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Significance Understanding the evolution, dispersals, behaviors, and ecologies of early African Homo sapiens requires accurate geochronological placement of fossils and artifacts. We introduce open-air occurrences of such remains in sediments of the Middle Awash study area in Ethiopia. We describe the stratigraphic and depositional contexts of our discoveries and demonstrate the effectiveness of recently developed uranium-series dating of ostrich eggshell at validating and bridging across more traditional radioisotopic methods (14C and 40Ar/39Ar). Homo sapiens fossils and associated Middle Stone Age artifacts are placed at >158 and ∼96 ka. Later Stone Age occurrences are dated to ∼21 to 24 ka and ∼31 to 32 ka, firmly dating the upper portion of one of the longest records of human evolution., The Halibee member of the Upper Dawaitoli Formation of Ethiopia’s Middle Awash study area features a wealth of Middle and Later Stone Age (MSA and LSA) paleoanthropological resources in a succession of Pleistocene sediments. We introduce these artifacts and fossils, and determine their chronostratigraphic placement via a combination of established radioisotopic methods and a recently developed dating method applied to ostrich eggshell (OES). We apply the recently developed 230Th/U burial dating of OES to bridge the temporal gap between radiocarbon (14C) and 40Ar/39Ar ages for the MSA and provide 14C ages to constrain the younger LSA archaeology and fauna to ∼24 to 21.4 ka. Paired 14C and 230Th/U burial ages of OES agree at ∼31 ka for an older LSA locality, validating the newer method, and in turn supporting its application to stratigraphically underlying MSA occurrences previously constrained only by a maximum 40Ar/39Ar age. Associated fauna, flora, and Homo sapiens fossils are thereby now fixed between 106 ± 20 ka and 96.4 ± 1.6 ka (all errors 2σ). Additional 40Ar/39 results on an underlying tuff refine its age to 158.1 ± 11.0 ka, providing a more precise minimum age for MSA lithic artifacts, fauna, and H. sapiens fossils recovered ∼9 m below it. These results demonstrate how chronological control can be obtained in tectonically active and stratigraphically complex settings to precisely calibrate crucial evidence of technological, environmental, and evolutionary changes during the African Middle and Late Pleistocene.

The Advanced Topographic Laser Altimeter System (ATLAS) was launched aboard the Ice Cloud and land‐Elevation Satellite‐2 (ICESat‐2) satellite in September 2018. ATLAS is a single wavelength (532 nm) lidar system designed to acquire high resolution measurements of the earth's surface while also obtaining atmospheric backscatter from molecules, clouds, and aerosols. Because ATLAS is optimized for altimetry, the atmospheric data acquired is unique in many respects and requires non‐standard analysis techniques. For example, the high repetition rate laser limits the vertical extent of the profiles to just 14 km and causes atmospheric scattering from above 15 km to be added to the scattering in the lower 0–14 km profile. In addition, the limited vertical range of the acquired profiles renders it difficult to compute the magnitude of the solar background and hinders the application of standard calibration techniques. Despite these limitations, methods have been developed to successfully produce data products that have value to the atmospheric community for cloud and aerosol research and are currently available at the National Snow and Ice Data Center (NSIDC). In this paper we describe the ICESat‐2 atmospheric channel and the methods used to process the ATLAS raw photon count data to obtain calibrated backscatter and higher level products such as layer heights and type, blowing snow, column optical depth and apparent surface reflectance.

The Woranso–Mille (WORMIL) area in the west-central Afar, Ethiopia, contains several Pliocene basalt flows, tuffs, and fossiliferous volcaniclastic beds. We present whole-rock major- and trace-element data including REE, and Sr–Nd–Pb isotope ratios from these basalts to characterize the geochemistry, constrain petrogenetic processes, and infer mantle sources. Six basalt groups are distinguished stratigraphically and geochemically within the interval from ~ 3.8 to ~ 3 Ma. The elemental and isotopic data show intra- and inter-group variations derived primarily from source heterogeneity and polybaric crystallization ± crustal inputs. The combined Sr–Nd–Pb isotope data indicate the involvement of three main reservoirs: the Afar plume, depleted mantle, and enriched continental lithosphere (mantle ± crust). Trace element patterns and ratios further indicate the basalts were generated from spinel-dominated shallow melting, consistent with significantly thinned Pliocene lithosphere in western Afar. The on-land continuation of the Aden rift into western Afar during the Pliocene is reexamined in the context of the new geochemistry and age constraints of the WORMIL basalts. The new data reinforce previous interpretations that progressive rifting and transformation of the continental lithosphere to oceanic lithosphere allows for increasing asthenospheric inputs through time as the continental lithosphere is thinned.

The practice of anesthesiology has grown remarkably safer, but complications ranging from minor to catastrophic may still occur. Improvements in patient safety have made surgery and anesthesia both commonplace and routine. The perioperative process, which classically began with admission to the hospital the night before the day of surgery, has been condensed into the efficient and increasingly cost-effective process of outpatient surgery. Patient confidence and comfort with this process has much to do with openness and communication on behalf of anesthesiologists and surgeons which did not always exist.

