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1.
《Continental Shelf Research》2006,26(17-18):2050-2072
A 5-yr data set of near-bed current and suspended-sediment concentration measured within 2 m of the seabed in 60-m water depth has been analyzed to evaluate the interannual variability of physical processes and sediment transport events on the Eel River continental shelf, northern California. This data set encompasses a wide range of shelf conditions with winter events characterized as: Major Flood (1996/97), strong El Niño (1997/98), strong La Niña (1998/99), and Major Storm (1999/00). Data were collected at a site located 25 km north of the Eel River mouth, on the landward edge of the mid-shelf mud deposit. During the winter months sediment resuspension is forced primarily by near-bed oscillatory flows, and sediment transport occurs both as suspended load and as gravity-driven (fluid-mud) flows. Winter conditions that caused periods of increased sediment transport existed on average for 142 d yr−1 over the total record, ranging between 89 d in the Major Flood year (1996/97) and 171 d in the La Niña year (1998/99). Hourly averaged values of significant wave height varied between 0.5 and 10.7 m and hourly averaged values of near-bed orbital velocities ranged between 0 and 125 cm s−1. During the five winters, sediment threshold conditions were exceeded an average of 35% of the time, ranging from 19% in the Major Flood year (1996/97) to 52% in the La Niña year (1998/99). Mean concentration of suspended sediment, measured at 30 cmab, ranged from values close to 0–8 g l−1. Among winters, major sediment flux events exhibited different patterns due to varying combinations of physical processes including river floods, waves, and shelf circulation. Within winters, the major period of sediment flux varied from a 3-d fluid mud event (Major Flood winter) to a 50-d period of persistent southerlies (El Niño winter) and a winter of continuous storm cycles (La Niña winter). Winter-averaged suspended-sediment concentration appeared to vary in response to river discharge, while total sediment flux responded to storm intensity. The net sediment flux appeared to depend on timing of river discharge and shelf conditions. On the Eel River shelf, the mid-shelf mud deposit apparently is not emplaced by deposition from the river plume, but by secondary processes from the inner shelf including off-shelf transport of sediment suspensions and gravity-driven fluid-mud flows. Thus, these inner-shelf processes redistribute sediment supplied by the Eel River (a point source) making the inner shelf a line source of sediment that forms and nourishes the mid-shelf deposit. Large-scale shelf circulation patterns and interannual variability of the physical forcing are also important in determining the locus of the mid-shelf deposit, and both are influenced by climate variations. Post-depositional alteration of the deposit also depends on the subsequent shelf conditions following major floods.  相似文献   

2.
《Continental Shelf Research》1998,18(9):1039-1056
The detailed three-dimensional structure of the Kuroshio frontal eddy along the shelf edge of the East China Sea is revealed by the CTD, ADCP, and satellite-tracked drifter observations. The length and width of the cold core of the Kuroshio frontal eddy are about 60 and 40 km, respectively, and its phase speed is about 30 cm s-1. The calculated buoy tracks with the use of the observed ADCP data well reproduce the observed tracks of satellite-tracked drifters around the frontal eddy. The observed maximum horizontal velocity around this frontal eddy are 40 cm s-1 and the center of this eddy shifts offshore in the deep layer. Nutrient is advected onshore across the shelf edge by passing of this frontal eddy while it is advected offshore without the frontal eddy at the shelf edge.  相似文献   

3.
《Continental Shelf Research》2007,27(3-4):489-505
Sediment accumulation over the past century on the continental shelf near the Po delta varies with distance from the most active distributary channels. Near the Pila and Goro distributaries, sediment accumulation is rapid (1–4 cm yr−1) and occurs in pulses. In these areas, the seabed is dominated by physical sedimentary structures that can be related to flood sedimentation. Between the two distributaries and in the southern portion of the dispersal system, sediment accumulation is slower (rates reach a minimum of 0.23 cm yr−1 at ∼50 km from the Pila mouth) and steady-state, reflecting more continuous dispersal of sediment during non-flood periods. Sedimentary strata in these locations are composed of finer (clayey silt), mottled sediment. The similarity in the spatial distribution of long-term (100-yr) sediment accumulation to deposition resulting from the 2000 flood event suggests that the Po shelf is flood-dominated.About half of the sediment delivered by the Po River on a 100-yr time scale can be accounted for in the seabed deposit within ∼50 km of the Pila mouth. The remaining sediment is likely transported southward by the prevailing circulation, and this sediment coalesces with inputs from the Apennine Rivers.  相似文献   

