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1.
Annual sedimentary layers in core A from the Yangtze River (Changjiang) subaqueous delta were identified based on high-resolution biogenic silica (BSi) and grain-size records. The contents of BSi, the sediment grain-size and the sensitive grain-size fractions of <13.14 μm and 13.14-250 μm illustrated steady coincident fluctuations with the depth profile of core A. These fluctuations were inferred to reflect seasonal changes in physical and biological processes. For example, variations in the content of BSi likely represent changes in primary production in flood versus dry seasons. It in flood seasons was several times higher than that in dry seasons. The amounts of <13.14 and 13.14-250 μm fractions were complimentary to each other and co-varied with the BSi amount. Therefore, the BSi, grain-size and sensitive fractions can be used as proxies for the annual sedimentation. From 1992 to 2005, the estimated sedimentary rate based on the annual layer varied from 1.0 to 3.5  cm/a (mean of 2.07  cm/a), and these values varies with the annual sediment discharge from the Yangtze River to the sea. The significant seasonal variations in the water discharge, sediment load, ocean hydrodynamics and primary production, which are common in the area affected by the East Asian monsoon, are likely responsible for the formation of the annual sedimentary lamination.  相似文献   

2.
The sedimentology of the floor of the Amvrakikos Gulf, a river influenced, semi-enclosed relatively shallow-silled embayment, lying along the northeastern Hellenic coast of the Ionian Sea (eastern Mediterranean Sea), is investigated with respect to its origin (terrigenous and/or biogenic), the prevailing oceanographic conditions and human interference. Nearshore (water depths approximately <10 m) sediments, especially along the northern margin of the Gulf, consist mostly of biogenic sands, as the result of water exchange between the freshwater lagoonal waters and the surface waters of the Gulf. An exception to this is the mouth area of the Arachthos River, which is dominated by the terrigenous riverine sediment influx. The offshore (water depths >10 m) bottom surficial sediments are fine-grained (silty and clayey) of terrigenous origin (>70%); this is attributed to the inter-seasonal, strong two-layer stratification of the water column in the Gulf which restricts benthic productivity by inhibiting the downward flux of surface eutrophic waters and the development of nearbed disoxic conditions in water depths >40 m. River damming has reduced also the riverine terrigenous sediment supply; this is more profound in the case of the Arachthos River where not only the deltaic evolution has been affected, but also the textural character of the seabed sediments of the mouth area has been altered; this is expected to influence the benthic communities of prodeltaic surficial sediment.  相似文献   

3.
The accumulation of extraterrestrial 3He, a tracer for interplanetary dust particles (IDPs), in sediments from the Ontong Java Plateau (OJP; western equatorial Pacific Ocean) has been shown previously to exhibit a regular cyclicity during the late Pleistocene, with a period of ∼100 ka. Those results have been interpreted to reflect periodic variability in the global accretion of IDPs that, in turn, has been linked to changes in the inclination of Earth’s orbit with respect to the invariable plane of the solar system. Here we show that the accumulation in OJP sediments of authigenic 230Th, produced by radioactive decay of 234U in seawater, exhibits a 100-ka cyclicity similar in phase and amplitude to that evident in the 3He record. We interpret the similar patterns of 230Th and 3He accumulation to reflect a common origin within the ocean-climate system. Comparing spatial and temporal patterns of sediment accumulation against regional patterns of biological productivity and against the well-established pattern of CaCO3 dissolution in the deep Pacific Ocean leads to the further conclusion that a common 100-ka cycle in accumulation of biogenic, authigenic and extraterrestrial constituents in OJP sediments reflects the influence of climate-related changes in sediment focusing, rather than changes in the rate of production or supply of sedimentary constituents.  相似文献   

