Stable Isotope Studies of Vertebrate Fossils from Polar Localities: Paleoclimatic and Paleoecological Insights and Parallels Between Late Cretaceous and Eocene Time Periods

Carbon and oxygen isotope ratios of tooth enamel are related to those of ingested food and water, and can thus be used to study animal biology and behavior. Isotopic offsets are observed for enamel from both coexisting herbivorous dinosaurs and mammals, thus indicating that dietary niche partitioning was common in the Arctic during both late Cretaceous and Eocene time. Isotopic offsets among herbivores and carnivores can be used to infer common prey of the latter. A comparison of isotopic data from the same taxa living above the Arctic Circle and at lower latitudes is consistent with animals living in polar regions all year.

Focusing on oxygen isotope data, oxygen isotope gradients inferred from enamel are shallower than at present. Intra-tooth/seasonal variations in oxygen isotope ratios in polar regions during both time periods is greater than at lower latitudes for each time slice, but is much less than at present in both cases. Both observations are consistent with fossil evidence for amplified warming in polar regions compared to the tropics, and thus shallower temperature gradients for both the late Cretaceous and Eocene time periods.

Diagenetic patterns in isotopic data provide insight into mechanisms of amplified polar warming during hothouse time periods. Carbon isotope ratios of dentine, which is more susceptible to alteration than enamel, are much higher than associated enamel at polar localities, with values reaching +5 per mil. These high values suggest methane production/stratospheric cloud formation at high latitudes during both times.