4.
The 1.0 Ma Kidnappers supereruption (~ 1200 km3 DRE) from Mangakino volcanic centre, Taupo Volcanic Zone, New Zealand, produced a large phreatomagmatic fall deposit followed by an exceptionally widespread ignimbrite. Detailed sampling and analysis of glass shards and mineral phases have been undertaken through a proximal 4.0 m section of the fall deposit, representing the first two-thirds of erupted extra-caldera material. Major and trace element chemistries of glass shards define three distinct populations (types A, B and C), which systematically change in proportion through the fall deposit and are inferred to represent three magma types. Type B glass and biotite first appear at the same level (~ 0.95 m above base) in the fall deposit suggesting later tapping of a biotite-bearing magma. Plagioclase and Fe–Ti oxide compositions show bimodal distributions, which are linked to types A and B glass compositions. Temperature and pressure (T–P) estimates from hornblende and Fe–Ti oxide equilibria from each magma type are similar and therefore the three magma bodies were adjacent, not vertically stacked, in the crust. Most hornblende model T–P estimates range from 770 to 840 °C and 90 to 170 MPa corresponding to storage depths of ~ 4.0–6.5 km. Hornblende model T–P estimates coupled with in situ trace element fingerprinting imply that the magma bodies were individually well mixed, and not stratified. Compositional gaps between the three glass compositional types imply that no mixing between these magmas occurred. We interpret these data, coupled with the systematic changes in shard compositional proportions through the fall deposit, to reflect that three independent melt-dominant bodies of magma contributed large (A, ~ 270 km3), medium (B, ~ 90 km3) and small (C, ~ 40 km3) volumes (as reflected in the fall deposits) and were systematically tapped during the eruption. We propose that the systematic evacuation of the three independent magma bodies implies that there was tectonic triggering and linkage of eruptions. Our results show that supereruptions can be generated by near simultaneous multiple eruptions from independent magma chambers rather than the evacuation of a large single unitary magma chamber.  相似文献   

5.
The Ganzi-Yushu-Xianshuihe Fault Zone (GYXFZ) is a typical active strike-slip fault that has triggered many large historic earthquakes, including the 2010 Mw 6.9 Yushu earthquake in the central Tibetan Plateau. This fault zone extends for ca. 800 km from the central Tibetan Plateau to its southeastern margin and varies in trend from WNW-ESE in the northwestern segment of the fault zone to NNW-SSE in the southeastern segment, having the geometry of an arc projecting northeastwards. In this study, we present evidence for the systematical sinistral deflection and/or offset of the Yangtze River and its branch stream channels and valleys along the GYXFZ. Topographic analysis of three-dimensional (3D) perspective images constructed using digital elevation model (DEM) data, 0.5 m-resolution WorldView and GeoEye images, and 15 m-resolution Landsat-Enhanced Thematic Mapper (ETM+) images, together with analysis of geological structures, reveals the following: (i) the main river channels and valleys of the Yangtze River drainage system show systematic sinistral deflections and/or offsets along the GYXFZ; (ii) various amounts of sinistral offset have accumulated on the tributary stream channels, valleys, and gullies of the Yangtze River along the fault, with a linear relation, D = aL, between the upstream length L from the deflected point and the offset amount D with a certain coefficient a; (iii) the maximum amount of sinistral offset is up to ca. 60 km, which was accumulated in the past 13–5 Ma; and (iv) the long-term average strike-slip rate is ca. 4.6–12 mm/year. Geological and geomorphic evidence, combined with geophysical data, demonstrates that the GYXFZ is currently active as one of the major seismogenic faults in the Tibetan Plateau, dominated by left-lateral strike-slip motion. Our findings supply important evidence for the tectonic evolution of strike-slip faults in the Tibetan Plateau since the Eurasia-India continental collision.  相似文献   

