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51.
Numerical models constitute the most advanced physical-based methods for modeling complex ground water systems. Spatial and/or temporal variability of aquifer parameters, boundary conditions, and initial conditions (for transient simulations) can be assigned across the numerical model domain. While this constitutes a powerful modeling advantage, it also presents the formidable challenge of overcoming parameter uncertainty, which, to date, has not been satisfactorily resolved, inevitably producing model prediction errors. In previous research, artificial neural networks (ANNs), developed with more accessible field data, have achieved excellent predictive accuracy over discrete stress periods at site-specific field locations in complex ground water systems. In an effort to combine the relative advantages of numerical models and ANNs, a new modeling paradigm is presented. The ANN models generate accurate predictions for a limited number of field locations. Appending them to a numerical model produces an overdetermined system of equations, which can be solved using a variety of mathematical techniques, potentially yielding more accurate numerical predictions. Mathematical theory and a simple two-dimensional example are presented to overview relevant mathematical and modeling issues. Two of the three methods for solving the overdetermined system achieved an overall improvement in numerical model accuracy for various levels of synthetic ANN errors using relatively few constrained head values (i.e., cells), which, while demonstrating promise, requires further research. This hybrid approach is not limited to ANN technology; it can be used with other approaches for improving numerical model predictions, such as regression or support vector machines (SVMs). 相似文献
52.
53.
Contributors to chromium isotope variation of meteorites 总被引:3,自引:0,他引:3
Liping Qin Conel M.O’D. Alexander Mary F. Horan 《Geochimica et cosmochimica acta》2010,74(3):1122-400
We report the results of a comprehensive, high precision survey of the Cr isotopic compositions of primitive chondrites, along with some differentiated meteorites. To ensure complete dissolution of our samples, they were first fused with lithium borate-tetraborate at 1050-1000 °C. Relative to the NIST Cr standard SRM 3112a, carbonaceous chondrites exhibit excesses in 54Cr/52Cr from 0.4 to 1.6ε (1ε = 1 part in 10,000), and ordinary chondrites display a common 54Cr/52Cr deficit of ∼0.4ε. Analyses of acid-digestion residues of chondrites show that carbonaceous and ordinary chondrites share a common 54Cr-enriched carrier, which is characterized by a large excess in 54Cr/52Cr (up to 200ε) associated with a very small deficit in 53Cr/52Cr (<2ε). We did not find 54Cr anomalies in either bulk enstatite chondrites or in leachates of their acid-digestion residues. This either requires that the enstatite chondrite parent bodies did not incorporate the 54Cr anomaly carrier phase during their accretion, or the phase was destroyed by parent body metamorphism. Chromium in the terrestrial rocks and lunar samples analyzed here show no deviation from the NIST SRM 3112a Cr standard. The eucrite and Martian meteorites studied exhibit small deficits in 54Cr/52Cr. The 54Cr/52Cr variations among different meteorite classes suggest that there was a spatial and/or temporal heterogeneity in the distribution of a 54Cr-rich component in the inner Solar System.We confirm the correlated excesses in 54Cr/52Cr and 53Cr/52Cr for bulk carbonaceous chondrites, but the new data yield a steeper slope (∼6.6) than that reported in Shukolyukov and Lugmair (2006). The correlated excesses may affect the use of the Mn-Cr chronometer in carbonaceous chondrites. We could not confirm the bulk carbonaceous chondrite Mn-Cr isochron reported by Shukolyukov and Lugmair (2006) and Moynier et al. (2007), mostly because we find much smaller total variations in ε53Cr (∼0.2). All bulk chondrites have small ε53Cr excesses (up to 0.3) relative to the Earth, most likely reflecting the sub-chondritic Mn/Cr ratio of the Earth. The ε53Cr variations in chondrites do seem to grossly correlate with Mn/Cr and yield an initial Solar System 53Mn/55Mn value of 5.4(±2.4) × 10−6, corresponding to an absolute age of 4566.4 (±2.2) Ma.Nuclear interactions with cosmic rays result in coupled excesses in ε54Cr and ε53Cr with a ∼4:1 ratio in phases with high Fe/Cr. These are most dramatically demonstrated in the iron meteorite Carbo, showing excesses in ε54Cr of up to 140ε. These new results show that the Mn-Cr chronometer should be used with caution in samples/minerals with high Fe/Cr and long cosmic ray exposure ages. 相似文献
54.