4.
The deltas of large rivers have become the focus of many research studies due to their vulnerability in times of expected sea level rise. One major delta in peril is the Ganges-Brahmaputra Delta (Bangladesh) due to its low elevation and high subsidence rate. In this study, high-resolution seismic and sediment echosounder data of the subaqueous part of the delta are analyzed by an integrated seismo-acoustic stratigraphic approach.Transparent Units (TUs) are the most prominent, continuously traceable architectural elements within the sigmoidal clinoform. TUs consist of homogenized sediments and are interpreted to be formed by liquefaction flows generated by subduction-related earthquakes of the nearby plate-boundary. The uppermost TUs are related to the historical well known major earthquakes which occurred in 1762, 1897 and 1950. Using the TUs as time marks the mean annual storage rates for the intervals could be assessed. A direct comparison of echosounder data gathered in 1994, 1997 and 2006 is used as another method to estimate the last decade mean annual storage rate.In general, the averaged decadal to centennial mean annual storage rates in the foreset beds have significantly decreased within the last ∼300 years from 22% to 18% and down to 13.8% of the present total annual fluviatile suspension supply to the delta (109 t a−1). This decrease, however, is not evenly distributed along the foreset beds. In the western clinoform mean annual storage rates slightly decreased during the last ∼300 years, whereas in the eastern clinoform the mean annual storage rate of the last decade is one-third of the initial value in the late eighteenth century.The variation of clinoform slope angles generally coincides with the local accumulation rates but a variable degree of asymmetry gives some new insights in the future trend of the subaqueous delta development. The sink of sediment in the western clinoform depocentre landward of the inflection point, where shape changes from convex to concave, is producing an increase in concavity.The decline in monsoon precipitation over the last centuries is assumed to have significantly contributed to the decreased mean annual storage rate in the eastern clinoform where the convexity of the clinoform increases seaward of the inflection point. It is very likely that the whole depocentre of the fluvial sediment load is shifting toward the western subaqueous delta and to the Swatch of No Ground Canyon and, thereby, increasing the export to the deep-sea fan.  相似文献   

5.
A large number of sediment cores collected during 2005-2010 from the Taiwan Strait were analyzed for radionuclides (210Pb, 137Cs and 7Be) to elucidate sedimentation dynamics in this all-important gateway linking two largest marginal seas in the western Pacific (namely, the South China Sea and the East China Sea). Apparent sediment accumulation rates derived from 210Pb and 137Cs profiles vary from <0.1 to >2 cm/yr, averaging ∼0.4 cm/yr and showing a spatial pattern closely related to hydrodynamics and sediment source-to-sink pathways. Spatial-temporal variation of 7Be activity in surface sediments off Taiwan’s west coast indicates episodic deposition of flood layers and their mobility from river estuaries toward the north. In conjunction with particle size distribution in surface sediments and the structure of sediment strata revealed by sub-bottom echo images; the radionuclide data can be used to outline three different sediment source-to-sink dispersal systems. Based on sediment loads of surrounding rivers and the distribution of sediment accumulation rates, lateral transport is required to account for the budget and size distribution of sediments in the strait.  相似文献   

6.
The biogeochemistry of methane in the sediments of Lake Caviahue was examined by geochemical analysis, microbial activity assays and isotopic analysis. The pH in the water column was 2.6 and increased up to a pH of 6 in the deeper sediment pore waters. The carbon isotope composition of CH4 was between − 65 and − 70‰ which is indicative for the biological origin of the methane. The enrichment factor ε increased from − 46‰ in the upper sediment column to more than − 80 in the deeper sediment section suggesting a transition from acetoclastic methanogenesis to CO2 reduction with depth. In the most acidic surface layer of the sediment (pH < 4) methanogenesis is inhibited as suggested by a linear CH4 concentration profile, activity assays and MPN analysis. The CH4 activity assays and the CH4 profile indicate that methanogenesis in the sediment of Lake Caviahue was active below 40 cm depth. At that depth the pH was above 4 and sulfate reduction was sulfate limited. Methane was diffusing with a flux of 0.9 mmol m− 2 d− 1 to the sediment surface where it was probably oxidized. Methanogenesis contributed little to the sediments carbon budget and had no significant impact on lake water quality. The high biomass content of the sediment, which was probably caused by the last eruption of Copahue Volcano, supported high rates of sulfate reduction which probably raised the pH and created favorable conditions for methanogens in deeper sediment layers.  相似文献   

