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1.
Sediment-level oscillations with heights of about 6 cm and shore-normal lengths of order 10 m have been measured in the swash zone of a high-energy, coarse-sand beach. Crests of oscillations were shore parallel and continuous alongshore. The oscillations were of such low steepness (height-to-length ratio approximately 0.006) that they were difficult to detect visually. The period of oscillation ranged between 6 and 15 min and decreased landward across the swash zone. The sediment-level oscillations were progressive landward with an average migration rate in the middle to upper swash zone of 0.8 m min−1. Migration was caused mostly by erosion on the seaward flank of the crest of an oscillation during a period of net seaward sediment transport. Thus, the observed migration was a form migration landward rather than a migration involving net landward sediment transport. The observed sediment-level oscillations were different than sand waves or other swash-zone bedforms previously described.  相似文献   

2.
Estimates of time-integrated values of total (ITVF) and net (INVF) sediment volume flux and the associated changes in bed elevation and local slope were determined for a crescentic outer nearshore bar in Kouchibouguac Bay, New Brunswick, Canada, for eight discrete storm events. A 100 × 150 m grid of depth-of-activity rods spaced at 10 m intervals was used to monitor sediment behaviour on the seaward slope, bar crest and landward slope during the storms, at which time winds, incident waves and near-bed oscillatory currents were measured. Comparisons between storm events and between these events and a longer-term synthetic wave climatology were facilitated using hindcast wave parameters. Strong positive correlations between storm-wave conditions (significant height and total cumulative energy) and total volume flux contrasted strongly with the zero correlation between storm-wave conditions and net volume flux. ITVF values ranged up to 1646 m3 for the experimental grid and were found to have power function relations with significant wave height (exponent 2) and cumulative wave wave energy (exponent 0.4); values of INVF ranged from 0 up to 100 m3 for the same grid indicating a balance of sediment volume in the bar form through time. Sediment reactivation increased linearly with decreasing depth across the seaward slope and bar crest reaching maxima of 20 cm for the two largest storms; bed elevation, and thus slope, changes were restricted to the bar crest and upper landward slope with near zero morphological change on the seaward slope. The latter represents a steady-state equilibrium with null net transport of sediment under shoaling waves. Measurements of the asymmetry of orbital velocities close to the bed show that the energetics approach to predicting beach slope of Inman and Bagnold (1963) is sound. Gradients predicted vary from 0.01 to 0.03 for a range of angles of internal friction appropriate to the local sediment (tan ø = 0.3–0.6). These compare favorably with the measured seaward slope of 0.015 formed under average maximum orbital velocities of 1.12 m s−1 (landward) and 1.09 m s−1 (seaward) recorded during the period of the largest storm waves.  相似文献   

3.
Evaluation of velocity data on water movements over the New Zealand continental shelf has revealed that the mean circulation by itself is too slow to induce transport of bottom sediments. Tides generally have higher velocities, but are still not the main transporting agent except in the tide‐dominated Cook and Foveaux. Straits. Waves have the potential to stir sediments on the inner and middle shelf (less than about 70 m deep) during annual storms, and probably down to 130 m depth during the maximum 25‐y storm.

For sediment transport to take place, energies of at least two of the major water movements would have to complement one another. Optimum conditions for transport probably occur during storm periods when wave‐suspended sediment is readily moved by tides and the mean circulation.

The direction of transport is mainly along the continental shelf and is largely in response to prevailing weather patterns coincident with the direction of the mean circulation and strongly reinforced by the appropriate phase of the tide.  相似文献   

