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International audience; The Miocene rotation of Sardinia (Western Mediterranean) remains poorly constrained despite a wealth of paleomagnetic data, primarily due to poor chronostratigraphic control. However, this rotation is contemporaneous with the opening of the Liguro-Provençal back-arc oceanic basin, and its history is key to understanding the kinematics of opening of the Western Mediterranean. We address this issue through paleomagnetic and 40Ar/39Ar geochronological investigations of Miocene volcanic sequences in Sardinia. Precise age control allows secular variation of the geomagnetic field to be evaluated. These data provide constraints on the rotational history of this continental microplate; Sardinia rotated 45° counterclockwise with respect to stable Europe after 20.5 Ma (Aquitanian), which is a marked increase over the estimate of 30° derived from prior paleomagnetic studies. Rotation was essentially complete by 15 Ma. About 30° of rotation occurred between 20.5 and 18 Ma (Burdigalian), corresponding to the period of maximum volcanic activity in Sardinia. The observed rotation validates palinspastic models derived from a morphological fit of basin margins, and indicates high rates of opening (up to 9 cm yr− 1 in the southern part of the basin) between 20.5 and 18 Ma.

The Woranso-Mille paleontological study area is located in the central Afar region of Ethiopia, about 360 km northeast of the capital, Addis Ababa. Some parts of this area have been paleontologically known since the 1970s. However, most of the fossiliferous areas were discovered by surveys conducted in the region between 2002 and 2004. By the end of the 2006 field season, a total of 17 vertebrate localities had been designated, and more than 1000 vertebrate fossil specimens collected. Among these specimens, there are more than 20 fossil hominid craniodental and postcranial remains, including one partial skeleton, of Pliocene age (3.5–3.8 Ma). Research at this study area has thus far focused on the geology and paleontology of the early Pliocene deposits along the Mille river and adjacent areas located between the towns of Mille and Chifra. Exposed sediments in the new fossiliferous area are mostly silty sand and silty clay horizons interbedded with a number of volcanic tuffs and basaltic flows suitable for 40Ar/39Ar radioisotopic dating. The total stratigraphic section is about 50 m thick with a minimum age of ~3.5 Ma. The study area also has deposits of early to middle Pleistocene age, although no locality has been designated to date. The new Woranso-Mille paleontological study area provides a crucial temporal window into the time during which Australopithecus anamensis (3.9–4.2 Ma) appears to have given rise to Australopithecus afarensis (3.0–3.6 Ma). Radiometric dates have thus far yielded a minimum age of 3.5 Ma for the hominid localities and this conforms well with a biochronological age estimate of 3.6–3.8 Ma. The associated fauna, particularly the abundance of fossil cercopithecids and presence of diverse aquatic fossil vertebrates, indicates a relatively closed, wooded habitat probably associated with a paleo-Mille river.

KSD-VP-1/1, a partial skeleton of Australopithecus afarensis , was excavated from Pliocene strata at Korsi Dora, 3.3 km southeast of the confluence of the Waki and Mille rivers in the northwestern part of the Woranso-Mille paleoanthropological research site. A tuff collected from ~2.7 m below the fossil horizon, at the bottom of a trench dug 25 m to the east of the fossil excavation, yielded an 40Ar/39Ar age of 3.60 ± 0.03 Ma for anorthoclase feldspar. Strata in the trench and the fossil excavation site comprise a single normal magnetozone interpreted as part of the normal subchron C2An.3n, immediately above the Gauss/Gilbert paleomagnetic transition. Geologic mapping and tephrochemical analyses combined with paleomagnetic data place the fossil horizon and the trench section into local and regional stratigraphic context by constraining the partial skeleton to be younger than the Kilaytoli tuff (KT), a ~4 m thick vitric ash with an anorthoclase feldspar age of 3.570 ± 0.014 Ma. This unit is widely recognized at Korsi Dora, in collection areas north of the Waki-Mille confluence and outside the field area. The KT correlates with the Lokochot Tuff of the Omo-Turkana Basin in Kenya. Sedimentological features of the mudstone and sandstone in and near the excavation site are consistent with deposition in a floodplain or floodplain lake proximal to a stream channel.

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 Lake Malawi, East Africa (9-15° S). Over the Quaternary, we observe dramatic shifts in effective moisture, resulting in large-scale changes in one of the world's largest lakes and most diverse freshwater ecosystems. Results show evidence for 24 lake level drops of more than 200 m during the Late Quaternary, including 15 lowstands when water levels were more than 400 m lower than modern. A dramatic shift is observed at the Mid-Pleistocene Transition (MPT), consistent with far-field climate forcing, which separates vastly different hydroclimate regimes before and after ∼800,000 years ago. Before 800 ka, lake levels were lower, indicating a climate drier than today, and water levels changed frequently. Following the MPT high-amplitude lake level variations dominate the record. From 800 to 100 ka, a deep, often overfilled lake occupied the basin, indicating a wetter climate, but these highstands were interrupted by prolonged intervals of extreme drought. Periods of high lake level are observed during times of high eccentricity. The extreme hydroclimate variability exerted a profound influence on the Lake Malawi endemic cichlid fish species flock; the geographically extensive habitat reconfiguration provided novel ecological opportunities, enabling new populations to differentiate rapidly to distinct species.

Tephra geochemistry and (40)Ar/(39)Ar geochronology are reported for the Waki-Mille area in the northwestern part of the Woranso-Mille paleoanthropological project area in the west central Afar region of Ethiopia. Previous studies documented dentognathic fossils that are morphologically intermediate between Australopithecus anamensis and Australopithecus afarensis and some that are attributed to Australopithecus afarensis. Additional dentognathic remains from the study area were assigned to the newly identified species Australopithecus deyiremeda. These fossil hominin taxa were recovered from volcanic and sedimentary strata containing tuffs ranging in age from more than 3.77 million years ago (Ma) to less than 3.469 Ma. One of the tuffs was correlated based on geochemistry, feldspar mineralogy, and age to the Lokochot Tuff of the Omo-Turkana Basin of southern Ethiopia and Kenya. Variations in major and minor element abundances in volcanic glass demarcate ten geochemically distinct tuffs and tuff sequences, including three that are geochemically similar to widespread regional tuffs, specifically the Lomogol, Lokochot, and β- Tulu Bor/Sidi Hakoma tuffs. A new (40)Ar/(39)Ar age for the Waki Tuff, which is geochemically similar to the Lomogol Tuff, is 3.664 ± 0.016 Ma. Other tuffs in the Waki-Mille area are geochemically dissimilar to regional tuffs documented to date. Identification of tuffs based on character, stratigraphic position, and geochemistry refines local stratigraphic correlations and delineates the geographic distributions of precisely dated fossiliferous levels within the Waki-Mille area.