6.
In the Australian landscape larg stores of soluble salt are present naturally. In many cases it is attributable to salts entrapped as marine sediment in earlier geological time. At the district level, the need for information on the presence of saline subsurface material is increasing, particularly for its application to salinity hazard assessment and environmental management. This is the case in irrigated areas, where changes in hydrology can result in secondary salinisation. To reduce the expense, environmental studies use a regression relationship to make use of more readily observed measurements (e.g. electromagnetic (EM) data) which are strongly correlated with the variable of interest. In this investigation a methodology is outlined for mapping the spatial distribution of average subsurface (6–12 m) salinity (ECe — mS m? 1) using an environmental correlation with EM34 survey data collected across the Bourke Irrigation District (BID) in the Darling River valley. The EM34 is used in the horizontal dipole mode at coil configurations of 10 (EM34-10), 20 (EM34-20), and 40 (EM34-40). A multiple-linear regression (MLR) relationship is established between average subsurface ECe and the three EM34 signal data using a forward modeling stepwise linear modeling approach. The spatial distribution of average subsurface salinity generally reflects the known surface expression of point-source salinisation and provides information for future environmental monitoring and natural resource management. The generation of EM34 data on various contrived grids (i.e. 1, 1.5, 2. 2.5 and 3 km) indicates that in terms of accuracy, the data available on the 0.5 (RMSE = 188) and 1 km (RMSE = 283) grid are best, with the least biased predictions achieved using 1 (ME = ? 1) and 2 km (ME = 12) grids. Viewing the spatial distribution of subsurface saline material showed that the 0.5 km spacing is optimal, particularly in order to account for short-range spatial variation between various physiographic units. The Relative Improvement (RI) shows that increasing EM grids from 1, 1.5, 2, 2.5 to 3 km gave RI of ? 53, ? 100%, ? 107%, ? 128% and ? 140%, respectively. We conclude that at a minimum a 1 km grid is needed for reconnaissance EM34 surveying.  相似文献   

7.
《Continental Shelf Research》2005,25(9):1097-1114
South of the eastern end of Long Island (Montauk Point) along the Eastern U.S. coast, a coastal density front forms between the buoyant outflow plume of the Long Island Sound (LIS) and the denser shelf waters offshore. During a 2-day cruise in April 2002, measurements of the density and velocity structure of this front were obtained from high-resolution CTD and ADCP data. Transects show the front intersecting the bottom inshore of the 30 m isobath and shoaling offshore. Variability in the location of the front is small offshore of the 40 m isobath, yet tidal excursions of the front along the bottom are significant (5 km) inshore of this depth.The frontal structure of the LIS plume was similar to observations of bottom-trapped coastal density fronts and shelf break fronts. A coastal jet in the along front direction was the main feature of the mean velocity field and was found to be in thermal wind balance with the mean density field. Stronger than expected offshore velocities near the surface, most likely a result of wind forcing, were the only exception to these similarities. In addition, analysis of temperature and salinity gradients along isopycnals gives evidence of secondary cross-frontal circulation and detachment of the bottom boundary layer. Characteristics of the LIS plume are used to evaluate recent analytical models of bottom-trapped coastal density fronts and bottom-advected plume theory, finding good agreement.  相似文献   

8.
The Masaya Caldera Complex has been the site of three highly explosive basaltic eruptions within the last six thousand years. A Plinian eruption ca. 2 ka ago formed the widespread deposits of the Masaya Triple Layer. We distinguish two facies within the Masaya Triple Layer from each other: La Concepción facies to the south and Managua facies to the northwest. These two facies were previously treated as two separated deposits (La Concepción Tephra and the Masaya Triple Layer of Pérez and Freundt, 2006) because of their distinct regional distribution and internal architectures. However, chemical compositions of bulk rock, matrix and inclusion glasses and mineral phases demonstrate that they are the product of a single basaltic magma batch. Additionally, a marker bed containing fluidal-shaped vesicular lapilli allowed us to make a plausible correlation between the two facies, also supported by consistent lateral changes in lithologic structure and composition, thickness and grain size.We distinguish 10 main subunits of the Masaya Triple Layer (I to X), with bulk volumes ranging between 0.02 and 0.22 km3, adding up to 0.86 km3 (0.4 km3 DRE) for the entire deposit. Distal deposits identified in two cores drilled offshore Nicaragua, at a distance of ～ 170 km from the Masaya Caldera Complex, increase the total tephra volume to 3.4 km3 or ～ 1.8 km3 DRE of erupted basaltic magma.Isopleth data of five major fallout subunits indicate mass discharges of 106 to 108 kg/s and eruption columns of 21 to 32 km height, affected by wind speeds of < 2 m/s to ～ 20 m/s which increased during the course of the multi-episodic eruption. Magmatic Plinian events alternated with phreatoplinian eruptions and phreatomagmatic explosions generating surges that typically preceded breaks in activity. While single eruptive episodes lasted for few hours, the entire eruption probable lasted weeks to months. This is indicated by changes in atmospheric conditions and ash-layer surfaces that had become modified during the breaks in activity. The Masaya Triple Layer has allowed to reconstruct in detail how a basaltic Plinian eruption develops in terms of duration, episodicity, and variable access of external water to the conduit, with implications for volcanic hazard assessment.  相似文献   

