			Home About us Contact

Impact Site (impact + site)

Distribution by Scientific Domains

Physics and Astronomy	50%

Selected Abstracts

LANDSCAPE-SCALE ANALYSIS AND MANAGEMENT OF CUMULATIVE IMPACTS TO RIPARIAN ECOSYSTEMS: PAST, PRESENT, AND FUTURE,

JOURNAL OF THE AMERICAN WATER RESOURCES ASSOCIATION, Issue 6 2001
Eric D. Stein
ABSTRACT: Analyses of cumulative impacts to riparian systems is an important yet elusive goal. Previous analyses have focused on comparing the number of hectares impacted to the number of hectares restored, without addressing the loss of riparian function or the effect of the spatial distribution of impacts. This paper presents an analysis of the spatial distribution of development-related impacts to riparian ecosystems, that were authorized under Section 404 of the Clean Water Act. Impacts on habitat structure, contiguity, and landscape context were evaluated using functional indices scaled to regional reference sites. Impact sites were mapped using GIS and analyzed for spatial associations. Positive spatial autocorrelation (i.e. clustering of impact sites) resulted from the piecemeal approach to impact assessment, which failed to prevent cumulative impacts. Numerous small projects in close proximity have resulted in adverse impacts to entire stream reaches or have fragmented the aquatic resources to a point where overall functional capacity is impaired. Additionally, the ecological functions of unaffected areas have been diminished due to their proximity to degraded areas. A proactive approach to managing cumulative impacts is currently being used in Orange County, California as part of a Corps of Engineers sponsored Special Area Management Plan (SAMP). The SAMP process is evaluating the ecological conditions and physical processes of the study watersheds and attempting to plan future development in a manner that will guard against cumulative impacts. [source]

A study of juvenile rat spinal cord injury

JOURNAL OF NEUROCHEMISTRY, Issue 2002
J. M. Wingrave
Greater than 5% of all spinal cord injuries (SCI) in the US occur in people younger than 16, although a minority, children will require extended attention during their lifetime. While facing increased mortality in the initial 24 h after trauma, children with incomplete injuries seem to have a greater capacity for recovery of function compared to adults suggesting that there is a difference in injury tolerance in the young over the adult. Knowledge of the factors involved in this difference would not only increase understanding of SCI, but also potentiate new avenues for SCI treatment. Yet there has not been a model for the study of youth SCI. For these reasons, we developed a model of SCI in juvenile rats equivalent to an adult injury of 25 g cm force (GCF). To do so, we recorded spinal cord masses of Sprague,Dawley rats at 21, 30, 45, and 60 days of age, compared them to adult cord masses, and assembled a conversion factor that provides youth injuries comparable to adult. To investigate the pathophysiology in juvenile SCI, two cord segments, 1 cm long, were removed from animals 24 h following injury. One segment was centered at the impact site, the other immediately caudal. After homogenization, the samples were assayed by Western blot analysis for calpain content and degradation of 68K Neuro-Filament Protein (68K NFP), a neuronal structural protein. mCalpain expression, a neutral protease previously implicated in secondary SCI, was reduced in juvenile animals relative to adult cohorts. The degradation of 68K NFP was also found to be reduced in juvenile animals. From these analyses, it seems plausible that calpain expression and pathogenic activity is abated in the setting of young rat SCI. Acknowledgements:, Supported by grants from NIH-NINDS. [source]

Trace element concentrations in the Mexico-Belize ejecta layer: A link between the Chicxulub impact and the global Cretaceous-Paleogene boundary

METEORITICS & PLANETARY SCIENCE, Issue 11 2007
Jane Wigforss-Lange
The ejecta deposits consist of a lower spheroid bed, containing clay and dolomite spheroids, and an upper diamictite bed with boulders and clasts of limestone and dolomite. The matrix of both beds is composed of clay and micritic dolomite. The rare earth element (REE) compositions in the matrix of both units show strong similarities in concentrations and pattern. Furthermore, the Zr/TiO2 scatter plot shows a linear correlation indicating one source. These results indicate that the basal spheroid bed has the same source and was generated during the same event as the overlying diamictite bed, which lends support to a single-impact scenario for the Albion Formation ejecta deposits. The elevated concentrations of non-meteoritic elements such as Sb, As, U, and Zn in the matrix of the lower spheroid bed are regarded to have been derived from the sedimentary target rocks at the Chicxulub impact site. The positive Eu and Ce anomalies in clay concretion and in the matrix of the lower part of the spheroid bed in Albion Island quarry is probably related to processes involved in the impact, such as high temperature and oxidizing conditions. Analogous trace element anomalies have been reported from the distal Cretaceous-Paleogene (K/T) boundary clay layer at different sites. Thus, the trace element signals, reported herein, are regarded to support a genetic link between the Chicxulub impact, the ejecta deposits along the Mexico-Belize border, and the global K/T boundary layer. [source]

Earth Impact Effects Program: A Web-based computer program for calculating the regional environmental consequences of a meteoroid impact on Earth