Sedimentary basins in eastern Africa preserve a record of continental rifting and contain important fossil assemblages for interpreting hominin evolution. However, the record of hominin evolution between 3 and 2.5 million years ago (Ma) is poorly documented in surface outcrops, particularly in Afar, Ethiopia. Here we present the discovery of a 2.84- to 2.58-million-year-old fossil and hominin-bearing sediments in the Ledi-Geraru research area of Afar, Ethiopia, that have produced the earliest record of the genus Homo. Vertebrate fossils record a faunal turnover indicative of more open and probably arid habitats than those reconstructed earlier in this region, which is in broad agreement with hypotheses addressing the role of environmental forcing in hominin evolution at this time. Geological analyses constrain depositional and structural models of Afar and date the LD 350-1 Homo mandible to 2.80 to 2.75 Ma.

The fluviolacustrine sedimentary sequence of the Chemeron Formation exposed in the Barsemoi River drainage, Tugen Hills, Kenya, contains a package of five successive diatomite/fluvial cycles that record the periodic development of freshwater lakes within the axial portion of the Central Kenya Rift. The overwhelming abundance in the diatomite of planktonic species of the genera Aulacoseira and Stephanodiscus, and the virtual absence of benthic littoral diatoms and detrital material indicate areally extensive, deep lake systems. A paleomagnetic reversal stratigraphy has been determined and chronostratigraphic tie points established by 40 Ar/ 39 Ar dating of intercalated tuffs. The sequence spans the interval 3.1–2.35 Ma and bears a detailed record of the Gauss/Matuyama paleomagnetic transition. The 40 Ar/ 39 Ar age for this boundary of 2.589±0.003 Ma can be adjusted to concordance with the Astronomical Polarity Time Scale (APTS) on the basis of an independent calibration to 2.610 Ma, 29 kyr older than the previous APTS age. The diatomites recur at an orbital precessional interval of 23 kyr and are centered on a 400-kyr eccentricity maximum. It is concluded that these diatomite/fluvial cycles reflect a narrow interval of orbitally forced wet/dry climatic conditions that may be expressed regionally across East Africa. The timing of the lacustrine pulses relative to predicted insolation models favors origination of moisture from the northern Africa monsoon, rather than local circulation driven by direct equatorial insolation. This moisture event at 2.7–2.55 Ma, and later East African episodes at 1.9–1.7 and 1.1–0.9 Ma, are approximately coincident with major global climatic and oceanographic events.

Accurate 40Ar/39Ar dating of Quaternary and Holocene samples requires a standard (neutron fluence monitor) of appropriate age. This standard should be isotopically homogenous at the scale of analysis, and should be well intercalibrated with other standards. Several such standards have been proposed, including sanidine from the ∼1.19 Ma Alder Creek Rhyolite (ACs;Turrin et al., 1994 [Turrin, B.D., Donnely-Nolan, J.M., Hearn, B.C. Jr., 1994. 40Ar/39Ar ages from the rhyolite of Alder Creek, California: Age of the Cobb Mountain Normal-Polarity Subchron revisited. Geology 22, 251–254]). This sanidine is satisfactorily homogeneous, well intercalibrated with Fish Canyon sanidine (FCs; Renne et al., 1998 [Renne, P.R., Swisher, C.C., Deino, A.L., Karner, D.B., Owens, T.L., DePaolo, D.J., 1998. Intercalibration of standards, absolute ages and uncertainties in 40Ar/39Ar dating. Chem. Geol. 145, 117–152]), and is linked to the astronomically calibrated magnetic polarity time scale, since the Alder Creek Rhyolite lava is the type occurrence of the Cobb Mountain event (CM). We collected 408 kg of this rock, from which we anticipate being able to extract more than 8 kg of >425 μm sanidine crystals. Analyses of sanidine phenocryts from the ACs-2 from three separate irradiations and two distinct grain sizes yield ages ranging from 1.190±0.004 to 1.194±0.003 Ma1 with a weighted mean age of 1.193±0.001 Ma (MSWD=0.74, n=225; based on FCs=28.02 Ma; Renne et al., 1998 [Renne, P.R., Swisher, C.C., Deino, A.L., Karner, D.B., Owens, T.L., DePaolo, D.J., 1998. Intercalibration of standards, absolute ages and uncertainties in 40Ar/39Ar dating. Chem. Geol. 145, 117–152]). The value of the intercalibration factor (Renne et al., 1998 [Renne, P.R., Swisher, C.C., Deino, A.L., Karner, D.B., Owens, T.L., DePaolo, D.J., 1998. Intercalibration of standards, absolute ages and uncertainties in 40Ar/39Ar dating. Chem. Geol. 145, 117–152]) is RFCsACs-2=0.04229±0.00012 (n=8), identical with that of the original ACs measurements (0.04229±0.00012; Renne et al., 1998 [Renne, P.R., Swisher, C.C., Deino, A.L., Karner, D.B., Owens, T.L., DePaolo, D.J., 1998. Intercalibration of standards, absolute ages and uncertainties in 40Ar/39Ar dating. Chem. Geol. 145, 117–152]). The age of ACs-2 is in agreement with the recent astronomical calibration of the CM event (1.215 to 1.190 Ma; [Channell, J.E.T., Mazaud, A., Sullivan, P., Turner, S., Raymo, M.E., 2002. Geomagnetic excursions and paleointensities in the Matuyama Chron at Ocean Drilling Program Sites 983 and 984 (Iceland Basin). J. Geophys. Res. 107 (6), 10.1029/2001JB000491.]). The apparent absence of xenocrysts and excess argon suggests that multigrain analyses are feasible and therefore the amount of material can be tailored to experimental requirements. Aliquots from various grain sizes will be available upon request. Zircon is also potentially available, which may be of interest to (U–Th)/He researchers.

Laser-fusion 40Ar/39Ar analysis of four bentonite horizons produces the first absolute ages for the 860-m-thick Kaiparowits Formation and resolves previous age uncertainty caused by ambiguous biostratigraphy. A late Campanian (Judithian) age of ca. 76.1–74.0 Ma is determined, resulting in a high-resolution temporal framework for the richly fossiliferous formation. Detailed stratigraphic correlation reveals that the Kaiparowits Formation is contemporaneous with many of the most important vertebrate fossil-bearing formations in the Western Interior Basin, and with other well-studied strata across Utah and southeastern Wyoming, including portions of the Book Cliffs sequence. The Judithian age determination and correlations for the Kaiparowits Formation presented here provide a new chronological basis for addressing questions relating to mammal biostratigraphy, vertebrate evolution, biodiversity and paleobiogeography (e.g., dinosaur provincialism) in the Cretaceous Western Interior Basin.

