Fe−Mn nodule field indicated by GLORIA,north of the Puerto Rico Trench     

Kathryn M. Scanlon  Douglas G. Masson 《Geo-Marine Letters》1992,12(4):208-213

A 2,500-km² area of sea floor on the south-eastern flank of the Greater Antilles Outer Ridge north of the Puerto Rico Trench displays anomalously high acoustic backscattering properties on GLORIA longrange sidescan-sonar data. Previously collected dredges, bottom photographs, and sediment cores indicate the presence of Fe–Mn nodules within the area of high backscatter. We were able to map the extent of the inferred nodule field on the basis of acoustic property contrast between the nodule-covered sediment and the surrounding nodule-free sediment.  相似文献   

Waves, currents, sediment flux and morphological response in a barred nearshore system     

Brian Greenwood  Douglas J Sherman   《Marine Geology》1984,60(1-4):31-61

A shore-normal array of seven, bi-directional electromagnetic flowmeters and nine surface piercing, continuous resistance wave staffs were deployed across a multiple barred nearshore at Wendake Beach, Georgian Bay, Canada, and monitored for a complete storm cycle. Time-integrated estimates of total (ITVF) and net (INVF) sediment volume flux together with bed elevation changes were determined using depth-of-activity rods.

The three bars, ranging in height from 0.10 to 0.40 m accreted during the storm (0.03 m), and the troughs were scoured (0.05 m). Sediment reactivation depths reached 0.14 m and 12% of the nearshore control volume was mobilized. However, the INVF value for the storm was less than 1% of the control volume revealing a near balance in sediment volume in the bar system. Landward migration of the inner, crescentic and second, sinuous bars occurred in association with an alongshore migration of the bar form itself; the outermost, straight, shore-parallel bar remained fixed in location.

The surf zone was highly dissipative throughout the storm (ε = 3.8 × 10²–192 × 10²) and the wave spectrum was dominated by energy at the incident frequency. Spectral peaks at frequencies of the first harmonic and at one quarter that of the incident wave were associated with secondary wave generation just prior to breaking and a standing edge wave, respectively. The former spectral peak was within the 95% confidence band for the spectrum while the latter contributed not more than 10% to the total energy in the surface elevation spectrum even near the shoreline.

During the storm wave height exceeded 2 m (H_s) and periods reached 5 s (T_{p k}): orbital velocities exceeded 0.5 m s⁻¹ (u_{rm s}) and were above the threshold of motion for the medium-to-fine sands throughout the storm. Shore-parallel flows in excess of 0.4 m s⁻¹ were recorded with maxima in the troughs and minima just landward of the bar crest.

The rate and direction of sediment flux is best explained by the interaction of antecedent bed slopes with spatial gradients in the mean and asymmetry of the shore-normal velocity field. These hydrodynamic parameters represent “steady” flows superimposed on the dominantly oscillatory motion and assumed a characteristic spatial pattern from the storm peak through the decay period. Increases spatially in the magnitudes of both the mean flows and flow asymmetries cause an increasing net transport potential (erosion); decreases in these values spatially cause a decreasing net transport potential and thus deposition. These transport potentials are increased or decreased through the gravity potential induced by the local bed slope. Shore-parallel flow was important in explaining sediment flux and morphological change where orbital velocities, mean flows and flow asymmetries were at a minimum.  相似文献   

Seasonal changes in speciation of dissolved chromium in the surface Sargasso Sea   总被引：1，自引：0，他引：1  

Douglas P. Connelly  Peter J. Statham  Anthony H. Knap 《Deep Sea Research Part I: Oceanographic Research Papers》2006,53(12):1975-1988

The concentrations of Cr (III) and Cr (VI) were determined in the upper 500 m of the Sargasso Sea at different times of the year. Total Cr concentrations ranged between 2.5 and 4.5 nM, and the predominant species of Cr was the thermodynamically favoured Cr (VI). Cr (III) concentrations were greatest during periods of high biological activity, as indicated by strong correlations between the ratio of Cr (III):Cr (VI) and both bacterial biomass and primary productivity. Whilst the oxidation state varied substantially over the seasonal cycle, the upper water column inventory of Cr was relatively constant. The long residence time of Cr in the surface Sargasso Sea estimated with respect to atmospheric inputs (1410–2240 years) is indicative of the largely conservative behaviour of this element. Deep particle fluxes of Cr are similar or somewhat greater than atmospheric inputs, and this downward flux of Cr is well correlated with organic carbon fluxes from the upper Sargasso Sea. The long residence time coupled with the seasonal changes in oxidation state of Cr implies rapid seasonal recycling of Cr between redox states, with only slow removal via particles to the deep ocean at this site.  相似文献   

