Highly siderophile element and 187Re‐187Os isotopic systematics of ungrouped achondrite Northwest Africa 7325: Evidence for complex planetary processes     

Gregory J. Archer  Richard J. Walker  Anthony J. Irving 《Meteoritics & planetary science》2019,54(5):1042-1050

The abundances of highly siderophile elements (HSE; including Re, Os, Ir, Ru, Pt, and Pd) and ¹⁸⁷Re‐¹⁸⁷Os isotopic systematics were determined for two fragments from ungrouped achondrite NWA 7325. Rhenium‐Os systematics are consistent with closed‐system behavior since formation or soon after. The abundances of the HSE were therefore largely unaffected by late‐stage secondary processes such as shock or terrestrial weathering. As an olivine gabbro cumulate, this meteorite has a bulk composition consistent with derivation from a body that produced a core, mantle, and crust. Also consistent with derivation from a body that produced a core, both fragments of NWA 7325 have HSE abundances that are highly depleted compared to bulk chondrites. One fragment has ~0.002× CI chondrite Ir and relative HSE abundances similar to bulk chondrites. The other fragment has ~0.0002× CI chondrite Ir and relative HSE abundances that are fractionated compared to bulk chondrites. The chondritic relative HSE abundances of the fragment characterized by higher HSE abundances most likely reflect the addition of exogenous chondritic material during or after crystallization by surface impacts. The HSE in the other fragment is likely more representative of the parent body crust. One formation model that can broadly account for the HSE abundances in this fragment is multiple episodes of low‐pressure metal‐silicate equilibration, followed by limited late accretion and mantle homogenization. Given the different HSE compositions of the two adjoining fragments, this meteorite provides an example of the overprint of global processes (differentiation and late accretion) by localized impact contamination.  相似文献   

Hydrologic artifact dispersals at Pingasagruk,North Coast,Alaska     

Gregory A. Reinhardt 《Geoarchaeology》1993,8(6):493-513

Situated on a coastal sand bar, Pingasagruk is a habitation site with prehistoric and historic components. Data from systematic surface sampling led to analyses that show different artifact distribution patterns on and off this site and suggest the dispersals are due to storms and erosion, in addition to human activity. To explain the disparate distributions of cultural materials, this study uses a model of hydrologic artifact dispersal and, possibly, differential sorting. Differences in distribution patterns at Pingasagruk highlight the importance of considering how cultural and noncultural transformation processes affect the occurrence of sites and their artifacts. © 1993 John Wiley & Sons, Inc.  相似文献   

Significance of infiltrated clays within the Lower Permian Yellow Sands of north-east England     

Gregory D. Price 《Geological Journal》1996,31(2):189-195

The Permian Yellow Sands are preserved as nine aeolian ridges in north-east England. The sands are soft and friable and locally contain large amounts of interstitial detrital clays, which are interpreted as the product of early mechanical clay infiltration. Features within the sands considered to be due to mechanical clay infiltration include grain coatings, meniscus and geopetal fabrics. The infiltrated clays mask crystal nucleation sites on grain surfaces and often amalgamate into lenses or form continuous clay-rich layers. Calcite and quartz cement growth has consequently been limited and often confined to the coarser, generally clay-free horizons. The meniscus and geopetal fabrics were formed early during accretion of the dunes and helped to bind and stabilize the sediment, which restricted reworking during the Zechstein transgression to the uppermost few decimeters.  相似文献   

Improvement in the calculation of anti-Stokes energy transfer and experimental justification based on Er_0.01Yb_xY_{1 - 0.01 - x}VO₄ crystal     

陈晓波  卢建  张蕴芝  徐小灵  冯宝华  王策  Gregory J. Salamo  杨国建 《海洋学报(英文版)》2010,29(9)

The improvement on the calculation of anti-Stokes energy transfer rate is studied in the present work. The additional proportion coefficient between Stokes and anti-Stokes light intensities of quantum Raman scattering theory as compared with the classical Raman theory is introduced to successfully describe the anti-Stokes energy transfer. The theoretical formula for the improvement on the calculation of anti-Stokes energy transfer rate is derived for the first time in this study. The correctness of introducing coefficient exp{ΔE / kT} from well-known Raman scatter theory is demonstrated also. Moreover, the experimental lifetime measurement in Er_0.01Yb_xY_{1 - 0.01 - x}VO₄ crystal is performed to justify the validity of our important improvement in the original phonon-assisted energy transfer theory for the first time.  相似文献   

Spatio-temporal patterns and changes in environmental attitudes and place attachment in Gauteng,South Africa     

Simangele Dlamini  Solomon G.Tesfamichael  Gregory D.Breetzke  Tholang Mokhele 《地球空间信息科学学报》2021,24(4):666-677

In this study, we employed a number of geospatial techniques to examine the spatio-temporal patterns and changes of environmental attitudes and place attachment...  相似文献   

Leveraging modern climatology to increase adaptive capacity across protected area networks     

Jennifer E. Davison  Lisa J. Graumlich  Erika L. Rowland  Gregory T. Pederson  David D. Breshears 《Global Environmental Change》2012,22(1):268-274

