Abstract: Pre- or post-depositional heating of fossil biominerals will impact resulting amino acid dates, but the study on the effects on their amino acids of dry artificial heating of molluscs has not yet been fully tested. Here we report a study on three mollusc species to show how amino acid content can potentially be used to identify archaeological samples that have been subjected to artificial heating. The identification of markers of exposure to high temperature could be useful to reject compromised samples for amino acid racemisation dating. In order to identify markers of heat exposure, we artificially heated modern shell samples belonging to three marine molluscs found in abundance at archaeological sites, namely Patella vulgata, Phorcus lineatus and Littorina littorea. The markers found were compared with data from an archaeological dataset for these three species. Our results indicate that there is unlikely to be a single marker indicating artificial heating of a sample, especially for archaeological samples, which have been exposed to a wide variety of complex conditions that cannot be simulated in an isothermal experiment. However in general, we observed changes in shell composition which were indicative of heating: higher %Glx and %Ala and lower %Asx and %Ser for a given Asx D/L value. Moreover, samples heated at high temperatures showed higher than expected Glx and Ala D/L values for a given Asx D/L value. Likewise, the amino acid content of the shells decreased with heating time and temperature. Furthermore, shells artificially heated at > 300 °C had additional compounds evident in the chromatogram, and a poor baseline, thus causing a loss of resolution of late eluting amino acids. Of note, samples heated at 500 °C (5 min) and 400 °C (10?20 min) preserved negligible amino acid content. Archaeological samples that deviated from the expected unheated trend were therefore identified as having been heated.

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