In search of the Earth‐forming reservoir: Mineralogical,chemical, and isotopic characterizations of the ungrouped achondrite NWA 5363/NWA 5400 and selected chondrites          下载免费PDF全文

Christoph Burkhardt  Nicolas Dauphas  Haolan Tang  Mario Fischer‐Gödde  Liping Qin  James H. Chen  Surya S. Rout  Andreas Pack  Philipp R. Heck  Dimitri A. Papanastassiou 《Meteoritics & planetary science》2017,52(5):807-826

High‐precision isotope data of meteorites show that the long‐standing notion of a “chondritic uniform reservoir” is not always applicable for describing the isotopic composition of the bulk Earth and other planetary bodies. To mitigate the effects of this “isotopic crisis” and to better understand the genetic relations of meteorites and the Earth‐forming reservoir, we performed a comprehensive petrographic, elemental, and multi‐isotopic (O, Ca, Ti, Cr, Ni, Mo, Ru, and W) study of the ungrouped achondrites NWA 5363 and NWA 5400, for both of which terrestrial O isotope signatures were previously reported. Also, we obtained isotope data for the chondrites Pillistfer (EL6), Allegan (H6), and Allende (CV3), and compiled available anomaly data for undifferentiated and differentiated meteorites. The chemical compositions of NWA 5363 and NWA 5400 are strikingly similar, except for fluid mobile elements tracing desert weathering. We show that NWA 5363 and NWA 5400 are paired samples from a primitive achondrite parent‐body and interpret these rocks as restite assemblages after silicate melt extraction and siderophile element addition. Hafnium‐tungsten chronology yields a model age of 2.2 ± 0.8 Myr after CAI, which probably dates both of these events within uncertainty. We confirm the terrestrial O isotope signature of NWA 5363/NWA 5400; however, the discovery of nucleosynthetic anomalies in Ca, Ti, Cr, Mo, and Ru reveals that the NWA5363/NWA 5400 parent‐body is not the “missing link” that could explain the composition of the Earth by the mixing of known meteorites. Until this “missing link” or a direct sample of the terrestrial reservoir is identified, guidelines are provided of how to use chondrites for estimating the isotopic composition of the bulk Earth.  相似文献   

Quantification of the effects of nitrates,phosphates and chlorides on soil stabilization with lime and cement     

Olivier Cuisinier  Tangi Le Borgne  Dimitri Deneele  Farimah Masrouri 《Engineering Geology》2011,117(3-4):229-235

Despite the scant quantitative data available in the literature, it has been hypothesized that some chemical compounds can have deleterious effects on soil stabilization with lime and cements (e.g., nitrates, phosphates and chlorides). This study intends to assess their influence on soil stabilization quantitatively. An original experimental procedure was followed. Selected soils were mixed with a potential deleterious compound at a concentration representative of what can be found in the field. The performance of the different mixtures in terms of soil stabilization was then assessed by performing mechanical tests on samples submitted to several curing conditions (temperature and humidity). The results showed that the tested compounds are likely to alter the soil stabilization processes and thus lower the mechanical performance of the stabilized soil. The results also showed that it is not possible to determine a single threshold value for the compounds considered because their influence on soil stabilization is also a function of the nature of the soil (silt or fine sand), the type of cement (CEM I or CEM II) and the curing conditions.  相似文献   

Instance‐based learning compared to other data‐driven methods in hydrological forecasting     

Dimitri P. Solomatine  Mahesh Maskey  Durga Lal Shrestha 《水文研究》2008,22(2):275-287

Data‐driven techniques based on machine learning algorithms are becoming popular in hydrological modelling, in particular for forecasting. Artificial neural networks (ANNs) are often the first choice. The so‐called instance‐based learning (IBL) has received relatively little attention, and the present paper explores the applicability of these methods in the field of hydrological forecasting. Their performance is compared with that of ANNs, M5 model trees and conceptual hydrological models. Four short‐term flow forecasting problems were solved for two catchments. Results showed that the IBL methods often produce better results than ANNs and M5 model trees, especially if used with the Gaussian kernel function. The study showed that IBL is an effective data‐driven method that can be successfully used in hydrological forecasting. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

A perfectly matched layer absorbing boundary condition for the second-order seismic wave equation   总被引：24，自引：0，他引：24  

Dimitri Komatitsch  Jeroen Tromp 《Geophysical Journal International》2003,154(1):146-153

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Ground motion prediction equations based on shallow crustal earthquakes in Georgia and the surrounding Caucasus          下载免费PDF全文

Jorjiashvili Nato  Shengelia Ia  Godoladze Tea  Gunia Irakli  Akubardia Dimitri 《地震科学（英文版）》2022,35(6):497-509