7.
From 2002 through 2004, time-series sediment trap samples were collected from a depth of 410 m in Cuenca Alfonso, Bahía de La Paz, on the SW coast of the Gulf of California. The instrument recorded the impact of the local passage of hurricanes “Ignacio” (24–26 August) and “Marty” (21–23 September) in 2003. These two events accounted for 82% of the total rainfall measured in 2003, equivalent to the annual average precipitation in years without hurricanes. Mean total mass fluxes (TMFs) of 2.88 and 3.58 g m−2 d−1 were measured during the week of each hurricane as well as the following week. This may have been enough to produce a lamina in the underlying sediment with characteristics peculiar to such events. The terrigenous component was particularly abundant, with notably higher concentrations of Fe, Sc, Co and Cs and REEs. In contrast, TMFs throughout 2002–2004 (excluding the hurricane periods) averaged only 0.73 g m−2 d−1 and had a larger marine biogenic component. The extraordinary elemental fluxes during the 29 days of hurricane-influenced sedimentation represented a great proportion of the totals over an entire “normal” year: Co (67.8%) >Sc (62.6) >Fe (59.6) >Cs (53.4)>Lu (51.5)>La (51.3)>Yb (51.0)>Ce (49.5) >Tb (48.4) >Sm (44.7)>Cr (36.5) >Ca (31.0)>Eu (25.4%). The terrigenous fraction was calculated using (a) TMF minus the sum of CaCO3, biogenic silica and organic matter and (b) the ratio of Sc in the trap samples to the average in the Earth's crust. The latter was consistently smaller, but the two methods offered similar results following hurricanes (78% vs. 63%, respectively). For normal sedimentation, however, the difference method yielded values twice as large as the Sc method (58% vs. 30%) This suggests that the mineralogy of the terrigenous fraction may also vary, with unsorted dessert soil being carried to sea by the powerful flash floods associated with hurricanes. Eolian supply of particles, particularly Sc-free quartz grains, possibly from beyond the limited fluvial drainage basin, apparently dominates normal sedimentation.  相似文献   

8.
Impure reworked evaporitic sandstones, preserved on Meridiani Planum, Mars, are mixtures of roughly equal amounts of altered siliciclastic debris, of basaltic provenance (40 ± 10% by mass), and chemical constituents, dominated by evaporitic minerals (jarosite, Mg-, Ca-sulfates ± chlorides ± Fe-, Na-sulfates), hematite and possibly secondary silica (60 ± 10%). These chemical constituents and their relative abundances are not an equilibrium evaporite assemblage and to a substantial degree have been reworked by aeolian and subaqueous transport. Ultimately they formed by evaporation of acidic waters derived from interaction with olivine-bearing basalts and subsequent diagenetic alteration. The rocks experienced an extended diagenetic history, with at least two and up to four distinct episodes of cementation, including stratigraphically restricted zones of recrystallization and secondary porosity, non-randomly distributed, highly spherical millimeter-scale hematitic concretions, millimeter-scale crystal molds, interpreted to have resulted from dissolution of a highly soluble evaporite mineral, elongate to sheet-like vugs and evidence for minor synsedimentary deformation (convolute and contorted bedding, possible teepee structures or salt ridge features). Other features that may be diagenetic, but more likely are associated with relatively recent meteorite impact, are meter-scale fracture patterns, veins and polygonal fractures on rock surfaces that cut across bedding. Crystallization of minerals that originally filled the molds, early cement and sediment deformation occurred syndepositionally or during early diagenesis. All other diagenetic features are consistent with formation during later diagenesis in the phreatic (fluid saturated) zone or capillary fringe of a groundwater table under near isotropic hydrological conditions such as those expected during periodic groundwater recharge. Textural evidence suggests that rapidly formed hematitic concretions post-date the primary mineral now represented by crystal molds and early pore-filling cements but pre-date secondary moldic and vug porosity. The second generation of cements followed formation of secondary porosity. This paragenetic sequence is consistent with an extended history of syndepositional through post-depositional diagenesis in the presence of a slowly fluctuating, chemically evolving, but persistently high ionic strength groundwater system.  相似文献   

