Sediments from the Magadi Basin (south Kenya Rift) preserve a one-million-year palaeoenvironmental record that reflects interactions between climatic, volcanic and tectonic controls. Climate changes ...that impacted sedimentation include wet-dry cycles on variable timescales and an overall progressive trend towards greater aridity. Volcanic influences involved inputs of tephra to the basin, significant inflow of geothermal fluids, and the effects of weathering, erosion and transportation of clastics from trachyte and basalt terrains. Tectonic controls, which were often step-like, reflect the influence of faults that provided pathways for fluids and which controlled accommodation space and drainage directions.
Intensified aridity and evaporative concentration resulted in salinity and pH increasing with time, which led to a change from calcite deposition in mildly saline lakes before 380 ka to the later formation of zeolites from reactions of volcaniclastic debris with highly alkaline lake and pore water. After 105 ka, hyperalkaline conditions led to trona accumulation and increasingly variable rare earth elements (REEs). The presence of mixed saline and freshwater diatom taxa between 545 and 16 ka indicates climate variability and episodic inputs of fresh water to saline lakes. Calcrete formed in lake marginal settings during semi-arid periods.
Tectonic controls operated independently of climate, but they interacted together to determine environmental conditions. Aquatic deposition was maintained during periods of increasing aridity because fault-controlled ambient and geothermal springs continued to flow lakewards. This recharge, in turn, limited pedogenesis: palaeosols are common in other rift floor sequences. Trona formed when aridity and evapoconcentration increased, but its precipitation also reflects increased magmatic CO2 that ascended along faults. Basin fragmentation and north-south fractures caused loss of cross-rift (east-west) drainage from rift-marginal basalts, resulting in reduced transition metals after 545 ka. The Magadi Basin demonstrates how a careful reconstruction of these complex tectono-climatic interactions is essential for accurate palaeoenvironmental reconstruction in continental rifts and in other tectonic settings.
•Progressive increase in palaeolake alkalinity and salinity over the past million years.•Middle Pleistocene to Holocene increase in aridity was interrupted by climate-induced wetter episodes.•Diatom floras indicate episodically meromictic palaeolakes.•Step-like changes in sedimentation reflect faulting and drainage diversion.•Faulting tapped deep groundwater releasing Si-enriched deep fluids and mantle CO2 to produce abundant chert and thick trona deposits.
Two cores were recovered from the Lake Magadi and Nasikie Engida Basins in the south Kenya Rift. Core MAG14-2A (194 m) contains a middle Pleistocene to Holocene record, whereas core NAS15/19 (4.36 m) ...covers only the late Holocene. Surficial sediments from springs and shallow-water sites were sampled in both basins. MAG14-2A rests on trachyte dated at 1.08 Ma. Diatoms are rare in the oldest sediments, but well preserved after about 545 ka, documenting a trend from less to more saline water. Core MAG14-2A contains fifteen facies, five of which are diatomaceous. In contrast, NAS15/19 is dominated by two facies, each containing well-preserved diatoms. Both sequences are distinct from others of similar age in the Kenya Rift in lacking pedogenic horizons, reflecting the location of Lake Magadi and Nasikie Engida in a tectonic sump where aquatic environments were maintained by geothermal and meteoric springs. Canonical Correspondence Analysis distinguishes three assemblages in the modern surface muds of Lake Magadi and Nasikie Engida, but with no pre-Holocene counterparts. Eleven diatom zones are recognised in MAG14-2A: Zones D2 to D10 contain rare to common diatoms dominated by
Aulacoseira granulata
and its varieties,
Aulacoseira agassizii, Thalassiosira faurii
,
Thalassiosira rudolfi
and
Cyclotella meneghiniana
. Individual samples commonly include a mixture of benthic and planktonic taxa and saline and freshwater species. These assemblages indicate waters that ranged between pH 7.4 and 11.5 with conductivities of ~ 300 to > 25,000 μS cm
−1
. Correlations with the neighbouring Olorgesailie and Koora Basins indicate four major environmental phases that affected the south Kenya Rift during the last million years with fresh to moderately saline water, or land surfaces, developing during Phase I (1000 to 750 ka). These environments gave way to generally wetter conditions and freshwater lakes in all basins during Phase II (~ 750 to 500 ka). Phase III (~ 500 to 325 ka) was characterised by drier conditions with paleoenvironments becoming more variable and dry during Phase IV (325 ka to present).
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.
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.
Petrographic, XRD, EDS, SEM and SIMS microanalysis demonstrate that Pleistocene Magadi cherts formed primarily from the precipitation of an amorphous siliceous gel and occasionally from hydrous ...sodium silicates, such as magadiite or kenyaite. Brine simulations and oxygen isotope analysis support the formation of these cherts from highly evaporated brines at modern lake temperatures near surface.Fossils within chert, including, microorganisms, diatoms, gastropods, ostracods, and plant and insect fragments provide valuable paleoenvironmental information, including the presence of alkaliphilic fungal communities found in chert. Identifiable crystal habits in evaporite pseudomorphs of trona and gaylussite suggest saline conditions were present throughout much of the basin history.Labyrinth patterns discovered in Magadi cherts indicate the precipitation of silica gels in the saline-alkaline Lake Magadi system near the surface in conjunction with chert formation. Hele-Shaw cell experiments conducted to replicate Magadi labyrinth patterns, however some of these patterns in nature occurred along non-horizontal planes. The orientations of the Magadi patterns imply invasion of air from above if confined to open cracks or infiltration from all sides if the gels dried and cracked in the vadose zone. Cherts developed from siliceous gel precursors have elevated d18O values, from +41‰ to +47‰ indicating the formation in the presence of brines. Such brines, with total dissolved solids of ~ 220,000 to 300,000 mg/L, exist in the modern Magadi basin during dry periods. Cherts with magadiite and other hydrous sodium silicates precursors form brines with lower salinities than cherts formed from gels. Cementing chalcedony and megaquartz crystals in chert have lower d18O values that require formation from waters at elevated temperatures, which suggests a hydrothermal origin. Finally, siliceous mudstone cherts show relatively wide ranges of d18O, from +37‰ to +46‰, which suggests formation from subsurface brines.Based on this new information, we have further classified the origins of chert in Lake Magadi, Kenya and extended the presence of a high salinity paleoenvironment to ~500 ka before present.
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