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Preliminary results from year one of the study showed that many elements typically considered as “immobile” during alteration of tephra can be significantly depleted when alteration occurs under saline-alkaline conditions. This limits their usefulness for “fingerprinting” tephra from alkaline lake basins that have undergone zeolitic diagenesis. The second year of research focused instead on the overall preservation of the bulk major element composition of tephra even after significant zeolitic alteration in a closed-basin environment, as elements present in the original glass are redistributed between co-existing clays and zeolites. For example, in altered tephra samples dominated by zeolites, co-existing clays are enriched in Fe, Ti, and Mg, elements not easily accommodated within zeolites. Samples of a single tephra layer preserved across a lake center through lake margin transect of a saline-alkaline paleolake deposit were analyzed by SEM, EPMA, XRD, and XRF in order to track changes in mineral assemblage, mineral composition, morphology, and bulk composition. The results of this part of the project are presented in a manuscript recently accepted for publication in the journal Clays and Clay Minerals.

I also analyzed altered tephra samples collected (in 2007 and 2008) in a neighboring Pleistocene paleolake basin (Peninj, in the modern Lake Natron basin to the north) for comparison. Tephra at Peninj are more altered than at Olduvai: no glass is preserved at any site visited, and titanomagnetite minerals are also completely altered. The authigenic mineral assemblage is also different from that at Olduvai, with analcime and erionite dominating the zeolites (rather than phillipsite). Overall, phenocrysts vary little in composition between most Peninj tephra, leaving only a few that can be mineralogically fingerprinted. The complete lack of glass also makes it impossible to ascertain the exact starting compositions, which will make it difficult to assess element mobility within this system. A manuscript detailing the authigenic and primary volcanic mineral assemblages and phenocryst compositions of the Peninj tephra was submitted to Quaternary Research and is currently in revision.

An outgrowth of this project has been my continued research into the unusual occurrence of the potassium iron sulfate mineral jarosite in the Olduvai paleolake sediments. This mineral is generally associated with acidic, water-limited conditions on Earth and Mars, so its presence in an otherwise saline-alkaline lacustrine deposit is unexpected. I now suspect that this jarosite formed very recently as an alteration product of oxidized lacustrine pyrite. The results of this part of the project were presented at the Lunar and Planetary Science Conference in Houston, since jarosite is commonly used as an indicator of predominantly acidic conditions on Mars. A manuscript outlining these results was submitted to Journal of Geophysical Research and is currently in revision.

I have also continued my research on Olduvai Gorge tephrostratigraphy using the composition of phenocrysts more resistant to alteration. I am now focusing on the younger Olduvai beds (specifically Bed II, more recent that 1.75 Ma). The tephra are highly altered and unfortunately have similar phenocryst compositions, making it difficult to develop geochemical or mineralogical fingerprints for these tephra. These tephra are also more contaminated than those studied previously (in better-preserved Olduvai Bed I), thus bulk composition is less likely to help distinguish them. Despite these limitations, two important marker tephra, the Bird Print Tuff and Tuff IID, have more distinctive phenocryst compositions and morphologies and can thus be used for site-to-site correlation. A preliminary tephrostratigraphic framework for Bed II, based on these fingerprints and physical mapping, was presented at the Geological Society of America annual meeting (Portland) and at the Paleoanthroplogy Society meeting (St. Louis).

I was invited to give a talk on my project-related XRF research at a “Practical XRF Application in Industry” Symposium in London, Ontario, in June 2010.

I visited both Olduvai and Peninj again in July-August 2010, collecting additional altered tephra samples to build upon research already in progress. These samples are currently being prepared for analysis by XRD and XRF.