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Variations in the temporal and spatial distribution of solar radiation caused by orbital changes provide a partial explanation for the observed long-term fluctuations in African lake levels. The understanding of such relationships is essential for designing climate-prediction models for the tropics. Our assessment of the nature and timing of East African climate change is based on lake-level fluctuations of Lake Naivasha in the Central Kenya Rift (0°55′S 36°20′E), inferred from sediment characteristics, diatoms, authigenic mineral assemblages and 17 single-crystal 40Ar/39Ar age determinations. Assuming that these fluctuations reflect climate changes, the Lake Naivasha record demonstrates that periods of increased humidity in East Africa mainly followed maximum equatorial solar radiation in March or September. Interestingly, the most dramatic change in the Naivasha Basin occurred as early as 146 kyr BP and the highest lake level was recorded at about 139–133 kyr BP. This is consistent with other well-dated low-latitude climate records, but does not correspond to peaks in Northern Hemisphere summer insolation as the trigger for the ice-age cycles. The Naivasha record therefore provides evidence for low-latitude forcing of the ice-age climate cycles. [Copyright &y& Elsevier]

The South Kenya Rift is comprised of a series of N-S-oriented grabens with sediments that preserve an approximate one-million-year environmental history that reflects the interplay of climate, tectonism and volcanism. This study attempts to disentangle the relative roles of these major controls by comparing the geochemical records preserved in three sedimentary basins. The study focuses on the Koora Basin using bulk geochemical data in a 139-m-long core. This record is then compared with geochemical data and environmental histories from a 196-m-long core at Magadi and outcrops in the Olorgesailie Basin. Four climatic phases (1000–850; 850–470; 470–400; 400–0 ka) are recognised at Koora, which can also be distinguished in the Magadi and Olorgesailie Basins. However, inter-basin contrasts also suggest that additional, non-climatic factors influenced these geochemical histories, particularly during four intervals. These include 1) the Magadi Transition (MT; ∼770–700 ka), 2) the Magadi Tectonic Event (MTE; ∼540 ka), 3) the Koora Instability Period (KIP; ∼325–180 ka), and 4) the Trona Precipitation Period (TPP; ∼105–0 ka). Prior to the MT, Zr/TiO 2 , La/Lu, Mo, As, V and Na/Ca in Magadi and Koora cores were similar but afterwards diverged. Major reductions in transition metals at Magadi during the MTE reflect tectonically-induced cross-rift drainage diversion. This contrasts with the Koora and Olorgesailie basins where these metals were constant from ∼1000 to 300 ka. The KIP represents a significant increase in volcanic inputs to the Koora Basin and increased geochemical variability. Bromine (Br), which reflects peralkaline volcanic activity and/or evaporative concentration, is elevated during the KIP at Koora but is below detection limits in the rest of the Koora core. Br in the Magadi core does not correlate with that in the Koora record, suggesting contrasting accumulation processes. The TPP represents a phase of trona precipitation at Magadi but not at Koora. This difference partly reflects increased magmatic CO 2 rising along faults in the Magadi basin during a period of increasing aridity. Rare-earth element patterns indicate a major change at Magadi with many anomalies after about 325 ka to the present, caused by the development of hypersaline waters, which did not occur at Koora or Olorgesailie. The geochemical data from the three basins help to partially separate climatic controls from those related to volcanism, tectonism and local geomorphology. • Lake level changes in South Kenya Rift basins show only partial correlations with global glacial-interglacial cycles. • Geochemical and mineralogical contrasts between basins reflect four local and regional tectono-volcanic events • Bromine concentrations indicate volcanic sources and evaporative concentration • Rare earth elements anomalies reflect trachytic and basaltic source rocks and the impact of high lake alkalinities • Trona deposition reflects the combined effect of increasing aridity and magmatic CO 2 inputs • Transition metal stratigraphies reflect drainage diversions caused by faulting [ABSTRACT FROM AUTHOR]