9.
《Continental Shelf Research》2006,26(17-18):2029-2049
A field experiment was carried out in Massachusetts Bay in August 1998 to assess the role of large-amplitude internal waves (LIWs) in resuspending bottom sediments. The field experiment consisted of a four-element moored array extending from just west of Stellwagen Bank (90-m water depth) across Stellwagen Basin (85- and 50-m water depth) to the coast (24-m water depth). The LIWs were observed in packets of 5–10 waves, had periods of 5–10 min and wavelengths of 200–400 m, and caused downward excursions of the thermocline of as much as 30 m. At the 85-m site, the current measured 1 m above bottom (mab) typically increased from near 0 to 0.2 m/s offshore in a few minutes upon arrival of the LIWs. At the 50-m site, the near-bottom offshore flow measured 6 mab increased from about 0.1 to 0.4–0.6 m/s upon arrival of the LIWs and remained offshore in the bottom layer for 1–2 h. The near-bottom currents associated with the LIWs, in concert with the tidal currents, were directed offshore and sufficient to resuspend the bottom sediments at both the 50- and 85-m sites. When LIWs are present, they may resuspend sediments for as long as 5 hours each tidal cycle as they travel westward across Stellwagen Basin. At 85-m water depth, resuspension associated with LIWs is estimated to occur for about 0.4 days each summer, about the same amount of time as caused by surface waves.  相似文献   

10.
To study the occurrence characteristics of equatorial spread-F irregularities and their latitudinal extent, simultaneous digital ionosonde data (January–December 2001) from Trivandrum (8.2°N), Waltair (17.7°N) and Delhi (28.6°N) and 4 GHz scintillation data from Sikandarabad (26.8°N) and Chenglepet (10.4°N), and 250 MHz scintillation data from Bhopal (23.2°N) for equinoxes period are analysed. It is noted that except summer months, occurrence of spread F is always maximum at Trivandrum, minimum at Delhi and moderate at Waltair. During equinoxes and winter months. Their occurrences at higher latitude station are always conditional to their prior occurrences at lower latitudes indicating their association with the generation of equatorial plasma bubble and associated irregularities. Scintillation occurrences also follow the similar pattern. During the summer months, the spread-F occurrences are highest at equatorial location Trivandrum, moderate at Delhi and minimum at Waltair and seem to be caused by irregularities generated locally especially over Delhi.To gain forecasting capability, night-to-night occurrences of spread-F/scintillation at these locations are examined in relation to post sunset rise of h’F and upward ExB drift velocity over the magnetic equator using Trivandrum ionosonde data. It is noted that except the summer months, the spread-F at Trivandrum, Waltair and Delhi are observed only when equatorial ExB (h’F) is more than about 15 m/s (325 km), 20 m/s (350 km) and 25 m/s (375 km), respectively. With these threshold values their corresponding success rate of predictions are more than 90%, 50% and 15% at the respective locations. Whereas in the case of GHz scintillations near equator are observed only when ExB (h’F) is more than 15 m/s (325 km), whereas for low latitude, the same should be 30 m/s (400 km) and their success rate of prediction is about 90% and 30%, respectively. The intensity of 4 GHz scintillation at low latitude is also found to be positively correlated with equatorial upward ExB drift velocity values, whereas correlation is poor with that of equatorial scintillations. In conclusions, near magnetic equator threshold values of ExB or h’F can be successfully used for the night-to-night prediction of spread-F/scintillations occurrences, whereas these are necessary but not sufficient for their prediction at higher latitudes. For that some other controlling parameters like background electron density, neutral winds, gravity waves, etc. should also be examined.  相似文献   

11.
Inversion of local earthquake travel times and joint inversion of receiver functions and Rayleigh wave group velocity measurements were used to derive a simple model for the velocity crustal structure beneath the southern edge of the Central Alborz (Iran), including the seismically active area around the megacity of Tehran. The P and S travel times from 115 well-located earthquakes recorded by a dense local seismic network, operated from June to November 2006, were inverted to determine a 1D velocity model of the upper crust. The limited range of earthquake depths (between 2 km and 26 km) prevents us determining any velocity interfaces deeper than 25 km. The velocity of the lower crust and the depth of the Moho were found by joint inversion of receiver functions and Rayleigh wave group velocity data. The resulting P-wave velocity model comprises an upper crust with 3 km and 4 km thick sedimentary layers with P wave velocities (Vp) of ～5.4 and ～5.8 km s?1, respectively, above 9 km and 8 km thick layers of upper crystalline crust (Vp ～6.1 and ～6.25 km s?1 respectively). The lower crystalline crust is ～34 km thick (Vp ～ 6.40 km s?1). The total crustal thickness beneath this part of the Central Alborz is 58 ± 2 km.  相似文献   

