Mechanisms of lipid preservation in archaeological clay ceramics revealed by mass spectrometry imaging

Simon Hammann, David J Scurr, Morgan R Alexander, Lucy J E Cramp

Research output: Contribution to journalArticle (Academic Journal)peer-review

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Abstract

Traces of lipids, absorbed and preserved for millennia within the inorganic matrix of ceramic vessels, act as molecular fossils and provide manifold information about past people’s subsistence, diet and rituals. It is widely assumed that lipids become preserved after adsorption into nano- to micrometer sized pores, but to this day the distribution of these lipids in the ceramics was virtually unknown, which severely limits our understanding about the process of lipid preservation. Here we use secondary ion mass spectrometry (SIMS) imaging for the first direct in situ analysis of lipids absorbed in 700 – 2000 year old archaeological pottery. After sectioning from larger sherds, wall cross sections of smaller fragments were used for SIMS analysis. Lipids were found in relatively large zones of 5 – 400 µm in diameter which does not support the notion of absorption only into individual nanometer-scale pores but indicates that more macroscopic structures in the ceramics are involved in lipid preservation as well. Furthermore, lipids were found concentrated on calcium carbonate inclusions in the ceramics, which suggests that precipitation of fatty acids as calcium salts is an important aspect of lipid preservation in archaeological samples. This has important implications for analytical methods based on extraction of lipids from archaeological ceramics and needs to be considered to maximize the yield and available information from each unique sample. Significance Statement Lipids, preserved in archaeological ceramics, offer unique insights into past people's diets and cultural practices. Yet, the fundamental question of how lipids are preserved for millennia is not clear. This study describes the first direct visualisation of lipids in wall cross sections of archaeological potsherds using mass spectrometry imaging. Lipids were distributed unevenly in zones of >200 μm size, indicating that macroscopic structures and properties, and not only small pores, facilitate lipid preservation. We also show the formation of calcium fatty acid salts as an additional aspect of lipid preservation. These findings have important implications for the selection of appropriate methods for lipid analysis in potsherds and show that the analysis of lipid distribution patterns could improve and refine interpretations.

Keywords

  • lipid
  • archaeology
  • mass spectrometry imaging
  • SIMS

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