Evaluating sodium hydroxide usage for stable isotope analysis of prehistoric human tooth dentine

Analyzing carbon and nitrogen stable isotopes (δ13C and δ15N) in serial samples of human tooth dentine can aid in reconstructing life history events such as weaning and diet. As dentine does not remodel after formation, it retains the isotopic signatures of the foods ingested during a tooth's development, allowing investigation of the diet consumed during this time. Microsampling human archaeological tissues is becoming increasingly popular but no consensus has been reached on the best method to remove soil humates from such small samples. It is important to remove these humates, as they can alter collagen δ13C values. This study presents an adjustment to a commonly used method for removing humates from bone collagen samples, the sodium hydroxide soak. Here we compare dentine microsamples from five modern unburied teeth that received the usual 20 hour NaOH soak to microsamples from an archaeological tooth for which NaOH treatment time was reduced to 6 h. The results show that microsamples from modern material tolerate the standard NaOH treatment well despite their tiny size. In the archaeological tooth, the six hour treatment was sufficient to remove humates without damaging the collagen of the small and fragile prehistoric dentine microsamples. Even in these trial samples, the δ13C and δ15N values provide some interesting insights into dietary changes during development, underscoring the benefits of analysis at the intra-individual level.