Strong ground motions caused by earthquakes with magnitudes ranging from 3.5 to 6.9 and hypocentral distances of up to 300 km were recorded by local broadband stations and three-component accelerograms within Georgia’s enhanced digital seismic network. Such data mixing is particularly effective in areas where strong ground motion data are lacking. The data were used to produce models based on ground-motion prediction equations (GMPEs), one benefit of which is that they take into consideration information from waveforms across a wide range of frequencies. In this study, models were developed to predict ground motions for peak ground acceleration and 5%-damped pseudo-absolute-acceleration spectra for periods between 0.01 and 10 s. Short-period ground motions decayed faster than long-period motions, though decay was still in the order of approximately 1/r. Faulting mechanisms and local soil conditions greatly influence GMPEs. The spectral acceleration (SA) of thrust faults was higher than that for either strike-slip or normal faults but the influence of strike-slip faulting on SA was slightly greater than that for normal faults. Soft soils also caused significantly more amplification than rocky sites.  相似文献   

Oxygenation episodes on the continental shelf of central Peru: Remote forcing and benthic ecosystem response     

Dimitri Gutirrez  E. Enríquez  S. Purca  L. Quipúzcoa  R. Marquina  G. Flores  M. Graco 《Progress in Oceanography》2008,79(2-4):177

The interplay between the oxygen minimum zone and remotely-forced oxygenation episodes determines the fate of the benthic subsystem off the Central Peruvian coast. We analyzed a 12 year monthly time-series of oceanographic and benthic parameters at 94 m depth off Callao, Central Peru (12°S), to analyze: (i) near-bottom oxygen level on the continental shelf in relation to dynamic height on the equator (095°W); and (ii) benthic ecosystem responses to oxygen change (macrobiotic infauna, meiofauna, and sulphide-oxidizing bacteria, Thioploca spp.). Shelf oxygenation episodes occurred after equatorial dynamic height increases one month before, consistent with the propagation of coastal trapped waves. Several but not all of these episodes occurred during El Niños. The benthic biota responded to oxygenation episodes by undergoing succession through three major ecological states. Under strong oxygen deficiency or anoxia, the sediments were nearly defaunated of macro-invertebrates and Thioploca was scarce, such that nematode biomass dominated the macro- and meiobiotas. When frequency of oxygenation events reduced the periods of anoxia, but the prevailing oxygen range was 10–20 μmol L⁻¹, mats of Thioploca formed and dominated the biomass. Finally, with frequent and intense (>40 μmol L⁻¹) oxygenation, the sediments were colonized by macrofauna, which then dominated biomass. The Thioploca state evolved during the 2002–2003 weak EN, while the macrofauna state was developed during the onset of the strong1997–1998 EN. Repeated episodes of strong oxygen deficiency during the summer of 2004, in parallel with the occurrence of red tides in surface waters, resulted in the collapse of Thioploca mats and development of the Nematode state. Ecological interactions may affect persistence or the transition between benthic ecosystem states.  相似文献   

Learning from data for wind-wave forecasting   总被引：1，自引：0，他引：1  

Ahmadreza Zamani  Dimitri Solomatine 《Ocean Engineering》2008,35(10):953-962

Along with existing numerical process models describing the wind-wave interaction, the relatively recent development in the area of machine learning make the so-called data-driven models more and more popular. This paper presents a number of data-driven models for wind-wave process at the Caspian Sea. The problem associated with these models is to forecast significant wave heights for several hours ahead using buoy measurements. Models are based on artificial neural network (ANN) and instance-based learning (IBL) .To capture the wind-wave relationship at measurement sites, these models use the existing past time data describing the phenomenon in question. Three feed-forward ANN models have been built for time horizon of 1, 3 and 6 h with different inputs. The relevant inputs are selected by analyzing the average mutual information (AMI). The inputs consist of priori knowledge of wind and significant wave height. The other six models are based on IBL method for the same forecast horizons. Weighted k-nearest neighbors (k-NN) and locally weighted regression (LWR) with Gaussian kernel were used. In IBL-based models, forecast is made directly by combining instances from the training data that are close (in the input space) to the new incoming input vector. These methods are applied to two sets of data at the Caspian Sea. Experiments show that the ANNs yield slightly better agreement with the measured data than IBL. ANNs can also predict extreme wave conditions better than the other existing methods.  相似文献   

Trophic modeling of the Northern Humboldt Current Ecosystem, Part I: Comparing trophic linkages under La Niña and El Niño conditions   总被引：1，自引：1，他引：0  

