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  • 1
    Electronic Resource
    Electronic Resource
    Amsterdam : Elsevier
    Journal of Human Evolution 20 (1991), S. 67-75 
    ISSN: 0047-2484
    Source: Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics: Biology
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Amsterdam : Elsevier
    Journal of Human Evolution 20 (1991), S. 67-75 
    ISSN: 0047-2484
    Source: Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics: Biology
    Type of Medium: Electronic Resource
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  • 3
  • 4
    Publication Date: 1997-10-14
    Print ISSN: 0027-8424
    Electronic ISSN: 1091-6490
    Topics: Biology , Medicine , Natural Sciences in General
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  • 5
    Publication Date: 2015-12-24
    Description: The transport of moisture in the tropics is a critical process for the global energy budget and on geologic timescales, has markedly influenced continental landscapes, migratory pathways, and biological evolution. Here we present a continuous, first-of-its-kind 1.3-My record of continental hydroclimate and lake-level variability derived from drill core data from...
    Print ISSN: 0027-8424
    Electronic ISSN: 1091-6490
    Topics: Biology , Medicine , Natural Sciences in General
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  • 6
    Publication Date: 2014-11-04
    Description: Late Cenozoic climate history in Africa was punctuated by episodes of variability, characterized by the appearance and disappearance of large freshwater lakes within the East African Rift Valley. In the Baringo-Bogoria basin, a well-dated sequence of diatomites and fluviolacustrine sediments documents the precessionally forced cycling of an extensive lake system between 2.70 Ma and 2.55 Ma. One diatomite unit was studied, using the oxygen isotope composition of diatom silica combined with X-ray fluorescence spectrometry and taxonomic assemblage changes, to explore the nature of climate variability during this interval. Data reveal a rapid onset and gradual decline of deepwater lake conditions, which exhibit millennial-scale cyclicity of ~1400–1700 yr, similar to late Quaternary Dansgaard-Oeschger events. These cycles are thought to reflect enhanced precipitation coincident with increased monsoonal strength, suggesting the existence of a teleconnection between the high latitudes and East Africa during this period. Such climatic variability could have affected faunal and floral evolution at the time.
    Print ISSN: 0091-7613
    Electronic ISSN: 1943-2682
    Topics: Geosciences
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  • 7
    Publication Date: 2013-08-02
    Description: The Indian Peak–Caliente caldera complex and its surrounding ignimbrite field were a major focus of explosive silicic activity in the eastern sector of the subduction-related southern Great Basin ignimbrite province during the middle Cenozoic (36–18 Ma) ignimbrite flareup. Caldera-forming activity migrated southward through time in response to rollback of the subducting lithosphere. Nine partly exposed, separate to partly overlapping source calderas and an equal number of concealed sources compose the Indian Peak–Caliente caldera complex. Calderas have diameters to as much as 60 km and are filled with as much as 5000 m of intracaldera tuff and wall-collapse breccias. More than 50 ignimbrite cooling units, including 22 of regional (〉100 km 3 ) extent, are distinguished on the basis of stratigraphic position, chemical and modal composition, 40 Ar/ 39 Ar age, and paleomagnetic direction. The most voluminous ash flows spread as far as 150 km from the caldera complex across a high plateau of limited relief—the Great Basin altiplano, which was created by late Paleozoic through Mesozoic orogenic deformation and crustal thickening. The resulting ignimbrite field covers a present area of ~60,000 km 2 in east-central Nevada and southwestern Utah. Before post-volcanic