“Amount Effect” recorded in oxygen isotopes of Late Glacial horse ( Equus) and bison ( Bison) teeth from the Sonoran and Chihuahuan deserts, southwestern United States

Abstract

Stable oxygen isotopes from fossils, both vertebrate and invertebrate, or inorganic sedimentary minerals frequently have been used to make interpretations about ancient global climates. Oxygen isotope values measured from terrestrial vertebrates or sedimentary carbonates provide information about paleotemperature and amounts of precipitation at a particular site. In general, these inferences are made on the centennial to millennial scale. Serial, i.e. ontogenetic, sampling of Equus and Bison tooth enamel provides climatic data on the scale of months to a few years. We present models showing how annual environmental patterns of δ 18O would be replicated in the tooth enamel of Equus and Bison. Changes in δ 18O due to shifts in temperature (in terrestrial environments δ 18O increases with increased temperature) and amount and timing of precipitation (increased precipitation may result in a decrease in δ 18O; a phenomenon called the “Amount Effect”) are archived in both Equus and Bison teeth. The input signal is better resolved in Bison due to its more rapid enamel growth. Modeled patterns are compared with actual data from modern sites in the Sonoran and Chihuahuan deserts of the southwestern United States. The isotopic patterns from modern teeth agree with that predicted from known variations in meteoric water. Known floral and faunal assemblages as well as computer models suggest summer rain for the Chihuahuan Desert of Late Glacial time (15,000–10,000 years ago), but little summer rain for the Sonoran Desert at that same time. Serial data from fossil teeth show a clear pattern interpreted to represent the Amount Effect that includes increased summer rains in the Chihuahuan Desert and only minor summer rains in the Sonoran Desert.