4.
Fine sediment dynamics were recorded in February 2007 in coastal waters of the Great Barrier Reef during a moderate flood of the Tully River. An estuarine circulation prevailed on the inner continental shelf with a surface seaward velocity peaking at 0.1 m s−1 and a near-bottom landward flow peaking at 0.05 m s−1. Much of the riverine mud originating from eroded soils was exported onto a 10 km wide coastal strip during the rising stage of the river flood in the first flush. In coastal waters, suspended sediment concentration peaked at 0.2 kg m−3 near the surface and 0.4 kg m−3 at 10 m depth during calm weather, and 0.5 kg m−3 near the surface and 2 kg m−3 at 10 m depth during strong winds when bottom sediment was resuspended. Diurnal irradiance at 4 m depth was almost zero for 10 days. The sedimentation rate averaged 254 (±33) g m−2 d−1 over the 28-day study period, and concentrations of dissolved and particulate nutrients originating from the river were high. The observed low irradiance would have prevented coral photosynthesis, while the sedimentation rate would have been lethal to some juvenile corals. The mud may ultimately be minnowed out over long periods, however, flushing of the mud occurs at time scales much longer than the flood event and the mud is likely to affect coral physiology for significant periods after the flood has subsided. The data show the need to better control erosion on farmed land for the conservation of coral reefs on the inner shelf of the Great Barrier Reef.  相似文献   

5.
The focus of this study is the analysis of coastal sand barriers and associated coastal lagoons on the inner continental shelf of the Gulf of Valencia (western Mediterranean), based on two W–E seismic profiles recorded seaward of the Albufera de Valencia coastal lagoon. Seismic facies identified include a number of coastal sand barriers with landward lagoons draped by contemporary continental shelf deposits. The barrier systems have been grouped into two sedimentary systems tracts, the older one corresponding to a prograding/aggrading highstand systems tract involving at least four paleo-coastal sand barrier/lagoon systems, followed landward by a transgressive systems tract comprising three such systems. All the systems have been allocated a Tyrrhenian age, the formation of individual barrier systems having been associated with successive sea-level stillstands, and their present-day position being explained by the very high regional subsidence rate. In summary, this study demonstrates that the Quaternary stratigraphic record of the Gulf of Valencia inner continental shelf is composed of littoral sand facies, in particular coastal sand barrier and lagoon deposits. These findings are in agreement with corresponding observations on other continental shelves of the western Mediterranean, showing that the formation of coastal sand barriers was a characteristic feature of this region during the Quaternary.  相似文献   

6.
The Cabo Frio region in the state of Rio de Janeiro, southeast coast of Brazil, is characterized by a local coastal upwelling system and converging littoral sediment transport systems that are deflected offshore at Cabo Frio, as a consequence of which a thick cross-shelf sediment deposit has developed over time. To investigate the evolution of this muddy deposit, geophysical, sedimentological and geochemical data from four sediment cores (3.8–4.1 m in length) recovered in water depths between 88 and 141 m were analyzed. The high-resolution seismic data show variable sediment thicknesses ranging from 1 to 20 m, comprising two sedimentary units separated by a high-impedance layer at a depth of about 10 m below the seafloor at the coring sites. According to the available age datings, the upper sedimentary unit is late Pleistocene to Holocene in age, whereas the lower unit (not dated) must, by implication, be entirely Pleistocene in age. The boomer-seismic reflection signal can be divided into three echo-types, namely transparent (inner shelf), stratified (middle shelf) and reflective (outer shelf), each type seemingly related to the local sediment composition. The upper 4 m of the upper sedimentary unit is dominated by silty sediment on the middle shelf, and by upward-fining sediments (silty sand to sandy silt) on the inner and outer shelf. The downcore trends of P-wave velocity, gamma-ray density and acoustic impedance are largely similar, but generally reversed to those of water and organic carbon contents. Total organic carbon contents increase with decreasing mean grain size, periodic fluctuations suggesting temporal changes in the regional hydrodynamics and primary productivity fuelled by the local upwelling system. The reconstruction of sedimentation rates in the course of the Holocene is based on 35 AMS age datings of organic material recovered from variable downcore depths. These range from a maximum of 13.3 cm/decade near the base of the inner shelf core (7.73–7.70 ka BP) to generally very low values (<0.11 cm/century) over the last thousand years in all cores. Over the last 6 ka there appear to have been three distinct sedimentation peaks, one between 6 and 5 ka BP, another between 4 and 3 ka PB, and one around 1 ka BP. Due to different time intervals between dates, not every peak is equally well resolved in all four cores. Based on the similar sedimentology of the inner and outer shelf cores, an essentially identical sedimentation model is proposed to have been active in both cases, albeit at different times. Thus, already during the last glacial maximum, alongshore sediment transport was deflected offshore by a change in shoreline orientation caused by the Cabo Frio structural high. The source of terrigenous material was probably a barrier-island complex that was subsequently displaced landward in the course of sea-level rise until it stabilized some 6.5 ka BP along the modern coast.  相似文献   