The White River Ash, a prominent stratigraphic marker bed in Alaska (USA) and Yukon (Canada), consists of multiple compositional units belonging to two geochemical groups. The compositional units are characterized using multiple criteria, with combined glass and ilmenite compositions being the best discriminators. Two compositional units compose the northern group (WRA-Na and WRA-Nb), and two units are present in the eastern group (WRA-Ea and the younger, WRA-Eb). In the proximal area, the ca. 1900 yr B.P. (Lerbekmo et al., 1975) WRA-Na displays reverse zoning in the glass phase and systematic changes in ilmenite composition and estimated oxygen fugacity from the base to the top of the unit. The eruption probably tapped different magma batches or bodies within the magma reservoir with limited mixing or mingling between them. The 1147 cal yr B.P. (calibrated years, approximately equivalent to calendric years) (Clague et al., 1995) WRA-Ea eruption is only weakly zoned, but pumices with different glass compositions are present, along with gray and white intermingled glass in individual pumice clasts, indicating the presence of multiple magmatic bodies or layers. All White River Ash products are high-silica adakites and are sourced from the Mount Churchill magmatic system.

[1] Plagioclase and spinel lherzolite thermometry and barometry are applied to an extensive geochemical dataset of young (

Beginning in the 1960s, geological and paleoanthropological exploration of the Ethiopian rift system’s basins have led to the discovery and assembly of the most comprehensive record of human biological and technological change during the last 6 million years. The hominid fossils, including partial skeletons, were primarily discovered in the Afar Rift, the Main Ethiopian Rift, and in the Omo Basin of the broadly rifted zone of SW Ethiopia. The paleoanthropological research areas within the SW Afar Rift that have yielded many diverse hominid species and the oldest stone tools are, from north to south, Woranso-Mille (aff. Ardipithecus and Au. afarensis), Hadar (Au. afarensis, Homo sp.), Dikika (Au. afarensis), Gona (Ar. kadabba, Ar. ramidus, H. erectus, and oldest stone tools), Middle Awash (Ar. kadabba, Ar. ramidus, Au. anamensis, Au. afarensis, Au. garhi, H. erectus, H. rhodesiensis, H. sapiens idaltu, and the oldest paleo-butchery locality), and Galili (Au. afarensis). Additional hominid remains were discovered at Melka Kunture on the banks of the Awash River near its source along the western margin of the central part of the Main Ethiopian Rift (H. erectus), at Konso (H. erectus and A. boisei), and at the southern end of the MER, and in the Omo Basin (Au. anamensis, Au. afarensis, Au. aethiopicus, Au. boisei, H. habilis, and H. erectus). Distal and sometimes proximal tephra units interbedded within fossilifeous sedimentary deposits have become key elements in this work by providing chronological and correlative control and depositional contexts. Several regional tephra markers have been identified within the northern half of the eastern African rift valley in Ethiopia and Kenya, and in marine sediments of the Gulf of Aden Rift and the NW Indian Ocean. Out of the many regional tephra stratigraphic markers that range in age from the early Pliocene (3.97 Ma) to the late Pleistocene (0.16 Ma), the Sidi Hakoma Tuff (SHT) has been more widely identified and thoroughly characterized than any of the others. An age of 3.446 ± 0.041 Ma was determined on the SHT according to the most recent calibration, and it is the only regional stratigraphic marker whose source has been traced to a buried caldera in the central sector of the Main Ethiopian Rift. This paper describes new SHT occurrences and presents chemical and chronological results in the context of a broader review of the importance of this key marker. Moreover, the geographic distributions, probable dispersal mechanisms, and importance of regional tephra units in determining the tectonic and sedimentological processes in the different rift basins of the eastern African rift valleys are considered.