extension, ignimbrites had an estimated aggregate volume of ~33,000 km 3 . At least seven of the largest cooling units were produced by super-eruptions of more than 1000 km 3 . The largest, at 5900 km 3 , originally covered an area of 32,000 km 2 to outflow depths of hundreds of meters. Outflow ignimbrite sequences comprise as many as several cooling units from different sources with an aggregate thickness locally reaching a kilometer; sequences are almost everywhere conformable and lack substantial intervening erosional debris and angular discordances, thus manifesting a lack of synvolcanic crustal extension. Fallout ash in the mid-continent is associated with two of the super-eruptions. Ignimbrites are mostly calc-alkalic and high-K, a reflection of the unusually thick crust in which the magmas were created. They have a typical arc chemical signature and define a spectrum of compositions that ranges from high-silica (78 wt%) rhyolite to andesite (61 wt% silica). Rhyolite magmas were erupted in relatively small volumes more or less throughout the history of activity, but in a much larger volume after 24 Ma in the southern part of the caldera complex, creating ~10,000 km 3 of ignimbrite. The field has some rhyolite ignimbrites, the largest of which are in the south and were emplaced after 24 Ma. But the most distinctive attributes of the Indian Peak–Caliente field are two distinct classes of ignimbrite: 1. Super-eruptive monotonous intermediates. More or less uniform and unzoned deposits of dacitic ignimbrite that are phenocryst rich (to as much as ~50%) with plagioclase 〉 biotite quartz hornblende 〉 Fe-Ti oxides ± sanidine, pyroxene, and titanite; apatite and zircon are ubiquitous accessory phases. These tuffs were deposited at 31.13, 30.06, and 29.20 Ma in volumes of 2000, 5900, and 4400 km 3 , respectively, from overlapping, multicyclic calderas. A unique, and possibly kindred, phenocryst-rich latite-andesite ignimbrite with an outflow volume of 1100 km 3 was erupted at 22.56 Ma from a concealed source caldera to the south. 2. Trachydacitic Isom-type tuffs. Also relatively uniform but phenocryst poor (〈20%) with plagioclase 〉〉 clinopyroxene orthopyroxene Fe-Ti oxides 〉〉 apatite. These alkali-calcic tuffs are enriched in TiO 2 , K 2 O, P 2 O 5 , Ba, Nb, and Zr and depleted in CaO, MgO, Ni, and Cr, and have an arc chemical signature. Magmas were erupted from a concealed source immediately after and just to the southeast of the multicyclic monotonous intermediates. Most of their aggregate outflow volume of 1800 km 3 was erupted from 27.90 to 27.25 Ma. Nothing like this couplet of distinct ignimbrites, in such volumes, have been documented in other middle Cenozoic volcanic fields in the southwestern U.S. where the ignimbrite flareup is manifest. Magmas were created in unusually thick crust (as thick as 70 km) where large-scale inputs of mantle-derived basaltic magma powered partial melting, assimilation, mixing, and differentiation processes. Dacite and some rhyolite ignimbrites were derived from relatively low-temperature (700–800 °C), water-rich magmas that were a couple of log units more oxidized than the quartz-fayalite-magnetite (QFM) oxygen buffer at depths of ~8–12 km. In contrast to these "main-trend" magmas, trachydacitic Isom-type magmas were derived from drier and hotter (~950 °C) magmas originating deeper in the crust (to as deep as 30 km) by fractionation processes in andesitic differentiates of the mantle magma. "Off-trend" rhyolitic magmas that are both younger and older than the Isom type but possessed some of their same chemical characteristics possibly reflect an ancestry involving Isom-type magmas as well as main-trend rhyolitic magmas. Andesitic lavas extruded during the flareup but mostly after 25 Ma constitute a roughly estimated 12% of the volume of silicic ignimbrite, in contrast to major volcanic fields to the east, e.g., the Southern Rocky Mountain field, where the volume of intermediate-composition lavas exceeds that of silicic ignimbrites.
