Crustal compositions exposed by impact craters in the Tyrrhena Terra region of Mars: Considerations for Noachian environments     

A. Deanne Rogers 《Earth and Planetary Science Letters》2011,301(1-2):353-364

Crater ejecta blankets distinguished by differences in thermophysical properties from the target material were investigated in Tyrrhena Terra, Mars. Approximately one third of craters exhibit spectral differences from target material in Mars Global Surveyor Thermal Emission Spectrometer (TES) data. Craters that exhibit these spectral differences from the target material do not strongly correlate with phyllosilicate exposures detected in near infrared data from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM). Shock processes cannot fully explain the observed spectral trends. Rather, the subtle spectral differences observed in TES data are likely due to differences in plagioclase and/or high-silica phase abundance relative to pyroxene abundance, with the crater ejecta exhibiting lower pyroxene abundance than target materials. These mineralogic trends are consistent with several scenarios, none of which can be ruled out from existing observations: 1) vertical variations in primary lithology (considered least likely), 2) subsurface alteration exposed by impact, 3) syn- or post-impact alteration associated with the impact process, such as devitrification of melt glass materials or alteration of highly shocked materials, and 4) light surface alteration that preferentially altered/removed plagioclase and/or high-silica glass relative to pyroxene and subsequent exposure of less-altered materials from the subsurface by impact. Scenario 2 is consistent with phyllosilicate detections; Scenarios 1, 3 and 4 are consistent with CRISM phyllosilicate detections in some impact craters if phyllosilicate mineral abundance is volumetrically small. Results from this work suggest that some signatures of alteration may be escaping detection by near infrared measurements; alteration environments would be best interpreted through a combination of near infrared and thermal infrared data sets, rather than near infrared detections alone.  相似文献   

Nearshore Tsunami Inundation Model Validation: Toward Sediment Transport Applications     

Alex Apotsos  Mark Buckley  Guy Gelfenbaum  Bruce Jaffe  Deepak Vatvani 《Pure and Applied Geophysics》2011,168(11):2097-2119

Model predictions from a numerical model, Delft3D, based on the nonlinear shallow water equations are compared with analytical results and laboratory observations from seven tsunami-like benchmark experiments, and with field observations from the 26 December 2004 Indian Ocean tsunami. The model accurately predicts the magnitude and timing of the measured water levels and flow velocities, as well as the magnitude of the maximum inundation distance and run-up, for both breaking and non-breaking waves. The shock-capturing numerical scheme employed describes well the total decrease in wave height due to breaking, but does not reproduce the observed shoaling near the break point. The maximum water levels observed onshore near Kuala Meurisi, Sumatra, following the 26 December 2004 tsunami are well predicted given the uncertainty in the model setup. The good agreement between the model predictions and the analytical results and observations demonstrates that the numerical solution and wetting and drying methods employed are appropriate for modeling tsunami inundation for breaking and non-breaking long waves. Extension of the model to include sediment transport may be appropriate for long, non-breaking tsunami waves. Using available sediment transport formulations, the sediment deposit thickness at Kuala Meurisi is predicted generally within a factor of 2.  相似文献   

Arsenic in sediments, groundwater, and streamwater of a glauconitic Coastal Plain terrain, New Jersey, USA—Chemical “fingerprints” for geogenic and anthropogenic sources     

Julia L. Barringer  Pamela A. Reilly  Dennis D. Eberl  Alex E. Blum  Jennifer L. Bonin  Robert Rosman  Barbara Hirst  Marzooq Alebus  Kimberly Cenno  Miroslawa Gorska 《Applied Geochemistry》2011,26(5):763-776

Glauconite-bearing deposits are found worldwide, but As levels have been determined for relatively few. The As content of glauconites in sediments of the Inner Coastal Plain of New Jersey can exceed 100 mg/kg, and total As concentrations (up to 5.95 μg/L) found historically and recently in streamwaters exceed the State standard. In a major watershed of the Inner Coastal Plain, chemical “fingerprints” were developed for streambed sediments and groundwater to identify contributions of As to the watershed from geologic and anthropogenic sources. The fingerprint for streambed sediments, which included Be, Cr, Fe and V, indicated that As was predominantly of geologic origin. High concentrations of dissolved organic C, nutrients (and Cl⁻) in shallow groundwater indicated anthropogenic inputs that provided an environment where microbial activity released As from minerals to groundwater discharging to the stream. Particulates in streamwater during high flow constituted most of the As load; the chemical patterns for these particulates resembled the geologic fingerprint of the streambed sediments. The As/Cr ratio of these suspended particles likely indicates they derived not only from runoff, but from groundwater inputs, because As contributed by groundwater is sequestered on streambed sediments. Agricultural inputs of As were not clearly identified, although chemical characteristics of some sediments indicated vehicle-related inputs of metals. Sediment sampling during dry and wet years showed that, under differing hydrologic conditions, local anthropogenic fingerprints could be obscured but the geologic fingerprint, indicating glauconitic sediments as an As source, was robust.  相似文献   