9.
Observations of sediment dispersal from the Santa Clara River of southern California during two moderately sized river discharge events suggest that river sediment rapidly formed a negatively buoyant (hyperpycnal) bottom plume along the seabed within hours of peak discharge. An array of acoustic and optical sensors were placed at three stations 1 km from the Santa Clara River mouth in 10-m water depth during January–February 2004. These combined observations suggest that fluid mud concentrations of suspended sediment (>10 g/l) and across-shore gravity currents (∼5 cm/s) were observed in the lower 20–40 cm of the water column 4–6 h after discharge events. Gravity currents were wave dominated, rather than auto-suspending, and appeared to consist of silt-to-clay sized sediment from the river. Sediment mass balances suggest that 25–50% of the discharged river sediment was transported by these hyperpycnal currents. Sediment settling purely by flocs (∼1 mm/s) cannot explain the formation of the observed hyperpycnal plumes, therefore we suggest that some enhanced sediment settling from mixing, convective instabilities, or diverging plumes occurred that would explain the formation of the gravity currents. These combined results provide field evidence that high suspended-sediment concentrations from rivers (>1 g/l) may rapidly form hyperpycnal sediment gravity currents immediately offshore of river mouths, and these pathways can explain a significant portion of the river-margin sediment budget. The fate of this sediment will be strongly influenced by bathymetry, whereas the fate of the remaining sediment will be much more influenced by ocean currents.  相似文献   

10.
Recent research on the Mississippi margin indicates notable seasonal variation in seabed dynamics. During years with minimal tropical-system activity, sediments initially deposited from late spring to early fall are remobilized by wind-driven currents and wave energy during extra-tropical weather systems in the winter. This research reveals the profound significance of tropical cyclones on Louisiana Shelf sedimentation. The amount of material delivered to and advected across the shelf by recent tropical cyclones is considerably larger than that related to winter storm systems. In Fall 2004, the river-dominated shelf of Louisiana was impacted by three tropical systems in less than a month, including Hurricane Ivan. Ivan, with maximum sustained winds in excess of 74 m s−1 (144 knots) and a minimum measured central pressure of 910 mbar, was the eighth most intense Atlantic hurricane on record at the time. In order to assess the impact these tropical systems had on the continental margin west of the Mississippi delta, seabed samples were collected from box cores in October 2004 and analyzed for particle-reactive radionuclides 234Th, 7Be, and 210Pb. Radiochemical data and observations from X-radiographs indicate event-driven sediment deposits ranged from 4 to 30 cm on the shelf and 2–6 cm in the Mississippi Canyon. These deposits exhibit distinct radiochemical signatures and differ visually and texturally from the underlying sediment. The well-developed physical stratification and graded nature of the deposits observed in core X-radiographs suggests that the sediment could have been deposited from sediment-gravity flows. Inventories of 7Be and 7Be/234Thxs ratios reveal this series of cyclones transported considerably more material to the outer shelf and slope than periods of minimal tropical-system activity. When compared to seasonal depositional rates created by winter storms, tropical-cyclone-related event deposits on the middle and outer shelf are up to an order of magnitude greater in thickness. The number and thickness of these event deposits decrease with distance from the delta and suggest that only the most severe tropical systems are likely capable of redistributing significant quantities of sediment to more distal portions of the shelf and slope. These severe-event-driven deposits may account for as much as 75% of the sediment burial budget on decadal time scales within Mississippi Canyon. Higher than average tropical cyclone activity, predicted by the National Hurricane Center over the next decade, may be the major mechanism controlling sediment transport and deposition on the Mississippi River continental shelf and in Mississippi Canyon.  相似文献   

