Late Pleistocene C4 plant dominance and summer rainfall in the southwestern United States from isotopic study of herbivore teeth

Patterns of climate and C-4 plant abundance in the southwestern United States during the last glaciation were evaluated from isotopic study of herbivore tooth enamel. Enamel delta(13)C values revealed a substantial eastward increase in C-4 plant consumption for Mammuthus spp., Bison spp., Equus spp., and Camelops spp. The delta(13)C values were greatest in Bison spp. (-6.9 to +1.7 parts per thousand) and Mammuthus spp. (-9.0 to +0.3 parts per thousand), and in some locales indicated C-4-dominated grazing. The delta(13)C values of Antilocaprids were lowest among taxa (-12.5 to -7.9 parts per thousand) and indicated C-3 feeding at all sites. On the basis of modern correlations between climate and C-4 grass abundance, the enamel data imply significant summer rain in parts of southern Arizona and New Mexico throughout the last glaciation, Enamel delta(18)O values range from +19.0 to +31.0 parts per thousand and generally increase to the east. This pattern could point to a tropical or subtropical source of summer rainfall. At a synoptic scale, the isotope data indicate that interactions of seasonal moisture, temperature, and lowered atmospheric pCO(2) determined glacial-age C-4 abundance patterns. (C) 1998 University of Washington.