7.
Southwestward volume transport (referred to 1,500 db) out of the Gulf of Alaska seaward of the continental shelf in May 1972 was 12.5 Sv, and nearly 3/4 of this flow occurred within 50 km of the shelf edge. Mean geostrophic velocities of about 50 cm s–1 occurred in a band 20 km wide, which extended 500 km along the shelf edge; a maximum velocity of 98 cm s–1 (nearly 2 knots) was obtained. Bottom flow along the inshore part of the shelf as determined by seabed drifters was generally onshore at 0.5 cm s–1. Evidence is presented of a large cyclonic gyre on the shelf encompassing the Portlock and Albatross Banks, perturbations in surface flow along the shelf edge, and relations between coastal tidal heights and fluctuations in geopotential topography at the shelf edge.  相似文献   

8.
The purpose of this study was to determine if and how a large, modern estuarine system, situated in the middle of an ancient carbonate platform, has affected its adjacent inner shelf both in the past during the last, post-glacial sea-level rise and during the present. An additional purpose was to determine if and how this inner shelf seaward of a major estuary differed from the inner shelves located just to the north and south but seaward of barrier-island shorelines. Through side-scan sonar mosaicking, bathymetric studies, and ground-truthing using surface grab samples as well as diver observations, two large submarine sand plains were mapped – one being the modern ebb-tidal delta and the other interpreted to be a relict ebb-tidal delta formed earlier in the Holocene. The most seaward portion of the inner shelf studied consists of a field of lobate, bathymetrically elevated, fine-sand accumulations, which were interpreted to be sediment-starved 3D dunes surrounded by small 2D dunes composed of coarse molluscan shell gravel. Additionally, exposed limestone hardbottoms supporting living benthic communities were found as well. This modern shelf sedimentary environment is situated on a large, buried shelf valley, which extends eastward beneath the modern Tampa Bay estuary. These observations plus the absence of an incised shelf valley having surficial bathymetric expression, and the absence of sand bodies normally associated with back-tracking estuarine systems indicate that there was no cross-shelf estuarine retreat path formed during the last rise in sea level. Instead, the modern Tampa Bay formed within a mid-platform, low-relief depression, which was flooded by rising marine waters late in the Holocene. With continued sea-level rise in the late Holocene, this early embayment was translated eastward or landward to its present position, whereby a larger ebb-tidal delta prograded out onto the inner shelf. Extensive linear sand ridges, common to the inner shelves to the north and south, did not form in this shelf province because it was a low-energy, open embayment lacking the wave climate and nearshore zone necessary to create such sand bodies. The distribution of bedforms on the inner shelf and the absence of seaward-oriented 2D dunes on the modern ebb-tidal delta indicate that the modern estuarine system has had little effect on its adjacent inner shelf.  相似文献   

9.
Current velocity and suspended sediment concentration measurements at anchor stations in the downstream extremity of the Gironde estuary indicate that during periods of high river discharge, a significant amount of suspended sediment is transported out of the estuary onto the adjacent continental shelf. The vertical profile of the residual (non-tidal) suspended sediment flux is similar to that of the residual current velocity, with a net upstream flux near the bottom and an overlying seaward-directed transport. The overall, depth-integrated result is a net seaward transport of suspended sediment out of the estuary. It appears that this net seaward transport varies directly with tidal amplitude.Aerial photography and water sampling indicate that during high river inflow, the downstream extremity of the turbidity maximum extends onto the continental shelf at ebb tide. The tidal and coastal current patterns of the inlet and inner shelf induce a northward transport of the turbid estuarine water, and at each tidal cycle, a certain amount of suspended sediment leaves the estuary; part of this sediment is deposited in a silt and clay zone on the continental shelf.  相似文献   