11.
Low tide rainfall may represent an important but little studied process affecting sediment fluxes on intertidal mudflats. In this study, we simulated rainfall events on an intertidal mudflat (median grain size=18.4 μm) of low slope (1 in 300) then quantified effects on sediment erodibility. Treatments consisted of a high (4.1 mm min−1 for 6 min) and low (0.36 mm min−1 for 60 min) rain intensity, chosen to match naturally occurring events and experiments were conducted seasonally (May and August) to encompass variations in ambient sediment stability. Changes in bed elevation due to rainfall were estimated using marked rods and sediment erodibility parameters (mass of sediment eroded at a flow velocity of 0.3 m s−1 (ME-30, g m−2) and critical erosion velocity (Ucrit, m s−1)) were determined in annular flumes (bed area=0.17 m2). Ambient/control sediment erodibility in May (ME-30=211 g m−2, Ucrit=0.18 m s−1) was higher than in August (ME-30=30 g m−2, Ucrit=0.26 m s−1) and was correlated with changes in biological activity. In May, surface sediment was influenced by high densities of the bioturbating snail Hydrobia ulvae (1736 ind. m−2) and low biomass of the sediment stabilising microphytobenthos (5.7 μg chlorophyll a cm−2). In contrast, in August H. ulvae densities were low (52 ind. m−2) and microphytobenthic biomass higher (9.2 μg chlorophyll a cm−2). The high rain treatment caused a decrease in bed elevation of between 1.5 mm (May) and 4.4 mm (August) and significantly reduced sediment organic content and microphytobenthic biomass. Rainfall increased sediment erodibility; compared to ambient sediments ME-30 increased by a factor of 1.4× in May and 8.8× in August and caused a 10–30% decline in Ucrit. The seasonal difference in treatment effect was due to the change in ambient sediment stability. The low rain treatment in August had no effect on bed elevation, microphytobenthic biomass or sediment erodibility. In May, the same treatment caused a reduction in bed elevation (0.5 mm) and microphytobenthic biomass but counter-intuitively, a decrease in sediment erodibility (ME-30 was reduced by 40%, Ucrit increased by 5%) compared to controls. We attribute this result to removal by rainfall of easily eroded surface flocs and biogenic roughness which resulted in an underlying sediment with a smoother surface and greater resistant to erosion. Results suggest that high intensity rain events may destabilise intertidal sediments making them more susceptible to erosion by returning tidal currents and that the sediment eroded during such events may represent a considerable fraction (up to 25%) of the seasonal variation in shore elevation. The impact of natural rain events are likely to vary considerably due to variations in droplet size, intensity and duration and the interaction with ambient sediment stability.  相似文献   

12.
Two gravity cores collected off the modern Huanghe (Yellow River) delta in the southern Bohai Sea were analyzed for grain size, the total organic carbon (TOC)/total nitrogen (TN) ratio, color diffraction, magnetic susceptibility, 14C dating and 137Cs and 210Pb isotope contents to clarify changes in the sedimentary environment during the Holocene. In particular, the effect of natural and artificial river-course shifts of the Huanghe on the Bohai Sea sediment was investigated. A peat layer, scouring surface and sharp changes in the grain size, TOC/TN ratio, sediment color (L?, a?) and magnetic susceptibility were identified and are likely to be due to the early-Holocene sea-level rise resulting in environmental changes from coastal to shelf environments in the Bohai Sea. After the sea level reached its maximum at 6-7 ka BP, the lateral shifts in the river course of the Huanghe formed 10 superlobes, and superlobe 7 (11-1048 AD) and superlobe 10 (1855-present) of the Huanghe delta affected the core sites. The northern site of BH-239 has been more affected by the Huanghe since the middle Holocene. Notably, in the superlobe 10 period, the reshaping of the northern Huanghe delta due to an artificial river-course shift from northward to eastward in 1976 (e.g., a∼10 km shoreline retreat due to coastal erosion) was recorded in the core sediments, particularly in terms of the TOC/TN ratio, sediment color (L? and a?) and magnetic susceptibility, owing to the huge sediment supply from coastal erosion of the former river mouth area.  相似文献   

