UNIVERSITY OF ARIZONA
With the support of the National Science Foundation, an interdisiplinary team at the University of Arizona will greatly expand work in progress on characterizing (1) sources of turquoise in the southwestern USA and northern Mexico; (2) sources of native copper in north America; (3) sources of copper in North and West Africa; and (4) glass trade beads of the Muslim era in Africa. The team consists of an archaeologist (David Killick), two geochemists (Joaquin Ruiz and John Chesley) and three graduate students (Thomas Fenn, Lisa Molofsky and Alyson Thibodeau).
Some of the inorganic materials that were highly valued by past societies are both geologically scarce and unevenly distributed across the earth's surface. For example, Mesopotamia and the West African Sahel both have few copper ore deposits, and thus the populations of these regions had to obtain almost all of the copper that they consumed through long-distance trade. Turquoise only forms around copper ore deposits in semi-arid areas, but is often recovered from archaeological sites in regions where there are no local sources, such as Mesopotamia and central Mexico. These materials (and others, like obsidian, tin and jade) are therefore potentially valuable indicators of the extent, duration and scale of prehistoric contacts between distant regions.
Actual proof of long-distance exchange can only be obtained by matching the chemical or mineralogical composition of archaeological artefacts to the composition of specific geological sources of these materials (the field of provenance analysis). A necessary first step is to demonstrate that all potential sources of these materials can in fact be distinguished from each other on chemical or mineralogical evidence. The methods most commonly employed for provenance analysis (trace element ratios, optical petrography and x-ray diffraction) are not likely to be fully successful in distinguishing copper and turquoise deposits, which are highly variable aggregates of copper minerals and host rocks. This study will rely instead upon the measurement of isotopic ratios of lead and strontium. Ratios of these isotopes are less variable within particular deposits than are trace element ratios, are not altered by weathering or smelting, and can often be related to regional geological processes (e.g. the assembly of subcontinents from terranes). Copper isotope ratios will also be used as a third line of evidence in some cases.
The broader impacts of these studies will depend upon the empirical findings. If turquoise sources can be successfully discriminated from each other, archaeologists will have a powerful new way of investigating prehistoric interaction between Mexico and the southwestern USA. Similarly, if the geological sources of copper alloys and glass beads in archaeological sites in West Africa can be reliably inferred from isotopic and chemical composition, archaeologists will be able to study in some detail the early history of contacts between the Muslim world and sub-Saharan Africa - a topic of very wide popular interest, given that one-third of the population of Africa today are Muslims.