The 3.119 ± 0.010 Ma Chefe Donsa phreatomagmatic deposits on the shoulder of the Ethiopian Rift mark the northern termination of the Silti-Debre Zeyit Fault Zone, a linear zone of focused extension within the modern Ethiopian Rift. These peralkaline pumice fragments and glass shards span a wide range of glass compositions but have a restricted phenocryst assemblage dominated by unzoned sanidine. Glass shards found within the ash occupy a far more limited compositional range (75–76 wt% SiO2) in comparison with the pumice (64–75 wt% SiO2), which is rarely mingled. Thermodynamic modeling shows that liquids broadly similar to the least evolved glass composition can be achieved with 50–60 % fractionation of moderately crustally contaminated basalt. Inconsistencies between modeled solutions and the observed values of CaO and P2O5 highlight the important role of fluorine in stabilizing fluor-apatite and the limitations of current thermodynamic models largely resulting from the scarce experimental data available for the role of fluorine in igneous phase stability. On the basis of limited feldspar heterogeneity and crystal content of pumice at Chefe Donsa, and the difficulties of extracting small volumes of Si-rich melt in classical fractional crystallization models, we suggest a two-step polybaric process: (1) basaltic magma ponds at mid-upper-crustal depths and fractionates to form a crystal/magma mush. Once this mush has reached 50–60 % crystallinity, the interstitial liquid may be extracted from the rigid crystal framework. The trachytic magma extracted at this step is equivalent to the most primitive pumice analyzed at Chefe Donsa. (2) The extracted trachytic liquid will rise and continue to crystallize, generating a second mush zone from which rhyolite liquids may be extracted. Some of the compositional range observed in the Chefe Donsa deposits may result from the fresh intrusion of trachyte magma, which may also provide an eruption trigger. This model may have wider application in understanding the origin of the Daly Gap in Ethiopian magmas—intermediate liquids may not be extracted from crystal-liquid mushes due to insufficient crystallization to yield a rigid framework. The wide range of glass compositions characteristic of the proximal Chefe Donsa deposits is not recorded in temporally equivalent tephra deposits located in regional depocenters. Our results show that glass shards, which represent the material most likely transported to distal depocenters, occupy a limited compositional range at high SiO2 values and overlap some distal tephra deposits. These results suggest that distal tephra deposits may not faithfully record the potentially wide range in magma compositions present in a magmatic system just prior to eruption and that robust distal–proximal tephra correlations must include a careful analysis of the full range of materials in the proximal deposit.

The mid-Miocene Santa Rosa–Calico volcanic field (SC) of northern Nevada provides an outstanding example of the role open-system magmatic processes play in producing calc-alkaline and tholeiitic andesite–dacite magmas in an intracontinental setting. SC volcanism commenced at ∼ 16.7 Ma and is associated with the initial manifestations of the Yellowstone hotspot, the Columbia River–Steens flood basalt event(s), and the formation of the Northern Nevada rift. Locally a diverse package of magmatic products ranging from tholeiitic basalt to high-Si rhyolite was produced during an ∼ 2 myr duration. Within this package are the products of at least four distinct intermediate composition magmatic systems that may represent as much as 40% of the SC volcanic pile. These help differentiate the SC from contemporaneous Oregon Plateau volcanic fields (e.g. McDermitt, Lake Owyhee, Northwest Nevada) that are dominated by bimodal basalt–rhyolite assemblages. All SC intermediate units are characterized by textural and mineralogic complexities including xenoliths and xenocrysts of local crust and crystal clots of plagioclase ± clinopyroxene ± orthopyroxene ± oxide. SC intermediate units are dominantly tholeiitic, but include lava flows with transitional to calc-alkaline affinities. Relative to locally erupted Steens Basalt, SC intermediate lava flows have similar elemental enrichments and depletions, but dissimilar Sr and Nd isotopic compositions. These isotopic differences, coupled with the abundant disequilibrium features and variable incompatible element ratios, indicate that open system magmatic processes played a major role in the genesis of the intermediate units. SC silicic magmas were produced primarily via upper crustal melting of chemically and isotopically heterogeneous Cretaceous granitoid. Interaction between fractionating mafic Steens flood basalt magmas and the more evolved crustal melts ± assimilation of local upper crust provides a general template for the generation of SC tholeiitic and calc-alkaline intermediate magmas. Between system heterogeneities primarily reflect magma- versus assimilation-dominated mixing. These complex processes and the resulting production and eruption of SC intermediate composition melts were stimulated by incipient, focused lithospheric extension and driven by the virtually continuous local availability of upwelling Steens Basalt, over at least a 2 myr duration. These results demonstrate that at least some of the mid-Miocene and younger calc-alkaline magmas that are found across the Pacific Northwest and are unrelated to subduction processes, were formed as a direct result of open-system processes affecting regional flood basalt magmas, rather than just melting of previously enriched (via subduction) lithospheric mantle as suggested by previous workers.

Sheep Creek tephra (SCt) consists of inflated, rhyolitic pumice with abundant plagioclase and hornblende, lesser amounts of ferrian ilmenite and magnetite, and trace quantities of basaltic hornblende, augite, quartz, apatite, and zircon. It has an adakitic composition; the glass has low Y, Yb, and high La/Yb and Sr/Y values. Previously, compositional differences between samples were known to exist and thought to be due to derivation from a compositionally zoned magma involving several closely spaced eruptions. SCt was recognized as a useful stratigraphic marker across central Alaska and western Yukon with an age of ∼190 ka. New OSL dates on a SCt occurrence in western Yukon are ∼80 ka and force a reassessment of the earlier work. SCt is now known to be made up of several stratigraphic units: SCt-F (Fairbanks, Alaska; ∼200 ka), SCt-CC (Canyon Creek, Alaska), and in the western Yukon, SCt-C, SCt-K (∼80 ka), and SCt-A. Beds C, K, and A must be similar in age because of their close stratigraphic association at Ash Bend in Yukon. Compositional and temporal controls, including Sr isotopes, suggest Mount Drum in the Wrangell volcanic field as the source of the SCt beds. Revision of the age of the SCt beds in western Yukon means that the Reid Glaciation of Yukon is likely of MIS 6 age and probably correlates with the Delta Glaciation of Alaska. Further, the buried forest bed just below SCt-K in the Klondike goldfields must be of MIS 5 age, not MIS 7, as previously thought.

