Effects of Pretreatment on the Isotopic Composition of Structural Carbonate in Ray-Finned Fish Teeth

The carbon and oxygen stable isotope ratios (δ¹³C and δ¹⁸O) of carbonate contained in fish tooth dentin have the potential to offer insights into environmental and ecological conditions, making them promising targets for analysis in fossil specimens. However, the tooth dentin of extant specimens contains significant amounts of organic matter that can decompose or produce gases that interfere with isotope measurements. Therefore, a well-developed pretreatment protocol for removing organic components prior to isotopic analysis is essential.

In this study, we developed pretreatment methods for removing organic matter to enable the isotopic analysis of structural carbonates in tooth dentin. Using teeth of the half-smooth golden pufferfish (Lagocephalus spadiceus) as specimens, we applied a total of eight physical and chemical pretreatment protocols and evaluated their effects on carbonate isotopic composition.

Treatments with neutral or alkaline pancreatin solutions effectively removed organic matter without altering the δ¹³C values of the carbonate. In contrast, no pretreatment method was identified that kept δ¹⁸O values entirely unaffected. However, treatment with the neutral pancreatin solution resulted in sufficient removal of organic matter and only minimal, consistent changes in δ¹⁸O.

Treatments with neutral or alkaline pancreatin solutions are recommended for the analysis of δ¹³C in carbonate within dentin. Although no method for ideal organic matter removal without changing δ¹⁸O values was identified, treatment with neutral pancreatin solution is recommended as the second-best option. Because of the small and consistent change in δ¹⁸O, it is possible to correct for δ¹⁸O alterations induced by pretreatment.