Models for the formation of the archetypal rift-rift-rift triple junction in the Afar depression have assumed the synchronous development of the Red Sea-Aden-East African rift systems soon after flood basaltic magmatism at 31 Ma, but the timing of intial rifting in the northern sector of the East African rift system had been poorly constrained. The aims of our field, geochronology, and remote sensing studies were to determine the timing and kinematics of rifting in the 3rd arm, the Main Ethiopian rift (MER), near its intersection with the southern Red Sea rift. New structural data and 10 new SCLF 40 Ar/ 39 Ar dates show that extension in the northern Main Ethiopian rift commenced after 11 Ma, more than 17 My after initial rifting in the southern Red Sea and Gulf of Aden. The triple junction, therefore, could have developed only during the past 11 My, or 20 My after the flood basaltic magmatism. Thus, the flood basaltic magmatism and separation of Arabia from Africa are widely separated in time from the opening of the Main Ethiopian rift, which marks the incipient Nubia-Somalia plate boundary; triple junction formation is not a primary feature of breakup above the Afar mantle plume. The East African rift system appears to have propagated northward from the Mesozoic Anza rift system into the Afar depression to cut across Oligo-Miocene rift structures of the Red Sea and Gulf of Aden, in response to global plate reorganisations. Structural patterns reveal a change from 130jE-directed extension to 105jE-directed extension sometime in the interval 6.6 to 3 Ma, consistent with predictions from global plate kinematic studies. The along-axis propagation of rifting in each of the three arms of the triple junction has led to a NE-migration of the triple junction since 11 Ma. D 2004 Elsevier B.V. All rights reserved.

The Neapolitan Yellow Tuff (NYT) is the product of the largest known trachytic phreatoplinian eruption. It covered an area larger than 1000 km2 with an estimated volume of about 40 km3 of erupted magma. During the course of the eruption a caldera collapsed within the previously formed Campanian Ignimbrite caldera. The resulting nested structure strongly influenced the following volcanic activity in the Campi Flegrei caldera. As previous dating of the NYT does not converge toward a unique result, a new set of 40Ar/39Ar age determinations has been carried out to better constrain the age of the eruption. Two variants of the 40Ar/39Ar dating method were applied to determine the age of the NYT eruption: (1) single-crystal total fusion (SCTF), on an individual phenocryst of feldspar, and (2) laser incremental heating (LIH), on bulk aliquots of feldspar phenocrysts. The results of the SCTF analyses show that the overall sample weighted mean age, derived from the conventional age calculation, is 15.6±0.8 ka. A weighted mean of the isochron age is 15.3±1.2 ka (2σ), and has been assumed as the best indicator of age to be derived from the SCTF analyses. The LIH analyses results show that plateau ages vary from 15.4±0.5 to 14.5±0.5 ka. The overall weighted mean age of the isochron results is 14.9±0.4 ka (2σ). This result has been assumed as the reference age for the NYT eruption, and agrees with the SCTF age. The new age obtained for the NYT deposits is of great relevance for the understanding of the evolution and the present state of the Campi Flegrei caldera and collocates the NYT in a crucial stratigraphical position to date the climatic oscillations that occurred between the Late Glacial and the Holocene.

Lemudong’o is located on the western margin of the southern Rift Valley approximately 100 km west of Nairobi (Fig. 1), an area deeply incised by three major permanent river systems. Stratified lavas, air-fall and water-laid tuffs, alluvial, and fluviolacustrine sediments, and paleosols of Late Miocene to Late Pleistocene age crop out over a w25 50 km area. Wright (1967) reconstructed three paleolakes and shoreline facies, assumed to be Plio-Pleistocene in age, in the vicinity of an isolated Basement Complex inselberg. Radiometric dating demonstrates the paleolake deposits exposed at Lemudong’o are Late Miocene in age. During archaeological surveys and excavations in this region in 1995-96 (Kyule et al., 1997) and 1999–2002 (Ambrose et al., 2000; 2002; Hlusko et al., 2002), 55 new archaeological sites (Acheulean, Middle Stone Age, Later Stone Age, Neolithic and Iron Age), and several paleontological occurrences were discovered. Here we describe the preliminary results from research at the Late Miocene fossil site of Lemudong’o. The most productive Late Miocene paleontological site in the area is exposed in Lemudong’o Gorge, GvJh15, GvJh32 (Figs. 2 and 3). Lithologic units include paludal (marsh) and lake margin claystones, lacustrine diatomaceous silts and claystones, and coarser alluvial deposits with interstratified tuffs. Similar exposures occur within tens of kilometers, though their correlation to the Lemudong’o strata is not yet confirmed, and fossils are scarce and taxonomically non-diagnostic. Lemudong’o Gorge is a fault-controlled, deeply incised gully system bounded on the east by the Enkoria fault (Wright, 1967). Fossiliferous sediments are exposed at two localities approximately 500 m apart. Locality 1 (Lemudong’o 1, GvJh15, coordinates: 1(18.19S, 35(58.74E, approximate elevation 1600–1620 m) was discovered in 1994, and is located in the upper reaches of the main gully. It contains the higher levels of the depositional sequence, and the main fossiliferous * Corresponding author. Tel.: +1-217-244-4914; fax: +1-217-244-3490 E-mail addresses: ambrose@uiuc.edu (S.H. Ambrose), hlusko@uiuc.edu (L.J. Hlusko), mkyule@uonbi.ac.ke (D. Kyule), al@bgc.org (A. Deino), martin.williams@arts.adelaide.edu.au (M. Williams). Journal of Human Evolution 44 (2003) 737–742

Authigenic K-feldspar occurs widely in the North American Midcontinent in an alteration profile developed on uppermost Precambrian rocks. In the study areas of west-central Wisconsin and southeast Missouri, the alterationprofiles are kaolinitic and interpreted as former saprolite formed by weathering prior to deposition of Upper Cambrian sandstone. K-feldspar in granitic alteration profiles of west-central Wisconsin occurs either as coarse-grained red crystals and veins replacing kaolinite or as overgrowths and fine-grained crystals in pore space of partly altered granite. In a granitic profile of southeast Missouri, K-feldspar replaces primary feldspar and probably kaolinite. In a diabasic profile, K-feldspar is disseminated and forms veins in saprolite, and it replaces the outer 3-5 cm of core-stones. Dating was conducted with the 4 0 Ar/ 3 9 Ar method using incremental heating. Observed spectral types are plateaus (6), near plateaus (3), undulatory (14), and stair-step (5). Undulatory patterns are attributed chiefly to recoil, and integrated ages of these are usually within the error of plateau ages where present, indicating a balanced enrichment/depletion of 3 9 Ar due to recoil. Stair-step spectra are attributed to severe recoil. Plateau 4 0 Ar/ 3 9 Ar ages of 446-427 Ma (Late Ordovician and Silurian) were obtained from coarse-grained K-feldspar replacing kaolinite in Wisconsin, suggesting a duration of the alteration episode of ∼19 m.y. However, potentially accurate nonplateau age measurements (integrated and near-plateau ages) suggest that alteration continued for a considerable time thereafter, possibly to ca. 399-395 Ma (Early Devonian). Integrated 4 0 Ar/ 3 9 Ar ages of ca. 362 Ma (Late Devonian and/or Early Mississippian) were obtained from fine-grained K-feldspar. Plateau ages of ca. 450 (Late Ordovician) were obtained from K-feldspar of the diabase profile in Missouri; duration of K-feldspar alteration in this region was at least ∼16 m.y. and may have been as long as ∼52 m.y. The Late Devonian-Early Mississippian K-feldspar formed at temperatures of ∼100 °C from saline fluid averaging ∼10% NaCl equivalent and having a δ 1 8 O value of ∼3.5%. This brine may have originated from the Michigan Basin, East-Central Iowa Basin, and/or the Illinois Basin, coincident with the Acadian orogeny. The Middle Ordovician to Early Devonian dates of K-feldspar do not generally correspond in time with orogenic activity at the continental margin, and the mechanism(s) for forming the K-feldspar are uncertain. Fluid sources proposed for some or all of the K-feldspar include the Michigan Basin, Taconic mountains, Reelfoot Rift, Illinois Basin, and Midcontinent epeiric seas. Middle Ordovician K-feldspar of Missouri very likely formed at elevated temperatures, but evidence of fluid temperature is lacking for other pre-Late Devonian K-feldspar.

