干旱区生态环境知识资源中心

Geochemical tools, including the analysis of stable isotopes from fossil mammals, are often used to infer regional climatic and environmental differences. We have further developed an oxygen isotope aridity index and used oxygen (delta O-18) isotope values and carbon (delta C-13) isotope values to assess regional climatic differences between the southeastern and southwestern United States during the Pleistocene. Using data collected from previously published studies, we assigned taxa to evaporation-sensitivity categories by quantifying the frequency and magnitude of aridity index values (i.e., an average taxon delta O-18 value minus a site specific proboscidean delta O-18 value). Antilocapridae, Camelidae, Equidae, and Cervidae were identified as evaporation-sensitive families, meaning that a majority of their water comes from the food they eat, thus indicating that they are more likely to capture changing climatic conditions. Bovidae, Tayassuidae, and Tapiridae were identified as less sensitive families, possibly because of increased or more variable drinking behavior. While it is difficult to tease out individual influences on delta O-18 values in tooth enamel, the use of an aridity index will provide a more in-depth look at relative aridity in the fossil record. Greater aridity index values in the Southwest suggest a drier climate than in the Southeast during the Pleistocene, and delta C-13 values suggest that diet does not determine evaporation sensitivity. The combination of more-positive delta C-13 values and the lack of forest indicator taxa in the Southwest suggest that landscapes were more open than in the Southeast. Inferred higher aridity in the Southwest may indicate that aridity or seasonal aridity/precipitation, not temperature or pCO(2), was a greater driver of C-4 abundance during the Pleistocene. Collectively, these data suggest that regional climatic and environmental interpretations can be improved by using an aridity index and a more detailed understanding of mammalian paleobiology.