Submersible mountedin situ geotechnical instrumentation     

Douglas N. Lambert 《Geo-Marine Letters》1982,2(3-4):209-214

A series of new miniaturizedin situ geotechnical instruments were developed and subsequently field tested with the Deep Sea Research Vessel (DSRV)Alvin during October of 1980 in various sedimentary features on the U.S. East Coast continental slope and upper rise within the Wilmington Geotechnical Corridor. These instruments are herein described and include a cone penetrometer, resistivity/conductivity probe, miniature piezometer, an inclinometer and a small diameter coring system. The various drive systems, controls and data recording systems are also briefly discussed.  相似文献   

Bottom processes, morphology, and geotechnical properties of the continental slope south of Baltimore Canyon     

Bonnie A. McGregor  Richard H. Bennett  Douglas N. Lambert 《Applied Ocean Research》1979,1(4):177-187

The continental slope south of Baltimore Canyon seaward of the coasts of Delaware and Maryland has a different morphology and sedimentary structure than adjacent portions of the continental margin. Ridges of sediment 600 m thick and transverse to the slope contain many unconformities that can be traced from ridge to ridge. The age of the sediment is inferred to be late tertiary to recent with the morphology related to a major drainage system. Physical properties of a suite of sediment cores display a pattern that varies in relationship to the morphology and depositional environment. Sedimentary structures and low shear strengths indicate instability of surficial sediments present on the upper slope and can be correlated with regions where the seismic reflection profiles show slumping has occurred. A veneer of sand overlying the general silty clay of the area is present on the upper slope and on the ridges indicating sand spillover from the shelf with a recent change in deposition pattern.  相似文献   

The reflection spectrum of liquid sulfur: Implications for Io     

Robert M. Nelson  David C. Pieri  Stephen M. Baloga  Douglas B. Nash  Carl Sagan 《Icarus》1983,56(3):409-413

The spectral reflectance from 0.38 to 0.75 μm of a column of liquid sulfur has been measured at several temperatures between the melting point (～118°C) and 173°C. Below 160°C the spectral reflectance was observed to vary reversibly as a function of temperature, independent of the previous thermal history of the column. Once the temperature exceeded 160°C, the spectrum would not change given a subsequent decrease in temperature. The spectral reflectance of the liquid-sulfur column at all temperatures was very low (10–19%). Combining this information with Voyager spectrophotometry of Jupiter's satellite Io, it is concluded that liquid sulfur at any temperature on Io's surface would be classified as a “black area” according to the standards used by the Voyager imaging team in their spectrophotometric analysis (L. Soderblom, T. V. Johnson, D. Morrison, E. Danielson, B. L. Smith, J. Veverka, A. Cook, C. Sagan, P. Kupferman, D. Pieri, J. Mosher, C. Avis, J. Gradie, and T. Clancy (1980). Geophys. Res. Lett.7, 963–966).  相似文献   

Spectral reflectance of Martian areas during the 1973 opposition: Photoelectric filter photometry 0.33–1.10 μm     

Thomas B. McCord  Robert L. Huguenin  Douglas Mink  Carle Pieters 《Icarus》1977,31(1):25-39

Reflectance spectra of 26 Martian areas (200–400 km in diameter) that were measured during the 1973 opposition are presented. They were measured through 25 narrow-band interference filters between 0.33 and 1.10 μm, using a photoelectric filter photometer at the Mauna Kea 230-cm telescope. There were many more bright and dark areas observed than during previous oppositions, and for the first time spectra were obtained of dust clouds and areas of mixed and intermediate albedo. The bright areas and dust clouds were all apparently composed of the same mineralogic unit. The dark area spectra differed substantially from the bright area and dust cloud spectra, and they showed major regional variations. The spectra of mixed and intermediate albedo areas had absorption bands seen in both bright and dark area spectra, and did not display any unique new features: thus they were apparently not compositionally unique, but rather were probably composed of mixtures of high albedo dust and dark area soils.  相似文献   