Macro benthic community assemblage before and after the 2007 tsunami and earthquake at Paracas Bay,Peru     

Betina J. Lomovasky  Fausto N. Firstater  Alex Gamarra Salazar  Jaime Mendo  Oscar O. Iribarne 《Journal of Sea Research》2011,65(2):205-212

Macrobenthic soft bottom community assemblages were studied from December 2006 to December 2007 at Paracas Bay (Atenas Beach, 13°45' S, 76°17' W, Peru), including the period of the tsunami of 15 August 2007, providing a unique opportunity to assess the effects of this type of natural disturbance in soft bottom marine ecosystems. The results show that the tsunami affected the soft bottom community assemblage by changing the sediment granulometry, the biomass and abundance of epifaunal species, and by increasing the silt proportion. The event affected all functional groups, resulting in an increased numbers and biomass of filter feeder and grazers that were followed soon after by predators during the post-tsunami period. A similar pattern was observed for biomass and the abundance of infaunal species without changing the functional groups. The slow recovery observed after four months was likely related to the loss of sediment stability. In summary, there were differences in the structure and composition of soft bottom macrofaunal assemblages before and after the earthquake and posterior tsunami disturbance, forcing benthic communities to initiate re-colonisation processes.  相似文献   

Impacts of diverted freshwater on dissolved organic matter and microbial communities in Barataria Bay, Louisiana, U.S.A     

Bianchi TS  Cook RL  Perdue EM  Kolic PE  Green N  Zhang Y  Smith RW  Kolker AS  Ameen A  King G  Ojwang LM  Schneider CL  Normand AE  Hetland R 《Marine environmental research》2011,72(5):248-257

Here we present results of an initial assessment of the impacts of a water diversion event on the concentrations and chemical composition of dissolved organic matter (DOM) and bacterioplankton community composition in Barataria Bay, Louisiana U.S.A, an important estuary within the Mississippi River Delta complex. Concentrations and spectral properties of DOM, as reflected by UV/visible absorbance and fluorescence, were strikingly similar at 26 sites sampled along transects near two western and two eastern areas of Barataria Bay in July and September 2010. In September 2010, dissolved organic carbon (DOC) was significantly higher (568.1-1043 μM C, x=755.6+/-117.7 μM C, n=14) than in July 2010 (249.1-577.1 μM C, x=383.7+/-98.31 μM C, n=14); conversely, Abs254 was consistently higher at every site in July (0.105-0.314) than in September (0.080-0.221), averaging 0.24±0.06 in July and 0.15±0.04 in September. Fluorescence data via the fluorescence index (FI450/500) revealed that only 30% (8 of 26) of the July samples had an FI450/500 above 1.36, compared to 96% (25 of 26) for the September samples. This indicates a more terrestrial origin for the July DOM. Bacterioplankton from eastern sites differed in composition from bacterioplankon in western sites in July. These differences appeared to result from reduced salinities caused by the freshwater diversion. Bacterioplankton communities in September differed from those in July, but no spatial structure was observed. Thus, the trends in bacterioplankton and DOM were likely due to changes in water masses (e.g., input of Mississippi River water in July and a return to estuarine waters in September). Discharge of water from the Davis Pond Freshwater Diversion (DPFD) through Barataria Bay may have partially mitigated some adverse effects of the oil spill, inasmuch as DOM is concerned.  相似文献   