This study focuses on Middle Miocene tholeiitic flood basalt lava flows from the Oregon Plateau, northwestern USA (Steens Basalt), and is the first to comprehensively document and evaluate their morphology. Field observations of flows from several sections within and proximal to the main exposures at Steens Mountain have been supplemented with textural and geochemical data, and are used to offer preliminary insights into their emplacement. Compound pahoehoe flows of variable thickness appear to be common throughout the study area, laterally and vertically. These tend to be plagioclase phyric and the morphology and disposition of constituent flow lobes are quite similar to those from other provinces such as Hawaii and the Snake River Plain. Classic a’a flows with brecciated upper and basal crusts are not abundant, but by no means uncommon. Flows with characters different from typical pahoehoe and a’a are also common. Such flows display a range in morphology; flows with preserved upper crusts but brecciated basal crusts, as well as those displaying well-developed flow-top breccias and preserved basal crusts (rubbly pahoehoe) are observed. The Steens Basalt appears to display greater morphological and textural diversity at the outcrop scale than that described for some other flood basalt provinces. The abundant compound pahoehoe flows (often rich in plagioclase phenocrysts) were likely emplaced during slow but sustained eruptive episodes; their constituent lobes show clear evidence for endogenous growth. The relatively aphyric flows with brecciated surfaces (including a’a) hint at higher strain rates and/or higher viscosity, probably caused by higher effusion rates. A couple of sections are characterized by compositionally similar, but morphologically different flows that were possibly part of the same eruption. While differences in pre-eruptive topography could explain this, it is also possible that certain physical parameters changed substantially and abruptly during eruption and that such changes were accompanied by differentiation processes within the plumbing system. It is possible that such observations indicate temporal fluctuations within complex magmatic and eruptive systems, and deserve closer scrutiny.

The Santa Rosa-Calico volcanic field (SC) of northern Nevada is a complex, multi-vent mid-Miocene eruptive complex that formed in response to regional lithospheric extension and flood basalt volcanism. Santa Rosa-Calico volcanism initiated at ∼16.7 Ma, concurrent with regional Steens-Columbia River flood basalt activity and is characterized by a complete compositional spectrum of basalt through high-silica rhyolite. To better understand the relationships between upwelling mafic magmatism, coeval extension, and magmatic system development on the Oregon Plateau we have conducted the first compre- hensive study of Santa Rosa-Calico silicic volcanism. Detailed stratigraphic-based field sampling and mapping illustrate that silicic activity in this volcanic field was primarily focused along its eastern and western margins. At least five texturally distinct silicic units are found in the western Santa Rosa-Calico volcanic field, including abun- dant lava flows, near vent deposits, and shallow intrusive bodies. Similar physical features are found in the eastern portion of the volcanic field where four physically distinct units are present. The western and eastern Santa Rosa- Calico units are characterized by abundant macro- and microscopic disequilibrium textures, reflecting a complex petrogenetic history. Additionally, unlike other mid-Mio- cene Oregon Plateau volcanic fields (e.g. McDermitt), the Santa Rosa-Calico volcanic field is characterized by a paucity of caldera-forming volcanism. Only the Cold Springs tuff, which crops out across the central portion of the volcanic field, was caldera-derived. Major and trace element geochemical variations are present within and between eastern and western Santa Rosa-Calico silicic units and these chemical differences, coupled with the observed disequilibrium textures, illustrate the action of open-system petrogenetic processes and melt derivation from heterogeneous source materials. The processes and styles of Santa Rosa-Calico silicic magmatism are linked to three primary factors, local focusing of and thermal and material contributions from the regional flood basalt event, lithospheric extension within the northern portion of the Northern Nevada rift, and interaction of mid-Miocene silicic magmas with pre-Santa Rosa-Calico lithosphere. Similar processes and styles of mid-Miocene silicic volcanism likely occurred across the Oregon Plateau in regions characterized by both focused lithospheric exten- sion and localized mafic magmatism.

