Effect of formic acid treatment on carbon and nitrogen stable isotope ratios in sperm whale teeth dentine.

Rationale: Stable isotope analysis of growth layers in sperm whale teeth dentine can provide valuable insight into individual long-distance displacements and diet. Although treating teeth half-sections with formic acid and rubbing their surface with a graphite pencil improves growth layer visibility and reduces sampling error, previous studies mostly used untreated half-sections as the effect that this treatment may have on stable isotope ratios in dentine is unknown. The present study investigates the treatment effect on stable C and N isotope ratios in sperm whale teeth dentine.

Methods: In the teeth of 30 sperm whales, we analysed and compared samples of powdered dentine obtained from (a) untreated half-sections, (b) half-sections etched with formic acid and rubbed with a graphite pencil and (c) half-sections etched with formic acid from which the graphite pencil rubbing had been cleansed off. C and N values were compared between the three sample groups.

Results: We found significant differences in values of both elements between untreated and etched samples, with a mean increase of 0.2% in δ C and δ N values in the etched samples. No significant differences were found between etched samples with graphite rubbing versus those without it. Significant linear regression models were calculated to predict untreated δ C and δ N values from the values obtained in the etched half-sections with limited precision.

Conclusions: We show for the first time that formic acid etching has a clear effect on C and N values in sperm whale teeth dentine. The developed models permit the estimation of untreated values from etched half-sections, thus enabling the use of the latter in stable isotope analysis. However, as treatment procedures may vary between studies, it is advisable that similar predictive models are developed case-by-case to ensure comparability of results.