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The evaluation of groundwater geochemistry of transboundary aquifers has an important role in the management and sustainable use of groundwater resources in arid and semi-arid regions. This study presents the results of an integrated hydrogeochemical study of the Bulal transboundary aquifers located in the Borena zone, along the Ethiopia-Kenya border. The main aquifers are basic volcanic rocks, subordinate felsic pyroclastic deposits, and metamorphic basement rocks. These aquifers are overlain by alluvio-lacustrine sediments, eluvial soil and regolith. The groundwaters are dominated by Na+ and HCO 3 − and mixed water types. Silicate weathering and evaporation are the dominant hydrogeochemical processes controlling the chemistry of the groundwaters in most parts of the catchment. However, dissolution of evaporitic minerals might have also played a significant role in adding major ions into the groundwater system. Groundwater in the volcanic aquifers are Na–HCO 3 , Ca–Mg–HCO 3 , Na–Ca–HCO 3 and Na–Cl–SO 4 water types while Ca–Mg–HCO 3 , Na–Ca–Cl–SO 4 and Ca–Mg–SO 4 water types are dominant in the metamorphic basement aquifers. The groundwater in the catchment mainly evolved from Na–HCO 3 in the west, Ca–Mg–HCO 3 in the north, and Na–Ca–Cl–SO 4 in the central east and southeast to Na–Ca–HCO 3 type in the southern low-lying areas along the groundwater flow direction. The study has significant implications for the management of groundwater quality in the region. • Groundwater chemistry is investigated using an integrated approach. • Ca–Mg–HCO 3 , Na–Ca–Cl–SO 4 and Na–Ca–HCO 3 are the dominant hydrochemical facies. • Groundwater chemistry is governed by silicate weathering and evaporation. • The study has significant implications for managing groundwater quality. [ABSTRACT FROM AUTHOR]

There is ongoing debate concerning whether or not changes in the eastern African climate, both long-term and short-term, affected the evolution, dispersal, cultural development, and technological innovations of Homo sapiens – and if so, in what way. We present the wavelet spectral analysis results of a ∼620 kyr record of environmental change from the Chew Bahir (CHB) basin in the southern Ethiopian rift, approximately 120 km from the Omo-Kibish fossil locality, which boasts one of the oldest documented appearances of H. sapiens. Our results indicate that the long-term wet-dry changes in the eastern African climate recorded in the CHB sediments were mainly caused by changes in orbital eccentricity, with relatively dry but variable climates occurring during eccentricity minima within the 400 kyr eccentricity cycle, and increased precipitation, interspersed with distinctly dryer phases associated with orbital precession, during eccentricity maxima. Such insolation-forced precipitation changes would have affected the habitat of H. sapiens in the region; the transition from Acheulean to Middle Stone Age (MSA) documented in the Olorgesailie Basin of southern Kenya coincides with a distinct eccentricity minimum with reduced precipitation and repeated abrupt climatic transitions. In contrast, at the time of the subsequent, first documented occurrence of H. sapiens in eastern Africa the climate was distinctly wetter and less variable. • Orbitally controlled variation in insolation have caused wet-dry climate cycles in the CHB over the las 620 kyrs. • The climate of the CHB was characterized by a long term drying trend coinciding with an increase in variability. • Short term increases of variability correspond to changes in eccentricity translating into fast climate transitions. • During the transition from Acheulean to MSA technologies the climate of the CHB was highly variable. [ABSTRACT FROM AUTHOR]

