Bosumtwi impact structure,Ghana: Geochemistry of impactites and target rocks,and search for a meteoritic component     

Xiongxin Dai  Daniel Boamah  Christian Koeberl  Wolf Uwe Reimold  Gordon Irvine  Iain McDonald 《Meteoritics & planetary science》2005,40(9-10):1493-1511

Abstract— Major and trace element data, including platinum group element abundances, of representative impactites and target rocks from the crater rim and environs of the Bosumtwi impact structure, Ghana, have been investigated for the possible presence of a meteoritic component in impact‐related rocks. A comparison of chemical data for Bosumtwi target rocks and impactites with those for Ivory Coast tektites and microtektites supports the interpretation that the Bosumtwi structure and Ivory Coast tektites formed during the same impact event. High siderophile element contents (compared to average upper crustal abundances) were determined for target rocks as well as for impactites. Chondrite‐normalized (and iron meteorite‐normalized) abundances for target rocks and impactites are similar. They do not, however, allow the unambiguous detection of the presence, or identification of the type, of a meteoritic component in the impactites. The indigenous siderophile element contents are high and possibly related to regional gold mineralization, although mineralized samples from the general region show somewhat different platinum‐group element abundance patterns compared to the rocks at Bosumtwi. The present data underline the necessity of extensive target rock analyses at Bosumtwi, and at impact structures in general, before making any conclusions regarding the presence of a meteoritic component in impactites.  相似文献   

The use of ablation‐dominated medial moraines as samplers for 10Be‐derived erosion rates of glacier valley walls,Kichatna Mountains,AK     

Dylan J. Ward  Robert S. Anderson 《地球表面变化过程与地形》2011,36(4):495-512

We use cosmogenic ¹⁰Be concentrations in amalgamated rock samples from active, ice‐cored medial moraines to constrain glacial valley sidewall backwearing rates in the Kichatna Mountains, Alaska Range, Alaska. This dramatic landscape is carved into a small ～65 Ma granitic pluton about 100 km west of Denali, where kilometer‐tall rock walls and ‘cathedral’ spires tower over a radial array of over a dozen valley glaciers. These supraglacial landforms erode primarily by rockfall, but erosion rates are difficult to determine. We use cosmogenic ¹⁰Be to measure rockwall backwearing rates on timescales of 10³–10⁴ years, with a straightforward sampling strategy that exploits ablation‐dominated medial moraines. A medial moraine and its associated englacial debris serve as a conveyor system, bringing supraglacial rockfall debris from accumulation‐zone valley walls to the moraine crest in the ablation zone. We discuss quantitatively several factors that complicate interpretation of cosmogenic concentrations in this material, including the complex scaling of production rates in very steep terrain, the stochastic nature of the rockfall erosion process, the unmixed nature of the moraine sediment, and additional cosmogenic accumulation during transport of the sediment. We sampled medial moraines on each of three glaciers of different sizes and topographic aspects. All three moraines are sourced in areas with identical rock and similar sidewall relief of ～1 km. Each sample was amalgamated from 25 to 35 clasts collected over a 1‐km longitudinal transect of each moraine. Two of the glaciers yield similar ¹⁰Be concentrations (～1·6–2·2 × 10⁴ at/g) and minimum sidewall slope‐normal erosion rates (～0·5–0·7 mm/yr). The lowest ¹⁰Be concentrations (8 × 10³ at/g) and the highest erosion rates (1·3 mm/yr) come from the largest glacier in the range with the lowest late‐summer snowline. These rates are reasonable in an alpine glacial setting, and are much faster than long‐term exhumation rates of the western Alaska Range as determined by thermochronometric studies. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

Preliminary estimates of regolith generation and mobility in the Susquehanna Shale Hills Critical Zone Observatory,Pennsylvania, using meteoric 10Be     