Marilyn B. Vogel David J. Des Marais Mary N. Parenteau Linda L. Jahnke Kendra A. Turk Michael D.Y. Kubo 《Sedimentary Geology》2010,223(3-4):265-280
Gypsum (CaSO4·2H2O) deposits from a range of sedimentary environments at Guerrero Negro, Baja California Sur, Mexico were investigated for microscale texture and composition in order to differentiate features formed under substantial microbial influence from those for which microbial effects were relatively minor or absent. Gypsum deposits were classified according to their sedimentary environment, textures, crystal habit, brine composition and other geochemical factors. The environments studied included subaqueous sediments in anchialine pools and in solar salterns, as well as subsurface sediments of mudflats and saltpans. Gypsum that developed in the apparent absence of biofilms included crystals precipitated in the water column and subsedimentary discs that precipitated from phreatic brines. Subsedimentary gypsum developed in sabkha environments exhibited a sinuous microtexture and poikilitically enclosed detrital particles. Water column precipitates had euhedral prismatic habits and extensive penetrative twinning. Gypsum deposits influenced by biofilms included bottom nucleated crusts and gypsolites developing in anchialine pools and saltern ponds. Gypsum precipitating within benthic biofilms, and in biofilms within subaerial sediment surfaces provided compelling evidence of biological influences on crystal textures and habits. This evidence included irregular, high relief surface textures, accessory minerals (S°, Ca-carbonate, Sr/Ca-sulfate and Mg-hydroxide) and distinctive crystal habits such as equant forms and crystals having distorted prism faces. 相似文献
55.
Juraj Majzlan Peter Glasnák Robert A. Fisher Mary Anne White Michel B. Johnson Brian Woodfield Juliana Boerio-Goates 《Physics and Chemistry of Minerals》2010,37(9):635-651
Jarosite phases are common minerals in acidic, sulfate-rich environments. Here, we report heat capacities (C
p) and standard entropies (S°) for a number of jarosite samples. Most samples are close to the nominal composition AFe3(SO4)2(OH)6, where A = K, Na, Rb, and NH4. One of the samples has a significant number of defects on the Fe sites and is called the defect jarosite; others are referred
to as A-jarosite. The samples, their compositions, and the entropies at T = 298.15 K are:
There are additional configurational entropies of 13.14 and 8.23 J mol−1 K−1 in defect and NH4-jarosite, respectively. A detailed analysis of the synchrotron X-ray diffraction patterns showed a large anisotropic peak
broadening for defect and NH4-jarosite. The fits to the low-temperature (approx. <12 K) C
p data showed that our samples can be divided into two groups. The first group is populated by the K-, Na-, Rb-, and NH4-jarosite samples, antiferromagnetic at low temperatures. The second group contains the H3O-jarosite (studied previously) and the defect jarosite. H3O- and defect jarosite are spin glasses and their low-T
C
p was fit with the expression C
p = γT + ΣB
j
T
j
, where j = (3, 5, 7, 9). The linear term is typical for spin glasses and the sum represents the lattice contribution to C
p. Surprisingly, the C
p of the K-, Na-, Rb-, and NH4-jarosite samples, which are usually considered to be antiferromagnetic at low temperatures, also contains a large linear
term. This finding suggests that even these phases do not order completely, but have a partial spin-glass character below
their Néel transition temperature. 相似文献
Sample | Chemical composition | S o/(J mol−1 K−1) |
---|---|---|
K-jarosite | K0.92(H3O)0.08Fe2.97(SO4)2(OH)5.90(H2O)0.10 | 427.4 ± 0.7 |
Na-jarosite | Na0.95(H3O)0.05Fe3.00(SO4)2(OH)6.00 | 436.4 ± 4.4 |
Rb-jarosite | RbFe2.98(SO4)2(OH)5.95(H2O)0.05 | 411.9 ± 4.1 |
NH4-jarosite | (NH4)0.87(H3O)0.13Fe3.00(SO4)2(OH)6.00 | 447.2 ± 4.5 |
Defect jarosite | K0.94(H3O)0.06Fe2.34(SO4)2(OH)4.01(H2O)1.99 | 412.7 ± 4.1 |
56.