Excavation in the previously little-explored western portion of Olduvai Gorge indicates that hominid land use of the eastern paleobasin extended at least episodically to the west. Finds included a dentally complete Homo maxilla (OH 65) with lower face, Oldowan stone artifacts, and butchery-marked bones dated to be between 1.84 and 1.79 million years old. The hominid shows strong affinities to the KNM ER 1470 cranium from Kenya ( Homo rudolfensis ), a morphotype previously unrecognized at Olduvai. ER 1470 and OH 65 can be accommodated in the H. habilis holotype, casting doubt on H. rudolfensis as a biologically valid taxon.

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.

Finding Homo nearly 3 million years ago The fossil record of humans is notoriously patchy and incomplete. Even so, skeletal remains and artifacts unearthed in Africa in recent decades have done much to illuminate human evolution. But what is the origin of the genus Homo ? Villmoare et al. found a fossil mandible and teeth from the Afar region in Ethiopia. The find extends the record of recognizable Homo by at least half a million years, to almost 2.8 million years ago. The morphological traits of the fossil align more closely with Homo than with any other hominid genus. DiMaggio et al. confirm the ancient date of the site and suggest that these early humans lived in a setting that was more open and arid than previously thought. Science , this issue p. 1352 , p. 1355

Unusual monotonous intermediate ignimbrites consist of phenocryst-rich dacite that occurs as very large volume (>1000 km 3 ) deposits that lack systematic compositional zonation, comagmatic rhyolite precursors, and underlying plinian beds. They are distinct from countless, usually smaller volume, zoned rhyolite–dacite–andesite deposits that are conventionally believed to have erupted from magma chambers in which thermal and compositional gradients were established because of sidewall crystallization and associated convective fractionation. Despite their great volume, or because of it, monotonous intermediates have received little attention. Documentation of the stratigraphy, composition, and geologic setting of the Lund Tuff – one of four monotonous intermediate tuffs in the middle-Tertiary Great Basin ignimbrite province – provides insight into its unusual origin and, by implication, the origin of other similar monotonous intermediates. The Lund Tuff is a single cooling unit with normal magnetic polarity whose volume likely exceeded 3000 km 3 . It was emplaced 29.02±0.04 Ma in and around the coeval White Rock caldera which has an unextended north–south diameter of about 50 km. The tuff is monotonous in that its phenocryst assemblage is virtually uniform throughout the deposit: plagioclase>quartz≈hornblende>biotite>Fe–Ti oxides≈sanidine>titanite, zircon, and apatite. However, ratios of phenocrysts vary by as much as an order of magnitude in a manner consistent with progressive crystallization in the pre-eruption chamber. A significant range in whole-rock chemical composition (e.g., 63–71 wt% SiO 2 ) is poorly correlated with phenocryst abundance. These compositional attributes cannot have been caused wholly by winnowing of glass from phenocrysts during eruption, as has been suggested for the monotonous intermediate Fish Canyon Tuff. Pumice fragments are also crystal-rich, and chemically and mineralogically indistinguishable from bulk tuff. We postulate that convective mixing in a sill-like magma chamber precluded development of a zoned chamber with a rhyolitic top or of a zoned pyroclastic deposit. Chemical variations in the Lund Tuff are consistent with equilibrium crystallization of a parental dacitic magma followed by eruptive mixing of compositionally diverse crystals and high-silica rhyolite vitroclasts during evacuation and emplacement. This model contrasts with the more systematic withdrawal from a bottle-shaped chamber in which sidewall crystallization creates a marked vertical compositional gradient and a substantial volume of capping-evolved rhyolite magma. Eruption at exceptionally high discharge rates precluded development of an underlying plinian deposit. The generation of the monotonous intermediate Lund magma and others like it in the middle Tertiary of the western USA reflects an unusually high flux of mantle-derived mafic magma into unusually thick and warm crust above a subducting slab of oceanic lithosphere.

A fossil hominid temporal bone (KNM-BC 1) from surface exposures at Baringo Paleontological Research Project site BPRP#2 in the Chemeron Formation outcropping in a tributary drainage of the Kapthurin River west of Lake Baringo, Kenya has been attributed to Homo sp. indet. K-feldspar phenocrysts from lapilli tuffs bracketing the inferred fossiliferous horizon yield single-crystal(40)Ar/(39)Ar ages of 2.456+/-0.006 and 2.393+/-0.013 Ma. These age determinations are supported by stratigraphically consistent ages on higher tuff horizons and from nearby sections. In addition, new(40)Ar/(39)Ar ages on tuffaceous units near the base and top of the formation along the Kapthurin River yield 3.19+/-0.03 and 1.60+/-0.05 Ma respectively. The base of the formation along the Kapthurin River is thus approximately 0.5 Ma younger than the uppermost Chemeron Formation strata exposed at Tabarin, 23 km to the north-northwest. The upper half of the formation along the Kapthurin River was deposited at an average rate of approximately 11 cm/ka, compared to 21-23 cm/ka at Tabarin.

Altered volcanic ash beds in the Middle Jurassic Temple Cap and Carmel Formations in southwestern Utah record a pulse of active arc-related volcanism between 166 and 171 Ma. A second pulse between 148 and 155 Ma has previously been documented in the Upper Jurassic Morrison Formation. Volcanic and volcaniclastic rocks of these same ages have also been identified closer to or within the arc in California in the Inyo Mountains, the Cowhole Mountains, the Palen Mountains, and the central Mojave Desert. The upper part of the volcaniclastic Mount Wrightson Formation and the strata of Cobre Ridge in southern Arizona are ca. 170 Ma in age and appear to be time correlative with the Middle Jurassic formations in southwestern Utah. The altered ash beds found in the Temple Cap and Carmel Formations typically contain phenocrysts of sanidine, quartz, biotite, apatite, zircon, and titanite. Plagioclase was likely present originally in all of the ashes, but was removed by alteration and is now found only in the Temple Cap red beds. Quartz and sanidine are absent in two crystal-poor ash beds that contain two pyroxenes, hornblende, and biotite. Although major and trace element concentrations in the ash beds have been substantially modified, compositions of relict phenocrysts reveal that the magmas were calc-alkaline rhyolites to andesites. Two-pyroxene, two-feldspar, biotite, and biotite-apatite thermometers suggest that crystallization occurred at temperatures ranging from 740 to 910 8C. Hornblende geobarometry yields pressures of 1‐2 kilobars for the two ash beds that contain the appropriate buffer assemblage. The mafic silicates all have moderately high Mg/Fe ratios. This fact, combined with the presence of hornblende, biotite, and titanite, suggests that the phenocrysts crystallized at high oxygen fugacities similar to those of the granites of the batholiths of California. The ash probably erupted from a low-lying arc cut by strikeslip faults in what is now southern California and western Nevada. Major Jurassic unconformities occur near or within the ash-bearing formations in southwestern Utah. Laser-fusion singlecrystal 40 Ar/ 39 Ar measurements have defined the ages of the unconformities and the associated volcanism. The age of the J-1 unconformity, found at the base of the Temple Cap Formation in southwestern Utah, is older than ca. 170.5 Ma. The J-2 unconformity, which lies between the Temple Cap and Carmel Formations, formed between ca. 169 and 168 Ma. The origin of these unconformities is still unclear, but may be related to the Middle Jurassic pulse of magmatism and the oblique plate convergence along the western margin of North America. The age range of ash beds in the Carmel Formation between 166.3 and 168.0 6 ;0.5 Ma is consistent with a Bajocian-Bathonian boundary of ca. 166 Ma.