Nicole West  Eric Kirby  Paul Bierman  Dylan Rood 《Applied Geochemistry》2011

This study seeks to quantify the rate and timing of regolith generation in the Critical Zone at the Susquehanna Shale Hills Critical Zone Observatory (SSHO). Meteoric ¹⁰Be depth profiles were determined using measurements from 30 hillslope soil and bedrock core samples in an effort to constrain ¹⁰Be inventories. The SSHO is located in the temperate climate zone of central Pennsylvania and comprises a first-order watershed developed entirely on a Fe-rich, organic-poor, Silurian-aged shale. Two major perturbations to the landscape have occurred at SSHO in the geologically recent past, including significant and sustained periglacial activity until after the retreat of the Laurentide ice sheet (～21 ka) and deforestation during early colonial land-use. Bulk soil samples (n = 16) were collected at three locations along a planar hillslope on the southern ridge of the catchment, representing the ridge top, mid-slope and valley floor. Rock chip samples (n = 14) were also collected from a 24 m deep core drilled into the northern ridge top. All meteoric ¹⁰Be concentration profiles show a declining trend with depth, with most of the ¹⁰Be retained in the uppermost decimeters of the soil. Meteoric ¹⁰Be inventories are higher at the mid-slope and valley floor sample sites, at 3.71 ± 0.02 × 10¹⁰ at/cm² and 3.69 ± 0.02 × 10¹⁰ at/cm², than at the ridge top site (1.90 ± 0.01 × 10¹⁰ at/cm²). The ¹⁰Be inventory at the convex ridge top site implies a minimum residence time of ～10.6 ka, or if erosion is steady, an erosion rate of 19.4 ± 0.2 m/My.  相似文献   

Contaminant remediation decision analysis using information gap theory     

Dylan R. Harp  Velimir V. Vesselinov 《Stochastic Environmental Research and Risk Assessment (SERRA)》2013,27(1):159-168

Decision making under severe lack of information is a ubiquitous situation in nearly every applied field of engineering, policy, and science. A severe lack of information precludes our ability to determine a frequency of occurrence of events or conditions that impact the decision; therefore, decision uncertainties due to a severe lack of information cannot be characterized probabilistically. To circumvent this problem, information gap (info-gap) theory has been developed to explicitly recognize and quantify the implications of a severe lack of information in decision making. This paper presents a decision analysis based on info-gap theory developed for a contaminant remediation scenario. The analysis provides decision support in determining the fraction of contaminant mass to remove from the environment. An info-gap uncertainty model is developed to characterize uncertainty due to a lack of information concerning the contaminant flux. The info-gap uncertainty model groups nested, convex sets of functions defining contaminant flux over time based on their level of deviation from a nominal contaminant flux. The nominal contaminant flux defines a best estimate of contaminant flux over time based on existing, though incomplete, information. A robustness function is derived to quantify the maximum level of deviation from nominal that still ensures compliance for alternative decisions. An opportuneness function is derived to characterize the possibility of meeting a desired contaminant concentration level. The decision analysis evaluates how the robustness and opportuneness change as a function of time since remediation and as a function of the fraction of contaminant mass removed.  相似文献   

Directional anisotropy in thermal infrared measurements over Toulouse city centre during the CAPITOUL measurement campaigns: first results   总被引：2，自引：0，他引：2  

J.-P. Lagouarde  M. Irvine 《Meteorology and Atmospheric Physics》2008,102(3-4):173-185

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Predation and competition effects on the size diversity of aquatic communities     

Xavier D. Quintana  Matías Arim  Anna Badosa  José María Blanco  Dani Boix  Sandra Brucet  Jordi Compte  Juan J. Egozcue  Elvira de Eyto  Ursula Gaedke  Stéphanie Gascón  Luis Gil de Solá  Kenneth Irvine  Erik Jeppesen  Torben L. Lauridsen  Rocío López-Flores  Thomas Mehner  Susana Romo  Martin Søndergaard 《Aquatic Sciences - Research Across Boundaries》2015,77(1):45-57

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Saturn's Rings: Azimuthal variations,phase curves,and radial profiles in four colors     

W.T. Thompson  K. Lumme  W.M. Irvine  W.A. Baum  L.W. Esposito 《Icarus》1981,46(2):187-200