13.
Selenium concentrations have been measured in sediment, fucoid macroalgae and macroinvertebrates from four estuaries of SW England (Yealm, Plym, Looe, Fal). Sediment concentrations ranged from about 0.4 μg g−1 in the Yealm to 1.49 μg g−1 at one site in the Plym. Concentrations in Fucus vesiculosus (0.05–0.31 μg g−1) and F. ceranoides (0.05–0.51 μg g−1) were significantly lower than corresponding concentrations in sediment but there was no correlation between algal and sediment concentrations. Selenium concentrations in Littorina littorea (∼4 μg g−1), Hediste diversicolor (2.82–12.68 μg g−1), Arenicola marina (∼17 μg g−1) and Scrobicularia plana (1.18–6.85 μg g−1) were considerably higher than concentrations in macroalga or sediment, suggesting that Se is effectively accumulated from the diet. Although Se concentrations in some invertebrates exceed toxicity thresholds for the diet of predacious birds and fish, no specific evidence for Se toxicity exists in these estuaries.  相似文献   

14.
Water and surface sediment samples were analyzed for butyltins (TBT, DBT, MBT) from various ports along the east and west coast of India. The total butyltin (TB) in water samples varied between ∼1.7 and 342 ng Sn l−1, whereas for sediments it varied between below detection limit to 14861 ng Sn g−1 dry weight of sediment. On an average Chennai port recorded the highest level of butyltins in the sediments while Paradip recorded the highest level of butylins in the waters. A fairly good relationship between the TB in the sediment and overlying water samples, as well as between organic carbon and TB, implicates the importance of adsorption/desorption process in controlling the levels of TBT in these port areas. In India the data on organotin pollution is very sparse; most of the port areas have been surveyed for butyltins for the first time during this study.  相似文献   

15.
Drifting sediment traps were deployed at 9 stations in May-June (ice-covered conditions) and July-August (ice-free conditions) 2004 in the Chukchi Sea to investigate the variability in export fluxes of biogenic matter in the presence and absence of sea ice cover. Measurements of chlorophyll-a (Chl-a), particulate organic carbon (POC), particulate nitrogen (PN), phytoplankton, zooplankton fecal pellets, and the stable carbon isotope composition (δ13C) of the sinking material were performed along Barrow Canyon (BC) and a parallel shelf-to-basin transect from East Hanna Shoal (EHS) to the Canada Basin. POC export fluxes were similarly high in the presence (378±106 mg C m−2 d−1) and in the absence of ice cover (442±203 mg C m−2 d−1) at the BC stations, while fluxes were significantly higher in the absence (129±98 mg C m−2 d−1) than in the presence of ice cover (44±29 mg C m−2 d−1) at the EHS stations. The C/N ratios and δ13C values of sinking organic particles indicated that POC export fluxes on the Chukchi continental shelf were mostly composed of freshly produced labile material, except at the EHS stations under ice cover where the exported matter was mostly composed of refractory material probably advected into the EHS region. Chl-a fluxes were higher under ice cover than in ice-free water, however, relatively low daily loss rates of Chl-a and similar phytoplankton carbon fluxes in ice-covered and ice-free water suggest the retention of phytoplankton in the upper water column. An increase in fecal pellet carbon fluxes in ice-free water reflected higher grazing pressure in the absence of ice cover. Elevated daily loss rates of POC at the BC stations confirmed other indications that Barrow Canyon is an important area of carbon export to the basin and/or benthos. These results support the conclusion that there are large spatial and temporal variations in export fluxes of biogenic matter on the Chukchi continental shelf, although export fluxes may be similar in the presence and in the absence of ice cover in highly productive regions.  相似文献   

