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Abstract Detail



The Miocene vegetation and environment of Western North America

Harris , Elisha B [1], Stromberg, Caroline [2], Sheldon, Nathan [3], Smith, Selena [4].

Dynamics of the spread of grasslands during the middle Miocene in the Northern Rocky Mountains.

The mid-Miocene Climatic Optimum (MMCO) was a warming event 17-14.5 Ma ago characterized by a transient shift to higher atmospheric CO2 levels and warmer, possibly wetter climatic conditions. The MMCO was also an important period of biotic modernization, particularly in North America, with accelerated spread of grass-dominated ecosystems with both C3 and C4 grasses present. Faunal analyses from eastern Idaho during the mid-Miocene have shown turnover from faunas dominated by oreodonts to those dominated by equids, and paleosol morphology data have suggested a return to woodland communities during this time. Previous work using a small dataset of phytolith samples pointed to an opening of landscapes during the late early-middle Miocene associated with the spread of C3-dominated grass communities. However, we still lack a detailed record of vegetation change necessary for a more thorough documentation of vegetation response to this global climatic event. Here, we analyze phytolith assemblages and δ13C records from paleosol organic matter to track long-term changes in vegetation composition and structure in the Railroad Canyon Sequence (RCS) of eastern Idaho during the MMCO. Samples were collected from late Hemingfordian to late Barstovian paleosols preserved in the RCS. Our phytolith results suggest that C3 grasses dominated local assemblages throughout the RCS and that there were no major changes in relative abundance of forest indicators during this time (forest indicators never comprise more than 28% of phytolith assemblages). Grass phytolith assembages consisted primarily of C3 grasses but potential C4 grasses were present as early as the late Hemingfordian. Potential C4 grass phytoliths vary in relative abundances (conservative estimate 7–40% of all phytoliths) through the section, indicating spatial heterogeneity over short time scales. Additionally, the relative abundance of C4 grasses appears to decrease over time, which might be correlated with climatic cooling following the peak of mid-Miocene warming. Paleosol δ13C results indicate 0–26% C4 vegetation, and no consistent pattern of change through time. Overall, these data suggest that the MMCO did not result in any major changes in vegetation structure in eastern Idaho, which is inconsistent with previously published paleosol morphology data. These results suggest that additional studies looking at high-resolution floral change across the MMCO are needed to fully understand vegetation response to global warming during this time. 


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1 - University of Washington, Biology, 24 Kincaid Hall, Box 351800, Seattle, WA, 98195, USA
2 - University Of Washington, Department Of Paleobotany, 24 Kincaid Hall, Box 351800, Seattle, WA, 98195-1800, USA, 206-619-2152
3 - University of Michigan, Earth and Environmental Sciences, 2534 CC Little Building, Ann Arbor, MI, 48109, USA
4 - University Of Michigan, Department Of Geology, 1100 North University Avenue, 2534 CC Little Building, Ann Arbor, MI, 48109, USA

Keywords:
Phytolith
Middle Miocene Climatic Optimum
Vegetation structure
Railroad Canyon Sequence
Northern Rocky Mountains.

Presentation Type: Symposium or Colloquium Presentation
Session: C2
Location: Salmon/Boise Centre
Date: Tuesday, July 29th, 2014
Time: 1:45 PM
Number: C2002
Abstract ID:658
Candidate for Awards:None


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