12.
The distributions of 41 polychlorinated biphenyls (PCBs) were determined in the aqueous phase, suspended particulate matter (SPM), and sediment of the Daliao River estuary in Liaodong Bay, Bohai Sea (China). The total PCB concentrations ranged from 5.51 to 40.28 ng L−1 in the surface water, from 6.78 to 66.55 ng L−1 dry weight in the SPM, and from 0.83 to 7.29 ng g−1 dry weight in the sediment. The PCB concentrations in water, SPM, and sediment were moderate relative to those reported for other estuary and marine systems around the world. Sedimentary PCB concentrations decreased offshore due to the active deposition of laterally transported river-borne particles. The predominance of the highly chlorinated congeners for the water, SPM, and sediment samples are an indication of either a lack of degradation or the presence of nearby or recent releases into the environment.  相似文献   

13.
The molecular distribution and compound specific stable carbon and hydrogen isotope ratios were measured on solvent extractable hydrocarbons from the Late Paleoproterozoic McArthur River, or “Here's Your Chance” (HYC) Pb/Zn/Ag ore deposit in the Northern Territory of Australia. Five samples were collected from the McArthur River mine on a northeast–southwest transect in order to sample a gradient of hydrothermal alteration. One sample was taken from the unmineralized W-Fold Shale unit immediately below the HYC ore deposit. δD of n-alkanes, branched alkanes and bulk aromatic fractions were measured, and δ13C of n-alkanes, polynuclear aromatic hydrocarbons (PAHs) and bulk kerogen were measured to assess the isotopic effects of a varying degree of hydrothermal alteration on different components of HYC organic matter (OM). Relative to n-alkanes in Barney Creek Formation sediments that did not undergo mineralization, HYC n-alkanes are enriched in deuterium (D) by 50–60‰. This is likely to be a result of equilibrium hydrogen exchange during ore genesis with a highly D-enriched fluid that originated in an evaporitic basin. Trends with distance along the sample transect are ambiguous, but from the northernmost to southernmost point, n-alkanes are less D-enriched, and PAHs are less abundant and less 13C-enriched. This could be due to decreasing hydrothermal alteration effects, decreasing delivery of highly altered OM by the mineralizing fluid, or both. The carbon isotopic composition of HYC PAHs is inconsistent with a Barney Creek Fm source, but consistent with an origin in the underlying Wollogorang Formation. PAHs are 13C-depleted relative to n-alkanes, reflecting a kerogen source that was 13C-depleted compared to n-alkanes, typical for Precambrian sediments. PAHs are more 13C-depleted with increasing molecular weight and aromaticity, strengthening the case for a negative isotopic effect associated with aromatization in ancient sediments. Together, these data are consistent with a an ore deposition model in which fluids originated in an evaporitic deposit lower in the basin and interacted with metals and OM in the Tawallah Group at temperatures above 250 °C and a depth of ~ 6 km before ascending along a flower structure associated with the Emu Fault and cooling to 200 ± 20 °C before reaching Barney Creek sediments.  相似文献   