The timing and petrogenesis of mid-Miocene flood basalt volcanism in the northwest United States has been extensively addressed, yet the chemical characteristics and temporal details of the Steens Basalt, exposed on the Oregon Plateau, are poorly defined. Steens Basalt volcanism has generally been accepted to have occurred at ∼16.6 Ma, coeval and/or just prior to the onset of Columbia River Basalt Group volcanism to the north. New major and trace element analyses and nine 40 Ar/ 39 Ar ages ranging from 15.51±0.28 to 16.58±0.18 Ma were obtained on Oregon Plateau flood basalt lava flows from stratigraphic sections in close proximity to Steens Mountain. Additionally, new 40 Ar/ 39 Ar ages were obtained on the uppermost and thirty-first lava flow down from the top of the ∼1 km section of Steens Basalt exposed at Steens Mountain and yield eruption ages of 16.59±0.10 and 16.55± 0.10 Ma, respectively. Field relations between these basalt sections suggest that multiple eruptive centers were present in the vicinity of Steens Mountain. The chemical and chronologic data presented in this study illustrate that flood basalts with the “Steens Basalt” chemical signature erupted across the southern Oregon Plateau over a much greater timespan than what is typically quoted for the Steens Mountain type section. These data suggest that the main volume of Steens Basalt volcanism erupted over at least an ∼1 m.y. duration from eruptive loci in the vicinity of Steens Mountain, while likely much less voluminous volcanism and lithospheric input of mafic magma appears to have occurred for N2 Ma across the Oregon Plateau. These new geochemical and geochronological constraints verify a common temporal link between Steens, Malheur Gorge-region, and Columbia River Basalt Group (CRBG) volcanism. This direct temporal link requires that petrogenetic and tectonic models of mid-Miocene northwestern U.S. flood basalt volcanism recognize that the northern (Columbia Plateau) and southern (Oregon Plateau) portions of this mid-Miocene basalt province were erupting simultaneously during portions of the regional event. © 2007 Elsevier B.V. All rights reserved.

A substantial revision to the age of the Chorora Formation, Ethiopia, constraining the deposits to around 8 million years old and forming a revised age constraint for the human–gorilla lineage split. The Chorora Formation, at the southern margin of the Afar rift in Ethiopia, has been regarded as providing a fossil record approximately 10.5 million years old. In 2007, Gen Suwa et al. reported the discovery of a fossil relative of the gorilla in the Chorora Formation. Thought to have been between 10 and 10.5 million years old, Chororapithecus abyssinicus was seen a primitive member of the gorilla clade. Now Suwa and colleagues report new field observations, geochemical, magnetostratigraphic and radioisotopic data, consistent with a substantial revision of the age of the Chorora Formation to around 8 million years old. This lifts Chororapithecus from a time when apes were common in Eurasia, to one in which evidence for fossil apes is scarce. The attribution to the gorilla lineage looks all the more important as it helps constrain the split between gorillas and the lineage leading to hominins and chimpanzees, and suggests that this split occurred in Africa. The palaeobiological record of 12 million to 7 million years ago (Ma) is crucial to the elucidation of African ape and human origins, but few fossil assemblages of this period have been reported from sub-Saharan Africa. Since the 1970s, the Chorora Formation, Ethiopia, has been widely considered to contain ~10.5 million year (Myr) old mammalian fossils1,2,3,4,5,6,7. More recently, Chororapithecus abyssinicus, a probable primitive member of the gorilla clade6, was discovered from the formation. Here we report new field observations and geochemical, magnetostratigraphic and radioisotopic results that securely place the Chorora Formation sediments to between ~9 and ~7 Ma. The C. abyssinicus fossils are ~8.0 Myr old, forming a revised age constraint of the human–gorilla split. Other Chorora fossils range in age from ~8.5 to 7 Ma and comprise the first sub-Saharan mammalian assemblage that spans this period. These fossils suggest indigenous African evolution of multiple mammalian lineages/groups between 10 and 7 Ma, including a possible ancestral-descendent relationship between the ~9.8 Myr old Nakalipithecus nakayamai8 and C. abyssinicus. The new chronology and fossils suggest that faunal provinciality between eastern Africa and Eurasia had intensified by ~9 Ma, with decreased faunal interchange thereafter9,10,11,12. The Chorora evidence supports the hypothesis of in situ African evolution of the Gorilla–Pan–human clade, and is concordant with the deeper divergence estimates of humans and great apes based on lower mutation rates of ~0.5 × 10−9 per site per year (refs 13, 14, 15).

Developing thymocytes undergo maturation while migrating through the thymus and ultimately emigrate from the organ to populate peripheral lymphoid tissues. The process of thymic emigration is controlled in part via receptor-ligand interactions between the chemokine stromal-derived factor (SDF)-1, and its cognate receptor CXCR4, and sphingosine 1-phosphate (S1P) and its receptor S1PR. The precise mechanism by which S1P/S1PR and CXCR4/SDF-1 contribute to thymic emigration remains unclear. We proposed that S1P-dependent and -independent mechanisms might coexist and involve both S1P-induced chemoattraction and SDF-1-mediated chemorepulsion or fugetaxis of mature thymocytes. We examined thymocyte emigration in thymi from CXCR4-deficient C57BL/6 embryos in a modified assay, which allows the collection of CD62Lhigh and CD69low recent thymic emigrants. We demonstrated that single-positive (SP) CD4 thymocytes, with the characteristics of recent thymic emigrants, failed to move away from CXCR4-deficient fetal thymus in vitro. We found that the defect in SP CD4 cell emigration that occurred in the absence of CXCR4 signaling was only partially overcome by the addition of the extrathymic chemoattractant S1P and was not associated with abnormalities in thymocyte maturation and proliferative capacity or integrin expression. Blockade of the CXCR4 receptor in normal thymocytes by AMD3100 led to the retention of mature T cells in the thymus in vitro and in vivo. The addition of extrathymic SDF-1 inhibited emigration of wild-type SP cells out of the thymus by nullifying the chemokine gradient. SDF-1 was also shown to elicit a CXCR4-dependent chemorepellent response from fetal SP thymocytes. These novel findings support the thesis that the CXCR4-mediated chemorepellent activity of intrathymic SDF-1 contributes to SP thymocyte egress from the fetal thymus.

