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

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, di-et, 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 virtu-ally unknown, which severely limits our understanding about the process of lipid preservation. Here we use secondary ion mass spectrometry (SIMS) imaging for direct in situ analysis of lipids absorbed in 700-to 2,000-y-old archaeological pottery. After sec-tioning from larger sherds, wall cross-sections of smaller frag-ments were used for SIMS analysis. Lipids were found in relatively large zones of 5-to 400-mu m 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 archae-ological samples. This has important implications for analytical methods based on extraction of lipids from archaeological ce-ramics and needs to be considered to maximize the yield and avail-able information from each unique sample.