Laboratory Experiments

The AMG lab has pioneered the use of the isotopic composition of tooth enamel new dietary proxy for reconstructing food webs and investigating the trophic ecology of extant and extinct taxa. In this section we describe the results of a series of controlled feeding experiments in which we evaluated the incorporation of the dietary N isotopic signal in tooth enamel. 

Controlled feeding experiments

Nitrogen isotope ratios (δ15N) are a well-established tool for investigating the dietary and trophic behavior of animals in terrestrial and marine food webs. To date, δ15N values in fossils have primarily been measured in collagen extracted from bone or dentin, which is susceptible to degradation and rarely preserved in deep time (>100,000 years). In contrast, tooth enamel organic matter is protected from diagenetic alteration by the mineral structure of hydroxyapatite and thus is often preserved over geological time. However, due to the low nitrogen content (<0.01 %) of enamel, the measurement of its nitrogen isotopic composition has been prevented by the analytical limitations of traditional methods. In a recent article, we used a novel application of the oxidation-denitrification method that allows measurement of δ15N values in tooth enamel (Leichliter et al. 2021). (δ15Nenamel). This method involves the oxidation of nitrogen in enamel-bound organic matter to nitrate followed by bacterial conversion of nitrate to N2O, and requires ≥100 times less nitrogen than traditional approaches. 

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N isotopic composition of mature enamel in rats for plant-, insect- and meat-based diet groups

In addition, to demonstrate that δ15Nenamel values record diet and trophic behavior, we conducted a series of controlled feeding experiments with rats and guinea pigs (n = 37) in collaboration with the group of Prof. Thomas Tütcken at the JGU in Mainz. The nitrogen isotope composition of enamel reflects diet with an enrichment of around 2–4‰. δ15Nenamel values differ significantly between dietary groups and clearly record a shift from pre-experimental to experimental diet. Moreover, the small sample size required (≤5 mg) by this method permits analyses of sample size-limited, diagenetically robust tooth enamel, and, as such it represents a promising new dietary proxy for reconstructing food webs and investigating the trophic ecology of extant and extinct taxa.