Nathan M. Rabideaux

Progressive aridification in East Africa over the last half million years and implications for human evolution

Significance Previous research hypotheses have related hominin evolution to climate change. However, most theories lack basin-scale evidence for a link between environment and hominin evolution. This study documents continental, core-based evidence for a progressive increase in aridity since about 575 ka in the Magadi Basin, with a significant change from the Mid-Brunhes Event (∼430 ka). Intense aridification in the Magadi Basin corresponds with faunal extinctions and changes in toolkits in the nearby Olorgesailie Basin. Our data are consistent with climate variability as an important driver in hominin evolution, but also suggest that intensifying aridity may have had a significant influence on the origins of modern Homo sapiens and the onset of the Middle Stone Age. Evidence for Quaternary climate change in East Africa has been derived from outcrops on land and lake cores and from marine dust, leaf wax, and pollen records. These data have previously been used to evaluate the impact of climate change on hominin evolution, but correlations have proved to be difficult, given poor data continuity and the great distances between marine cores and terrestrial basins where fossil evidence is located. Here, we present continental coring evidence for progressive aridification since about 575 thousand years before present (ka), based on Lake Magadi (Kenya) sediments. This long-term drying trend was interrupted by many wet–dry cycles, with the greatest variability developing during times of high eccentricity-modulated precession. Intense aridification apparent in the Magadi record took place between 525 and 400 ka, with relatively persistent arid conditions after 350 ka and through to the present. Arid conditions in the Magadi Basin coincide with the Mid-Brunhes Event and overlap with mammalian extinctions in the South Kenya Rift between 500 and 400 ka. The 525 to 400 ka arid phase developed in the South Kenya Rift between the period when the last Acheulean tools are reported (at about 500 ka) and before the appearance of Middle Stone Age artifacts (by about 320 ka). Our data suggest that increasing Middle- to Late-Pleistocene aridification and environmental variability may have been drivers in the physical and cultural evolution of Homo sapiens in East Africa.

A Mineralogical and Geochemical Analysis of Bed I in Olduvai Gorge, Tanzania

Olduvai Gorge is located in northern Tanzania (3°S, 35.35°E) on the Serengeti Plain and on the margin of the East Africa Rift System. Seventy-nine sediment samples were collected during the 2015 field season from a 6-meter high section of lacustrine sediment (~1.9-1.86 Ma). Samples were collected at ~15 cm intervals from 50 cm to 580 cm. The purpose of these analyses are to determine the chemistry of clay minerals and whether, or not, authigenic clays can be used as paleoenvironmental proxies. Bulk minerology was obtained by XRD analyses of random-oriented powder. Samples were analyzed for 15 minutes from 10 to 65 degrees 2θ. An abundance of K feldspars were found and a few samples contained zeolites (phillipsite). Calcite is common, whereas dolomite is rare. XRD analyses of oriented clays were conducted on additional Bed I sediments, located ~ 300 meters to the east. The samples were analyzed air-dried and ethylene glycol solvated from 3 to 45 degrees 2θ for 15 minutes in order to identify the major clay mineral phases. Previous work has shown that the major clay phases are authigenic illite, smectite and illite/smectite. Preliminary results from this study are consistent with these findings. Samples were also analyzed from 59 to 63 degrees 2θ for 30 minutes. The 060 peak positions varied from 1.510 Å to 1.522 Å. This likely corresponds to variations in octahedral Mg from 1.5 to 2.5 atoms per half formula unit. This variation in Mg suggests fluctuation in salinity of paleo Lake Olduvai over the ~35 kyr period represented by the ~5 m thick lake section. To conclude, preliminary results do suggest that clay chemistry and authigenic clay minerals can be used as paleoenvironmental proxies.