The effects of thin layer sand renourishment on tidal marsh processes: Masonboro Island, North Carolina     

Alex L. Croft  Lynn A. Leonard  Troy D. Alphin  Lawrence B. Cahoon  Martin H. Posey 《Estuaries and Coasts》2006,29(5):737-750

The objective of this study was to determine if the placement of dredged material on sediment-starved back barrier marshes in southeastern North Carolina could offset submergence without negatively affecting function. Clean sediment was placed in thickness from 0 to 10 cm, on deteriorated and non-deteriorated marsh plots. Original stem densities were greater, in non-deteriorated plots (256 stems m⁻²) compared to deteriorated sites (149 stems m⁻²). By the second growing season (after sediment additions), stem densities in the deteriorated plots (308 stems m⁻²) approached levels in the non-deteriorated plots (336 stems m⁻²). Sediment additions to, both nos-deteriorated and deteriorated plots resulted in a higher redox potential with plots receiving the most sediment exhibiting the highest E_h values. In deteriorated plots, placement of dredged material had the greatest effect on plant density, but also affected soil oxidation-reduction potential and sediment deposition (or mobility). Following sediment placement, substrate texture and composition incrementally returned to prefill conditions due to a combination of bioturbation and sedimentation. Where infaunal differences occurred, they were generally less abundant in deteriorated plots, but responses to sediment addition were variable. Sediment addition had little effect on the non-deteriorated plots, suggesting that the disposal of certain types of dredged material in marshes may be useful to mitigate the effects of marsh degradation without adversely affecting non-deteriorating marsh.  相似文献   

Lunar ranging instrument for Chandrayaan-1     

J. A. Kamalakar  K. V. S. Bhaskar  A. S. Laxmi Prasad  R. Ranjith  K. A. Lohar  R Venketeswaran  T. K. Alex 《Journal of Earth System Science》2005,114(6):725-731

Lunar Laser Ranging Instrument (LLRI) proposed for the first Indian lunar mission Chandrayaan-1 is aimed to study the topography of the Moon’s surface and its gravitational field by precisely measuring the altitude from a polar orbit around the Moon. Altimetry data close to the poles of the Moon would also be available from the instrument, which was not covered by earlier missions. This instrument supplements the terrain mapping camera and hyperspectral imager payloads on Chandrayaan-1. The instrument consists of a diode pumped Nd:YAG pulsed laser transmitter having 10 nsec pulse width and a receiver system. The receiver system features 17 cm diameter Ritchey—Chrétien collecting optics, Si Avalanche Photo Detector (APD), preamplifiers, constant fraction discriminators, time-of-flight measurement unit and spacecraft interface. Altimeter resolution of better than 5 m is targeted. The received signal strength of LLRI depends on laser pulse backscatter from the Moon’s surface. Moon’s surface being a poor reflector, the choice of receiver size and its type and the selection of detector play an important role in getting a good signal-to-noise ratio and in turn achieving the target resolution. At the same time, the spacecraft puts a limitation on payload size and weight. This paper discusses the proposed LLRI system for Chandrayaan-1 and signal-to-noise ratio estimation.  相似文献   

The Seismic Project of the National Tsunami Hazard Mitigation Program     

David H. Oppenheimer  Alex N. Bittenbinder  Barbara M. Bogaert  Raymond P. Buland  Lynn D. Dietz  Roger A. Hansen  Stephen D. Malone  Charles S. McCreery  Thomas J. Sokolowski  Paul M. Whitmore  Craig S. Weaver 《Natural Hazards》2005,35(1):59-72

