AUTHIGENIC MINERALS: LOCALITY 80, OLDUVAI GORGE, TANZANIA

Mineralogic study of altered volcanic tuffs is intended to shed light on climatic and water-mineral chemistry of hominin habitats during the period 1.92 to 1.80 Ma in Paleolake Olduvai basin, Tanzania. The Basin holds Pliocene-Pleistocene keys to understanding human race evolution. While research shows cyclic patterns in regional climate, definitive ages and correlations with changes in hominin occupation, habitat, and behavior remain unclear.

This work tests the hypothesis that the alteration styles of volcanic Tuffs IA through IF deposited in the center of the paleolake varied over time due to changes in salinity/alkalinity in the lake basin. X-ray diffraction of samples collected at Locality 80 identified several primary authigenic minerals. The tuffs are thoroughly altered, mostly to K-feldspars and zeolites, as well as clay minerals not the subject of this study. Based on Berry's (2012) geochemical interpretations, tuffs were deposited during times of both high salinity/alkalinity and relatively dilute conditions. Tuffs can be divided into four groups (1 to 5 sample-members each) deposited during peak dilution and two at peak salinity/alkalinity (2 sample-members each). Authigenic silicates may be expected only at peak salinity conditions, but they occur in the whole section. One 5-member group altered in the rise to and during the Tuff IA high salinity and high alkalinity period, however, only one of those was to a high K mineral (orthoclase) - the others to calcite (3) and analcime (1). Ongoing investigations seek a reason for these departures from normal expectations.

The age model based on Deino (2012) for this effort revealed that using a revised sedimentation rate of 23.8 cm/Ka for the period beginning with the deposition of Tuff IA up section 181 cm improved the stratigraphic fit for Hay's 1976 stratigraphic column, over the previously used 22 cm/Ka. Deino's sedimentation factors for interpolating ages for other parts of the section produced almost perfect fits for the stratigraphic column.

Final results will aid understanding of how the paleolake's internal processes functioned, reacted to external forces, and influenced humans' ancestors.