Jorge Tam  Marc H. Taylor  Vernica Blaskovic  Pepe Espinoza  R. Michael Balln  Erich Díaz  Claudia Wosnitza-Mendo  Juan Argüelles  Sara Purca  Patricia Ayn  Luis Quipuzcoa  Dimitri Gutirrez  Elisa Goya  Noemí Ochoa  Matthias Wolff 《Progress in Oceanography》2008,79(2-4):352

The El Niño of 1997–98 was one of the strongest warming events of the past century; among many other effects, it impacted phytoplankton along the Peruvian coast by changing species composition and reducing biomass. While responses of the main fish resources to this natural perturbation are relatively well known, understanding the ecosystem response as a whole requires an ecotrophic multispecies approach. In this work, we construct trophic models of the Northern Humboldt Current Ecosystem (NHCE) and compare the La Niña (LN) years in 1995–96 with the El Niño (EN) years in 1997–98. The model area extends from 4°S–16°S and to 60 nm from the coast. The model consists of 32 functional groups of organisms and differs from previous trophic models of the Peruvian system through: (i) division of plankton into size classes to account for EN-associated changes and feeding preferences of small pelagic fish, (ii) increased division of demersal groups and separation of life history stages of hake, (iii) inclusion of mesopelagic fish, and (iv) incorporation of the jumbo squid (Dosidicus gigas), which became abundant following EN. Results show that EN reduced the size and organization of energy flows of the NHCE, but the overall functioning (proportion of energy flows used for respiration, consumption by predators, detritus and export) of the ecosystem was maintained. The reduction of diatom biomass during EN forced omnivorous planktivorous fish to switch to a more zooplankton-dominated diet, raising their trophic level. Consequently, in the EN model the trophic level increased for several predatory groups (mackerel, other large pelagics, sea birds, pinnipeds) and for fishery catch. A high modeled biomass of macrozooplankton was needed to balance the consumption by planktivores, especially during EN condition when observed diatoms biomass diminished dramatically. Despite overall lower planktivorous fish catches, the higher primary production required-to-catch ratio implied a stronger ecological impact of the fishery and stresses the need for precautionary management of fisheries during and after EN. During EN energetic indicators such as the lower primary production/total biomass ratio suggest a more energetically efficient ecosystem, while reduced network indicators such as the cycling index and relative ascendency indicate of a less organized state of the ecosystem. Compared to previous trophic models of the NHCE we observed: (i) a shrinking of ecosystem size in term of energy flows, (ii) slight changes in overall functioning (proportion of energy flows used for respiration, consumption by predators and detritus), and (iii) the use of alternate pathways leading to a higher ecological impact of the fishery for planktivorous fish.  相似文献   

Pliocene to Quaternary deformation in the Var Basin (Nice, SE France) and its interpretation in terms of “slow-active” faulting     

Victorien Bauve  Yann Rolland  Guillaume Sanchez  Gérard Giannerini  Dimitri Schreiber  Michel Corsini  Jean-Louis Perez  Adrien Romagny 《Swiss Journal of Geoscience》2012,105(3):361-376

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Reactive Transport Modeling of Subaqueous Sediment Caps and Implications for the Long-Term Fate of Arsenic, Mercury, and Methylmercury   总被引：1，自引：0，他引：1  

Brad A. Bessinger  Dimitri Vlassopoulos  Susana Serrano  Peggy A. O’Day 《Aquatic Geochemistry》2012,18(4):297-326

A 1-D biogeochemical reactive transport model with a full set of equilibrium and kinetic biogeochemical reactions was developed to simulate the fate and transport of arsenic and mercury in subaqueous sediment caps. Model simulations (50?years) were performed for freshwater and estuarine scenarios with an anaerobic porewater and either a diffusion-only or a diffusion plus 0.1-m/year upward advective flux through the cap. A biological habitat layer in the top 0.15?m of the cap was simulated with the addition of organic carbon. For arsenic, the generation of sulfate-reducing conditions limits the formation of iron oxide phases available for adsorption. As a result, subaqueous sediment caps may be relatively ineffective for mitigating contaminant arsenic migration when influent concentrations are high and sorption capacity is insufficient. For mercury, sulfate reduction promotes the precipitation of metacinnabar (HgS) below the habitat layer, and associated fluxes across the sediment–water interface are low. As such, cap thickness is a key design parameter that can be adjusted to control the depth below the sediment–water interface at which mercury sulfide precipitates. The highest dissolved methylmercury concentrations occur in the habitat layer in estuarine environments under conditions of advecting porewater, but the highest sediment concentrations are predicted to occur in freshwater environments due to sorption on sediment organic matter. Site-specific reactive transport simulations are a powerful tool for identifying the major controls on sediment- and porewater-contaminant arsenic and mercury concentrations that result from coupling between physical conditions and biologically mediated chemical reactions.  相似文献