Human-driven changes in the global environment pose an increasingly urgent challenge for the management of ecosystems that is made all the more difficult by the uncertain future of both environmental conditions and ecological responses. Land managers need strategies to increase regional adaptive capacity, but relevant and rapid assessment approaches are lacking. To address this need, we developed a method to assess regional protected area networks across biophysically important climatic gradients often linked to biodiversity and ecosystem function. We plot the land of the southwestern United States across axes of historical climate space, and identify landscapes that may serve as strategic additions to current protected area portfolios. Considering climate space is straightforward, and it can be applied using a variety of relevant climate parameters across differing levels of land protection status. The resulting maps identify lands that are climatically distinct from existing protected areas, and may be utilized in combination with other ecological and socio-economic information essential to collaborative landscape-scale decision-making. Alongside other strategies intended to protect species of special concern, natural resources, and other ecosystem services, the methods presented herein provide another important hedging strategy intended to increase the adaptive capacity of protected area networks.  相似文献   

Attenuation of compressional waves in peridotite measured as a function of temperature at 200 MPa   总被引：3，自引：0，他引：3  

Hiroki Sato  I. Selwyn Sacks  Tsutomu Murase  Gregory Muncill  Hiroyuki Fukuyama 《Pure and Applied Geophysics》1988,128(1-2):433-447

A technique has been developed to determine attenuation in rocks at high temperature using a gas-media, high-pressure apparatus. A pulse transmission technique and a spectral ratio method are used to study compressional seismic properties of rocks. Seismic waves are transmitted to and from the sample through buffer rods of mullite. The effect of seismic wave reflections within the sample assembly are cancelled out by taking ratios of the spectra measured at different temperatures. In order to obtain good signal-to-noise ratio for resolving the attenuation at high pressure and temperature, special care is taken in the sample assembly and the ultrasonic coupling between the sample, buffer rods and transducers. A very tight connection of the sample-buffer rod-transducer is essential for obtaining high frequency signals (>300 kHz) at high temperature. A small mass is attached to each outside end of the transducer to drive low frequency signals (<250 kHz) into the sample. Before attenuation measurements, the sample and the buffer rods are tightly compacted in a platinum tube at high pressure and room temperature to ensure pressure seal of the sample assembly. The frequency range of measurement covers 50 to 450 kHz for the sample. Attenuation is very small in the buffer rod compared to the sample for the entire temperature range of the study. Because of the small attenuation, a wide frequency band of 50 kHz to 3.2 MHz can be covered for investigating the attenuation in the buffer rod. The technique has been used to measure attenuation at high confining pressure, and temperatures including sub- and hyper-solidus of upper mantle rocks. Therefore, effects of partial melting on attenuation can be studied.The method is applied to the attenuation measurement in a peridotite as a function of temperature to 1225°C at 200 MPa confining pressure. At high temperature, signal amplitude decays more rapidly at high frequency than at low frequency, from which attenuation (andQ) can be determined using a spectral ratio method. No frequency dependence ofQ is resolved for both the sample and the buffer rod over the entire temperature and frequency ranges of the measurement. The results show thatQ decreases rapidly with increasing temperature even in the temperature range below the solidus of peridotites. Such temperature sensitivity ofQ is probably more useful to probe thermal structure in the upper mantle than that of conductivity at temperatures below the solidus. The results in this study are compared with available seismic velocity, electrical conductivity and solidus data for peridotites, suggesting that there is no discontinuous change in both mechanical and electrical properties of peridotites at the solidus temperature. Even at hypersolidus temperatures, it appears that velocity drops and conductivity increases continuously (not abruptly) with increasing melt fraction. This implies that mechanical and electrical properties of the upper mantle will gradually change at the boundary where the geotherm crosses the solidus.  相似文献   

Issues of Spatial and Temporal Variability in Climate Impact Studies     

Gregory J. Carbone 《The Professional geographer》1995,47(1):30-40

The accuracy of impact estimates relating climate change to regional-scale agricultural production is constrained by the temporal and spatial resolution of climate change projections. Several techniques have been used to compensate for these limitations in order to provide reasonable estimates of the impact of climate change on crop yield. One approach assumes that variability over time can substitute for spatial variability, thereby reducing the need to estimate the impacts at a spatially dense network of stations—an assumption that has not been generally tested. This study evaluates this assumption using methods similar to those employed in the climate impact literature. The findings suggest that current practices are generally defensible if the goal is to provide a range of possible crop responses to climate change. However, the results also show that the assumption is highly sensitive to specific interactions at the soil-plant-atmosphere interface and, consequently, does not hold under certain circumstances.  相似文献   

Comparing Human and Neural Network Learning of Climate Categories     

Robert Lloyd  Gregory Carbone 《The Professional geographer》1995,47(3):237-250

This study compares how humans and neural networks classify climate types. Human subjects were asked to classify climates from monthly temperature and precipitation patterns. To model their learning process, the same data were used to produce input vectors that trained a pattern associator neural network. Both human subjects and the neural network classified climates accurately after 10 rounds of supervised learning. The neural network successfully modeled the rate of human learning and the ability to learn specific climate categories. Moreover, the neural network weights used to classify climates correspond to distinct visual characteristics in temperature and precipitation. These results suggest that neural networks can model the formation of visual categories.  相似文献   

Book review     

Charles Fairhurst  Renata Dmowska  Gregory L. Duckworth  Brian J. Mitchell  David V. Wiltschko  Charles G. Sammis 《Pure and Applied Geophysics》1985,123(5):791-803

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