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671.
Many pterodactyloids had wingspans that vastly exceed those of any other flying animals, living or extinct. Ornithocheiroids and neoazhdarchidans were particularly large pterosaurs that regularly attained wingspans over 4 m. Certain members of these groups – Pteranodontia and Azhdarchidae – attained the largest wingspans of any pterosaurs, making them the largest flying animals known. 相似文献
672.
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
673.
Michael Wiedmer David R. Montgomery Alan R. Gillespie Harvey Greenberg 《Quaternary Research》2010,73(3):413-424
Geomorphic, stratigraphic, geotechnical, and biogeographic evidence indicate that failure of a Pleistocene ice dam between 15.5 and 26 ka generated a megaflood from Glacial Lake Atna down the Matanuska Valley. While it has long been recognized that Lake Atna occupied ≥ 9000 km2 of south-central Alaska's Copper River Basin, little attention has focused on the lake's discharge locations and behaviors. Digital elevation model and geomorphic analyses suggest that progressive lowering of the lake level by decanting over spillways exposed during glacial retreat led to sequential discharges down the Matanuska, Susitna, Tok, and Copper river valleys. Lake Atna's size, ∼ 50 ka duration, and sequential connection to four major drainages likely made it a regionally important late Pleistocene freshwater refugium. We estimate a catastrophic Matanuska megaflood would have released 500–1400 km3 at a maximum rate of ≥ 3 × 106 m3 s− 1. Volumes for the other outlets ranged from 200 to 2600 km3 and estimated maximum discharges ranged from 0.8 to 11.3 × 106 m3 s− 1, making Lake Atna a serial generator of some of the largest known freshwater megafloods. 相似文献
674.
The integration of geographically referenced information into the conceptual frameworks and applied uses of the social sciences
and humanities has been an ongoing process over the past few centuries. It has gained momentum in recent decades with advances
in technologies for computation and visualization and with the arrival of new data sources. This article begins with an overview
of this transition, and argues that the spatial integration of information resources and the cross-disciplinary sharing of
analysis and representation methodologies are important forces for the integration of scientific and artistic expression,
and that they draw on core concepts in spatial (and spatio-temporal) thinking. We do not suggest that this is akin to prior
concepts of unified knowledge systems, but we do maintain that the boundaries to knowledge transfer are disintegrating and
that our abilities in problem solving for purposes of artistic expression and scientific development are enhanced through
spatial perspectives. Moreover, approaches to education at all levels must recognize the need to impart proficiency in the
critical and efficient application of these fundamental spatial concepts, if students and researchers are to make use of expanding
access to a broadening range of spatialized information and data processing technologies. 相似文献
675.
Michael Grodzicki G. Redhammer M. Reissner W. Steiner G. Amthauer 《Physics and Chemistry of Minerals》2010,37(1):11-23
The electronic and magnetic structure of the chain silicate hedenbergite (CaFe2+Si2O6) has been investigated by a number of experimental methods (neutron diffraction, Mössbauer spectroscopy, low temperature magnetic measurements), as well as by electronic structure calculations for clusters of different size in the local spin density approximation. The calculated size-converged spectroscopic data (d-d excitation energies, hyperfine parameters) are in quantitative agreement with the respective experimental values. The calculated magnetic coupling constants are about +25 cm?1 and ?4 cm?1 for intra-chain and inter-chain coupling, respectively. The latter value shows that weak superexchange via edges of silicon tetrahedra is well reproduced by the calculations, and it is in qualitative agreement with an observed metamagnetic transition at 4.2 K in an external magnetic field with an onset around 4 T but saturation is not achieved in fields up to 14.5 T. The large ferromagnetic intra-chain coupling is attributed to a nearly degenerate ground state. The ratio between the two magnetic coupling constants agrees with earlier estimates on similar compounds. Finally, it is demonstrated how the detailed discussion of the various exchange pathways contributes to an improved understanding of the connection between magnetic properties and the geometrical structure. 相似文献
676.
Adewole Michael Gbadebo Olusegun O. BabalolaFelicia L. Ajigbotesho 《Journal of Geochemical Exploration》2010
Selenium like any other element can both be essential and toxic depending upon the concentration at which it occurs in human diet. This paper assessed the level of this trace element in twenty different (20) types of locally produced food items around Abeokuta town and the blood obtained from the antecubital fossa vein of eighty-eight (88) healthy adults resident in Abeokuta. 相似文献
677.
