Post-Depositional Alteration of Calcium Carbonate Phases in Archaeological Ceramics: Depletion and Redistribution Effects

Abstract

The final stage in the life history of prehistoric pottery prior to archaeological recovery is usually the longest, and frequently the most dynamic. The remains of archaeological ceramics spend hundreds to thousands of years deposited within the upper layers of the earth’s crust where they encounter the same diagenetic environmental processes as the surrounding natural materials. Harsh conditions of subterranean environments induce physical stresses and chemical reactions, causing alterations of ceramic structure and composition. This is especially true of carbonate-rich ceramics, as carbonate phases are soluble when deposited within acidic environments. This paper examines common carbonate depletion and accretion effects of post-depositional environments on ancient ceramics from two rather different geological and archaeological contexts: Mesoamerica and the Mediterranean. Potters in both regions produce vessels with carbonate-rich materials—clays, calcite, limestone—that alter due to long exposure to low-pH sediments and continual water table fluctuations. Ceramic petrography is employed to identify traces of carbonate alterations within ceramic microstructure and to characterize fabrics. Elemental compositions of the same sherds are characterized through either scanning electron microscopy coupled with energy-dispersive spectrometry (SEM-EDS), inductively coupled plasma mass spectrometry and optical emission spectrometry (ICP-MS/OES) or neutron activation analysis (NAA). This method enabled comparison of the differing effects of post-depositional alteration of carbonate phases on bulk composition signatures commonly used to determine provenance.