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.

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

Keywords:
Miocene
stable isotopes
Paleobotany
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