OXYGEN ISOTOPIC COMPOSITION OF MODERN MAMMAL TEETH ASSOCIATED WITH LONGITUDE, ALTITUDE, AND HUMIDITY IN SOUTH AMERICA

Moist air masses traverse the northern portion of South America generally from east to west. Rayleigh distillation results in the progressive depletion of ¹⁸O from water vapor and precipitation as the air mass moves further inland. When the air mass rises up over the Andes it becomes even further depleted in ¹⁸O This pattern is evident with average d¹⁸O values close to 0‰ on the eastern coast of South America, values of around -5‰ in the lowlands just east of the Andes, and decreasing values to ~-15‰ during ascent to the Altiplano. The rock record provides proxies that reflect the isotopic ratio of surface waters, which are affected by the factors above plus evaporation. In a highly arid region like the Altiplano, evaporation would result in more positive d¹⁸O values in surface waters with respect to precipitation.

Oxygen isotopes measured from tooth enamel of large water-dependent herbivorous mammals in part records the isotopic composition of water that animals drank as their teeth were mineralizing. Here we test whether oxygen from tooth enamel reflects the d¹⁸O variations in precipitation and surface waters by analyzing teeth of modern ungulates along a transect from east to west across South America, centered on Bolivia. Isotopic ratios of oxygen from tooth enamel remain relatively constant across the lower elevation, forested parts of South America, but show strong shifts with increased altitude in the Andes. On the Altiplano, domesticated mammals appear to better reflect d¹⁸O in precipitation, whereas native mammals reflect higher d¹⁸O values. Animals that lived in hot conditions versus those that lived in cooler but highly arid conditions may be distinguished with the assistance of carbon isotopes from the same teeth. The degree of carbon fractionation in plants is influenced by climate, resulting in distinct carbon isotopic signatures.

These modern data are compared with similar data from the teeth of late Oligocene through Pliocene fossil herbivorous mammals collected throughout the Bolivian Altiplano. Values similar to lowland d¹⁸O values are observed in late Oligocene teeth. By late Miocene time, d¹⁸O signatures similar to those from the modern Altiplano are observed, thereby constraining the timing of environmental change associated with uplift of the Andes.