10.
Surface currents influenced by a wind-driven upwelling event in San Pedro Bay moved total suspended matter (TSM) confined to the inner shelf on 19 April 1978 seaward, so that by 27 April surface TSM had increased over the outer shelf. Near-bottom concentrations of TSM also increased across the shelf during this time. This is explained by sediment resuspended by large surface waves being advected from the inner shelf seaward by the mean flow after this flow had turned from southeasterly to southerly when upwelling ceased on 26 April. These complex shelf dynamics contribute to the off-shelf transport of mud to the slope and deep basins.  相似文献   

11.
The Middle–Late Miocene Utsira Formation of the North Sea Basin contains a fully preserved, regional marine sand deposit that records a stable paleogeographic setting of sand transport and accumulation within a deep, epeiric seaway which persisted for >8 Ma. The sediment dispersal system was defined by (1) input through a marginal prograding strandplain platform, coast-to-basin bypass, transport along a narrow strait, and accumulation in strait-mouth shoal complexes within a shelf sea; (2) a high-energy marine regime; (3) very low time-averaged rates of sediment supply and accumulation; and (4) consequent high sediment reworking ratio. Sand distribution and stratal architectures reflect regional along-strike sediment transport and local to sub-regional landward sediment transport. Plume-shaped, south-building, submarine sand shoals that formed along the recurved arc of the strandplain margin nourished the shoal system. Very low-angle sigmoid clinoforms and down-stepping, aggradational top sets are distinctive architectures of these strike-fed sand bodies. The combination of strong marine currents and slow but long-lived sand supply from the Shetland strandplain created regional, sandy shelf shoal depositional systems that individually covered 3,500 to 6,000 km2 of the basin floor. Defining attributes of the shelf shoal systems include their location within the basin axis, abundance of autochthonous sediment, and sandy marine facies composition. Diagnostic depositional architectures include the along-strike-dipping sigmoidal clinoforms, poly-directional low-angle accretionary bedding at both regional and local scales, and mounded depositional topography. Erosional features include regional hummocky, low-relief shelf deflation surfaces, broad, elongate scours and sub-circular scour pits.  相似文献   

12.
Abstract

The continental margin of northern Sinai and Israel, up to Haifa Bay, is the northeastern limb of the submarine Nile Delta Cone. It is made up predominantly of clastics from the Nile and its predecessors. The continental shelf and coastal plain of Israel are built of a series of shore-parallel ridges composed of carbonate-cemented quartz sandstone (locally named kurkar), a lithification product of windblown sands that were piled up into dunes during the Pleistocene. The drop in global sea level and regression during the last glacial period exposed the continental shelf to subaerial erosion and created a widespread regional erosional unconformity which is expressed as a prominent seismic reflector at the top of the kurkar layers. The subsequent Holocene transgression abraded much of the westernmost kurkar ridges, drowned their cores, and covered the previous lowstand deposits with marine sands, which were in turn covered by a sequence of sub-Recent clayey silts.

The Mediterranean coasts of Sinai and Israel are part of the Nile littoral cell. Since the building of the Aswan dams the sand supplied to Israel's coastal system is derived mainly from erosion of the Nile Delta and from sands offshore Egypt that are stirred up by storm waves. The sands are transported by longshore and offshore currents along the coasts of northern Sinai and Israel. Their volume gradually declines northward with distance from their Nile source. The longshore transport terminates in Haifa Bay where some sand is trapped, and the test escapes to deeper water by bottom currents and through submarine canyons, thus denying Nile-derived sand supply to the 40-km-long Akko-Rosh Haniqra shelf.

The sand balance along Israel's coastal zone is a product of natural processes and human intervention. Losses due to the outgoing longshore transport, seaward escape, and landward wind transport exceed the natural gains from the incoming longshore transport and the abrasion of the coastal cliffs. The deficit is aggravated by the construction of (1) seaward-projecting structures that trap sands on the upstream side and (2) offshore detached breakwaters that trap sands between themselves and the coast. The negative sand balance is manifested by the removal of sand from the seabed and the consequent exposure of archaeological remains that were hitherto protected by it.

The sediments that escape seaward from the longshore transport system form a 2.5- to 4-km-wide sandy apron adjacent to the shore that extends to where the water is 30–40 m deep. The apron's slope (0.5–0.8°) is steeper than the theoretical equilibrium slope for the median grain-size diameter in this zone (0.1–0.3 mm).

