2014 GSA Annual Meeting in Vancouver, British Columbia (19–22 October 2014)

Paper No. 292-6
Presentation Time: 9:30 AM


YOST, Chad L., Department of Geosciences, University of Arizona, 1040 E. 4th Street, Tucson, AZ 85721, COHEN, Andrew S., Department of Geosciences, University of Arizona, Tucson, AZ 85721, FEIBEL, Craig S., Earth and Planetary Sciences, Rutgers University, 610 Taylor Road, Piscataway, NJ 08854 and CAMPISANO, Christopher J., Institute of Human Origins, School of Human Evolution and Social Change, Arizona State University, Tempe, AZ 85287

Phytoliths are microscopic opal silica plant remains that can be deposited in soils and lake sediments after the plant-parts that produce them decay. Although phytoliths have been used extensively in archaeological studies, they are underutilized in limnogeological investigations. Pollen and phytoliths share some similarities in how they are extracted and quantified; however, there are significant differences in taphonomy, preservation and taxonomic resolution. A major strength of phytolith analysis is its ability to taxonomically resolve grasses (Poaceae) to subfamily, tribe and even lower levels of identification. In East Africa, the overwhelming majority of grasses can be categorized into four plant functional types (PFTs): C3 cool climate/shade tolerant (Pooideae), C3 wet/shade tolerant (Bambusoideae), C4 mesophytic (Panicoideae), and C4 xerophytic (Chloridoideae). Proportions of these grass PFTs can unambiguously be reconstructed from stratigraphic sequences using phytoliths. During the summer of 2013, the 216 m West Turkana drill core (2.0-1.4 Ma) was recovered by HSPDP researchers. Initial core descriptions, multi-sensor core logging and sampling at 32 cm resolution was completed in the fall of 2013 at LacCore (U of MN). Phytolith extraction at 96 cm resolution was recently completed. Organics and carbonates were removed from 1 cm3 samples using strong acid and base. Clays were removed using gravity settling and siliciclastics were removed using density separation. Phytolith preservation ranged from excellent to poor. Intervals with poor preservation exhibited evidence of dissolution, most likely due to alkaline lake conditions. Intervals with low phytolith recovery often suffered from high amounts of cryptotephra and authigenic minerals. Well-preserved and high phytolith concentrations were observed in the 35 to 65 mblf interval. Below this level, ~1 m sections yielded well-preserved and highly concentrated phytoliths and other silica microfossils. These sections were separated by longer intervals (~10+ m) where there appears to have been total biogenic silica dissolution. Phytolith analysis is ongoing, but has already demonstrated the presence of taxa underrepresented in pollen studies from contemporaneous contexts, thus highlighting the complimentary aspect of this work.