Phyllonite and metaquartzite clasts occur in early Pliocene or possibly Miocene sediment at the base of the University of Wisconsin Arctic Ocean core Fl-380. Single-crystal 40 Ar/ 39 Ar laserprobe ages derived from feldspars in sediment enclosing the clasts range from Middle and Late Proterozoic to Paleozoic. The clasts occur in sediment deposited 1–2 m.y. earlier than any previously reported central Arctic Ocean ice-rafted debris, and although an ice-rafted origin may be possible, a local source such as adjacent Alpha Ridge bedrock should also be considered.

The Agnano–Monte Spina tephra (AMST), dated at 4100 years BP by 40 Ar / 39 Ar and 14 C AMS techniques, is the product of the highest-magnitude eruption in the Campi Flegrei caldera (CFc) during its last epoch of activity (4800–3800 years BP). The sequence alternates magmatic and phreatomagmatic pyroclastic-fallout, -flow and -surge beds and bedsets. Two main pumice-fallout deposits with variable easterly-to-northeasterly dispersal axes are about 10 cm thick at 42 km from the vent area. High particle concentration pyroclastic currents were confined to the caldera depression; lower concentration flows overtopped the morphological boundary of the caldera and traveled at least 15 km over the surrounding plain. The unit is subdivided into six members, named A through F in stratigraphic sequence, based upon their sedimentological characteristics. Isopachs and isopleths maps suggest a vent location in the Agnano plain. A volcano-tectonic collapse begun during the course of the eruption, took place along the faults of the northeastern sector of the resurgent block within the CFc, and generated the Agnano plain. The early erupted trachytic magma had a homogeneous alkali–trachytic composition, whereas later-erupted magma shows small-scale hetereogeneities. Trace elements and Sr-isotope compositions, indicate that two isotopically distinct magmas, one alkali–trachytic and the other trachytic, were tapped and partially mixed during the eruption. The small volume (1.2 km3 DRE) of erupted magma and the structural position of the vent suggest that the eruption was fed by a dyke intruded along a normal fault in the sector of the resurgent block under a tensional stress regime.

New geochronological, geochemical, and Sr-isotopic data on volcanics erupted before the Campanian Ignimbrite (CI, 37 ka) and the Neapolitan Yellow Tuff (NYT, 12 ka) caldera-forming eruptions at Campi Flegrei (CF) have allowed us to investigate the behavior and temporal evolution of the Phlegraean magmatic system. The most prominent feature of the CF magmatic system was the existence of a large, trachytic magma chamber, episodically recharged, which fed eruptions for tens of thousands years before the CI and NYT eruptions. During the pre-CI caldera activity, magmas were episodically erupted from vents located outside the present caldera structure. These magmas ranged in composition from trachyte to alkali-trachyte, with Sr-isotope ratios increasing through time, and becoming identical to that of the CI magma, at about 44 ka ago. This suggests that the Phlegraean magmatic system before the CI eruption was acting as an open system. It was being progressively replenished by new batches of magma that mixed with the resident less radiogenic, fractionating trachytic magmas and was periodically tapped. The magma chamber evolution culminated in the catastrophic eruption of the voluminous (150 km3 DRE), chemically and isotopically zoned CI trachytic magmas, and in the resultant CI caldera formation. Subsequent to the CI eruption, during a period of moderate subaereal volcanic activity of about 20 ka duration, magmas predominantly trachytic to alkali-trachytic in composition and isotopically similar to the last emitted CI magma were erupted from vents located inside the CI caldera. The temporal trend shown by Sr-isotope ratios provides evidence for a new input of alkali-trachytic magma, at ca. 15 ka, with 87 Sr / 86 Sr ratio identical to that of the alkali-trachytic magma feeding the first phase of the NYT eruption. These data testify to the arrival in a short time span of a new trachytic to alkali-trachytic magma in the system, isotopically distinct from the CI magma, that gave rise about 3 ka later to eruption of the NYT (40 km3 DRE).

Five new stepwise-heating 40 Ar/ 39 Ar ages and one new high-sensitivity 14 C date of ash-fall and ash-flow deposits from late Quaternary silicic volcanoes in northern Central America document the eruption rates and frequencies of five major rhyodacite and rhyolite calderas (Atitlan, Amatitlan, Ayarza, Coatepe- que, and Ilopango) located north of the basalt, ande- site, and dacite stratovolcanoes of the Central Ameri- can volcanic front. These deposits form extensive time- stratigraphic horizons that intercalate regionally, and knowledge of dates and stratigraphy provides a valua- ble framework for age determinations of more local- ized volcanic and nonvolcanic events. The new data, es- pecially when integrated with previous stratigraphic and dating work, show that all five calderas erupted several times in the past 200 ka and, despite a lack of historic activity, should be considered as active centers that could produce highly explosive eruptions again. Because of their locations near the highly vulnerable economic hearts of Guatemala and El Salvador, the risks of eruptions from these calderas should be care- fully considered along with risks of major earthquakes and volcanic front volcanoes, which are much more fre- quent but inflict less severe and extensive damage. This

