STABLE ISOTOPES IN LARGE HERBIVORE TOOTH ENAMEL SHOW MID-MIOCENE SOAKING OF CENTRAL OREGON

Paleoclimate records over the last 30 Ma generally document cool and dry conditions worldwide, with a warm and wet greenhouse period during the mid-Miocene climatic optimum (MMCO) c. 14.5-17 Ma. The MMCO has been associated with an atmospheric CO₂ pulse similar to levels anticipated in the next century, thus understanding MMCO climate may help inform predictions of future climate. To characterize the impact of the MMCO on the climate and ecology of the interior Pacific NW, we collected oxygen and carbon isotope compositions from fossil tooth enamel of rhino, equid, camel, and proboscidean. Most significantly, carbon isotopes in tooth enamel reflect δ¹³C values of plants consumed, which in return depends on atmospheric δ¹³C and mean annual precipitation (MAP). Thus, by analyzing tooth enamel δ¹³C and correcting for atmospheric δ¹³C, we can recover MAP through time. Sections investigated include the Oligocene John Day Formation (30-28 Ma), the Miocene Mascall (15.6 Ma), Quartz Basin (c. 15.5 Ma), Sucker Creek (15.0 Ma), Red Basin (c. 13.7 Ma), and Drewsey (10.0 Ma) Formations, and the Pliocene Glenns Ferry Formation (Hagerman; 3.3 Ma). Localities are in central or eastern Oregon except the Hagerman site in Idaho. Prior to the MMCO (John Day, c. 29 Ma), MAP in the interior Pacific NW was 440±220 mm/yr. During the MMCO (Mascall, 15.6 Ma), MAP increased to 1050±500 mm/yr. All later data (≤15 Ma) show a decrease in MAP (125-300 mm/yr), implying arid conditions to the present. Oxygen isotopes are broadly constant through 10 Ma, implying no major changes in moisture source or upwind elevations (Cascade Ranges). Lower δ¹⁸O at Hagerman is consistent with increased distance from moisture sources and increasing elevations in the Cascade Range commencing c. 7 Ma. The increase in MAP during the high-CO₂ MMCO is generally consistent with some but not all models of future climate in the region. A better understanding of the MMCO worldwide promises useful validation of model methods.