Six sites in Ethiopia and Kenya, all adjacent to key paleoanthropological sites have been investigated as part of the Hominin Sites and Paleolakes Drilling Project (HSPDP), aiming at an enhanced understanding of climatic influences on human physical and cultural evolution. The recovered sediment cores from the six sites all together span the last ~3.5 Ma. The sediment core records archive environmental change during diverse milestones in human evolution, and times of dispersal and technological and cultural innovation. The 280 m-long Chew Bahir lacustrine record, recovered from a tectonically-bound basin in the southern Ethiopian rift in late 2014, covers the past ~600 ka of environmental history, a time period that includes the transition to the Middle Stone Age, and the origin and dispersal of modern Homo sapiens. Developing a continuous climate history based on sediment core composition is not straightforward because on one hand some indicators might only be preserved intermittently and on the other hand the physical and chemical properties of the sedimentary deposits do not have a linear relationship to climate. Here we present the first outcome of our ongoing work on deciphering direct paleoenvironmental information from authigenic mineral transformations in the long (~280 m) Chew Bahir sediment cores. First results suggest mineralogical and geochemical indicators record wet, dry and hyper-arid climate intervals. Preliminary work suggests that the most extreme evaporative phases are represented by authigenic mineral assemblages including Mg-enriched clays, low-temperature authigenic illite and euhedral analcime. SEM and Energy Dispersive Spectrometer (EDS) analyses confirm that the variations in the analcime abundance that are evident in the XRD dataset seem to be distinctly formed deltoidal icositetrahedrons. These are known to form in highly saline and alkaline brines (pH 9 and higher) and can be associated with pronounced arid phases. Understanding and determining the degree of authigenic mineral alteration in the Chew Bahir records will enable interpretation of µXRF-derived indicators (e.g. K correlated with aridity), and provide direct paleohydrologic data. Together with a well-constrained age-model, our growing understanding of site-specific proxy formation and the establishment of climate proxies for Chew Bahir will provide a robust environmental history on decadal to orbital timescales. [ABSTRACT FROM AUTHOR]

Many Plio-Pleistocene lake basins in eastern Africa associated with early hominins contained waters that were alkaline, enriched in silica due to weathering of volcaniclastics and hydrothermal activity, and evaporatively concentrated. Such conditions are favorable for the formation of silicate minerals precipitated from solution, or neoformed through alteration of detrital parent material. The most common authigenic silicates thus produced include the zeolite family of minerals and a number of clay minerals, particularly 2:1 clays of the illite and smectite families. Particularly in the absence of well-preserved paleoecological records, these minerals may provide important evidence of paleoenvironmental conditions. Stratigraphic variations in lacustrine authigenic silicates can give an indication of changes in cation proportions in paleolake waters over time. For example, in Pliocene deposits of the Baringo Basin, Kenya, fluctuations between phillipsite (K-rich) and analcime (Na-rich) facies suggest changing salinity levels because more saline waters have greater Na/K ratios. In one interval in the Chemeron Formation, analcimic muds are interbedded with freshwater diatomites, suggesting a link between high amplitude of environmental variability and peak orbital eccentricity ~2.6 Ma. In the Lake Magadi basin in southern Kenya, marked increases in zeolite abundance and greater dominance of Na-rich phases indicate hydrological closure and enhanced evaporative concentration suggesting a long-term increase in aridity accelerating ~380 ka. In short cores of the Chew Bahir basin of southern Ethiopia, greater analcime abundances and incipient low-temperature illitization of smectite (indicating greater K uptake in more saline waters) are associated with Late Pleistocene arid episodes such as the Younger Dryas. In the Olduvai Basin of northern Tanzania, fluctuations in the Al/Mg ratio of authigenic clay minerals record changes in salinity, as greater Mg is incorporated into clays during saline episodes. These clay mineral changes are accompanied by shifts in the oxygen isotope composition of structural oxygen, indicating equilibrium with variably saline waters. Both elemental and isotopic changes fluctuate over precessional timescales, suggesting orbital control of hydrologic balance ~1.8 Ma.Authigenic silicates can be an important source of paleolimnological data, particularly over the range of salinities between the relatively freshwater conditions under which biological proxies readily accumulate, and the extreme salinities associated with evaporites such as halite and trona (Na-carbonate). [ABSTRACT FROM AUTHOR]