1,721,084 research outputs found
Herbivore enamel carbon isotopic composition and the environmental context of Ardipithecus at Gona, Ethiopia
No full textArdipithecus fossils found in late Miocene and early Pliocene deposits in the Afar region of Ethiopia, along with Sahelanthropus tchadensis from Chad and Orrorin tugenensis from Kenya, are among the earliest known human ancestors and are considered to be the predecessors to the subsequent australopithecines (Australopithecus anamensis and Australopithecus afarensis). Current paleoenvironmental reconstructions suggest a wooded habitat for both Ardipithecus kadabba and Ardipithecus ramidus but more open and varied environments for other hominids living in Africa during the late Miocene and early Pliocene. To further evaluate the environmental context of Ardipithecus, we present stable carbon isotope data of 182 fossil herbivore teeth from Ardipithecus-bearing fossil deposits in the Gona Paleoanthropological Research Project area, in the Afar region of Ethiopia. The sampled teeth include representatives of all major fossil herbivore taxa and the majority of the mammalian biomass that lived in the same time and place as the hominids. When compared to extant herbivores from East Africa, the spectra of isotopic results from herbivores found in late Miocene Ar. kadabba and early Pliocene Ar. ramidus sites at Gona are most similar to isotopic values from extant herbivores living in bushland and grassland regions and dissimilar to those from herbivores living in closed-canopy forests, montane forests, and high-elevation grasslands. The tooth enamel isotopic data from fossil herbivores make it clear that Ardipithecus at Gona lived among a guild of animals whose diet was dominated by C_4 grass, and where there is no record of closed-canopy vegetation
Stratigraphy and geochronology of the late Miocene Adu-Asa Formation at Gona, Ethiopia
No full textThe Gona area includes many rich fossil localities that are of great consequence to the study of human evolution. The Adu-Asa Formation, containing the oldest of these fossils, consists of nearly 200 m of fossil-bearing sedimentary rocks in thin (≤30 m), laterally variable sections interlayered with abundant basaltic lava flows. These volcanic and sedimentary rocks dip gently to the east and are repeated by north-northwest–trending, mostly west-dipping normal faults that accommodate extension in the Afar Rift.
The volcanic rocks in the Adu-Asa Formation are strongly bimodal. Basaltic lavas and tuffs are abundant, but we have also identified a rhyolite center and seven different silicic, or dominantly silicic, tuffs. Of these tuff units, we were able to identify four major tuffs across the Adu-Asa Formation at Gona by combining geochemical comparisons with detailed stratigraphic sections through fossil-bearing deposits: the Sifi, the Kobo'o, the Belewa, and the Ogoti Tuffs. New ^(40)Ar/^(39)Ar dates of these and other tuffs, as well as basalt flows, indicate that the formation spans the period from 5.2 Ma to 6.4 Ma, although the oldest deposits within the Gona Paleoanthropological Research Project (GPRP) area have yet to be thoroughly surveyed. Known fossil localities within the Adu-Asa Formation at Gona are grouped into three temporal clusters, ranging in age from ca. 6.4 Ma to ca. 5.5 Ma
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Experimental developments and application of carbon-14 and in situ cosmogenic nuclide dating techniques
No full textGastropod shells are commonly preserved in the geologic record, but are often avoided for radiocarbon dating because some taxa incorporate ¹⁴C-deficient carbon during shell formation. To determine the potential of minute (3-10 mm in length) gastropods for ¹⁴C dating, we collected specimens en vivo from alluvium dominated by Paleozoic carbonates and adjacent to springs with ¹⁴C-deficient water present at the surface. We found that at least three taxa, Pupilla blandi, Euconulus fulvus, and Succineidae, do not incorporate ¹⁴C-deficient carbon during shell formation. We ¹⁴C dated fossil shells from these taxa recovered from the Coro Marl, a paleowetland deposit in the San Pedro Valley of southern Arizona. Our results indicate that the timing of high water table conditions in the valley occurred between >28 and 15.5 ka, which is consistent with moist conditions found in other paleoclimate records from the American Southwest. The summit area of Mauna Kea, Hawaii was covered intermittently by ice caps during the Late Pleistocene. The maximum extents of the last two ice caps (older and younger Makanaka) were similar, reaching ∼800 m below the summit. We have developed a new chronology for these glaciations using cosmogenic ³⁶Cl which shows that the ice caps began retreating from their maximum extents at 23.3 ± 2.3 ka and 13.0 ± 0.9 ka, respectively, broadly coincident with the last glacial maximum and the Younger Dryas chronozone. The potential for using in situ cosmogenic ¹⁴C to determine surface exposure ages of Holocene landforms, quantify erosion rates, and decipher complex exposure histories when used in conjunction with other cosmogenic nuclides is well known. Before this potential can be realized, however, protocols for isolating and extracting in situ ¹⁴C must be developed. Analytical techniques have been developed previously to isolate in situ ¹⁴C from quartz and carbonate. Although these minerals can be found in most places on Earth, they are usually absent from basaltic terrains. To fill this gap, we conducted numerous chemical pretreatment experiments and step-heated extractions aimed at isolating and extracting in situ ¹⁴C from olivine. Our results suggest that step-heated extractions alone may be sufficient to isolate in situ ¹⁴C from olivine
