SEASONAL AND SPATIAL VARIATIONS IN CARBON ISOTOPE RATIOS OF PLANTS IN A C3 WORLD AS INFERRED FROM MAMMALIAN TOOTH ENAMEL AND THEIR PALEOECOLOGICAL AND PALEOENVIRONMENTAL IMPLICATIONS
This study focuses on the Eocene time period and relies on the analysis of tooth enamel carbonate from Coryphodon, a large herbivorous mammal that lived in riverine environments. Intra-tooth variations in carbon isotope ratios of 1 to 2 per mil are observed for samples from the Bighorn Basin of Wyoming, and are weakly correlated with seasonal variations in oxygen isotope ratios. These carbon isotope variations may reflect seasonal changes in habitat and/or types of C3 plants eaten by Corphodon. Alternatively they may reflect the influence of temperature or water/nutrient availability on carbon isotope ratios of a single C3 plant type. In addition to seasonal variations, carbon isotope ratios of Coryphodon vary with latitude. Samples from San Jan Basin (~36 N paleolatitude), Bighorn Basin (~45 N) and Ellesmere (~75 N) have average values of 15.0, 13.0, and 12.5 per mil respectively. The resulting negative correlation between carbon isotope ratios and temperature/latitude is different than that observed for C3 plants at the present time, and may instead reflect the influence of biological productivity, precipitation, or water availability.
These results confirm that carbon isotope ratios of tooth enamel can vary significantly in a C3 world, and that seasonal and spatial patterns can provide information regarding mammalian habitat and environmental conditions of the past.