In 1997, the Federal Emergency Management Agency (FEMA), National Oceanic and Atmospheric Administration (NOAA), U.S. Geological Survey (USGS), and the five western States of Alaska, California, Hawaii, Oregon, and Washington joined in a partnership called the National Tsunami Hazard Mitigation Program (NTHMP) to enhance the quality and quantity of seismic data provided to the NOAA tsunami warning centers in Alaska and Hawaii. The NTHMP funded a seismic project that now provides the warning centers with real-time seismic data over dedicated communication links and the Internet from regional seismic networks monitoring earthquakes in the five western states, the U.S. National Seismic Network in Colorado, and from domestic and global seismic stations operated by other agencies. The goal of the project is to reduce the time needed to issue a tsunami warning by providing the warning centers with high-dynamic range, broadband waveforms in near real time. An additional goal is to reduce the likelihood of issuing false tsunami warnings by rapidly providing to the warning centers parametric information on earthquakes that could indicate their tsunamigenic potential, such as hypocenters, magnitudes, moment tensors, and shake distribution maps. New or upgraded field instrumentation was installed over a 5-year period at 53 seismic stations in the five western states. Data from these instruments has been integrated into the seismic network utilizing Earthworm software. This network has significantly reduced the time needed to respond to teleseismic and regional earthquakes. Notably, the West Coast/Alaska Tsunami Warning Center responded to the 28 February 2001 Mw 6.8 Nisqually earthquake beneath Olympia, Washington within 2 minutes compared to an average response time of over 10 minutes for the previous 18 years.  相似文献   

Strontium-90 in surface and groundwater of the reclamation systems at the Chernobyl exclusion zone     

Alex Shevchenko  Vadym Gudzenko  Igor Nasedkin  Edward Panasevitch 《Environmental Geology》2001,40(10):1177-1184

The left bank of the Pripiat' river, opposite of the Chernobyl Nuclear Power Plant (ChNPP) is covered with a developed network of drainage canals, built for the reclamation of the swampy floodplain. This area is highly contaminated after the disaster in 1986. Concentrations of the most mobile radionuclide, ⁹⁰Sr, are comparable with ¹³⁷Cs, and reach many MBq m^-2. Their ratio in the upper 10 cm of soil is about 1. The high surface contamination as well as precipitation and flooding have been responsible for the water pollution in this area. Some specifics of ⁹⁰Sr behaviour in surface and groundwater, controlled by the hydrological regime, are described.  相似文献   

Direct measurement of the Si–O bond length and orientational disorder in the high-temperature phase of cristobalite     

Martin T. Dove  David A. Keen  Alex C. Hannon  Ian P. Swainson 《Physics and Chemistry of Minerals》1997,24(4):311-317

Neutron total scattering measurements from powdered samples of cristobalite have been used to determine the local structure in both the tetragonal and cubic phases. The results for the cubic phase show directly that the Si–O bonds are tilted at an angle of around 17° to the unit cell [111] direction. It is striking that the structure of β-cristobalite over the range 5–10 Å is closer to that of silica glass than α-cristobalite, which suggests that the local structure of β-cristobalite is not likely to consist of domains with the structure of α-cristobalite. The measurements show a small thermal expansion of the Si–O bonds over the temperature range 570–950 K.  相似文献   

Long-term tendencies in the electoral behavior and the geography of voting in St. Petersburg: 1989–1995     

Konstantin E. Axenov  Alex G. Papadopoulos 《GeoJournal》1997,42(4):433-448

This paper studies the basis and nature of the long-term factors that may have influenced the electoral behavior of St. Petersburgers between 1989 and 1995 from the perspective of geography. Our analysis shows two distinct periods in St. Petersburgs recent electoral history marked by differing behavioral motivations of voters in the city. The first period was marked by only one ideological cleavage between marketers and hard-liners. In 1993 protest motivation managed to erode the predominance of ideology as the determining variable in voting behavior, and began to act as a new independent variable. This marked the beginning of the second period. Four major groupings of St. Petersburg electorate are described in terms of the basic ideological cleavages, party affiliations and geographical gravity centers.  相似文献   

