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

Abstract

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.

Aspects of human physical and behavioural evolution during the last 1 million years

“…deMenocal, 1995, and overall climatic variability (e.g. Potts, 1996Potts, , 1998Potts, , 2012, and to correlate these trends with notable instances of evolutionary change (Potts, 2013;Shultz and Maslin, 2013;Grove, 2014;Maslin et al, 2014Maslin et al, , 2015Levin, 2015;Trauth et al, 2015;Carotenuto et al, 2016;Burke et al, 2017;Owen et al, 2018;Buck et al, 2018).…”

Animal bioturbation preserved in Pleistocene magadiite at Lake Magadi, Kenya Rift Valley, and its implications for the depositional environment of bedded magadiite

“…Lake Magadi, a hypersaline (>300 g/kg TDS: Total Dissolved Solids) alkaline (pH: 10-11) soda lake, 0-2 m deep, lies in faulted volcanic terrain in the axial depression of the southern Kenya Rift 14-16 just south of the equator (1°53′S). Modern lake sediments, cores and Quaternary deposits exposed around its margins have provided details of the sedimentary facies, including thick deposits of trona [Na 3 (CO 3 )(HCO 3 )·2H 2 O] that underlie the modern lake floor 14,16,17 . The Pleistocene sediment record includes abundant bedded, nodular and intrusive chert (including dykes) of diverse origins and the rare sodium-silicate mineral, magadiite [NaSi 7 O 13 (OH) 3 ·4(H 2 O)] 18-23 .…”

“…The stratigraphic succession, exposed discontinuously around its margins, consists of the early to mid-Pleistocene Oloronga Beds (fluvial and lacustrine), which lie upon Magadi Trachyte (1.4-0.8 Ma) basement, and are locally overlain by calcrete 26 , the lacustrine Green Beds and associated intrusive chert (191-158 ka), the High Magadi Beds (HMB: ~25-9 ka: mainly lacustrine), and the Evaporite Series (<9 ka to present: lacustrine evaporites and organic muds) 17,21,27 . Sedimentary rocks younger than the Oloronga Beds are confined mainly to the axial N-S graben.Burrows were discovered in bedded magadiite deposits of the High Magadi Beds, a series of fluvial, lacustrine and volcaniclastic sediments up to ~6 m thick in outcrop, exposed up to ~12 m above modern Lake Magadi, that record former high lake-levels of terminal Pleistocene to early Holocene age in the Magadi Basin 14,[16][17][18][19]27,28 . The outcrop with ichnofossils lies at ~605 m elevation, ~100 m east of the base of the narrow (<1.5 km wide) N-S-trending Magadi horst that separates the southeastern sub-basin from the axial trona pan (Figs.…”

“…The trace fossils are present in the uppermost part of the lower member, the base of which is not exposed near the outcrops in the southeastern sub-basin. Thicker units of High Magadi Beds are present in cores recovered from the lake floor 14,17,27,[33][34][35][36] . The informal divisions of the HMB based on outcrop will likely be revised.X-ray diffraction (XRD) analyses show that the weakly lithified silt and mud above and below the several magadiite beds in outcrop are composed of the authigenic zeolite erionite [(Na 2 ,K 2 ,Ca) 2 Al 4 Si 14 O 36 ·15H 2 O], detrital K-feldspar (mainly anorthoclase) and amphibole, with variable amounts of authigenic analcime and quartz 33,37 .…”

“…The lake, which is fed by saline hot springs and seasonal runoff 15,25 , lies in a region with a strongly negative precipitation/evaporation budget. The stratigraphic succession, exposed discontinuously around its margins, consists of the early to mid-Pleistocene Oloronga Beds (fluvial and lacustrine), which lie upon Magadi Trachyte (1.4-0.8 Ma) basement, and are locally overlain by calcrete 26 , the lacustrine Green Beds and associated intrusive chert (191-158 ka), the High Magadi Beds (HMB: ~25-9 ka: mainly lacustrine), and the Evaporite Series (<9 ka to present: lacustrine evaporites and organic muds) 17,21,27 . Sedimentary rocks younger than the Oloronga Beds are confined mainly to the axial N-S graben.…”