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Abstract Detail

Paleobotanical Section

Drewicz, Amanda [1], Kohn, Matt [1], Fremd, Ted [2].

The high, dry Miocene of southeastern Oregon.

Global cooling and drying is documented in many regions from the mid-Miocene climatic optimum at 16 Myr ago through the late Pliocene at 3 Myr ago. In general, as temperatures decreased, open drier environments (grasslands) replaced closed wetter environments (C3 plants, forest) of the mid-Miocene. Researchers debate whether increased grassland environments were related to progressive global drying. In this study, we analyzed δ13C and δ18O values in mammalian vertebrate teeth from Miocene localities in southeastern Oregon to assess the degree of Miocene-Pliocene cooling and drying in the Pacific Northwest.   
Stable oxygen and carbon isotopes of fossil teeth are commonly used to investigate paleoclimate and paleoecology. Preserved δ18O and δ13C values of the carbonate component in tooth enamel apatite depend on local moisture and food sources accessed during tooth formation. In this study, enamel was analyzed for δ13C and δ18O values from rhino, equid, camel, and proboscidean specimens collected from the mid-Miocene Sucker Creek and Deer Butte Formations (SCF, DBF 14.8 to 15.5 Ma) and the late Miocene Drewsey Formation (DF, 9.95±0.25 Ma) in southeastern Oregon. Data were compared with tooth enamel isotope data from the late Pliocene Glenns Ferry Formation (c. 3 Ma) from southern Idaho.  
δ13C values showed negligible differences among sections and were relatively high: SCF = -9.6±1.3‰ (±2σ), DBF = -9.4±0.3‰, and DF = -9.4±2.2‰ (V-PBD). Application to published transfer functions for mean annual precipitation imply extremely dry conditions with MAP < 200 mm/yr. δ18O values were also similar, but were low relative to published data from central Oregon: SCF = 17.9±2.6‰, DBF = 19.6±1.9‰, and DF = 19.1±4.9‰ (V-SMOW). Isotope zoning in equid teeth from the DF showed minimal variations and are inconsistent with sinusoidal patterns indicative of seasonal changes in diet and water sources.  
Low δ18O values and lack of seasonality suggest water may have been derived from larger streams sourced from higher elevations. Slightly lower δ18O values in SCF compared to DF is attributed to spatial variation, as SCF is farther east. However, the lack of any trend in δ13C values relative to atmospheric δ13C implies an absence of progressive drying with time and that environments were dry throughout the mid- to late Miocene. Similarly high δ13C values in the Pliocene (low MAP) suggest dry conditions since the mid-Miocene. Thus, we conclude that although Miocene drying may have occurred elsewhere, dry environments have been long established in this region of the Pacific Northwest.

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1 - Boise State University, Geosciences, 1910 University Dr, Boise, ID, 83725, USA
2 - University of Oregon, Geological Sciences, 1585 E 13th Ave, Eugene, OR, 97403, USA

stable isotopes
Climate change.

Presentation Type: Poster:Posters for Sections
Session: P
Location: Eyrie/Boise Centre
Date: Monday, July 28th, 2014
Time: 5:30 PM
Number: PPB007
Abstract ID:765
Candidate for Awards:None

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