Forecasting ocean waves: Comparing a physics-based model with statistical models     

Gordon Reikard  W. Erick Rogers 《Coastal Engineering》2011

The literature on ocean wave forecasting falls into two categories, physics-based models and statistical methods. Since these two approaches have evolved independently, it is of interest to determine which approach can predict more accurately, and over what time horizons. This paper runs a comparative analysis of a well-known physics-based model for simulating waves near shore, SWAN, and two statistical techniques, time-varying parameter regression and a frequency domain algorithm. Forecasts are run for the significant wave height, over horizons ranging from the current period (i.e., the analysis time) to 15 h. Seven data sets, four from the Pacific Ocean and three from the Gulf of Mexico, are used to evaluate the forecasts. The statistical models do extremely well at short horizons, producing more accurate forecasts in the 1–5 hour range. The SWAN model is superior at longer horizons. The crossover point, at which the forecast error from the two methods converges, is in the area of 6 h. Based on these results, the choice of statistical versus physics-based models will depend on the uses to which the forecasts will be put. Utilities operating wave farms, which need to forecast at very short horizons, may prefer statistical techniques. Navies or shipping companies interested in oceanic conditions over longer horizons will prefer physics-based models.  相似文献   

Editor's note: Astron. Nachr. 9/2012     

Carsten Denker  Alex Feller  Wolfgang Schmidt  Oskar von der Lühe 《Astronomische Nachrichten》2012,333(9):789-789

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Determining meteoroid bulk densities using a plasma scattering model with high-power large-aperture radar data     

Sigrid Close  Ryan Volz  Rohan Loveland  Alex Macdonell  Patrick Colestock  Ivan Linscott  Meers Oppenheim 《Icarus》2012,221(1):300-309

We present an improved technique for calculating bulk densities of low-mass (<1 g) meteoroids using a scattering model applied to the high-density plasma formed around the meteoroid as it enters Earth’s atmosphere. These plasmas, referred to as head echoes, travel at or near the speed of the meteoroid, thereby allowing the determination of the ballistic coefficient (mass divided by physical cross-section), which depends upon speed and deceleration. Concurrently, we apply a scattering model to the returned signal strength of the head echo in order to correlate radar-cross-section (RCS) to plasma density and meteoroid mass. In this way, we can uniquely solve for the meteoroid mass, radius and bulk density independently. We have applied this new technique to head echo data collected in 2007 and 2008 simultaneously at VHF (160 MHz) and UHF (422 MHz) at ALTAIR, which is a high-power large-aperture radar located on the Kwajalein Atoll. These data include approximately 20,000 detections with dual-frequency, dual-polarization, and monopulse (i.e. angle) returns. From 2000 detections with the smallest monopulse errors, we find a mean meteoroid bulk density of 0.9 g/cm³ with observations spanning almost three orders of magnitude from 0.01 g/cm³ to 8 g/cm³. Our results show a clear dependence between meteoroid bulk density and altitude of head echo formation, as well as dependence between meteoroid bulk density and 3D speed. The highest bulk densities are detected at the lowest altitudes and lowest speeds. Additionally, we stipulate that the approximations used to derive the ballistic parameter, in addition to neglecting fragmentation, suggest that the traditional ballistic parameter must be used with caution when determining meteoroid parameters.  相似文献   

A support system for assessing local vulnerability to weather and climate     

Alex Coletti  Peter D. Howe  Brent Yarnal  Nathan J. Wood 《Natural Hazards》2013,65(1):999-1008

The changing number and nature of weather- and climate-related natural hazards is causing more communities to need to assess their vulnerabilities. Vulnerability assessments, however, often require considerable expertise and resources that are not available or too expensive for many communities. To meet the need for an easy-to-use, cost-effective vulnerability assessment tool for communities, a prototype online vulnerability assessment support system was built and tested. This prototype tool guides users through a stakeholder-based vulnerability assessment that breaks the process into four easy-to-implement steps. Data sources are integrated in the online environment so that perceived risks??defined and prioritized qualitatively by users??can be compared and discussed against the impacts that past events have had on the community. The support system is limited in scope, and the locations of the case studies do not provide a sufficiently broad range of sample cases. The addition of more publically available hazard databases combined with future improvements in the support system architecture and software will expand opportunities for testing and fully implementing the support system.  相似文献   

A reinvestigation of thrusting in Portugese Timor     

Alex E. Grady 《Australian Journal of Earth Sciences》2013,60(2):223-227

Field relations from a small area in the Maubisse region of Portuguese Timor fail to support the hypothesis of southward overthrusting of Permian rocks (Audley‐Charles, 1965) or the postulate that the Maubisse Formation represents a mid‐Tethys island group (Audley‐Charles et al., 1972).  相似文献