Geosciences

The effect of changing palaeoclimate and palaeoenvironment on human evolution during the
Pleistocene is debated, but hampered by few East African records directly associated with
archaeological sites prior to the Last Glacial Maximum. Middle to Late Pleistocene deposits on
the shoreline of eastern Lake Victoria preserve abundant vertebrate fossils and Middle Stone Age
artefacts associated with riverine tufas at the base of the deposits, which are ideal for palaeoenvironmental reconstructions. New data from tufas identified on Rusinga Island and on
the mainland near Karungu, Kenya are provided from outcrop, thin-sections, mineralogical,
stable isotopic and U-series dating analyses. Tufa is identified at four sites: Nyamita (94.0 ± 3.3
and 111.4 ± 4.2 ka); Kisaaka, Aringo (455 ± 45 ka); and Obware. The age ranges of these tufa
deposits demonstrate that spring-fed rivers were a recurrent, variably preserved feature on the
Pleistocene landscape for ca 360 kyr. Poor sorting of clastic facies from all sites indicates flashy,
ephemeral discharge, but these facies are commonly associated with barrage tufas, paludal
environments with δ13C values of ca 10‰ indicative of C3 plants and fossil Hippopotamus, all of
which indicate a perennial water source. Other tufa deposits from Nyamita, Obware and Aringo
have a mixed C3/C4 signature consistent with a semi-arid C4 grassland surrounding these springfed
rivers. The δ18O values of tufa from Nyamita are on average ca 1‰ more negative than
calcite precipitated from modern rainfall in the region, suggesting greater contribution of
depleted monsoonal input, similar to the Last Glacial Maximum. Microdebitage and surface
collected artefacts indicate that early modern humans were utilizing these spring-fed rivers. The
presence of spring-fed rivers would have afforded animals a reliable water source, sustaining a
diverse plant and animal community in an otherwise arid environment.

The effect of changing environment on the evolution of Homo sapiens is heavily debated, but few data are
available from equatorial Africa prior to the last glacial maximum. The Karungu deposits on the northeast coast
of Lake Victoria are ideal for paleoenvironmental reconstructions and are best studied at the Kisaaka site near
Karunga in Kenya (94 to N33 ka) where paleosols, fluvial deposits, tufa, and volcaniclastic deposits (tuffs) are exposed
over a ~2 km transect. Three well-exposed and laterally continuous paleosols with intercalated tuffs allow
for reconstruction of a succession of paleocatenas. The oldest paleosol is a smectitic paleo-Vertisol with saline and
sodic properties. Higher in the section, the paleosols are tuffaceous paleo-Inceptisols with Alfisol-like soil characteristics
(illuviated clay). Mean annual precipitation (MAP) proxies indicate little change through time, with an
average of 764 ± 108 mm yr−1 for Vertisols (CALMAG) and 813 ± 182 to 963 ± 182 mm yr−1 for all paleosols
(CIA-K). Field observations and MAP proxies suggest that Karungu was significantly drier than today, consistent
with the associated faunal assemblage, and likely resulted in a significantly smaller Lake Victoria during the late
Pleistocene. Rainfall reduction and associated grassland expansion may have facilitated human and faunal dispersals
across equatorial East Africa.

The cichlid fishes of the East African Great Lakes are the largest extant vertebrate radiation identified to date. These lakes and their surrounding waters support over 2,000 species of cichlid fish, many of which are descended from a single common ancestor within the past 10 Ma. The extraordinary East African cichlid diversity is intricately linked to the highly variable geologic and paleoclimatic history of this region. Greater than 10 Ma, the western arm of the East African rift system began to separate, thereby creating a series of rift basins that ...

Olduvai Gorge, Tanzania is known for its fossil and cultural record of early hominins. The archaeological records are typically found within pedogenically modified sediments, thus the interpretation of paleosols provides important paleoenvironmental context information. The Gorge contains a rich and diverse record of paleosols that vary spatially and temporally, however the stratigraphy can be divided into time slices using dated tuffs and studied in a paleolandscape context. Sediments were deposited in a semi-arid closed rift basin containing paleo Lake Olduvai, with volcanoes supplying volcaniclastic material to an alluvial fan system on the eastern side and quartzofeldspathic fluvial sediments derived from weathered basement rocks on the western side. The shallow saline-alkaline lake in the basin center and groundwater levels in the surrounding uplands fluctuated with Milankovitch-driven climatic (precession) cycles of w20,000 years. The rift basin paleolandscape (at w1.8 Ma) is reconstructed using sedimentology, stratigraphy and paleopedology. Standard field descriptions of physical and biogenic paleosol structures are combined with stable isotope ratios of carbonates, and whole rock geochemistry of sediments, to reveal diversity of paleosols within the basin. There are at least three distinct types of paleosols that record a paleocatena related to both landscape and drainage differences. Red tephra-rich Andisols developed on the volcaniclastic alluvial fan to the east of the paleolake, calcium-carbonate-rich, silty Aridisols developed on the interfluves and floodplain of the fluvial plain on the west, whereas clay-rich paleosols (Vertisols) developed on the lake margin and lake in the center of basin. Variances in geo-morphology, depositional environment, parent material, and depth to the water table are reflected in the development of distinctly different soil types that can provide key data needed for high-resolution reconstruction of the landscape known to be utilized by early hominins.

Here we report our recent discovery of a new obsidian source in central Armenia. Using portable XRF, we were able to chemically identify Ptghni obsidian as a previously unrecognized source on the same day that we first encountered it during our field surveys. Obsidian was found in alluvial-lacustrine sediments exposed within the Hrazdan Gorge, where it had been deposited after having eroded from an upstream source. These sediments were covered by mafic lavas and later exposed by downcutting of the Hrazdan River. Based on the stratigraphy of the gorge, the lava flows -- and, therefore, the sediments sandwiched between them -- predate 441 ka. The composition of Ptghni obsidian does not fit into the chemical trends of known sources in the Gegham and Tsaghkunyats ranges, so its precise volcanic origin remains unknown. Comparisons to unidentified artifacts in the literature revealed no matches, but obsidian sourcing work in Armenia has largely focused on the Holocene, when the Ptghni source might no longer have been accessible. The discovery of Ptghni obsidian is crucial for research into early hominin expansions given that it was an obsidian source available for use as toolstone by hominins during the Early and/or Middle Pleistocene.

A B S T R A C T Here we report our recent discovery of a new obsidian source in central Armenia. Using portable XRF, we were able to chemically identify " Ptghni " obsidian as a previously unrecognized source on the same day that we first encountered it during our field surveys. Obsidian was found in alluvial-lacustrine sediments exposed within the Hrazdan Gorge, where it had been deposited after having eroded from an upstream source. These sediments were covered by mafic lavas and later exposed by downcutting of the Hrazdan River. Based on the stratigraphy of the gorge, the lava flows – and, therefore, the sediments sandwiched between them – predate 441 ka. The composition of Ptghni obsidian does not fit into the chemical trends of known sources in the Gegham and Tsaghkunyats ranges, so its precise volcanic origin remains unknown. Comparisons to unidentified artifacts in the literature revealed no matches, but obsidian sourcing work in Armenia has largely focused on the Holocene, when the Ptghni source might no longer have been accessible. The discovery of Ptghni obsidian is crucial for research into early hominin expansions given that it was an obsidian source available for use as toolstone by hominins during the Early and/or Middle Pleistocene.