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Abstract Detail

Paleobotanical Section

Stromberg, Caroline [1], Sheldon, Nathan [2], Smith, Selena [3], Cotton, Jennifer [2], Hamer, Jessica [4].

Reconstructing late Miocene ecosystem dynamics in southwestern Montana using multiple lines of evidence (phytoliths, stable isotopes, trace fossils).

The spread of C3/C4 grasslands constituted a profound ecosystem change in North America. However, the details of this ecological transition remain obscure because different types of data indicate drastically dissimilar patterns of grassland development. For example, study of phytolith assemblages from southwestern Montana suggests that forest-woodland vegetation, with palms and grasses of unknown ecology in the understory persisted until the late early Miocene, when C3-dominated savanna-woodlands spread. In contrast, paleosol interpretations from many of the same sections indicate that sagebrush steppe and bunch grassland spread in the latest Eocene; short sod grassland vegetation arose in the earliest Miocene, but never became abundant in the region. To address this problem and evaluate differential taphonomic biases in commonly employed paleoecological data, we combine stable carbon isotopic paleovegetation data (% C4), phytolith assemblage data, and paleopedological information about bioturbation and rooting for reconstruction of early late Miocene habitats in southwestern Montana. A 34-meter section of the Sixmile Creek Formation at Timber Hills, Montana, containing 35 paleosols was studied. Preliminary phytolith analysis suggests variation in vegetation structure through time, from relatively closed forest near water, as indicated by frequent wetland indicators (e.g., sedge phytoliths, diatoms) towards the base of the section, to open grasslands with a minor woody element, and varying frequency of wetland forms higher up. Percentage C4 vegetation based on isotopic proxy data and phytolith analysis are roughly similar, although error margins are large due to current uncertainty in how well C4 photosynthesis can be inferred from grass phytolith shape. Root diameter also displays substantial variance, in particular at intermediate levels of bioturbation. In the current dataset the highest degrees of bioturbation are associated with more closed vegetation near water, whereas the largest root diameters tend to be linked to more open habitats, counter to expectation. These results suggest that although some paleovegetation data (phytoliths vs. isotopes) correspond fairly well, others (phytoliths vs. trace fossils) may suffer from different taphonomic biases, for example in time averaging (phytolith assemblages may form over longer time spans than preserved trace fossils). These relationships influences how different types of data reflect paleovegetation (e.g., in terms of temporal and spatial resolution and scale) and should be taken into account in paleoecological reconstruction.

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1 - University of Washington, Biology & Burke Museum, 24 Kincaid Hall, Box 351800, Seattle, WA, 98195-1800, USA
2 - University of Michigan, Earth and Environmental Sciences, 2534 CC Little Building, Ann Arbor, MI, 48109, USA
3 - University of Michigan, Earth & Environmental Sciences and Museum of Paleontology, 1100 North University Ave., 2534 CC Little Building, Ann Arbor, MI, 48109, USA
4 - Lampton School, Lampton Ave, Hounslow, TW3 4EP, UK

Trace fossils.

Presentation Type: Oral Paper:Papers for Sections
Session: 44
Location: Prince of Wales/Riverside Hilton
Date: Wednesday, July 31st, 2013
Time: 2:30 PM
Number: 44005
Abstract ID:783
Candidate for Awards:None

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