ASSESSING THE IMPACT OF DIAGENESIS ON CARBON ISOTOPE RATIOS OF DINOSAUR TOOTH ENAMEL: A CASE STUDY FROM THE HELL CREEK FORMATION IN NORTH DAKOTA

Carbon isotope studies of mammal tooth enamel have provided a great deal of information regarding Late-Tertiary environments and plant-animal interactions. To date, however, it has been difficult to study Mesozoic environments and ecology using dinosaur enamel because of unknown aspects of dinosaur physiology, the lack of modern analogs, and difficulties in determining the extent of diagenesis. To address these problems, we measured isotope ratios of carbonate from hadrosaurian and ceratopsian teeth of the Late Cretaceous Hell Creek Formation of North Dakota as well as associated sedimentary organic material.

Diagenesis was investigated by sampling enamel and dentine from the same teeth and comparing results to bone samples. Patterns in the isotopic data are consistent with better preservation of carbon isotope ratios in enamel relative to other phases and agree with structural differences between enamel and dentine. Nevertheless, the degree to which material has been altered is difficult to constrain by this method alone.

Diagenesis was also assessed by comparing carbon isotope ratios of dinosaur enamel to ratios of associated sedimentary organic material. Assuming the latter reflects herbivorous dinosaur diet, the offset between these two phases should be constant, as is observed in the case of mammals. For 5 of 6 sites this offset was in fact constant at ~17 per mil. At one locality, however, a larger difference of ~25 per mil was observed, and is interpreted to reflect diagenetic alteration of enamel carbonate. The dinosaur enamel-diet offset is greater than that for birds (~16 per mil) and mammals (~14 per mil) and reflects differences in physiology.

Lastly, a comparison of average isotope ratios between dinosaur taxa may also be used to study diagenesis. At several locations, average ratios for hadrosaurians and ceratopsians are significantly different. Such patterns cannot be accounted for by diagenetic processes given similarities in enamel thicknesses and structure and thus provide an additional means of monitoring diagenesis.

While none of these methods alone may be used to rule out the effects of diagenesis, a combined approach can provide a means of identifying large diagenetic changes in primary carbon isotope ratios. As a result, isotope data can be used to investigate dinosaur ecology of the Mesozoic.