Soil Phytolith Assemblages of the American Southwest: The Use of Historical Ecology in Taphonomic Studies
Creators: name: McNamee, Calla
Publisher: name: Graduate Studies
Year: 2013
Subjects: subject: Archaeology
subject: Geography
subject: Paleoecology
subject: American Southwest
subject: Phytoliths
subject: Historical Ecology
subject: Paleoenvironment

description: Phytolith analysis is a prominent microbotanical technique utilized in archaeological and palaeoenvironmental research. Opal phytoliths are silica bodies that form in and around the cells of plants and are often preserved within sediment due to their silica structure long after decomposition of the original organic material. This dissertation combines phytolith analysis with modern vegetation data and historical ecology to increase the interpretive potential of phytolith studies within the American Southwest. The research expands on phytolith analysis and its usefulness as an archaeological and environmental proxy in the American Southwest by 1) developing and presenting a regional comparative collection of common species 2) establishing modern phytolith analogues for Southwestern vegetation communities, and 3) examining processes of phytolith deposition and preservation. This research presents a comprehensive comparative collection of 159 prominent plants from the American Southwest. The morphotypes identified in the comparative collection support a classification of soil phytoliths into twelve categories: chloridoid, Eragrostis species, panicoid, pooid, aristidoid, C4 short cells, redundant short cells, other Poaceae, gymnosperm, eudicot, Agave, and Cyperaceae. Using this classification system, modern soil phytolith assemblages are examined from three vegetation zones: the ponderosa-pine bunchgrass community, the Sonoran desert grassland, and the Chihuahuan desert grassland. In order to assess the impacts of soil formation processes on phytolith assemblages, historical vegetation data from two of these three vegetation zones, the ponderosa pine bunchgrass community and the Sonoran desert grassland, are compared to soil phytolith assemblages collected through microsampled soil profiles. The results of this study show that phytolith assemblages can clearly differentiate the ponderosa pine bunchgrass community from the desert grassland communities. The desert grassland communities, however, cannot be differentiated from one another based on soil phytoliths. This research also shows that phytoliths can identify different micro-ecological niches within desert grassland environments. Incongruities between the modern vegetation record and the phytolith assemblage exist at all three locations. Three factors, differential phytolith production, plant biomass, and soil formation processes, contribute to these incongruities. The long term accumulation of phytolith forms, bioturbation, and translocation are the primary soil formation factors that impact soil phytolith assemblages in this area. The effects of bioturbation and translocation are more pronounced in the upper 4 cm of the soil profile and decrease below this depth. The long term movement and accumulation of phytoliths in a soil profile results in a background signal that pervades the assemblage. These processes do not fully obscure the phytolith record of vegetation change and, through micro-sampling, soil phytoliths can provide valuable information on changes in plant communities through time. This is especially true in settings where sedimentary input contributes to vertical soil growth. This research emphasizes the need for researchers interested in palaeoenvironmental reconstruction to understand phytolith production and sample context and to target locations conducive to the preservation of the phytolith record.