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Late Quaternary paleohydrology and surficial processes of the Atacama Desert, Chile: Evidence from wetland deposits and stable isotopes of soil salts
No full textThe origin of pedogenic salts in the Atacama Desert has long been debated. Possible salt sources include in situ weathering at the soil site, local sources such as aerosols from the adjacent Pacific Ocean or salt-encrusted playas, and extra-local atmospheric dust. To identify the origin of Ca and S in Atacama soil salts, we determined δ ³⁴S and ⁸⁷Sr/⁸⁶Sr values of soil gypsum/anhydrite and ⁸⁷Sr/⁸⁶Sr values of calcium carbonate along three east-west trending transects in the Atacama. Our results demonstrate the strong influence of marine aerosols on soil gypsum/anhydrite development in areas where marine fog penetrates inland. In areas where the Coastal Cordillera is >1200 m, however, coastal fog cannot penetrate inland and the contribution of marine aerosols to soils is greatly reduced. Salts in inland soils appear to originate from eolian redistribution of playa salts that are precipitated from evaporated ground water. This ground water has acquired its dissolved solids from water-rock interactions (both thermal and low-temperature) along flowpaths from recharge areas in the Andes. The spatial distribution of high-grade nitrate deposits appears to correspond with areas that receive the lowest fluxes of local dust, supporting arguments for an atmospheric source of nitrate. Ground water in the Atacama is derived from precipitation in the High Andes (>3500 m) that infiltrates soils and then flows down the Pacific slope of the Andes to feed aquifers within the hyperarid core of the Atacama Desert. At many locations, ground water surfaces and creates springs, marshes, and wetlands. In order to track late Quaternary fluctuations in ground-water recharge, paleowetland deposits at eight separate locations (between 18°-26°S) were mapped and dated. Over 200 AMS ¹⁴C dates on a variety of materials provide firm age control on these deposits. Replication of time-stratigraphic units from an assortment of hydrologic settings and varying distances from recharge areas in the Andes show that ground-water systems are responding to regional changes in climate and that response times are probably short (<1000 years). Results suggest that the wettest period represented by deposits was during the late Glacial/early Holocene (∼16-9.5 ka B.P.) and that a moderately wet period occurred during the mid-Holocene (8--3 ka B.P.). Major drops in Atacama water tables, due to regional drought, occurred between 9.5-8 and ∼3 ka B.P. The late Holocene was characterized by generally lower water tables than during the mid-Holocene and subject to more frequent water table drops. Fluctuations in tropical Pacific Sea Surface Temperatures, the Walker Circulation, and ENSO variability is thought to be the major control on precipitation over this region during the late Quaternary
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Applications of noble gas cosmogenic nuclides to geomorphology
No full textThe buildup of the cosmogenic nuclides ³He and ²¹Ne in surficial rocks permit exposure ages and erosion rates to be estimated. This dissertation extends the cosmogenic exposure technique to garnets, plagioclase with significant nonspallation ²¹Ne components, and alluvial fill terraces. Garnets from Nanga Parbat, Pakistan have low nucleogenic ³He and moderate radiogenic ⁴He concentrations. ³He exposure ages from garnets in glacial erratics indicate glacial advances at Nanga Parbat at about 16 ka and 55 ka. 3He in alluvial garnets suggests that denudation in small unglaciated basins proceeds 5 to 7 times slower than glacial erosion, and 10 to nearly 100 times slower than regional rock exhumation and surface uplift. Rocks older than several million years possess nucleogenic and mugenic ²¹Ne and ³He components that must be resolved for accurate exposure ages. These nonspallation components in plagioclase and clinopyroxene from the Miocene Columbia River Basalt Group are best isolated with shielded samples. Analyses of ⁴He, U, Th, and Li systematically underpredict the amount of nonspallation ²¹Ne and ³He present in shielded samples, probably because of mugenic production. Step heating experiments suggest that ²¹Ne diffusive loss from plagioclase is possible, but most samples do not exhibit such ²¹Ne loss. The ratio of ²¹Ne in plagioclase and ³He in clinopyroxene is generally constant after correction for the nonspallation component, indicating that little or no ²¹Ne loss has occurred. The last highly erosive floods at Grand Coulee occurred at about 21 ka, early in the cycle of Missoula flooding. Nuclide inheritance must be resolved for accurate exposure ages of stream fill terraces. Depth profiles of cosmogenic ²¹Ne in quartz from terraces on the Pajarito Plateau, northern New Mexico resolve nuclide inheritance. Three patterns of depth profiles are recognized: (1) downward decreasing; (2) downward increasing; and (3) uniform; types 2 and 3 are associated with cumulate deposits and bioturbation, respectively. Inheritance corrected exposure ages for the terraces agree with independent radiocarbon and soil development ages. Denudation rates estimated from the profiles are higher for fill terraces than for straths.This item was digitized from a paper original and/or a microfilm copy. If you need higher-resolution images for any content in this item, please contact us at [email protected] file replaced with corrected file October 2023
