2015 GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015)

Paper No. 198-10
Presentation Time: 10:50 AM

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


DREWICZ, Amanda E., Geosciences, Boise State University, 1910 University Dr, Boise, ID 83725, KOHN, Matthew J., Dept. of Geosciences, Boise State University, 1910 University Dr, Boise, ID 83725 and FREMD, Ted, John Day Fossil Beds National Monument, National Park Service, 32651 Hwy. 19, Kimberly, OR 97848, amandadrewicz@u.boisestate.edu

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.