Abstract
Chemical and mineral sourcing techniques are commonly employed in archaeology to reconstruct patterns of ceramic exchange and raw material procurement practices for the past, but the effects of post-depositional diagenesis are still often ignored despite a number of key studies warning that the composition of ceramics from archaeological deposits often diverges greatly from their original composition at the time of production. This current study on diagenesis derives from a large chemical and petrographic analysis of some of the earliest ceramics (1500–100 cal Before Common Era [BCE]) in the Basin of Mexico at a time when the development of ceramic trade networks helped to spread early stylistic canons across Mesoamerica. One important site, Altica, consistently presents ceramics that are high in barium. We use laser ablation-inductively coupled plazma—mass spectrometry (LA-ICP-MS) to map the cross-sections of several samples from this site with the idea that post-depositional intake of mobile cations would appear as enriched at the surfaces of the pottery and around pores. Conversely, cations that leach out of the fabric would exhibit depleted concentrations in those same areas. We find that barium and a suite of other elements (e.g., copper, zinc, lead, tin, arsenic, calcium, strontium, and vanadium) have been chemically altered due to nearly 3000 years of burial in the soil. We explore the implications of those revelations for properly sourcing archaeological ceramics found at the Altica site and provide additional guidance for archaeologists and geochemists who employ ceramic compositional sourcing elsewhere.
Highlights
Bulk chemical characterization—where a fragment of the sample is homogenized into a powder or solution prior to analysis—has historically been the most common method applied to sourcing analysis, but the heterogeneity of ceramics and their often sponge-like properties that take up chemicals from the burial matrix introduce variability that can deter successful source identification
We apply chemical mapping via laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) to tease apart the “good” from the “bad” sources of variability in a sample of archaeological ceramics previously characterized through the bulk chemical technique of neutron activation analysis (NAA)
As we demonstrate in this paper, surface burring might not be sufficient to remove the effects of chemical diagenesis that penetrate the ceramic body
Summary
Compositional sourcing of archaeological ceramics has become a staple method to recover lost information about the social relations of production and exchange in the past. The detrimental effects of chemical weathering on archaeological ceramics for sourcing studies have been recognized for decades [1], but most composition studies today continue to uncritically apply bulk chemistry to reconstruct social and economic behaviors without considering post-depositional changes. We apply chemical mapping via laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) to tease apart the “good” from the “bad” sources of variability in a sample of archaeological ceramics previously characterized through the bulk chemical technique of neutron activation analysis (NAA)
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