Climatic variability and the anchovy/sardine ratio in Hellenic waters     

I. Konstantinos Stergiou  Alex Lascaratos 《GeoJournal》1997,41(3):245-254

In the present work, the monthly anchovy (Engraulis encrasicolus) and sardine (Sardina pilchardus) commercial catches, the anchovy/sardine commercial catch ratio as well as different climatic parameters in Hellenic waters (air and sea surface temperature AIRT and SST, sea level pressure SLP, N-S wind component NSW, and wind speed cubed WISC) over the 1964 to 1989 period were analyzed using spectral and cross-spectral analyses. The monthly anchovy and sardine catches as well as the anchovy/sardine catch ratio all exhibited a pronounced seasonal cycle that is related to the behaviour of sardine and anchovy and the nature of the purse seine fishery. Spectral analysis of the monthly sardine catches and anchovy/sardine catch ratio in the Hellenic waters also revealed cycles of 2.7 yr whereas that of anchovy catches revealed a prominent peak at 4.6 yr and secondary ones at 3.6 and 1.9 yr. Cross-spectral analysis resolved significant coherence between the anchovy/sardine ratio and SLP at 2.7 yr and marginally with NSW and WISC at 2.7 and 5.3 yr, respectively. The corresponding phase spectra showed that increased SLP and NSW over the north Aegean Sea proceed and are associated with decreased anchovy/sardine ratio with a lag of about 2.5 months for SLP and 6 months for NSW whereas increased WISC over the north Aegean proceed and are associated with increased anchovy/sardine ratio with a lag of 28 months. Cross-spectral analysis also resolved significant coherence between the anchovy catches and AIRT and SLP at 2.7 yr, NSW at 1.8 yr and WISC at 5.3 yr. The corresponding phase spectra showed that increased SLP, AIRT and NSW over the north Aegean Sea proceed and are associated with decreased anchovy catches with a lag of about 2 months for SLP, 16 months for AIRT and 6 months for NSW whereas increased WISC over the same area proceed and are associated with increased anchovy catches with a lag of 30 months. In contrast, cross-spectral analysis resolved no significant coherence between sardine catches and all five climatic variables. Possible mechanisms underlying such patterns are discussed.  相似文献   

Comment on “open and closed compositional data in petrography” by E. H. T. Whitten     

Alex Woronow 《Mathematical Geosciences》1996,28(5):673-675

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Seasonal changes in D/H fractionation accompanying lipid biosynthesis in Spartina alterniflora   总被引：1，自引：0，他引：1  

Alex L. Sessions 《Geochimica et cosmochimica acta》2006,70(9):2153-2162

To investigate potential variability in the biosynthetic fractionation of hydrogen isotopes between environmental water and plant lipids, the cord grass Spartina alterniflora was sampled from a single location in a coastal marsh over a period of 16 months. Values of δD for a variety of lipids were measured by gas chromatography/pyrolysis/isotope ratio mass spectrometry. S. alterniflora grows partially submerged in seawater, so it has a virtually unlimited supply of water with nearly unvarying isotopic composition. Temporal changes in the δD values of lipids can thus be interpreted as representing mainly variations in biosynthetic fractionation. Fatty acids, n-alkanes, and phytol extracted from S. alterniflora have nearly constant δD values from ∼October through May, but exhibit marked decreases of up to 40‰ during summer months. These shifts in lipid δD values are interpreted as representing a change in the source of organic substrates, principally acetate, used for their biosynthesis. Lower summertime δD values for lipids are consistent with an increasing reliance on current photosynthate as feedstock for biosynthesis, whereas stored carbohydrate reserves are utilized more extensively during other times of the year. Regardless of the specific mechanism, the data emphasize that overall fractionations between water and plant lipids depend on biological as well as environmental variables, and that the biosynthetic fractionation is not necessarily constant even for a single plant. Because lipids such as fatty acids are present in all cells and turn over on timescales of weeks to months, measurements of δD values in fatty acids may also provide useful constraints for distinguishing biologic versus environmental controls on cellulose δD values in trees.  相似文献   

Sediment yield in South Africa — A preliminary geographical analysis     

Kim Boucher  Alex van Breda Weaver Dr. 《GeoJournal》1991,23(1):7-17

The study examines the spatial relationships between sediment yield and 15 independent environmental variables in 54 catchments in South Africa. Rooseboom's (1978) data on the sediment yield from the catchments were standardized for a single time period. Bivariate regression analyses reveal no simple relationships. Multivariate regression analyses conducted for the whole and various sub-areas of South Africa indicate that latitude and longitude are the primary variables affecting spatial variations in sediment yield. This may be as a result of latitude and longitude being surrogate variables reflecting variation in other environmental variables (e. g. geology, vegetation). Within the sub-areas, 43.4% to 97.8% of the variation in sediment yield is explained by the combined variation in a number of different environmental variables. This study highlights a need for the collection and analysis of more sediment yield data, which would allow the analyses to be refined, to predict sediment yields from ungauged catchments in South Africa.  相似文献