Preparation of bone carbonate for stable isotope analysis: the effects of treatment time and acid concentration

Abstract

While apatite carbonate δ 13C analysis is a useful tool in palaeodietary research, the vulnerability of carbonates to diagenesis necessitates careful sample preparation. The most common preparation method involves dilute acetic acid treatments to remove highly soluble mineral contaminants. However, there is variation in both treatment time and acid concentration. This study addresses the impact of such variation on carbonate δ 13C and δ 18O, sample loss in solution, and sample Fourier transform infrared spectroscopy (FTIR) characteristics. Samples of fresh and archaeological bone were treated with 1.0 M and 0.1 M acetic acid. Carbonate δ 13C and δ 18O, sample loss and FTIR characteristics were measured at treatment times of 4 to 24 h. The results suggest that 4 h of treatment suffice to remove highly soluble contaminants, and that longer treatment times can lead to sample recrystallization. Some samples treated with 1.0 M acid showed unacceptable loss or possible recrystallization, suggesting that 0.1 M acid treatment is preferable. However, the more dilute acid caused smaller shifts in δ 13C and δ 18O for all samples and treatment times. Thus, while 0.1 M acid appears to produce superior sample quality, it may not produce results directly comparable to those of 1.0 M acid treatments. This has important implications for the comparison of apatite carbonate stable isotope values reported in the literature.