Edward J. Phlips Susan Badylak Mary C. Christman Margaret A. Lasi 《Estuaries and Coasts》2010,33(2):498-512
This paper describes the results of 10 years of water quality monitoring in the Indian River Lagoon Florida, with special
emphasis on the relationships between trends in climatic conditions and the distribution, composition, and abundance of the
phytoplankton community. The Indian River Lagoon, which spans 220 km of Florida’s east coast, is a region of particular concern
because of the rapid rate of human development throughout the region and the hydrologically restricted character of the lagoon,
which heightens the potential for algal bloom. Water sampling was carried out on a monthly to twice-monthly basis at six sites
located in the northern and central lagoon. The 10-year study included both extended periods of below and above average rainfall.
A number of ecologically distinct regions exist within the lagoon, which differ considerably in water exchange properties
and watershed inputs. The northern lagoon is characterized by longer water residence times, lower phosphorus concentrations,
higher nitrogen concentrations, and more stable salinity conditions than the central lagoon. Mean phytoplankton biovolumes
were substantially higher at the sites in the northern lagoon than at the sites in the central lagoon, and algal blooms were
more common and intense in the former region. Inter-annual patterns of phytoplankton biovolume were also different in the
northern and central lagoon. In the northern lagoon, phytoplankton biovolumes were lowest during the drought period, from
the autumn of 1998 to the spring of 2001. By contrast, algal bloom events in the central lagoon were not only less frequent
but were not tied to periods of high rainfall. The most widespread and common bloom formers were the potentially toxic dinoflagellate
Pyrodinium bahamense var. bahamense and two centric diatoms, Dactyliosolen fragilissimus and Cerataulina pelagica. Many of the biovolume peaks observed over the study period were attributable to these three species. The results of time
series modeling of phytoplankton dynamics further highlighted the disparities between the two regions of the lagoon in terms
of the suite of parameters that best predict the observed trends in the biomass of phytoplankton. Overall, the outcome of
this initial modeling effort in the Indian River Lagoon suggests that time series approaches can help define the factors that
influence phytoplankton dynamics. 相似文献
57.
We study the numerical approximation on irregular domains with general grids of the system of poroelasticity, which describes fluid flow in deformable porous media. The flow equation is discretized by a multipoint flux mixed finite element method and the displacements are approximated by a continuous Galerkin finite element method. First-order convergence in space and time is established in appropriate norms for the pressure, velocity, and displacement. Numerical results are presented that illustrate the behavior of the method. 相似文献
58.
Julien Léger Raphaël Pik Bouchaïb Tibari Sébastien Ternois Mary Ford Chantal Peiffert Julien Mercadier 《Geostandards and Geoanalytical Research》2023,47(2):219-242
The in situ (U-Th-Sm)/He and U/Pb laser-ablation double-dating procedure is a valuable method that can provide a large dataset relatively efficiently in contrast with conventional bulk helium thermochronometry. In this study, we evaluate the potential age error associated with the double ablation procedure and report the in situ (U-Th-Sm)/He double-ablation dating of 249 zircons from the Fish Canyon Tuff locality. With LA-ICP-MS pseudo-depth profiling and 3D numerical modelling, we show that the concentric double-ablation procedure in minerals with U-Th-Sm zoning can generate a significant (U-Th-Sm)/He age error (positive or negative), resulting in over-scattering and/or an offset of the mean age. Pseudo-depth profiling is insufficient to predict the individual age error, partly because of the superimposed ablations. To evaluate the consequence of this inherent bias, we confront a synthetic age distribution to the error expected for U-Th-Sm zoned zircons analysed with double-ablation (U-Th-Sm)/He thermochronometry. As expected, a strong age bias causes the spreading of peak ages, downgrading the original signal. Yet, the throughput of the ablation-based method can allow intra- and inter-sample peak age identification and comparison, and the coupling of (U-Th-Sm)/He and U/Pb ages extends our ability to deconvolute a multimodal age spectrum. 相似文献
59.
In this work, we measure the performance of the fixed stress split algorithm for the immiscible water-oil flow coupled with linear poromechanics. The two-phase flow equations are solved on general hexahedral elements using the multipoint flux mixed finite element method whereas the poromechanics equations are discretized using the conforming Galerkin method. We introduce a rigorous calculation of the update in poroelastic properties during the iterative solution of the coupled system equations. The effects of the coupling parameter on the performance of the fixed stress algorithm is demonstrated in two field studies: the Frio oil reservoir and the Cranfield injection site.
相似文献