14.
Explosion deep seismic sounding data sections of high quality had been obtained with RV Meteor in the Reykjanes Iceland Seismic Project (RRISP77 [Angenheister, G., Gebrande, H., Miller, H., Goldflam, P., Weigel, W., Jacoby, W.R., Pálmason, G., Björnsson, S., Einarsson, P., Pavlenkova, N.I., Zverev, S., Litvinenko, I.V., Loncarecic, B., Solomon, S., 1980. Reykjanes Ridge Iceland Seismic Experiment (RRISP 77). J. Geophys. 47, 228–238]) which close an information gap near 62°N. Preliminary results were presented by Weigel [Weigel, W., 1980. Aufbau des Reykjanes Rückens nach refraktionsseismischen Messungen. In: Weigel, W. (Ed.), Reykjanes Rücken, Island, Norwegischer Kontinentalrand. Abschlusskolloquium, Hamburg zur Meteor-Expedition, vol. 45. DFG, Bonn, pp. 53–61], and here we report on the data and results of interpretation. Clear refracted phases to 90 km distance permit crustal and uppermost mantle structure to be modelled by ray tracing. The apparent P-wave velocities are around 4.5, 6–6.5, 7–7.6 and 8.2–8.7 km/s, but no wide-angle reflections have been clearly seen. Accompanying sparker reflection data reveal thin sediment ponds in the axial zone and up to 400 m thick sediments at 10 Ma crustal age. Ray tracing reveals the following model below the sediments: (1) a distinct, 1–2 km thick upper crust (layer 2A) with Vp increasing with age (to 10 Ma) from <3.4 to 4.9 km/s and with a vertical gradient of 0.1–0.2 km/s/km, (2) a lower crust or layer 3 beginning at depths of 2 (axis) to 4 km (10 Ma age) below sea level with 6.1–6.8 km/s and similar vertical gradients as above, (3) the lower crust bottoms at 5.2–9.5 km depth below sea level (0–10 Ma) with a marked discontinuity, underneath which (4) Vp rises from about 7.5–7.8 km/s (0–10 Ma) with a positive vertical gradient of, again, 0.1–0.2 km/s/km such that 8 km/s would be reached at 12 km and deeper near the axis. Our preferred interpretation is that the mantle begins at the distinct discontinuity (“Moho”), but a deeper “Moho” of Vp ≈ 8 km/s cannot be excluded. From Iceland southward to 60°N several experiments show a decrease of crustal thickness from 14 to 8 km. Velocity trends with age across the ridge reflect cooling and filling of cracks, and thickness trends probably suggest volcanic productivity variations as previously suggested.Gravity inversion concentrates on a profile across the ridge with the above seismic a priori information; with 0.2–0.5 km depth uncertainty it leads to a good fit (±2.5 mGal where seismic data exist). Best fitting densities are (in kg/m3) for sediments, 2180; upper crust, 2450–2570; lower crust, 2850–2940; mantle lithosphere, 3215–3240 with a deficit for an asthenospheric wedge of no more than −100 kg/m3. The morphological ridges and troughs superimposed on the SE ridge flank are partly correlated, partly anti-correlated with the Bouguer anomaly and suggest that variable crustal density variations accompany the morphology variations.  相似文献   

15.
A sand layer, interpreted as a hyperconcentrated flow deposit, was found interbedded between loess on a Yellow River terrace in Hukou, Shaanxi, China. The site is known as the Longwangchan Palaeolithic site. The deposits from the terrace were dated using optically stimulated luminescence (OSL). Two samples from the sand layer were dated to 30 ± 2 and 33 ± 3 ka using coarse-grained (CG) quartz, and to 82 ± 7 and 94 ± 5 ka using fine-grained (FG) quartz, respectively. The CG quartz OSL ages are believed to be reliable on the basis of the OSL ages of the bracketing loess samples. The overestimation of the FG quartz OSL ages of the sand samples is explained in terms of hyperconcentrated flow processes during sediment transport and deposition. The large difference between the CG and FG quartz OSL ages of the sand samples helps to better understand the formation of the hyperconcentrated flow deposit. The hyperconcentrated flow deposit in this study indicates an extreme river flood occurring at ～30 ka in this area. In addition, the stone artifacts found in the lower loess layer overlying the bedrock of the terrace were dated to ～47 ka.  相似文献   

16.
We report observations of a noctilucent cloud (NLC) over central Alaska by a ground-based lidar and camera on the night of 9–10 August 2005. The lidar at Poker Flat Research Range (PFRR), Chatanika (65°N, 147°W) measured a maximum integrated backscatter coefficient of 2.4×10?6 sr?1 with a peak backscatter coefficient of 2.6×10?9 m?1 sr?1 corresponding to an aerosol backscatter ratio of 120 at an altitude of 82.1 km. The camera at Donnelly Dome, 168 km southeast of PFRR, recorded an extensive NLC display across the sky with distinct filamentary features corresponding to wave structures measured by the lidar. The occurrence of the maximum integrated backscatter coefficient corresponded to the passage of a bright cloud band to the southwest over PFRR. The camera observations indicate that the cloud band had a horizontal width of 50 km and a length of 150 km. The horizontal scale of the cloud band was confirmed by medium-frequency radar wind measurements that reported mesopause region winds of 30 m/s to the southwest during the period when the cloud band passed over PFRR. Comparison of these measurements with current NLC microphysical models suggests a lower bound on the water vapor mixing ratio at 83 km of 7–9 ppmv and a cloud ice mass of 1.5–1.8×103 kg. Satellite measurements show that this NLC display occurred during a burst of cloud activity that began on 5 August and lasted for 10 days. This cloud appeared 10 days after a launch of the space shuttle. We discuss the appearance of NLCs in August over several years at this lower polar latitude site in terms of planetary wave activity and space shuttle launches.  相似文献   

