AUTHIGENIC CLAYS USED AS PROXIES FOR PALEOENVIRONMENTAL IDENTIFICATION

Paleoenvironmental reconstructions are critical in understanding environmental and climate change, and the relation of these changes to hominin evolution. Authigenic clays are a useful indicator of environmental change, as Mg-enrichment of clays occurs with increasing salinity. Using this proxy, we can identify fluctuations between saline and freshwater conditions in East African paleolake basins. Authigenic clays formed in Olduvai Basin during the late Pliocene to early Pleistocene. Between 1.9-1.7 Ma, Olduvai was a hydrologically closed basin into which Al- and Fe-rich detrital clays were deposited. Mg-rich smectite formed at times due to chemical alteration in saline, alkaline conditions. For this study, 40 samples were chosen from Locality 80 in the center of Paleolake Olduvai and analyzed to characterize the geochemistry of the authigenic material.

Submicron clay fractions were extracted through progressive centrifugation. In addition to XRD of oriented (air-dried and glycolated) clay slides, randomly oriented clay fraction 060 (hkl) peaks were analyzed to obtain measurement pertaining to the octahedral sheets. Trioctahedral phases were identified by 060 peaks which indicate the presence of Mg in the octahedral layer.

FTIR analyses were conducted on the samples from pressed KBr pellets. Observations support that the clays are mainly trioctahedral, although there are signs of a dioctahedral (Al- or Fe-) component. Ongoing studies include stable isotope analysis of structural water which may reflect paleosalinity. TEM and electron microprobe data will be collected for further geochemical characterization.

Preliminary data suggests the potential value of Si-O vibrations observed through FTIR for distinguishing between di- and tri-octahedral phases. There is a correlation between the bulk geochemistry results and 060 peaks, implying that the bulk composition is controlled by authigenic material. Peaks in both XRD and FTIR also show variable Mg- and Al- enrichment within the authigenic clays, suggesting a correlation between the geochemistry of the clay minerals and changes in paleoenvironmental conditions. These data indicate coupled mineralogical analyses are useful in paleoenvironmental reconstructions, and may aid our understanding of hominin evolution during the Plio-Pleistocene.