16.
The abundance and distribution of rare earth elements (REE) and their signatures in the Vigo Ria were studied from 50 samples of surface sediments and related to the geological formation in its watershed. The total amount of REE in the Ria is heterogeneous. It ranges from 220 mg kg−1 in the southern middle Ria margin in the vicinity of the Galiñeiro geological shore complex, which contains REE-enriched minerals, to 2 mg kg−1 near the Ria mouth due to dilution with high levels of carbonated biogenic particles (31% of Ca). Rare earth elements of the Ria sediments are considerably enriched in light-REE relative to heavy-REE (a LREE/HREE ratio of 9.7±1.6) and also show a slightly negative Eu-anomaly. Low European shale normalised REE patterns were distinguished in the innermost sediments of Vigo Ria, but were not correlated with Al. This suggests a minor contribution of REE from upstream freshwater inputs to the sediments in the middle Vigo Ria zone. Normalised REE ratios in the middle Ria imply that fine particles enriched in REE may be exported from the Ria to shelf mud patches and REE can be useful as sediment tracers of Ria input on the shelf.  相似文献   

17.
Sediment transport and the potential for erosion or deposition have been investigated on the Palos Verdes (PV) and San Pedro shelves in southern California to help assess the fate of an effluent-affected deposit contaminated with DDT and PCBs. Bottom boundary layer measurements at two 60-m sites in spring 2004 were used to set model parameters and evaluate a one-dimensional (vertical) model of local, steady-state resuspension, and suspended-sediment transport. The model demonstrated skill (Brier scores up to 0.75) reproducing the magnitudes of bottom shear stress, current speeds, and suspended-sediment concentrations measured during an April transport event, but the model tended to underpredict observed rotation in the bottom-boundary layer, possibly because the model did not account for the effects of temperature–salinity stratification. The model was run with wave input estimated from a nearby buoy and current input from four to six years of measurements at thirteen sites on the 35- and 65-m isobaths on the PV and San Pedro shelves. Sediment characteristics and erodibility were based on gentle wet-sieve analysis and erosion-chamber measurements. Modeled flow and sediment transport were mostly alongshelf toward the northwest on the PV shelf with a significant offshore component. The 95th percentile of bottom shear stresses ranged from 0.09 to 0.16 Pa at the 65-m sites, and the lowest values were in the middle of the PV shelf, near the Whites Point sewage outfalls where the effluent-affected layer is thickest. Long-term mean transport rates varied from 0.9 to 4.8 metric tons m−1 yr−1 along the 65-m isobaths on the PV shelf, and were much higher at the 35-m sites. Gradients in modeled alongshore transport rates suggest that, in the absence of a supply of sediment from the outfalls or PV coast, erosion at rates of ∼0.2 mm yr−1 might occur in the region southeast of the outfalls. These rates are small compared to some estimates of background natural sedimentation rates (∼5 mm yr−1), but do not preclude higher localized rates near abrupt transitions in sediment characteristics. However, low particle settling velocities and strong currents result in transport length-scales that are long relative to the narrow width of the PV shelf, which combined with the significant offshore component in transport, means that transport of resuspended sediment towards deep water is as likely as transport along the axis of the effluent-affected deposit.  相似文献   

