Late-Holocene sedimentation and sodium carbonate deposition in hypersaline, alkaline Nasikie Engida, southern Kenya Rift Valley

Continental rift systems are often characterized by geothermal activity and associated discharge of hot groundwater, which can substantially impact the water, solute and sediment budgets of rift-valley lakes. Hot-spring inflow can result in complex lake hydro- and geochemistry, but also buffers against the desiccation of closed-basin lakes in dry climate regimes. Consequently, hydrothermally fed lakes can potentially provide continuous sedimentary records from regions where other types of paleoenvironmental archives are lacking. This is illustrated by Nasikie Engida, a shallow hypersaline and alkaline (soda) lake in the semiarid Rift Valley of southern Kenya. Here, inflow of hot-spring water has maintained a shallow but permanent water body and continuous deposition of sediments through past climate episodes when many other lakes in the eastern (Gregory) branch of the East African Rift System stood dry. We present the first data on late-Holocene sedimentation in this remarkable lacustrine system, typified by authigenic nahcolite [NaH(CO3)] formation during part of its recorded history. Our data include measurements of bulk-sediment and mineralogical composition, clastic-mineral grain size and magnetic susceptibility. Analytical issues related to the large amount of salts within the sediments, both as crystals and in solution in the pore water, are discussed. We also present exploratory time series of the stable-isotope composition of bulk organic matter and authigenic nahcolite. Core lithostratigraphy and preliminary radiocarbon dating indicate that Nasikie Engida has accumulated finely laminated sediments continuously since ca. 2850 cal year BP, which is remarkable given its current maximum depth of only 1.6 m. Synsedimentary nahcolite appears abruptly similar to 2260 cal year BP and since then has been deposited regularly, in the form of distinct pure layers up to several cm thick. Its formation has been enabled by high pCO(2), supplied principally from geothermal sources, accumulating in a high-density brine. Although Nasikie Engida has likely been saline over the entire time span covered by the studied sediment sequence, variation in nahcolite deposition and bulk-sediment composition suggests multi-decadal to centennial oscillations in water-column stability and stream inflow driven by variation in climatic moisture balance. Further analyses of this paleoenvironmental archive may thus produce the first continuous climate-proxy record from a vast dry region of equatorial East Africa. Moreover, Nasikie Engida's extraordinary setting makes it a unique modern analogue for the interpretation of ancient nahcolite-bearing salt-lake deposits

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

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