Forensic Ar/Ar dating: a provenance study of Middle Stone Age obsidian artifacts from Ethiopia

Obsidian sourcing based on geochemistry is widely applied to reconstruct commerce patterns and dissemination of early hominid groups. Due to problems concerning potential intra-source variability or homogeneity of a geological source over large areas, parameters additional to this classical approach are desirable. Here we present the first ⁴⁰Ar/³⁹Ar investigation of Middle Stone Age (MSA) obsidian artifacts (i.e. debitage pieces) for this purpose, as well as of potential geological obsidian sources from Ethiopia. With the present pilot study we demonstrate that ⁴⁰Ar/³⁹Ar geochronology represents a reliable and powerful tool for archaeological provenance studies complementary to the geochemical approach and more recently applied techniques like fission track counting or Mössbauer spectroscopy. Two independent sets of debitage and potential source rock samples were dated by the ⁴⁰Ar/³⁹Ar method. Debitage pieces from the Kulkuletti excavation site and obsidian samples from the nearby obsidian outcrop Worja (central Ethiopia) exhibit basically identical age ranges as well as similar Ca/K and Cl/K ratios, supporting the debitage/source rock relationship previously inferred on geochemical grounds. A second small set of geochemically characterized debitage samples from the Porc Epic cave and potential source rocks from an obsidian outcrop at Kone (Eastern Ethiopia) however have different ⁴⁰Ar/³⁹Ar ages and Ca/K and Cl/K ratios. These results contradict the previous conclusion about a debitage/source rock relationship of the Porc Epic and Kone samples made on the basis of geochemistry [A. Negash, M.S. Shackley, Archaeometry 48 (2006) 1]. Additional sampling at the Kone outcrop as well as of other obsidian occurrences in the greater area is needed to unambiguously pinpoint the origin of these obsidian tools.     

Urban political ecology in late prehistory: New evidence from El Purgatorio,Peru

One of archaeology's greatest strengths is its reliance on interdisciplinary collaboration and the utilization of multiple lines of evidence to inform archaeological interpretation. For example, through an examination of faunal and floral remains, production and storage facilities, and the isotopic analysis of human skeletal remains one can develop a model for urban political ecology in ancient cultures. In this case study, the political ecology of the Casma capital city, El Purgatorio, Peru, is investigated in order to inform our interpretations and conclusions regarding Casma political, economic and social organization. The results indicate that Casma political ecology was firmly based in coastal resources and oriented towards supporting state-sponsored feasting and ritual activities, suggestive of a largely elite-controlled redistributive economy. In contrast to previous models characterizing this time period as one of factionalism and environmental stress, the data suggest that coastal cultural adaptations produced an era of widespread political and economic stability.