18.
Guajará Bay, located at the right margin of the Pará River estuary (Amazon) is formed in the confluence of Guamá and Acará–Moju rivers. It has low-depth zones (∼5 m) and deep channels (∼25 m). The ebb channel is located in the west section, where there is intense erosion of the margin. The flood channels and intertidal mudflats, which stretch out from north to south along the shore of the city of Belém do Pará, are in the east section. There are sandy (northwest) and muddy sedimentary deposits (east–southeast). Some 70% of Guajará Bay's bottom is covered by mud. The depositation of such muddy sediments and the formation of a point bar in the south section (Guamá River mouth) happen due to a decrease in the intensity of tidal currents to the south and of fluvial currents to the north. However, the hydrodynamic regime is high, which is proved by the low clay amounts. The sand deposits in the northwest section indicate strong tidal currents. The vast area of the bottom that is covered by mud (∼90 km2) and the intertidal mudflats (∼150 m wide) in Guajará Bay hint the extent of the contribution and sediments flow from Guamá and Acará–Moju rivers (drainage basin total area of ∼87,400 km2) to the Pará River estuary. The regular rainfall regime, typical of the Amazon region, keeps the considerable discharges of such rivers and their high turbidity (Secchi depth ?0.5 m) in the investigation area. Generally speaking, the low topography, the great fluvial subsidy and the action of tidal currents are the main controlling elements of the depositation and dispersion of sediments in Guajará Bay.  相似文献   

19.
Suspended sediments form an integral part of shelf sea systems, determining light penetration for primary production through turbidity and dispersion of pollutants by adsorption and settling of particles. The settling speed of suspended particles depends on their size and on turbulence. Here a method of determining particle size via remote sensing measurements of ocean colour and brightness has been applied to a set of monthly satellite images of the Irish Sea covering a full year (2006). The suspended sediment concentration was calculated from the ratio between green (555 nm) and red (665 nm) wavelengths in MODIS imagery. Empirical formulae were employed to convert suspended sediment concentrations and irradiance reflectance in the red part of the spectrum into specific scattering by mineral particles and floc size. A geographical pattern was evident in all images with shallow areas with fast currents having high year-average suspended sediment concentrations (7.6 mg l−1), high specific scattering (0.225 m2 g−1) and thus small particle sizes (143 μm). The reverse is true for deeper areas with slower currents, e.g. the Gyre southwest of the Isle of Man where turbidity levels are lower (3.3 mg l−1), specific scattering is lower (0.081 m2 g−1) and thus particle sizes are larger (595 μm) on average over a year. Temporal signals are also seen over the year in these parameters with minimum seasonal amplitudes (a factor 3.5) in the Turbidity Maximum and maximum seasonal amplitudes twice as large (a factor 7) in the Gyre. In the Gyre heating overcomes mixing in summer and stratification occurs allowing suspended sediments to settle out and flocs to grow large. The size of aggregated flocs is theoretically proportional to the Kolmogorov scale. This scale was calculated using depth, current, and wind speed data and compared to the size of flocculated particles. The proportionality changes through the year, indicating the influence of biological processes in summer in promoting larger flocs.  相似文献   

20.
Primary igneous anhydrite was first identified in 1982 El Chichón pumices. Analysis of the sulfur budget for the eruption provided compelling evidence that the pre-eruptive magma contained a significant gas phase at ∼ 7 km depth in order to account for the “excess gas release” of ∼ 5–9 million tons of SO2 to the stratosphere by the eruption. Primary igneous anhydrite and a larger “excess gas release” of ∼ 20 million tons of SO2 were noted for the significantly larger eruption of Mount Pinatubo in 1991, for which a separate gas phase at ∼ 7–9 km depth was also required by the sulfur budget. Pumices from both eruptions have mineral assemblages dominated by plagioclase and hornblende, with minor biotite, and show evidence for co-nucleation and mutual inclusions of anhydrite and apatite. Both magmas were also very water-rich and highly oxidized, with oxygen fugacities $1 log unit above the synthetic Ni–NiO buffer. Furthering the similarities between these two eruptions, ion-microprobe analyses of sulfur isotopic compositions of anhydrites in pumices from El Chichón and Mount Pinatubo both showed that individual crystals are isotopically homogeneous, but inter-crystalline variations in δ34S are well beyond analytical error.  相似文献   

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