    Electronic ISSN: 1553-040X
    Topics: Geosciences
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  • 8
    Publication Date: 2018
    Description: 〈span〉〈div〉Abstract〈/div〉The last half-million years of geological history of the East African Rift in southern Kenya is recorded by the Oltulelei Formation, which is newly described in this paper. Well-dated depositional cycles in this formation reveal how tectonics and climate shaped rift valley sedimentation, as well as the landscapes inhabited by human ancestors. The Oltulelei Formation (ca. 320 to ca. 36 ka) overlies the Olorgesailie Formation (1.2–0.5 Ma) on an erosional unconformity and is divided into three successive units, the Olkesiteti, Oltepesi, and Tinga Members. The stratotype for the formation is exposed in the Olorgesailie Basin, north and northwest of Mount Olorgesailie. The 〈sup〉40〈/sup〉Ar/〈sup〉39〈/sup〉Ar dates on interbedded tephras provide geochronological controls on the timing of three major depositional cycles. Lateral variability in the Oltulelei Formation shows that the Olorgesailie Basin and the northern Koora Graben formed three rift subbasins that were periodically connected and disconnected over the past 500 k.y. Erosional phases removed large volumes of sediment from the Olorgesailie Basin, requiring through-flowing drainage to a lower base level southwest and south of Mount Olorgesailie. Aggradational phases were primarily fluvial, with siliciclastic and volcaniclastic sediment filling valleys and burying dissected landscapes. Tufa, shallow lacustrine, and wetlands deposits also occur and indicate elevated water tables in fault-controlled sumps or blocked drainages. We propose that connections and disconnections between rift segments were driven by tectonic forces, primarily faulting of volcanic basement rocks and periodic influxes of volcanic sediment, combined with climatic conditions that enabled erosion and transport of large volumes of sediment into and out of the subbasins. The Oltulelei Formation preserves a record of a dynamic change in the physical landscape of the southern Kenya Rift, with major shifts in erosion versus deposition on time scales of 10〈sup〉4〈/sup〉–10〈sup〉5〈/sup〉 yr. This study provides new, well-calibrated information on sedimentation in active rift settings as well as an outcrop-based, three-dimensional, basin-scale geological history that can be integrated with emerging drill-core paleoclimate records from southern Kenya. The stratigraphic record preserved in the Oltulelei Formation advances understanding and poses new questions about how tectonics and climate shaped middle to late Pleistocene faunal turnover and the transition in southern Kenya from Acheulean to Middle Stone Age technology.〈/span〉
    Print ISSN: 0016-7606
    Electronic ISSN: 1943-2674
    Topics: Geosciences
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  • 9
    Publication Date: 2018-04-06
    Description: Development of the African Middle Stone Age (MSA) before 300,000 years ago raises the question of how environmental change influenced the evolution of behaviors characteristic of early Homo sapiens . We used temporally well-constrained sedimentological and paleoenvironmental data to investigate environmental dynamics before and after the appearance of the early MSA in the Olorgesailie basin, Kenya. In contrast to the Acheulean archeological record in the same basin, MSA sites are associated with a markedly different faunal community, more pronounced erosion-deposition cycles, tectonic activity, and enhanced wet-dry variability. Aspects of Acheulean technology in this region imply that, as early as 615,000 years ago, greater stone material selectivity and wider resource procurement coincided with an increased pace of land-lake fluctuation, potentially anticipating the adaptability of MSA hominins.
    Keywords: Anthropology
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Geosciences , Computer Science , Medicine , Natural Sciences in General , Physics
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  • 10
    Publication Date: 2018-04-06
    Description: The origin of the Middle Stone Age (MSA) marks the transition from a highly persistent mode of stone toolmaking, the Acheulean, to a period of increasing technological innovation and cultural indicators associated with the evolution of Homo sapiens . We used argon-40/argon-39 and uranium-series dating to calibrate the chronology of Acheulean and early MSA artifact–rich sedimentary deposits in the Olorgesailie basin, southern Kenya rift. We determined the age of late Acheulean tool assemblages from 615,000 to 499,000 years ago, after which a large technological and faunal transition occurred, with a definitive MSA lacking Acheulean elements beginning most likely by ~320,000 years ago, but at least by 305,000 years ago. These results establish the oldest repository of MSA artifacts in eastern Africa.
    Keywords: Anthropology, Geochemistry, Geophysics
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Geosciences , Computer Science , Medicine , Natural Sciences in General , Physics
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