and uncertainty in the interpretation of base and precious metal artifacts--silver and gold, as well as copper and lead--with respect to the origin of their raw materials, we have best evidence for intense local production of iron from ore types readily available on the territory of Sagalassos. Based on combined Sr and Pb isotope analysis and archaeological context, a large portion of the iron used and worked in the city seems to be clearly derived from recently found magnetite placer deposits and primary production sites south of Dereköy, in the Bey Daglari Massif between the valleys of Canakli and Aglasun. The magnetite placer sands show Rare Earth Element (REE) contents of up to the percent level, making them useful for Nd isotope analysis. Since there is a strong (geo)chemical coherence between REE elements and iron, Nd isotopes will be very sensitive to variations in petrologically different types of ores used for iron smelting. Therefore, Nd isotopes may turn out to be a most powerful tracer of ancient iron production. We intend to evaluate this using a set of iron artifact and ore samples that has already been well characterized by Sr and Pb isotopes.
4.) One main purpose of our July 2007 field campaign was the survey for clay deposits that may have delivered the material for local pottery workshops on a differing scale. The microscopic study of ceramic artifacts revealed that mixtures from different clay deposits on the territory may have been used for pottery. We have sampled a large number of different clay materials from all over the territory that will be characterized petrologically, mineralogically, and chemically by Ph.D. students Bert Neyt and Dennis Braekmans to get insights into the nature of local pottery activities throughout centuries. As different clay types derive from different lithologies via various weathering processes, radiogenic isotopes may also help in these studies not only to track point sources of clays, but also to quantify geochemically the proportions of mixed clays from different sources in the ceramics which may be difficult using other methods.