The 40Ar/39Ar dating method depends on accurate intercalibration between samples, neutron fluence monitors, and primary 40Ar/40K (or other external) standards. The 40Ar/39Ar age equation may be expressed in terms of intercalibration factors that are simple functions of the relative ages of standards, or equivalently are equal to the ratio of radiogenic to nucleogenic K-derived argon (40Ar/39ArK) values for one standard or unknown relative to another. Intercalibration factors for McClure Mountain hornblende (MMhb-1), GHC-305 biotite, GA-1550 biotite, Taylor Creek sanidine (TCs) and Alder Creek sanidine (ACs), relative to Fish Canyon sanidine (FCs), were derived from 797 analyses involving 11 separate irradiations with well-constrained neutronfluence variations. Values of the intercalibration factors are RFCsMMhb-1 = 21.4876 ± 0.0079; RFCsGA-1550 = 3.5957 ± 0.0038; RFCsTCs = 1.0112 ± 0.0010; RFCsACs = 0.04229 ± 0.00006, based on the mean and standard error of the mean resulting from four or more spatially distinct co-irradiations of FCs with the other standars. Analysis of 35 grains of GHC-305 irradiated in a single irradiation yields RFCsGHC-305 = 3.8367 ± 0.0143. Results at these levels of precision essentially eliminate intercalibration as a significant source of error in 40Ar/39Ar dating. Data for GA-1550 (76 analyses, 5 fluence values), TCs (54 analyses, 4 fluence values), FCs (380 analyses, 40 fluence values) and ACs (86 analyses, 11 fluence values) yield MSWD values showing that the between-grain dispersion of 40Ar∗/39ArK values is consistent with analytical errors alone, whereas MMhb-1 (167 analyses, 4 irradiations) and GHC-305 (34 analyses, 1 fluence value) are heterogeneous and therefore unsuitable as standards for small sample analysis. New K measurements by isotope dilution for two primary standards, GA-1550 biotite (8 analyses averaging 7.626 ± 0.016 wt%) and intralaboratory standard GHC-305 (10 analyses averaging 7.570 ± 0.011 wt%), yield values slightly lower and more consistent than previous data obtained by flame photometry, with resulting 40Ar/40K ages of 98.79 ± 0.96 Ma and 105.6 ± 0.3 Ma for GA-1550 and GHC-305, respectively. Combining these data with the intercalibration approach described herein and using GA-1550 as the primary standard (1.343 × 10−9 mol/g of 40Ar∗; [McDougall, I., Roksandic, Z., 1974. Total fusion 40Ar/39Ar ages using HIFAR reactor. J. Geol. Soc. Aust. 21, 81–89.]) yields ages of 523.1 ± 4.6 Ma for MMhb-1, 105.2 ± 1.1 Ma for GHC-305, 98.79 ± 0.96 Ma for GA-1550, 28.34 ± 0.28 Ma for TCs, 28.02 ± 0.28 for FCs, and 1.194 ± 0.012 Ma for ACs (errors are full external errors, including uncertainty in decay constants). Neglecting error in the decay constants, these ages and uncertainties are: 523.1 ± 2.6 Ma for MMhb-1, 105.2 ± 0.7 Ma for GHC-305, 98.79 ± 0.54 for GA-1550, 28.34 ± 0.16 Ma for TCs, 28.02 ± 0.16 Ma for FCs, and 1.194 ± 0.007 Ma for ACs. Using GHC-305 as the primary standard (1.428 ± 0.004 × 10−9 mol/g of 40Ar∗), ages are 525.1 ± 2.3 Ma for MMhb-1, 105.6 ± 0.3 Ma for GHC-305, 99.17 ± 0.48 Ma for GA-1550, 28.46 ± 0.15 Ma for TCs, 28.15 ± 0.14 Ma for FCs, and 1.199 ± 0.007 Ma for ACs, neglecting decay constant uncertainties. The approach described herein facilitates error propagation that allows for straightforward inclusion of uncertainties in the ages of primary standards and decay constants, without which comparison of 40Ar/39Ar dates with data from independent geochronometers is invalid. Re-examination of 40K decay constants would be fruitful for improved accuracy.

The potassium-argon (K-Ar) dating method has been widely used over the past 40 years to provide radioisotopic age control of hominid/hominoid evolutionary time scales. The wide appeal of the technique to paleoanthropology and archeology has been, in part, a result of its broad time range of applicability, from materials as young as a few thousand years old to an essentially unbounded upper age limit. Another reason for its appeal is the many geological circumstances in which datable materials are found. Beginning about two decades ago and accelerating into this decade, however, the conventional K-Ar technique has given way to 40Ar/39Ar dating as the method of preference. This technique is not only more precise and accurate when dating ideal materials, but also permits excellent ages to be obtained from situations that often stymie the conventional K-Ar technique, such as dating of contaminated tuffs and altered rocks. © 1998 Wiley-Liss, Inc.

Fossils from a large-bodied hominoid from early Miocene sediments of Uganda, along with material recovered in the 1960s, show features of the shoulder and vertebral column that are significantly similar to those of living apes and humans. The large-bodied hominoid from Uganda dates to at least 20.6 million years ago and thus represents the oldest known hominoid sharing these derived characters with living apes and humans.

The Whistler Squat Quarry (TMM 41372) of the lower Devil's Graveyard Formation in Trans-Pecos Texas is a middle Eocene fossil locality attributed to Uintan biochronological zone Ui1b. Specimens from the Whistler Squat Quarry were collected immediately above a volcanic tuff with prior K/Ar ages ranging from ∼47-50 Ma and below a tuff previously dated to ∼44 Ma. New 40Ar/39Ar analyses of both of the original tuff samples provide statistically indistinguishable ages of 44.88±0.04 Ma for the lower tuff and 45.04±0.10 Ma for the upper tuff. These dates are compatible with magnetically reversed sediments at the site attributable to C20r (43.505-45.942 Ma) and a stratigraphic position above a basalt dated to 46.80 Ma. Our reanalysis of mammalian specimens from the Whistler Squat Quarry and a stratigraphically equivalent locality significantly revises their faunal lists, confirms the early Uintan designation for the sites, and highlights several biogeographic and biochronological differences when compared to stratotypes in the Bridger and Uinta Formations. Previous suggestions of regional endemism in the early Uintan are supported by the recognition of six endemic taxa (26% of mammalian taxa) from the Whistler Squat Quarry alone, including three new taxa. The revised faunal list for the Whistler Squat Quarry also extends the biostratigraphic ranges of nine non-endemic mammalian taxa to Ui1b.

To test multiple regression models developed to predict seven variables of climate from leaf margin and size characters, leaf character data were collected from modern woodland and rainforest plant communities in Uganda. These localities were chosen for their structural similarity to two fossil sites for which climate values were desired. The test indicated that for these communities, the model accurately predicts mean annual temperature. However, the multiple regression models overestimate seasonality of temperature and mean annual and seasonal precipitation amounts for the two modern communities. Reasons for the models' inaccuracies may include, no African analog in the non-African database on which the model is based, too much intraspecific variability in leaf size (an important character in precipitation models), and relationships between leaf form and climate that may not be well defined by multiple regression. Mean annual temperatures were reconstructed for two Miocene paleobotanical localities from the Tugen Hills of the eastern rift valley, Kenya, using leaf form data (primarily margin characters). The results indicate that mean annual temperatures at Kabarsero (12.6 Ma), and Kapturo (6.7–7.2 Ma), were essentially equivalent to temperatures at those localities today and differed little from one another. The Kapturo paleofloral assemblage is a deciduous woodland interpreted as indicating less available or more seasonal moisture than the Kabarsero paleofloral assemblage, a wet forest with West African floral affinities. Laser-fusion 40 Ar/ 39 Ar dating of volcanic materials at Kapturo provides the first control of this paleoflora, bracketed by a 7.2 Ma trachyte at the base of the sedimentary sequence, and an age of 6.7 Ma on a reworked tuff overlying the fossiliferous horizon. The chronologic data places the paleofloral assemblage in stratigraphic context relative to other Tugen Hills units containing paleofloras and abundant vertebrate remains including hominoids.