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Fire frequency, nutrient concentrations and distributions, and δ¹³C of soil organic matter and plants in a southeastern Arizona grassland
No full textOver the past century, woody plants and shrubs have increased in abundance at the expense of grasslands in many semiarid regions. The availability and concentrations of nutrients influence the relative success of plants, but the effects of fire frequency on soil nutrients is unknown for semiarid grasslands. On the gunnery ranges of Fort Huachuca in southeastern Arizona, study sites were established to examine the effects of fire frequency on soil biogeochemistry, plant biochemistry, and δ¹³C values in soil organic matter (SOM). The sites were on homogeneous granitic alluvium where wildfire frequency history is known from 1973 to present and no cattle grazing has occurred in recent decades. Subplots represent fire frequencies of no burns, 3 fires per decade, and 5 fires per decade. The "no burn" plot has abundant C3 Prosopis velentina (mesquite) trees, whereas the burned plots are open C4-dominated grasslands with scattered mesquite trees. Prosopis trees have altered SOM pools by the concentration of plant nutrients and the addition of isotopically light shrub litter. Frequent fires have altered the basic geochemistry and nutrient availabilities of the soil, and the changes appear to be significant enough to affect plant growth. Soil pH increases with burning frequency, and TOC, total nitrogen, and plant-available phosphorus show significant increases on the infrequently burned plot. Burning is advantageous for preservation or restoration of grasslands, as total living grass biomass is greater on the two burned plots. Root biomass 11 is significantly lower on the "frequently burned" plot. Concentrations of the key nutrients nitrogen and phosphorus are reduced in plants on the burned sites compared to plants on the unburned site. Fires help re-distribute nutrients but evidence of nutrient concentrations and δ¹³C values are retained in SOM for many decades. Estimates of bulk carbon turnover rates range from 112 to 504 years. Evidence for modern C3 shrub expansion is found in the shift of SOM δ¹³C values from values characteristic of C4 grasses to C3 shrubs in surface soil layers. δ¹³CSOM values indicate that the Holocene and Late Pleistocene were dominated by C4 grasslands, and the pre-Late Pleistocene vegetation was a C4-grass savanna with abundant C3 plants
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Advances in the isotopic analysis of biogenic phosphates and their utility in ecophysiological studies of aquatic vertebrates
No full textDistinguishing marine and freshwater animals in the fossil record is a long-standing problem in paleontology. The physiological tolerances of extinct animals usually are inferred from environmental indicators and/or on the physiology of nearest living relatives. These types of evidence are often ambiguous and may be confounded by factors such as post-mortem transport and polymorphism in the living relatives. A solution to this problem is to combine these types of data with analyses of the oxygen isotope compositions of the phosphate (δ¹⁸O(p)) and the carbon isotope compositions of the carbonate (δ¹³C(sc)) of teeth and bones, to determine whether the ingested water and diet, respectively, were fresh or marine. The power of this approach is illustrated here in a study of the early evolution of cetaceans (whales, dolphins and porpoises). Changes in δ¹⁸O(p) and δ¹³C(sc) of the teeth and bones of early cetaceans documented here indicate that fully marine cetaceans existed by the middle Eocene and that some species exploited both marine and freshwater environments. This isotopic approach requires the avoidance of isotopically altered specimens. For this reason, the second component of this work deals with criteria for recognizing isotopically altered fossils. In contrast to one recent study, I found a positive correlation between δ¹⁸O(p) and δ¹⁸O(sc) not only in mammals but also in fish and reptiles. This correlation can be used as a test of whether the original isotopic composition is preserved in fossil specimens. Another approach to this problem is to make analyses of samples taken along growth transect of a fossil tooth or bone. Growth-transect analyses could resolve whether within-species isotopic variation represents differences in preservation or ontogenetic shifts in diet or habitat. In support of this goal, a new method for the analysis of phosphate oxygen is presented. This new method differs from all previous methods in that it involves no chemical reaction, but rather high-temperature (>725°C) equilibrium oxygen isotope exchange between CO₂ and Ag₃PO₄ As the amount of CO₂ is controlled by the analyst, small phosphate samples may be analyzed, making this method potentially useful for growth-transect analyses.This item was digitized from a paper original and/or a microfilm copy. If you need higher-resolution images for any content in this item, please contact us at [email protected] file replaced with corrected file October 2023
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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