17.
《Continental Shelf Research》2006,26(17-18):2260-2280
On October 3, 2002 Hurricane Lili made landfall on a previously studied region of the inner Louisiana shelf as a Category 2 storm with winds over 160 km/h. A week after the hurricane, major impacts of the storm were not evident in the water column except for the lower than expected inshore salinities (∼12 psu) for this time of year, which was characterized by low river discharge. Turbidity profiles were typical of those measured during previous investigations with suspended sediment concentrations >75 mg/L at inshore stations and <50 mg/L in surface waters and offshore. The implication is that the sediments resuspended during the hurricane settled soon after the storm passage. Water column particulate organic carbon (POC) concentrations ranged from 0.1 to over 2.0 mg/L, with the highest concentrations measured near the seabed and in the inshore portions of the study area. Suspended particles were characterized by low organic matter content (%POC of 0.5–2 wt%), low chlorophyll:POC ratios (Chl:POC<4 mg/g) and moderately elevated POC:particulate nitrogen ratios (POC:PN of 10–14 mol/mol), all suggesting their source was locally resuspended seabed sediment rather than from algal biomass or land-derived vascular plant detritus.Post hurricane sediment deposition throughout the study area resulted in a storm layer that ranged from <0.5 to 20 cm in thickness. In most locations sediment accumulation ranged from 3 to 10 cm. The storm deposits were generally composed of silty clays with a coarser, somewhat sandy 1–2 cm basal layer. Surface sediments from the storm layer were characterized by relatively high mineral surface areas (SA of 30–50 m2/g) and elevated OC contents (%OC of 1.0–2.0%). The dispersal of fine sediments following the hurricane resulted in marked changes in the SA and %OC values of surface sediments from offshore locations, which prior to the storm contained coarser, organic-poor particles (SA of 5–15 m2/g and %OC of 0.2–0.6%). The OC:SA and OC:N ratios of storm layer sediments ranged from 0.4 to 0.6 mg OC/m2 and from 10 to 12 mol/mol, respectively, and were comparable to those measured in surface sediments prior to the hurricane. Such similarities in the composition of the organic matter reinforce the idea that the source of the storm deposits was the finer fraction of resuspended seabed sediments, with little evidence for inputs from local land-derived sources or autochthonous algal production. Overall, the magnitude of sediment and organic matter deposition on the seabed after the storm greatly exceeded the annual inputs from the Atchafalaya River and coastal primary production. The combined effects of hurricane-driven erosion and post-storm deposition represent a major perturbation to the benthic community of the region, which is already subject to these types of disturbances due to the combined effects of peaks in river discharge and the passage of storm fronts.  相似文献   

18.
Magmas are transported through pre-existing fractures in many repeatedly erupting volcanoes. The study of this special process of magma transport is fundamentally important to understand the mechanisms and conditions of volcanic eruptions. In this paper, we numerically simulate the magma propagation process through a pre-existing vertical fracture in the crust by using the combined finite difference method (FDM), finite element method (FEM) and discontinuous deformation analysis (DDA) approach. FDM is used to analyze magma flow in the pre-existing fracture, FEM is used to calculate the opening of the fracture during magma intrusion, and DDA is used to deal with the contact of the closed fracture surfaces. Both two-dimensional (2D) and three-dimensional (3D) examples are presented. Parametric studies are carried out to investigate the influence of various physical and geometric parameters on the magma transport in the pre-existing fracture. We have considered magma chamber depth ranging from 7 km to 10 km under the crust surface, magma viscosity ranging from 2 × 10−2 to 2 × 10−7 MPa s, and the density difference between the magma and host rock ranging from 300 to 700 kg/m3. The numerical results indicate that (1) the fluid pressure p varies gradually along the depth, (2) the shape of the magma body during propagation is like a torch bar and its width ranges from 2 m to 4 m approximately in the 3D case and 10 m to 50 m in the 2D case for the same physical parameters used, (3) the crust surface around the pre-existing fracture begins to increase on both sides of the fracture, forms a trough between them, then gradually uplifts during the transport of the magma, and finally takes the shape of a crater when the magma reaches the surface. We have also examined the influence of physical and geometric parameters on the minimum overpressure for magma transport in the 3D case. The numerical results show that our numerical technique presented in this paper is an effective tool for simulating magma transport process through pre-existing fractures in the crust.  相似文献   

