To bleach or not to bleach? Comparing treatment methods for isolating biogenic carbonate

Stable carbon and oxygen isotope ratios in tooth enamel and bone hydroxyapatite are used to answer a variety of ecological questions. Researchers employ chemical treatments to reduce contaminants that may interfere with the isotope ratios in hydroxyapatite. Chemical treatment should remove exogenous as well as non lattice-bound molecules without affecting the isotopic composition of the biogenic carbonate or introducing secondary carbonate material. However, there is growing concern that some of the most frequently used chemicals may alter biogenic isotope ratios, thus compromising isotope data. Despite a considerable body of literature, there is still no consensus as to which chemical treatment methods are “the best”. Here we test the isotopic effects of a suite of chemical experimental treatments on modern and fossil enamel carbonate, including six published protocols. We also test the degree to which carbon and oxygen isotope ratios in bone and dentine are affected by commonly used treatment chemicals. We conclude that treatment with hydrogen peroxide followed by Ca-buffered acetic acid yields the “best” results. This combination results in less extreme carbon, oxygen, and weight percent carbonate values compared to other treatments. Although sequential treatment with sodium hypochlorite (bleach) and Ca-buffered acetic acid produces similar results for enamel, bleach is not recommended for tissues with a higher organic content, such as bone or dentine.