A 1.3-million-year-long climate history from the Lake Malawi basin in eastern Africa displays a trend towards progressively wetter conditions superimposed on strong 100,000-year eccentricity cycles of temperature and rainfall since the Mid-Pleistocene Transition around 900,000 years ago. Climate variability during the Pleistocene epoch is fairly well known for the marine realm and Antarctica. But for much of the land surface — and Africa in particular — past climate variability remains unclear. Thomas Johnson et al. present a record of aridity for the past 1.3 million years from Lake Malawi, southeast Africa, and show a trend towards progressively wetter conditions superimposed on strong 100,000-year eccentricity cycles of temperature and rainfall since the Mid-Pleistocene Transition around 900,000 years ago. Carbon dioxide and dust, but not global ice volume, appear to have influenced climate variations at Lake Malawi over the past 500,000 years. African climate is generally considered to have evolved towards progressively drier conditions over the past few million years, with increased variability as glacial–interglacial change intensified worldwide1,2,3. Palaeoclimate records derived mainly from northern Africa exhibit a 100,000-year (eccentricity) cycle overprinted on a pronounced 20,000-year (precession) beat, driven by orbital forcing of summer insolation, global ice volume and long-lived atmospheric greenhouse gases4. Here we present a 1.3-million-year-long climate history from the Lake Malawi basin (10°–14° S in eastern Africa), which displays strong 100,000-year (eccentricity) cycles of temperature and rainfall following the Mid-Pleistocene Transition around 900,000 years ago. Interglacial periods were relatively warm and moist, while ice ages were cool and dry. The Malawi record shows limited evidence for precessional variability, which we attribute to the opposing effects of austral summer insolation and the temporal/spatial pattern of sea surface temperature in the Indian Ocean. The temperature history of the Malawi basin, at least for the past 500,000 years, strongly resembles past changes in atmospheric carbon dioxide and terrigenous dust flux in the tropical Pacific Ocean, but not in global ice volume. Climate in this sector of eastern Africa (unlike northern Africa) evolved from a predominantly arid environment with high-frequency variability to generally wetter conditions with more prolonged wet and dry intervals.

There are numerous hypotheses linking climatic trends, events and variability to human origins, evolution and dispersal. Long palaeoenvironmental records from continental sites that may allow tests of these hypotheses are only now becoming available, but most are distant from fossil human sites. The Hominin Sites and Palaeolakes Drilling Project (HSPDP) aims to obtain long continuous sediment cores spanning critical intervals of evolutionary history from lacustrine sites close to globally significant hominin sites in East Africa. Together, the five sites – Northern Awash and Chew Bahir, Ethiopia; West Turkana, Baringo Basin and Lake Magadi, Kenya – will provide multi-proxy records spanning the last 4 million years. This will allow us to correlate and compare environmental changes to the more fragmentary record of evolution, dispersal, extinction and cultural innovation. The project team will evaluate models of climatic and tectonic forcing of environmental processes and landscape resources. We will test hypotheses linking climate variability to physical and cultural evolution. The project is supported by the International Continental Drilling Program (ICDP), NSF (USA), DFG (Germany) and by NERC (UK). Drilling began in June 2013 in Kenya. NERC funded research focuses on the Chew Bahir site in Ethiopia, where a survey of basin sediments using 2D electrical resistivity tomography was completed in October 2013 and drilling of a 400m core is scheduled for November 2014. A team of UK Quaternary scientists will work with German and Ethiopian colleagues to produce a multi-proxy record which is anticipated will cover the last c. 500,000 years. Detailed analysis will focus on identifying the nature of climate variability during the penultimate glacial-interglacial transition (Termination II: c. 125–135 ka), once an outline chronology has been established. Key proxies are: high-resolution geochemistry; isotopes (δ13C, δ18Ocarb, δ18Odiatom, δ18Oostracod); biomarkers (including GDGTs and compound specific leaf wax δ13C and δD); pollen; diatoms; ostracods and magnetic properties. The chronological framework will be constructed using radiocarbon, Post-IR IR stimulated luminescence and 40Ar-–39Ar dating along with tephrochronology, which will be used to correlate between sites where possible. The record of climatic and ecological change, from Chew Bahir, along with previous data from Lake Tana, will be used as model input to test the hypothesis that periods of high climatic variability correlate with key biological and cultural transitions.

High precision 40 Ar/ 39 Ar laser-microprobe ages of individual sanidines, 40 Ar/ 39 Ar plateau age spectra on bulk sanidine concentrates, U-Pb zircon ages, and zircon and apatite fission-track ages from three bentonites bracketing the Cenomanian-Turonian boundary in the Western Interior of the United States suggest an age for the boundary of 93.1 ± 0.3 (2σ. The lowermost bentonite comes from the Upper Cenomanian Sciponoceras gracile biozone, and gives a weighted mean laser-fusion single-crystal 40 Ar/ 39 Ar age of 93.50 ± 0.52 Ma (2σ, standard error of the mean, n = 14) for sanidine. The middle bentonite comes from the Upper Cenomanian Neocardioceras juddii biozone, accepted in both North America and Europe as the uppermost Cenomanian ammonite zone; it gives an average single-crystal 40 / 39 Ar age of 93.33 ± 0.50 Ma ( n = 29), a bulk-sample 40 Ar/ 39 Ar plateau age of 93.09 ± 0.34 Ma (2σ) for sanidine, and concordant 206 Pb/ 238 U and 207 Pb/ 235 U ages of 93.48 ± 0.32 Ma on zircon. The upper bentonite comes from near the base of the Turonian, immediately above the first occurrence of the basal Turonian bivalve Mytiloides and sanidines from it give an average single-crystal 40 Ar/ 39 Ar age of 93.46 ± 0.60 Ma ( n = 12) and a bulk-sample 40 Ar/ 39 Ar plateau age of 92.87 ± 0.34 Ma. The composition of these Cenomanian-Turonian bentonites from Colorado and Utah, the types of phenocrysts present, and the morphology of included zircons all indicate that the pre-alteration ash was rhyolitic and probably generated in a subduction setting involving a significant crustal component.

Single-crystal40Ar/39Ar age estimates of 392,000 ± 4000 to 330,000 ± 6000 yr from Lainyamok, a middle Pleistocene fossil locality in the southern Kenya rift, document the oldest evidence from sub-Saharan Africa of a diverse, large mammal fauna consisting entirely of extant species. The inferred age of this fauna implies an upper limit for extinction of species that characterize well-calibrated, mid-Pleistocene fossil assemblages in East Africa. For its age and species richness, the Lainyamok fauna is surprising for its lack of extinct forms (e.g., the bovinePelorovis) well documented in later faunal assemblages of East and South Africa. Definitive presence of the South African blesbok(Damaliscus dorcas)is also unexpected, especially as this alcelaphine bovid is the dominant large mammal in the Lainyamok fauna. These age estimates and the faunal composition at Lainyamok indicate that geographic ranges and taxonomic associations of extant largebodied mammals were susceptible to wide fluctuations in sub-Saharan Africa over the past 330,000 yr. This inference is consistent with the hypothesis of nonanalogue, or ephemeral, biotas believed to characterize late Quaternary ecosystems of northern continents.