Four-color photographic photometry of Saturn for the 1977–1979 apparitions has been analyzed to determine the dependence of ring brightness on wavelength, solar phase angle, ring particle orbital phase angle (azimuthal effect), declination of the Earth relative to the ring plane (tilt angle), and radial distance from Saturn. Azimuthal brightness variations up to ±20% relative to the ansae are clearly apparent for the maximum of ring A, but are not detectable for ring B or the outer portion of ring A. The shape of the intensity (I) versus orbital phase angle (θ) curve varies with ring tilt (B) and probably with wavelength, and shows 180° symmetry. As characterized by its slope near the ansae, this curve suggests that the azimuthal effect increases as B decreases from 26 to ≈11°. The phase curves l(α) for the ansae show very little dependence on ring tilt (26° > B > 6°), on wavelength, or on radial distance from Saturn; possibly the curves are somewhat steeper at the smallest tilt angles and for ring A relative to ring B. The radial profile of both rings becomes flatter with decreasing tilt angle and with decreasing wavelength. The latter effect is a natural result of the classical, many-particle-thick ring model.  相似文献   

Discussion on paper by K. M. Dempsey and H. M. Irvine     

Andrei Reinhorn  K. M. Dempsey  H. M. Irvine 《地震工程与结构动力学》1980,8(1):75-77

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Pressure dependence of the thermal Grüneisen parameter,with application to the earth's lower mantle and outer core     

Roderick D. Irvine  Frank D. Stacey 《Physics of the Earth and Planetary Interiors》1975,11(2):157-165

By considering high-temperature (classical) thermal oscillations of atoms in certain simple crystal structures with purely central interatomic forces, the treatment of anharmonic oscillations is generalised to random three-dimensional motion, yielding the Vashchenko and Zubarev relationship for the Grüneisen ratio γ at any pressure. If one-dimensional atomic oscillations only are considered the equation reduces to the Dugdale-MacDonald expression. To account for non-central forces additional terms must be introduced, giving: where f = 0 for purely central forces. Calculations of f in terms of the Poisson ratio for different crystal structures have not been made, but for many materials the central-force approximation suffices. This is believed to be true both for the outer core $\text{(}\text{γ}\text{≈1.4)}$ and for the close-packed structures of the lower mantle $\text{(}\text{γ}\text{≈1.0)}$. For the upper mantle non-central atomic forces are important and we have no estimate of ($\text{γ}$ independently of laboratory values for plausible minerals which suggest γ ≈ 0.8.  相似文献   

New constraints on the late Cenozoic incision history of the New River, Virginia     

Dylan J. Ward  James A. Spotila  Gregory S. Hancock  John M. Galbraith   《Geomorphology》2005,72(1-4):54-72

The New River crosses three physiogeologic provinces of the ancient, tectonically quiescent Appalachian orogen and is ideally situated to record variability in fluvial erosion rates over the late Cenozoic. Active erosion features on resistant bedrock that floors the river at prominent knickpoints demonstrate that the river is currently incising toward base level. However, thick sequences of alluvial fill and fluvial terraces cut into this fill record an incision history for the river that includes several periods of stalled downcutting and aggradation. We used cosmogenic ¹⁰Be exposure dating, aided by mapping and sedimentological examination of terrace deposits, to constrain the timing of events in this history. ¹⁰Be concentration depth profiles were used to help account for variables such as cosmogenic inheritance and terrace bioturbation. Fill-cut and strath terraces at elevations 10, 20, and 50 m above the modern river yield model cosmogenic exposure ages of 130, 600, and 600–950 ka, respectively, but uncertainties on these ages are not well constrained. These results provide the first direct constraint on the history of alluvial aggradation and incision events recorded by New River terrace deposits. The exposure ages yield a long-term average incision rate of 43 m/my, which is comparable to rates measured elsewhere in the Appalachians. During specific intervals over the last 1 Ma, however, the New River's incision rate reached 100 m/my. Modern erosion rates on bedrock at a prominent knickpoint are between 28 and 87 m/my, in good agreement with rates calculated between terrace abandonment events and significantly faster than 2 m/my rates of surface erosion from ancient terrace remnants. Fluctuations between aggradation and rapid incision operate on timescales of 10⁴− 10⁵ year, similar to those of late Cenozoic climate variations, though uncertainties in model ages preclude direct correlation of these fluctuations to specific climate change events. These second-order fluctuations appear within a longer-term signal of dominant aggradation (until 2 Ma) followed by dominant incision. A similar signal is observed on other Appalachian rivers and may be the result of sediment supply fluctuations driven by the increased frequency of climate changes in the late Cenozoic.  相似文献