THE EFFECTS OF WORK PLACE-RESIDENCE SEPARATION AND RIDE SHARING ON EMPLOYEE ABSENTEEISM: SURVEY AND PILOT STUDY     

Douglas J. Kocher  Thomas L. Bell 《The Professional geographer》1977,29(3):272-277

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The rupture process and tectonic implications of the great 1964 Prince William Sound earthquake     

Douglas H. Christensen  Susan L. Beck 《Pure and Applied Geophysics》1994,142(1):29-53

We have determined the rupture history of the March 28, 1964, Prince Williams Sound earthquake (M _w=9.2) from long-period WWSSNP-wave seismograms. Source time functions determined from the long-periodP waves indicate two major pulses of moment release. The first and largest moment pulse has a duration of approximately 100 seconds with a relatively smooth onset which reaches a peak moment release rate at about 75 seconds into the rupture. The second smaller pulse of moment release starts at approximately 160 seconds after the origin time and has a duration of roughly 40 seconds. Because of the large size of this event and thus a deficiency of on-scale, digitizableP-wave seismograms, it is impossible to uniquely invert for the location of moment release. However, if we assume a rupture direction based on the aftershock distribution and the results of surface wave directivity studies we are able to locate the spatial distribution of moment along the length of the fault. The first moment pulse most likely initiated near the epicenter at the northeastern down-dip edge of the aftershock area and then spread over the fault surface in a semi-circular fashion until the full width of the fault was activated. The rupture then extended toward the southwest approximately 300 km (Ruff andKanamori, 1983). The second moment pulse was located in the vicinity of Kodiak Island, starting at 500 km southwest of the epicenter and extending to about 600 km. Although the aftershock area extends southwest past the second moment pulse by at least 100 km, the moment release remained low. We interpret the 1964 Prince William Sound earthquake as a multiple asperity rupture with a very large dominant asperity in the epicentral region and a second major, but smaller, asperity in the Kodiak Island region.The zone that ruptured in the 1964 earthquake is segmented into two regions corresponding to the two regions of concentrated moment release. Historical earthquake data suggest that these segments behaved independently during previous events. The Kodiak Island region appears to rupture more frequently with previous events occurring in 1900, 1854, 1844, and 1792. In contrast, the Prince William Sound region has much longer recurrence intervals on the order of 400–1000 years.  相似文献   

Behavior of a colluvial slope located in Southeastern Brazil     

Mauricio Ehrlich  Douglas Pereira da Costa  Rafael Cerqueira Silva 《Landslides》2018,15(8):1595-1613

Colluvial deposits are commonly found throughout the Serra do Mar escarpment in Brazil. This article discusses predisposing geotechnical and geological factors related to the movement of a colluvium deposit located at km 29 of the Brazilian Federal Highway 116, Rio de Janeiro/RJ (BR-116/RJ). During the highway construction in mid-1976, an excavation of the toe of the slope was implemented, exacerbating preexisting movement. In order to understand the behavior of this deposit, field investigations, monitoring, and laboratory tests, as well as theoretical analyses were carried out. A subsoil survey in the area indicated the presence of a colluvium-filled paleo-thalweg. The site is located in a region with a specific geomorphology that favors significant sources of recharge to groundwater. The highly foliated rock has persistent parallel fractures, with a dip favorable to the groundwater flow in the direction of the slope. Statistical analyses were performed and good correlations were observed for rainfall, movements, groundwater table (GWT) levels, piezometer readings, and the discharge from deep horizontal drains (DHDs). The installation of the DHDs led to a GWT draw-down of 15 m, as well as reductions in movement and artesian pressure. The drains that presented the best performance were those installed in the colluvium-filled paleo-thalweg. Nevertheless, the present number of DHDs installed is not sufficient to permanently lower the GWT and stop the creep movement in the slope which was reactivated during a period of intense rainfall. Stability analysis indicates that a proper slope stability condition may be achieved by reducing the GWT level.  相似文献