The physico-chemical properties of water samples from the two athalassic endorheic lakes Bogoria and Nakuru in Kenya were analysed. Surface water samples were taken between July 2008 and October 2009 in weekly intervals from each lake. The following parameters were determined: pH, salinity, electric conductivity, dissolved organic carbon (DOC), the major cations (FAAS and ICP-OES) and the major anions (IC), as well as certain trace elements (ICP-OES). Samples of superficial sediments were taken in October 2009 and examined using Instrumental Neutron Activation Analysis (INAA) for their major and trace element content including rare earth elements (REE). Both lakes are highly alkaline with a dominance of Na > K > Si > Ca in cations and HCO 3  > CO 3  > Cl > F > SO 4 in anions. Both lakes also exhibited high concentrations of Mo, As and fluoride. Due to an extreme draught from March to October 2009, the water level of Lake Nakuru dropped significantly. This created drastic evapoconcentration, with the total salinity rising from about 20‰ up to 63‰. Most parameters (DOC, Na, K, Ca, F, Mo and As) increased with falling water levels. A clear change in the quality of DOC was observed, followed by an almost complete depletion of dissolved Fe from the water phase. In Lake Bogoria the evapoconcentration effects were less pronounced (total salinity changed from about 40‰ to 48‰). The distributions of REE in the superficial sediments of Lake Nakuru and Lake Bogoria are presented here for the first time. The results show a high abundance of the REE and a very distinct Eu depletion of Eu/Eu* = 0.33–0.45.

Major and trace element geochemistry of Lake Bogoria and Lake Nakuru, Kenya, during extreme draught.

The archaeology of East Africa during the last ~65,000 years plays a central role in debates about the origins and dispersal of modern humans, Homo sapiens. Despite the historical importance of the region to these discussions, reliable chronologies for the nature, tempo, and timing of human behavioral changes seen among Middle Stone Age (MSA) and Later Stone Age (LSA) archaeological assemblages are sparse. The Kisese II rockshelter in the Kondoa region of Tanzania, originally excavated in 1956, preserves a ≥ 6-m-thick archaeological succession that spans the MSA/LSA transition, with lithic artifacts such as Levallois and bladelet cores and backed microliths, the recurrent use of red ochre, and >5,000 ostrich eggshell beads and bead fragments. Twenty-nine radiocarbon dates on ostrich eggshell carbonate make Kisese II one of the most robust chronological sequences for understanding archaeological change over the last ~47,000 years in East Africa. In particular, ostrich eggshell beads and backed microliths appear by 46-42 ka cal BP and occur throughout overlying Late Pleistocene and Holocene strata. Changes in lithic technology suggest an MSA/LSA transition that began 39-34.3 ka, with typical LSA technologies in place by the Last Glacial Maximum. The timing of these changes demonstrates the time-transgressive nature of behavioral innovations often linked to the origins of modern humans, even within a single region of Africa.

/pdf/middle-and-later-stone-age-chronology-of-kisese-ii-jxzbvc143n.pdf

Middle and Later Stone Age chronology of Kisese II rockshelter (UNESCO World Heritage Kondoa Rock-Art Sites), Tanzania

The Cenozoic magmatism of East Africa: Part V – Magma sources and processes in the East African Rift

Volcanism across the North Tanzanian Divergence Zone (NTD), part of the East African Rift System, occurred episodically from the late Miocene to Recent. Here, we present a summary of previously published K–Ar and 40 Ar/ 39 Ar ages, new 40 Ar/ 39 Ar ages, and geochemical and Sr–Nd–Pb isotopic analyses on samples collected from several volcanoes distributed across the NTD: Burko, Monduli, Tarosero, Ketumbeine, Gelai, Kerimasi and Meru. The locus of volcanism over this period progressed from the southwestern portion of the NTD to the north and east, with a main pulse occurring at about 2.3 Ma, possibly marking the inception of a main rifting event. We model the source of the NTD volcanic rocks as a metasomatized subcontinental lithospheric mantle that includes minor and variable amounts of garnet and amphibole. REE data indicate variations in residual garnet content, consistent with varying depth of melt separation. Radiogenic isotopic data show systematic variations requiring the involvement of up to three components. Two alternative but not exclusive tectonic scenarios are proposed: one requiring the involvement of contributions from recent plume-related fluids, and one explaining the observed geochemical variations by melting of a lithosphere layered by multiple metasomatic events. Supplementary material: Details of analytical methods, operating system and calibration methods, a summary table of the recalculated 40 Ar/ 39 Ar and K–Ar NTD ages, a complete set of detailed release spectra analysis and dating figures, 40 Ar/ 39 Ar incremental heating data and analytical conditions, and examples of NTD calculated fractional crystallization modes are available at http://www.geolsoc.org.uk/SUP18813.

Magmatic activity across the East African North Tanzanian Divergence Zone

The East African Rift (EAR) has fascinated and challenged the geological imagination since its discovery nearly a
century ago. A new series of images showing the sequential development of faulting and volcanism along the Rift from 45 Ma
to present offers a regional overview of that development. The EAR is the latest phase of the extensive Phanerozoic rifting of the
East African continental plate, interwoven with the lithospheric fabrics knitted together during its complex Proterozoic past.
South of 5° S, the EAR variously follows or cuts across the Karoo rift trends; north of 5° S, it is almost totally within new or
reworked Neoproterozoic terranes, while the Karoo rifts are almost totally outside them. The compilations raise several aspects
of rift development seemingly in need of re-imagining, including tight-fit reconstructions of the Gulf of Aden, and the
projection of Mesozoic rifts from Yemen to Somalia. Overall, the rifting process does not accord well with a mechanistic
paradigm and is better imagined within the Prigoginian paradigm, which accepts instability and disorder within natural
processes such as mantle plumes. The structural complexity of Afar and its non-alignment with magnetic anomalies suggests
that the seafloor spreading process is, in its beginnings at least, more chaos than order.

Re-imagining and re-imaging the development of the East African Rift

This book is concerned essentially with the volcanoes of the Gregory Rift valley, the southern-most extension of the East Africa Rift system where rifting impinges onto the Tanzanian craton. The volcanism in this region has a distinctly different flavour to that occurring along the more northern parts of this rift, and is characterized by some extremely peralkaline rocks, and includes the unique natrocarbonatite lavas of Oldoinyo Lengai. The book provides some interesting historical notes regarding the geological investigation of the area and the initial recognition of the rift system by Suess. Although the bulk of the memoir consists of descriptions of individual volcanic centres and their petrogenesis, Dawson provides useful reviews of the structure and tectonics of the area including the rift-related sedimentary basins; the latter include the well-known Olduvai Basin and the Laetoli Beds.

Barry Dawson has been involved in geological and petrological studies of the Gregory Rift valley for fifty years and this volume …

The Gregory Rift Valley and Neogene-Recent Volcanoes of Northern Tanzania

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The Gregory Rift Valley and Neogene-Recent Volcanoes of Northern Tanzania