METEORITICS & PLANETARY SCIENCE, Issue 6 2005
Gareth S. COLLINS
This paper details the observations, assumptions and equations upon which the program is based. It describes our approach to quantifying the principal impact processes that might affect the people, buildings, and landscape in the vicinity of an impact event and discusses the uncertainty in our predictions. The program requires six inputs: impactor diameter, impactor density, impact velocity before atmospheric entry, impact angle, the distance from the impact at which the environmental effects are to be calculated, and the target type (sedimentary rock, crystalline rock, or a water layer above rock). The program includes novel algorithms for estimating the fate of the impactor during atmospheric traverse, the thermal radiation emitted by the impact-generated vapor plume (fireball), and the intensity of seismic shaking. The program also approximates various dimensions of the impact crater and ejecta deposit, as well as estimating the severity of the air blast in both crater-forming and airburst impacts. We illustrate the utility of our program by examining the predicted environmental consequences across the United States of hypothetical impact scenarios occurring in Los Angeles. We find that the most wide-reaching environmental consequence is seismic shaking: both ejecta deposit thickness and air-blast pressure decay much more rapidly with distance than with seismic ground motion. Close to the impact site the most devastating effect is from thermal radiation; however, the curvature of the Earth implies that distant localities are shielded from direct thermal radiation because the fireball is below the horizon. [source]

The clay mineralogy of sediments related to the marine Mjølnir impact crater

METEORITICS & PLANETARY SCIENCE, Issue 10 2003
Henning DYPVIK
It was formed about 142 ± 2.6 Myr ago by the impact of a 1,2 km asteroid into the shallow shelf clays of the Hekkingen Formation and the underlying Triassic to Jurassic sedimentary strata. A core recovered from the central high within the crater contains slump and avalanche deposits from the collapse of the transient crater and central high. These beds are overlain by gravity flow conglomerates, with laminated shales and marls on top. Here, impact and post-impact deposits in this core are studied with focus on clay mineralogy obtained from XRD decomposition and simulation analysis methods. The clay-sized fractions are dominated by kaolinite, illite, mixed-layered clay minerals and quartz. Detailed analyses showed rather similar composition throughout the core, but some noticeable differences were detected, including varying crystal size of kaolinite and different types of illites and illite/smectite. These minerals may have been formed by diagenetic changes in the more porous/fractured beds in the crater compared to time-equivalent beds outside the crater rim. Long-term post-impact changes in clay mineralogy are assumed to have been minor, due to the shallow burial depth and minor thermal influence from impact-heated target rocks. Instead, the clay mineral assemblages, especially the abundance of chlorite, reflect the impact and post-impact reworking of older material. Previously, an ejecta layer (the Sindre Bed) was recognized in a nearby well outside the crater, represented by an increase in smectite-rich clay minerals, genetically equivalent to the smectite occurring in proximal ejecta deposits of the Chicxulub crater. Such alteration products from impact glasses were not detected in this study, indicating that little, if any, impact glass was deposited within the upper part of the crater fill. Crater-fill deposits inherited their mineral composition from Triassic and Jurassic sediments underlying the impact site. [source]

VW Hyi: optical spectroscopy and Doppler tomography

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2006
Amanda J. Smith
ABSTRACT We present high-quality optical spectroscopy of the SU UMa-subtype dwarf nova, VW Hyi taken while the system was in quiescence. An S-wave is executed by the emission cores of the hydrogen Balmer lines and by the emission lines of He i, Ca ii, Fe ii and He ii. Using Doppler tomography, we show it originates in the accretion stream,disc impact region. The He ii emission is strongly phase-dependent, suggesting it originates exclusively within a hot cavity at the initial impact. We map the ionization structure of the stream,disc interaction region. One possible interpretation of this is that the Balmer hotspot lies downstream of the He ii hotspot in the outer accretion disc, with the He i and Ca ii hotspots at intermediate locations between the two. This suggests that Balmer emission is suppressed until material has cooled somewhat downstream of the impact site and is able to recombine. We favour a phase offset of 0.15 ± 0.04 between the photometric ephemeris and inferior conjunction of the mass donor. The white dwarf contributes significantly to the optical continuum, with broad Balmer absorption and narrow Mg ii ,4481 absorption clearly apparent. This latter feature yields the gravitational redshift: vgrav= 38 ± 21 km s ,1, so M1= 0.71+0.18,0.26 M,. This implies M2= 0.11 ± 0.03 M, and hence the donor is not a brown dwarf. A prominent Balmer jump is also observed. We note that the previously accepted system parameters for both VW Hyi and WX Hyi incorporate an algebraic error, and we provide a recalculated M1(q) plane for WX Hyi. [source]

Autonomous damage initiated healing in a thermo-responsive ionomer

POLYMER INTERNATIONAL, Issue 8 2010
Russell John Varley
Abstract The partially neutralised poly[ethylene- co -(methacrylic acid)] copolymer Surlyn 8940® (DuPont) ionomer exhibits damage-initiated healing during high-energy impact. This is attributed to the hierarchical structure of ionomers, arising from the presence of ionic aggregates and hydrogen bonding. This work investigates the mechanism of this process using novel techniques developed here. The ionomer's response to penetration has been found to consist of three consecutive events: an initial elastic response, an anelastic response and pseudo-brittle failure. In addition, the ultimate level of healing has been shown to be dependent upon the elastic response during impact as well as post-failure viscous flow. Increasing the local temperature at impact consistently increases elastic healing, although further improvements in healing are minor once the local temperature increases beyond the melting point. Below the order-to-disorder transition, microscopic investigations reveal severe plastic deformation while the lack of shape memory reduces the comparative level of elastic healing. Above this temperature, healing is facilitated by elastomeric behaviour at the impact site, while above the melting point a combination of elastomeric and viscous flow dominates. This work provides for the first time evidence of the consecutive healing events occurring during high-impact penetration for ionomers. The hierarchical structure of ionomers and its impact upon the microstructure have been shown to be critical to the process. Comparison of the mechanical response during impact with that of non-ionic polymers further highlights this. In addition, slow relaxational processes occurring post-impact are found to facilitate further recovery in mechanical properties. Copyright © 2010 Society of Chemical Industry [source]