The 200-m-thick fossiliferous Konso Formation and overlying terrace deposits, which crop out at the end of the southern sector of the Main Ethiopian Rift (MER), contain more than 30 distal tephra layers. Local and regional tephra correlations of more than 20 tephra units were established using major and trace element data of discrete and purified bulk glass samples within the Konso study area. Eleven correlative marker tuffs were recognized in stratigraphic sections of both the Konso Formation and the Omo-Turkana Basin sediments in southern Ethiopia and northern Kenya. The Turoha, Hope, Ivory, Bright White, and Boleshe Tuffs in the Konso Formation, and the Upper White Tuff in the overlying terrace deposits are securely correlated with the KBS, Akait, Lokapetamoi, Chari, Lower Nariokotome, and Silbo Tuffs of the Omo-Turkana Basin, using least mobile major elements (CaO, Fe2O3*, and TiO2) and geochronology. Preliminary correlations are also suggested between the Konso Formation distal tephra and proximal units of the Quaternary caldera-forming silicic centers in the central sector of the MER. The strongly peralkaline tuffs of the Konso Formation are compositionally similar to proximal eruptions mostly located along the Quaternary axial rift zone of the southern, central, and northern sectors of the MER. The tephra correlation provides information about the temporal and spatial features of the volcanic and tectonic processes recorded in the evolving basins. Thickness and sedimentation rate were determined for both the Konso Formation and the Omo-Turkana Basin sections, measured between the Turoha (=KBS; 1.91 ± 0.03 Ma) and the Bright White (=Chari; 1.40 ± 0.02 Ma) Tuffs. Although the lithostratigraphic sequence at the Konso study area is younger, sedimentation rate within the Konso Formation was comparable to that of the Koobi Fora Formation, higher in the Nachukui Formation, and lower in the Shungura Formation. Local and regional differences in thickness and sedimentation rates within the Plio–Pleistocene deposits of the Konso, Shungura, Nachukui, and Koobi Fora Formations may be attributed to erosion associated with increased topographic gradients maintained by continued subsidence mostly along axial fault zones of basins and uplift along the margins of the broadly rifted zone of the Omo-Turkana Basin and the southern sector of the MER.

We report geochemical data for nearly 400 new volcanic samples from the

The Middle Awash paleontological study area, located in the Afar Rift of Ethiopia, has yielded fossils spanning the last six million years. The geology and geochronology of the Mio-Pliocene sites of the study area have been refined and a reliable chronostratigraphy has been established by 40 Ar/ 39 Ar radiometric dating. The latest Miocene Adu-Asa Formation is divided into four members distinguished from each other by silicic and basaltic tuff marker horizons, most of which are dated basaltic tuffs. Radiometric dating has constrained the age of the Adu-Asa Formation to between 5.2-5.8 Ma. These dates are also supported by paleomagnetic results and biochronology. More than 2,000 fossil specimens were collected from the Adu-Asa Formation between 1992 and 2000. These fossils document 64 mammalian species belonging to 32 genera, 23 families, and 8 orders. This assemblage includes a number of new taxa. Included in the assemblage are First and Last Appearance Datums (FADs and LADs) of some groups, including the earliest record of the hominid genus Ardipithecus . Most of the taxa indicate a predominance of mesic and wooded habitat during the deposition of the Adu-Asa Formation. In these deposits, colobines, viverrids, mustelids, bovines, boselaphines, and tragelaphines are abundant, whereas alcelaphines are absent. Quantitative analyses of biogeographic relationships of the Middle Awash Late Miocene (MALM) mammalian fauna indicate stronger relationships with other African sites than with faunas from Eurasian sites. The MALM deposits have generated a critical dataset for analytic work on past environments, biogeographic relationships, and African vertebrate evolution. Moreover, the geographic position of the Middle Awash, coupled with its precise calibration and chronological span, make it a key section for interpreting latest Miocene faunal interchanges between Africa and Eurasia.

