Chemical Mapping to Evaluate Post-Depositional Diagenesis among the Earliest Ceramics in the Teotihuacan Valley, Mexico

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.

Editorial for Special Issue “Archaeological Ceramic Diagenesis”

“…Gilstrap et al [10] explore the diverse ways in which calcium-containing minerals can be altered in ceramics, including case studies from terrestrial and marine environments using petrographic and elemental analysis. Stoner and Shaulis [11] examine alterations in low-fired ceramics from the Valley of Mexico using profile mapping by LA-ICP-MS, demonstrating substantial Ba enrichment in the outer margins of some of the ceramics they study, as well as the uptake of a number of metallic elements. Golitko et al [12] examine the composition of dense surface encrustations found on Bronze Age ceramics from eastern Hungary using a combination of mineralogical and chemical methods of characterization, identifying soil carbonates and phosphates as the likely source of surface concretions.…”