Mapping block-and-ash flow hazards based on Titan 2D simulations: a case study from Mt. Taranaki,NZ 总被引:1,自引:0,他引:1
Jonathan N. Procter Shane J. Cronin Thomas Platz Abani Patra Keith Dalbey Michael Sheridan Vince Neall 《Natural Hazards》2010,53(3):483-501
Numerical models for simulation of mass flows are typically focussed upon accurately predicting the paths, travel times and
inundation from a single flow or collapse event. When considering catchment-based hazards from a volcano, this is complicated
by often being faced with several possible scenarios. Over the last 800 years at Mt. Taranaki/Egmont, a number of dome growth
and collapse events have resulted in the genesis and emplacement of block-and-ash flows (BAFs). Each BAF was directed northwestward
by a breach in the crater rim. The latest dome collapse events in the AD 1880s and AD 1755 inundated the northwestern flank
and had run-out lengths 10 km from source. Future activity of this type could have a devastating effect on the Taranaki region’s
communities, infrastructure and economy. Hazard planning has involved constructing volcanic hazard maps based upon the areas
inundated by past volcanic flows, with little consideration of present-day topography. Here, a numerical geophysical mass
flow modelling approach is used to forecast the hazards of future comparable BAF events on NW Mt. Taranaki. The Titan2D programme
encompasses a “shallow water”, continuum solution-based, granular flow model. Flow mechanical properties needed for this approach
include estimates of internal and basal friction as well as the physical dimensions of the initial collapse. Before this model
can be applied to Taranaki BAFs, the input parameters must be calibrated by simulating a range of past collapse events. By
using AD 1860 and AD 1755 scenarios, initial collapse volumes can be well constrained and internal and basal friction angles
can be evaluated through an iterative approach from previous run-out lengths. A range of possible input parameters was, therefore,
determined to produce a suite of potentially inundated areas under present-day terrain. A suite of 10 forecasts from a uniformly
distributed range were combined to create a map of relative probabilities of inundation by future BAF events. These results
were combined in a GIS package to produce hazard zones related to user-specified hazard thresholds. Using these input parameter
constraints, future hazard forecasts for this scale and type of event can also take into account changing summit and topographic
configurations following future eruptive or collapse events. 相似文献
678.
Antti Pulkkinen Michael Hesse Shahid Habib Luke Van der Zel Ben Damsky Fritz Policelli David Fugate William Jacobs Elizabeth Creamer 《Natural Hazards》2010,53(2):333-345
In this paper, central elements of the Solar Shield project, launched to design and establish an experimental system capable
of forecasting the space weather effects on high-voltage power transmission system, are described. It will be shown how Sun–Earth
system data and models hosted at the Community Coordinated Modeling Center (CCMC) are used to generate two-level magnetohydrodynamics-based
forecasts providing 1–2 day and 30–60 min lead-times. The Electric Power Research Institute (EPRI) represents the end-user,
the power transmission industry, in the project. EPRI integrates the forecast products to an online display tool providing
information about space weather conditions to the member power utilities. EPRI also evaluates the economic impacts of severe
storms on power transmission systems. The economic analysis will quantify the economic value of the generated forecasting
system. The first version of the two-level forecasting system is currently running in real-time at CCMC. An initial analysis
of the system’s capabilities has been completed, and further analysis is being carried out to optimize the performance of
the system. Although the initial results are encouraging, definite conclusions about system’s performance can be given only
after more extensive analysis, and implementation of an automatic evaluation process using forecasted and observed geomagnetically
induced currents from different nodes of the North American power transmission system. The final output of the Solar Shield
will be a recommendation for an optimal forecasting system that may be transitioned into space weather operations. 相似文献
679.
Phosphorus Speciation in Stream Bed Sediments from an Agricultural Watershed: Solid-Phase Associations and Sorption Behavior 总被引:1,自引:0,他引:1
The sorption behavior and solid-phase associations of phosphorus (P) in fine-grained sediments (<63 μm) from two upstream
tributaries and one downstream main stem site of the Spoon River in west-central Illinois were characterized to better understand
phosphorus bioavailability in this agriculturally dominated watershed. The P sorption affinities, as indicated by linear distribution
coefficients (K
d), of all sediments were 330–5,150 L/kg, and negatively correlated with equilibrium phosphorus concentration (EPCo) values, which ranged between 0.2 and 2.2 μM. pH values measured at the conclusion of the sorption experiments varied only
slightly (7.45–8.10) but were nonetheless strongly positively correlated to EPCo values, and negatively correlated to K
d values, suggesting the importance of pH to the observed sorption behavior. K
d values were generally lower and EPCo values higher at the main stem site than at the upstream tributary sites, suggesting dissolved reactive P (DRP) bioavailability
(specifically orthophosphate) increased downstream. The solid phase associations of P were operationally assessed with the
streamlined SEDEX (sedimentary extraction) procedure, and most sediment P (≥50%) was released during the step designed to
determine iron oxide–associated P. On average, 70–90% of the total sediment P pool was potentially bioavailable, as estimated
by the sum of the iron oxide-, authigenic carbonate-, and organic-associated P fractions. Considerable calcium was also extracted
from some sediments during the step designed to specifically remove iron oxide–associated P. It is hypothesized that the severe
drought conditions that persisted between April and October, 2005 allowed authigenic carbonates (perhaps partly amorphous)
to accumulate, and that these carbonates dissolved during the iron oxide extraction step. The extensive benthic algal populations
also present may have aided carbonate precipitation, which under more normal hydrologic conditions would be periodically flushed
downstream and replaced by fresh sediment. This suggests antecedent hydrologic conditions played a dominant role in the P
sorption and solid phase associations identified. 相似文献
680.
Yu-Chen Song Hai-Dong Meng Michael J. O’Grady Gregory M. P. O’Hare 《Computational Geosciences》2010,14(2):263-271
A clustering algorithm that is based on density and is adaptive density-reachable is developed and presented for arbitrary data point distributions in some real-world applications, especially in geophysical data interpretation. Through comparisons of the new algorithm and other algorithms, it is shown that the new algorithm can reduce the dependency of domain knowledge and the sensitivity of abnormal data points, that it can improve the effectiveness of clustering results in which data are distributed in different shapes and different densities, and that it can get a better clustering efficiency. The application of the new clustering algorithm demonstrates that data mining techniques can be used in geophysical data interpretation and can get meaningful and useful results, and that the new clustering algorithm can be used in other real-world applications. 相似文献