19.
《Continental Shelf Research》2006,26(17-18):2125-2140
Sediment delivered to coastal systems by rivers (15×109 tons) plays a key role in the global carbon and nutrient cycles, as deltas and continental shelves are considered to be the main repositories of organic matter in marine sediments. The Mississippi River, delivering more than 60% of the total dissolved and suspended materials from the conterminous US, dominates coastal and margin processes in the northern Gulf of Mexico. Draining approximately 41% of the conterminous US, the Mississippi and Atchafalaya river system deliver approximately 2×108 tons of suspended matter to the northern Gulf shelf each year. Unlike previous work, this study provides a comprehensive evaluation of sediment accumulation covering majority of the shelf (<150 m water depth) west of the Mississippi Delta from 92 cores collected throughout the last 15 years. This provides a unique and invaluable data set of the spatial and modern temporal variations of the sediment accumulation in this dynamic coastal environment.Three types of 210Pb profiles were observed from short cores (15–45 cm) collected on the shelf. Proximal to Southwest Pass in 30–100 m water depths, non-steady-state profiles were observed indicating rapid accumulation. Sediment accumulation rates in this area are typically >2.5 cm yr−1 (>1.8 g cm−2 yr−1). Kasten cores (∼200 cm in length) collected near Southwest Pass also indicate rapid deposition (>4 cm yr−1; >3 g cm−2 yr−1) on a longer timescale than that captured in the box cores. Near shore (<20 m), profiles are dominated by sediments reworked by waves and currents with no accumulation (the exception is an area just south of Barataria Bay where accumulation occurs). The remainder of the shelf (distal of Southwest Pass) is dominated by steady-state accumulation beneath a ∼10-cm thick mixed layer. Sediment accumulation rates for the distal shelf are typically <0.7 cm yr−1 (<0.5 g cm−2 yr−1). A preliminary sediment budget based on the distribution of 210Pb accumulation rates indicates that 40–50% of the sediment delivered by the river is transported out of the study region. Sediment is moved to distal regions of the shelf/slope through two different mechanisms. Along-isobath sediment movement occurs by normal resuspension processes west of the delta, whereas delivery of sediments south and southwest of the delta may be also be influenced by mass movement events on varying timescales.  相似文献   

20.
《Continental Shelf Research》2006,26(17-18):2241-2259
The Amazon River spawns a vast mobile mudbelt extending ∼1600 km from the equator to the Orinoco delta. Deposits along the Amazon–Guianas coastline are characterized by some of the highest Corg remineralization rates reported for estuarine, deltaic, or shelf deposits, however, paradoxically, except where stabilized by mangroves or intertidal algal mats, they are usually suboxic and nonsulfidic. A combination of tides, wind-driven waves, and coastal currents forms massive fluid muds and mobile surface sediment layers ∼0.5–2 m thick which are dynamically refluxed and frequently reoxidized. Overall, the seabed functions as a periodically mixed batch reactor, efficiently remineralizing organic matter in a gigantic sedimentary incinerator of global importance. Amazon River material entering the head of this dynamic dispersal system carries an initial terrestrial sedimentary Corg loading of ∼ 0.7 mg C m−2 particle surface area. Total Corg loading is lowered to ∼ 0.2 mg C m−2 in the proximal delta topset, ∼60–70% of which remains of terrestrial origin. Loading decreases further to 0.12–0.14 mg C m−2 (∼60% terrestrial) in mudbanks ∼600 km downdrift along French Guiana, values comparable to those found in the oligotrophic deepsea. DOC/ΣCO2 ratios in pore waters of French Guiana mudbanks indicate that >90% of metabolized organic substrates are completely oxidized. Within the Amazon delta topset at the head of the dispersal system, both terrestrial and marine organic matter contribute substantially to early diagenetic remineralization, although reactive marine substrate dominates (∼60–70%). The conditional rate constant for terrestrial Corg in the delta topset is ∼0.2 a−1. As sedimentary Corg is depleted during transit, marine sources become virtually the exclusive substrate for remineralization except very near the mangrove shoreline. The δ13C and Δ14C values of pore water ΣCO2 in mudbanks demonstrate that the primary source of remineralized organic matter within ∼1 km of shore is a small quantity of bomb signature marine plankton (+80‰). Thus, fresh marine organic material is constantly entrained into mobile deposits and increasingly drives early diagenetic reactions along the transit path. Relatively refractory terrestrial Corg is lost more slowly but steadily during sedimentary refluxing and suboxic diagenesis. Amazon Fan deposits formed during low sea level stand largely bypassed this suboxic sedimentary incinerator and stored material with up to ∼3X the modern high stand inner shelf Corg load (Keil et al., 1997b. Proceedings of the Ocean Drilling Program, Scientific Results. Vol. 155. pp. 531–537). Sedimentary dynamics, including frequency and magnitude of remobilization, and the nature of dispersal systems are clearly key controls on diagenetic processes, biogeochemical cycling, and global C storage along the continental margins.  相似文献   

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