The genesis, evolution and fate of Homo erectus have been explored palaeontologically since the taxon's recognition in the late nineteenth century. Current debate is focused on whether early representatives from Kenya and Georgia should be classified as a separate ancestral species ('H. ergaster'), and whether H. erectus was an exclusively Asian species lineage that went extinct. Lack of resolution of these issues has obscured the place of H. erectus in human evolution. A hominid calvaria and postcranial remains recently recovered from the Dakanihylo Member of the Bouri Formation, Middle Awash, Ethiopia, bear directly on these issues. These approximately 1.0-million-year (Myr)-old Pleistocene sediments contain abundant early Acheulean stone tools and a diverse vertebrate fauna that indicates a predominantly savannah environment. Here we report that the 'Daka' calvaria's metric and morphological attributes centre it firmly within H. erectus. Daka's resemblance to Asian counterparts indicates that the early African and Eurasian fossil hominids represent demes of a widespread palaeospecies. Daka's anatomical intermediacy between earlier and later African fossils provides evidence of evolutionary change. Its temporal and geographic position indicates that African H. erectus was the ancestor of Homo sapiens.

The Middle Awash study area of Ethiopia's Afar rift has yielded abundant vertebrate fossils (approximately 10,000), including several hominid taxa. The study area contains a long sedimentary record spanning Late Miocene (5.3-11.2 Myr ago) to Holocene times. Exposed in a unique tectonic and volcanic transition zone between the main Ethiopian rift (MER) and the Afar rift, sediments along the western Afar rift margin in the Middle Awash provide a unique window on the Late Miocene of Ethiopia. These deposits have now yielded the earliest hominids, described in an accompanying paper and dated here to between 5.54 and 5.77 Myr. These geological and palaeobiological data from the Middle Awash provide fresh perspectives on hominid origins and early evolution. Here we show that these earliest hominids derive from relatively wet and wooded environments that were modulated by tectonic, volcanic, climatic and geomorphic processes. A similar wooded habitat also has been suggested for the 6.0 Myr hominoid fossils recently recovered from Lukeino, Kenya. These findings require fundamental reassessment of models that invoke a significant role for global climatic change and/or savannah habitat in the origin of hominids.

The Sagantole Formation comprises more than 200 m of lacustrine, alluvial, and volcaniclastic sediments, plus compositionally bimodal tephras and basaltic lavas, exposed in a domelike horst named the Central Awash Complex in the southwestern Afar rift of Ethiopia. The Sagantole Formation is widely known for abundant vertebrate faunas, including the 4.4 Ma primitive hominid Ardipithecus ramidus . New lithostratigraphic data are used to subdivide the Sagantole Formation into the Kuseralee, Gawto, Haradaso, Aramis, Beidareem, Adgantole, and Belohdelie Members, in ascending order. The members are defined on the basis of lithologic differences and laterally continuous bounding tephras. 40 Ar/ 39 Ar dating of 12 intercalated volcanic units firmly establishes the age of the Sagantole Formation to be 5.6 to 3.9 Ma, significantly older than previous proposals based on erroneous correlations. Magnetostratigraphic data reveal eight paleomagnetic polarity zones, which can be correlated unambiguously with the Thvera, Sidufjall, Nunivak, and Cochiti Subchrons of the Gilbert Chron. Thus, by reference to the geomagnetic polarity time scale, seven additional chronological datums can be placed in the Sagantole Formation. With a total of 19 such datums, the age resolution anywhere in the Sagantole Formation is better than ±100 k.y., making this the best-dated Miocene–Pliocene succession in Africa.

Forty new K-Ar and 40 Ar / 39 Ar isotopic ages from the northern Main Ethiopian Rift (MER)–southern Afar transition zone provide insights into the volcano-tectonic evolution of this portion of the East African Rift system. The earliest evidence of volcanic activity in this region is manifest as 24–23 Ma pre-rift flood basalts. Transition zone flood basalt activity renewed at approximately 10 Ma, and preceded the initiation of modern rift margin development. Bimodal basalt–rhyolite volcanism in the southern Afar rift floor began at approximately 7 Ma and continued into Recent times. In contrast, post-subsidence volcanic activity in the northern MER is dominated by Mio-Pliocene silicic products from centers now covered by Quaternary volcanic and sedimentary lithologies. Unlike other parts of the MER, Mio-Pliocene silicic volcanism in the MER–Afar transition zone is closely associated with fissural basaltic products. The presence of Pliocene age ignimbrites on the plateaus bounding the northern MER, whose sources are found in the present rift, indicates that subsidence of this region was gradual, and that it attained its present physiography with steep escarpments only in the Plio-Pleistocene. Large 7–5 Ma silicic centers along the southern Afar and northeastern MER margins apparently formed along an E–W-oriented regional structural feature parallel to the already established southern escarpment of the Afar. The Addis Ababa rift embayment and the growth of 4.5–3 Ma silicic centers in the Addis Ababa area are attributed to the formation of a major cross-rift structure and its intersection with the same regional E–W structural trend. This study illustrates the episodic nature of rift development and volcanic activity in the MER–Afar transition zone, and the link between this activity and regional structural and tectonic features.