A newly discovered partial hominin foot skeleton from eastern Africa indicates the presence of more than one hominin locomotor adaptation at the beginning of the Late Pliocene epoch. Here we show that new pedal elements, dated to about 3.4 million years ago, belong to a species that does not match the contemporaneous Australopithecus afarensis in its morphology and inferred locomotor adaptations, but instead are more similar to the earlier Ardipithecus ramidus in possessing an opposable great toe. This not only indicates the presence of more than one hominin species at the beginning of the Late Pliocene of eastern Africa, but also indicates the persistence of a species with Ar. ramidus-like locomotor adaptation into the Late Pliocene.

Radiometric (K-Ar and Ar-40/Ar-39) age determinations of volcanic and volcaniclastic rocks, combined with structural, gravity, and seismic reflection data, are used to constrain the age of sedimentary strata contained within the seismically and volcanically active northern Malawi (Nyasa) rift and to characterize changes in basin and flank morphologies with time. Faulting and volcanism within the Tukuyu-Karonga basin began at approximately 8.6 Ma, when sediments were deposited in abroad, initially asymmetric lake basin bounded on its northeastern side by a border fault system with minor topographic relief. Extensions, primarily by a slip along the border fault, and subsequent regional isostatic compensation led to the development of a 5-km-deep basin bounded by broad uplifted flanks. Along the low-relief basin margin opposite border fault, younger stratigraphic sequences commonly onlap older wedge-shaped sequences, although their internal geometry is often progradational. Intrabasinal faulting, flankuplift, and basaltic and felsic volcanism from centers at the northern end of the basin became more important at about 2.5 Ma when cross-rift transfer faults developed to link the Tukuyu-Karonga basin to the Rukwa basin. Local uplift and volcanic construction at the northern end of the basin led to a southeastward shift in the basin's depocenter. Sequence boundaries are commonly erosional along this low-relief (hanging wall) margin and conformable in the deep lake basin. The geometry of stratigraphic sequences and the distribution of the erosion indicate that horizontal and vertical crustal movements both across and along the length of the rift basin led to changes in levels of the lake, irrespective of paleoclimatic fluctuations.

Secondary age standards are valuable in intra- and interlaboratory calibration. At present very few such standards are available for fission track dating that is older than Tertiary. Several altered volcanic ash beds occur in the Middle Jurassic Carmel Formation in southwestern Utah. The formation was deposited in a shallow marine/sabhka environment. Near Gunlock, Utah, eight ash beds have been identified. Sanidines from one of the ash beds (GUN-F) give a single-crystal laser-probe 40 Ar/ 39 Ar age of 166.3±0.8 Ma (2 σ ). Apatite and zircon fission track ages range from 152–185 Ma with typically 15–20 Ma errors (2σ). Track densities in zircons are high and most grains are not countable. Apatites are fairly common in most of the ash beds and have reasonable track densities ranging between 1.2–1.5 × 10 6 tracks/cm 2 . Track length distributions in apatites are unimodal, have standard deviations μ m, and mean track lengths of about 14–14.5 μm. High Cl apatites (F:Cl:OH ratio of 39:33:28) are particularly abundant and large in ash GUN-F, and are fairly easy to concentrate, but the concentrates contain some siderite, most of which can be removed by sieving. GUN-F shows evidence of some reworking and detriaal contamination based on older single grain 40 Ar/ 39 Ar analyses and some rounding of grains, but the apatite population appears to be largely uncontaminated. At present BJK has approximately 12 of apatite separate from GUN-F.

40Ar/39Ar dating of Pliocene tuffs from Laetoli, northern Tanzania, has refined the geochronological framework of the Laetolil Beds and overlying strata. Dated units include the Lower and Upper Laetolil Beds (4.36−3.85 Ma and 3.85−3.63 Ma, respectively), the Lower and Upper Ndolanya Beds (3.58 and 2.66 Ma, respectively), the Naibadad Beds (2.155−2.057 Ma), and the Olpiro Beds (

Accurate high-precision 40Ar/39Ar ages are limited, in part, by the degree of accuracy and precision of the measurement of the 36Ar atmospheric Ar contamination correction and the mass spectrometer mass fractionation bias (mass discrimination) correction. To improve the measurements of the low-level 36Ar signals, we have implemented digital ion-counting and multicollector data acquisition methods. The switch to the digital ion pulse counting method results in a tenfold improvement in the signal-to-noise ratio relative to analog electron multiplier measurements that are in general use in most 40Ar/39Ar laboratories. The use of ion pulse counting significantly improves low-level signal (36Ar) measurements. The improvement in low-level 36Ar measurements, however, comes at the cost of a reduced dynamic range of the electron multiplier detector, thus requiring the use of an alternate detector at times, such as a Faraday cup or analog multiplier for large signals. In turn, this requires accurate intercalibration of the detectors. Here we present a protocol that addresses these issues, one that closely tracks changes in the mass spectrometer mass discrimination and the detector intercalibration (IC) factor(s) during the time frame of an experiment, thereby improving measurement accuracy. A major advantage of our protocol is that the procedure uses the same aliquots of atmospheric Ar to monitor mass discrimination and detector IC factors, saving a significant amount of measurement time. In addition, this IC protocol may address the cause of reported inaccuracies in the measured isotopic ratio data on the “new” generation multicollector mass spectrometers. Further, we present a “time series protocol” that monitors any temporal drift in the mass spectrometer mass fractionation bias that can occur due to laboratory environmental changes.

Only one partial skeleton that includes both forelimb and hindlimb elements has been reported for Australopithecus afarensis . The diminutive size of this specimen (A.L. 288-1 ["Lucy"]) has hampered our understanding of the paleobiology of this species absent the potential impact of allometry. Here we describe a large-bodied (i.e., well within the range of living Homo ) specimen that, at 3.58 Ma, also substantially antedates A.L. 288–1. It provides fundamental evidence of limb proportions, thoracic form, and locomotor heritage in Australopithecus afarensis . Together, these characteristics further establish that bipedality in Australopithecus was highly evolved and that thoracic form differed substantially from that of either extant African ape.

The development of rise Cenozoic East African Rift System (EARS) profoundly re-shaped the landscape and significantly increased the amplitude of short-term environmental response to climate variation. In particular, the development of amplifier lakes in rift basins after three million years ago significantly contributed to this exceptional sensitivity of East Africa to climate change compared to elsewhere on the African continent. Amplifier lakes are characterized by tectonically-formed graben morphologies in combination with an extreme contrast between high precipitation in the elevated parts of the catchment and high evaporation in the lake area. Such amplifier lakes respond rapidly to moderate, precessional-forced climate shifts, and as they do so apply dramatic environmental pressure to the biosphere. Rift basins, when either extremely dry or lake-filled, form important barriers for migration, mixing and competition of different populations of animals and hominins. Amplifier lakes link long-term, high-amplitude tectonic processes and short-term environmental fluctuations. East Africa may have become the place where early humans evolved as a consequence of this strong link between different time scales. (C) 2010 Elsevier Ltd. All rights reserved.