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51.
Mary C. Bourke 《Icarus》2010,205(1):183-197
Barchan dune asymmetry refers to the extension of one barchan limb downwind. It is a common dune form on Earth and also occurs on Mars and Titan. A new classification of barchan limbs is presented where three types of limb morphology are identified: linear, kinked and beaded. These, along with other dune-scale morphological signatures, are used to identify three of the causes of barchan asymmetry on Mars: bi-directional winds, dune collision and the influence of inclined topography.The potential for specific dune asymmetric morphologies to indicate aspects of the formative wind regime on planetary surfaces is shown. For example, the placement of dune limbs can indicate the general direction and relative strength of formative oblique winds; an extreme barchan limb length may indicate a long duration oblique wind; a kinked limb may be evidence of the passage of a storm; beaded limbs may represent surface-wave instabilities caused by an increase in wind energy parallel to the dune. A preliminary application of these signatures finds evidence for bi-modal winds on Mars. However, these and other morphological signatures of wind direction and relative strength should be applied to planetary landforms with caution as more than one process (e.g., bi-modal winds and collision) may be operating together or sequentially on the dunefield. In addition, analysis should be undertaken at the dunefield scale and not on individual dunes. Finally, morphological data should be acquired from similar-scale dunes within a dunefield.In addition to bi-modal wind regimes on Mars, the frequent parallel alignment of the extended barchan limb to the dune suggests that dune collision is also an important cause of asymmetry on Mars. Some of the more complex dunefield patterns result from a combination of dune collision, limb extension and merging with downwind dunes.Dune asymmetric form does not inhibit dune migration in the Namib Desert or on Mars. Data from the Namib suggest that dune migration rates are similar for symmetric and asymmetric dunes. Further modeling and field studies are needed to refine our understanding of the potential range of limb and dune morphologies that can result from specific asymmetry causes. 相似文献
52.
There are few studies on the hydrogeology of sedimentary rock aquitards although they are important controls in regional ground water flow systems. We formulate and test a three-dimensional (3D) conceptual model of ground water flow and hydrochemistry in a fractured sedimentary rock aquitard to show that flow dynamics within the aquitard are more complex than previously believed. Similar conceptual models, based on regional observations and recently emerging principles of mechanical stratigraphy in heterogeneous sedimentary rocks, have previously been applied only to aquifers, but we show that they are potentially applicable to aquitards. The major elements of this conceptual model, which is based on detailed information from two sites in the Maquoketa Formation in southeastern Wisconsin, include orders of magnitude contrast between hydraulic diffusivity (K/S(s)) of fractured zones and relatively intact aquitard rock matrix, laterally extensive bedding-plane fracture zones extending over distances of over 10 km, very low vertical hydraulic conductivity of thick shale-rich intervals of the aquitard, and a vertical hydraulic head profile controlled by a lateral boundary at the aquitard subcrop, where numerous surface water bodies dominate the shallow aquifer system. Results from a 3D numerical flow model based on this conceptual model are consistent with field observations, which did not fit the typical conceptual model of strictly vertical flow through an aquitard. The 3D flow through an aquitard has implications for predicting ground water flow and for planning and protecting water supplies. 相似文献
53.
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
54.
55.
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
56.
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
57.
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 |
58.
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
59.
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
60.
We present a radiative transfer analysis of circumstellar dust shells around the Post-AGB stars HD 179821, HD 56126, HD 101584
and early R star HD 100764, using the code DUSTY. Parameters like mass-loss, shell inner radius, dust temperature, outflow
velocity etc., are derived for HD 179821and HD 56126 whose observed SED could be reproduced by our models.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献