Chemically primitive late Cenozoic tholeiitic basalts from the northwestern U.S.A. have Os-isotopic compositions more radiogenic than observed for most basalts from the ocean basins. This result is inconsistent with the simple petrogenetic model that explains the geographically correlated Sr-, Nd- and Pb-isotopic variation in these basalts as resulting solety from melting of metasomatized lithospheric mantle peridotite of varying age across this area. A magma source composed of a mixture of peridotite and pyroxenite/eclogite also fails because at the high percentage of mafic component required to explain the observed Nd—Os isotope systematics, this mixed source would not produce melts that match the major- and trace-element compositions of the observed olivine tholeiites. A more likely explanation for the observed isotopic compositions involves interaction of sublithospheric primary melts that are similar in composition to mid-ocean ridge basalt (MORB) with high-187Os/188Os materials in the lower crust or lithospheric mantle. Though not uniquely defined, mass-balance calculations suggest that the lithospheric component is mafic-potassic in composition, possibly a small-volume melt of mafic material in the lower crust or upper mantle. If the radiogenic Os is attributable to such interaction, the isotopic compositions of the tholeiites can be satisfied by 2–25% addition of the lithospheric component to a primitive MORB parental magma. These results show that the Os-isotope system in continental basalts can provide a clear distinction between magmas derived by melting isotopically evolved peridotitic lithosphere and those produced from a sublithospheric primary melt contaminated by mafic material in the lithosphere.

Sedimentary deposits in the Middle Awash research area of Ethiopia's Afar depression have yielded vertebrate fossils including the most ancient hominids known. Radioisotopic dating, geochemical analysis of interbedded volcanic ashes and biochronological considerations place the hominid-bearing deposits at around 4.4 million years of age. Sedimentological, botanical and faunal evidence suggests a wooded habitat for the Aramis hominids.

Fossils and artifacts recovered from the middle Awash Valley of Ethiopia's Afar depression sample the Middle Pleistocene transition from Homo erectus to Homo sapiens. Ar/Ar ages, biostratigraphy, and tephrachronology from this area indicate that the Pleistocene Bodo hominid cranium and newer specimens are approximately 0.6 million years old. Only Oldowan chopper and flake assemblages are present in the lower stratigraphic units, but Acheulean bifacial artifacts are consistently prevalent and widespread in directly overlying deposits. This technological transition is related to a shift in sedimentary regime, supporting the hypothesis that Middle Pleistocene Oldowan assemblages represent a behavioral facies of the Acheulean industrial complex.

Combined elemental and neodymium (Nd) isotope data for proximal and distal pyroclastic deposits from the Ethiopian volcanic province illustrate the utility of these parameters for establishing potential source regions for distal tephra and for correlation of widely-separated distal deposits. In the Ethiopian volcanic province, Nd isotopic compositions and, to some extent, elemental characteristics of volcanic products from distinct geographic/tectonic sub-provinces allow first-order indentification of geochemical domains. These domains are used to target potential source regions for the numerous distal tephra deposits exposed within the fossiliferous sedimentary sequences of the Afar and Omo-Turkana Basins. Pliocene distal deposits from the Hadar and Middle Awash regions of Afar, Ethiopia and the Omo-Turkana region of southern Ethiopia-northern Kenya are examined. Combined elemental and Nd isotope data are consistent with derivation of these tephra from sources within or adjacent to the central sector of the Main Ethiopian Rift or within the central to northern Afar. The data presented in this paper also point to the need for a better understanding of the physical and chemical characteristics of the eruptions giving rise to many of the distal deposits. Until information regarding the range of chemical and isotopic variability in these magmatic systems is available, it is difficult to link a specific set of geochemical characteristics to one particular eruption or vent.

Plio-Pleistocene silicic volcanoes, calderas, and their eruptive products are common throughout the Main Ethiopian Rift (MER). A compllation of KAr and 40 Ar 39 Ar dating results for the central sector of the MER demonstrate that the most voluminous and widespread ignimbrites of this region are Pliocene in age, between 4.2 and 3.5 Ma. These units occur in and around: (1) The Awasa and Wagebeta calderas, (2) the Guraghe and Munesa escarpments, where a major buried caldera is believed to occur in the adjacent rift floor, (3) the Addis Ababa-Nazret area, and (4) the foothills and shoulders of the northern sector of the Main Ethiopian Rift. The older units are overlain by less volumious and in most cases, rift-bound late Pliocene and Pleistocene pyroclastic rocks. The Pliocene ignimbrites in this area are likely to be the near-source equivalents of widespread a ash-fall units associated with fossil hominid deposits of the Turkana and Afar sedimentary basins.

Sediments containingArdipithecus ramiduswere deposited 4.4 million years ago on an alluvial floodplain in Ethiopia’s western Afar rift. The Lower Aramis Member hominid-bearing unit, now exposed across a >9-kilometer structural arc, is sandwiched between two volcanic tuffs that have nearly identical40Ar/39Ar ages. Geological data presented here, along with floral, invertebrate, and vertebrate paleontological and taphonomic evidence associated with the hominids, suggest that they occupied a wooded biotope over the western three-fourths of the paleotransect. Phytoliths and oxygen and carbon stable isotopes of pedogenic carbonates provide evidence of humid cool woodlands with a grassy substrate.