Despite eastern Africa being a key location in the emergence of Homo sapiens and their subsequent dispersal out of Africa, there is a paucity of long, well-dated climate records in the region to contextualize this history. To address this issue, we dated a ∼293 m long composite sediment core from Chew Bahir, south Ethiopia, using three independent chronometers (radiocarbon, 40Ar/39Ar, and optically stimulated luminescence) combined with geochemical correlation to a known-age tephra. The site is located in a climatically sensitive region, and is close to Omo Kibish, the earliest documented Homo sapiens fossil site in eastern Africa, and to the proposed dispersal routes for H. sapiens out of Africa. The 30 ages generated by the various techniques are internally consistent, stratigraphically coherent, and span the full range of the core depth. A Bayesian age-depth model developed using these ages results in a chronology that forms one of the longest independently dated, high-resolution lacustrine sediment records from eastern Africa. The chronology illustrates that any record of environmental change preserved in the composite sediment core from Chew Bahir would span the entire timescale of modern human evolution and dispersal, encompassing the time period of the transition from Acheulean to Middle Stone Age (MSA), and subsequently to Later Stone Age (LSA) technology, making the core well-placed to address questions regarding environmental change and hominin evolutionary adaptation. The benefits to such studies of direct dating and the use of multiple independent chronometers are discussed. • Four independent dating methods applied to ∼293 m lake core from southern Ethiopia. • Reveals 620 ka high-resolution sedimentary record near key fossil hominin sites. • Mean accumulation rate of 0.47 mm/a comparable to other African lacustrine sediments. • Accumulation rate fell to 0.1 mm/a during MIS 2, likely due to reduced sediment supply. • Use of multiple independent chronometers is a powerful approach in lake settings. [ABSTRACT FROM AUTHOR]

This study examines a one-million-year pollen record from a 194-m-long Lake Magadi core (HSPDP-MAG14-2A) in the south Kenya Rift Valley. The pollen indicate a general trend through the last 740 kyr from wetter conditions to generally drier environments. Grassland dominated with less common Podocarpus and Cyperaceae in a sparse flora between 1000 and 740 ka. Poaceae, woodland and herbaceous plants are common through the remaining core and abundant between 740 and 528 ka and after 200 ka. Pollen diversity increased after 200 ka. Podocarpus and Cyperaceae reached a peak abundance at ~575 ka with a subsequent decline that suggests a progressive increase in aridity, interrupted by wetter intervals. Podocarpus -dominated forests expanded and contracted many times during the Quaternary and document an anti-phased relationship with data from Lake Malawi. Similar anti-phased correlations are noted for herbaceous plants, suggesting that the two basins responded differently to the same climate or were influenced by contrasting climate regimes. Increases in macrocharcoal correlate with increasing pollen abundance and suggest wetter conditions. Data from the Magadi, Koora and Olorgesailie basins indicate similar trends and a dominant climate control on vegetation and habitats. Large lakes characterised all three basins at 740–528 ka with climate subsequently becoming drier, but with many wetter intervals. At various times the lakes expanded, contracted and dried out, except at Lake Magadi where spring inflows maintained lacustrine conditions through the late Quaternary. Faulting also contributed to fragmentation of the landscape and formation of a mosaic of habitats. An especially intense period of aridity at ~528–392 ka coincided with extinction of many large-bodied mammals and may have helped to drive a change from the use of Acheulean hand axes to the production of Middle Stone Age tools by 320 ka. After 200 ka pollen diversity increased substantially with a mix of montane, riparian and dry forest associations that were present in varying amounts through to ~4.2 ka at the core top. • Lake Magadi sediments provide a one-million-year record of environmental change. • Wetter environments and expanded lakes existed between 740 and 528 ka. • Aridity at 528–392 ka coincided with episode of mammalian extinctions. • Post mid-Brunhes Event climates were generally drier, but with wetter intervals. • Changes from Acheulian to MSA tools may reflect altered ecological resources. [ABSTRACT FROM AUTHOR]