The beach sands and the apron's surficial sands are well sorted. Their grain size decreases with distance from shore, from 0.2–0.3 mm nearshore to 0.11–0.16 mm by the drowned ridge. The coarse-grained fraction consists of skeletal debris (commonly 5–12% carbonate matter) and wave-milled kurkar grains (locally named zifzif). In deeper water, the basal sands underlying the fine-grained sediment cover consist of 1- to 30-cm layers whose composition ranges from silty sands to various types of sands (fine, medium, coarse, and gravelly) to zifzif. For the most part, they contain large amounts of skeletal debris (20–60%) and small fragments of kurkar.

Two types of kurkar rock were encountered offshore: a well-sorted, fine- to medium-grained (0.074–0.300 mm) lithified dune sand with variable amounts of carbonate cement, ranging from hard rock of low permeability to loose sand; and a porous sandstone made up predominantly of algal grains and skeletal debris (calcarenite).  相似文献   

13.
An open ocean shoreface typical of long, wave-dominated sandy coasts has been examined through a combination of extensive field measurements of wave and current patterns with computations of marine bedload transport and sedimentation. Sand transport on the upper shoreface is dominantly controlled by waves with only secondary transport by currents. Sand on the middle and lower shoreface, as well as the inner continental shelf is entrained by storm waves and transported by a complex pattern of bottom boundary layer currents.

Storm events have been studied and modeled for the shoreface off Tiana Beach, Long Island. The dominant effect of coastal frontal storms is to cause significant shore-parallel bedload transport with important shore-normal secondary components. These storms tend to result in net offshore transport of sand removed from the beach and surf zone systems. The bedload transport during a storm is convergent on the shoreface leading to accretion. Most accretion occurs on the upper shoreface with lesser deposits covering the middle and lower shoreface as well as the inner continental shelf. Longer-term equilibrium can be maintained by slow return of sand up the shoreface during non-storm conditions.

Annual and geologic time-scale budgets of shoreface sand transport and sedimentation yield equilibrium, net accretion or net deposition. The annual balance results from an integration of the event-scale bedload transport patterns and morphologic responses. These processes and responses have feedback mechanisms which stabilize the system over longer, but not geologic, time scales. Geologic time scale balances are controlled by relative sea level changes and relative availability of sediment supply with the event-scale shoreface and transporting processes providing the mechanism to produce the changes in long-term morphology and sedimentation patterns. In the area of study, the long-term pattern is one of net shoreface erosion, and the permanent loss of sand to the shelf floor.  相似文献   


14.
B. Manighetti  L. Carter 《Marine Geology》1999,160(3-4):271-300
Side-scan, seismic and surficial sediment data accompanied by current meter records highlight across-shelf sediment transport in Hauraki Gulf, an island-studded embayment off northern New Zealand. Calm weather currents are locally dominated by the tides, with periodic incursions of oceanic water from detached meanders of the East Auckland Current. Under these conditions, bedload transport occurs mainly in three 15–20 km-wide channels, where bathymetric intensification of the flow brings about near-bottom speeds of up to 82 cm s−1 for Colville Channel and 33–44 cm s−1 in Jellicoe and Cradock Channels. Surficial sediments are gravelly to muddy sand, winnowed in places, leaving a lag deposit of mainly biogenic carbonate gravel. Modelling results suggest that in Colville Channel, dominant fine to medium sand modes are mobile for 20–60% of the time, with a net eastward movement for fine sand. In Jellicoe and Cradock Channels, the prevailing direction of transport is southwards across the shelf, with sand mobile for up to 33% of the time. Oceanic incursions have the potential to boost flow in the western Gulf, however such incursions are transitory, and there is no measurable expression of oceanic water in the sedimentary record. Because of their association with prolonged periods of calm weather, the incursions are unlikely to accompany storm events, where their cumulative effect might be important for sediment transport. Near-bottom currents resulting from oceanic incursion may reinforce peak tides inside the Gulf by up to 2–4 cm s−1. Enhancement of prevailing water motions occurs during periods of extreme weather. During cyclone Drena (January 1997), measured flow speeds in Jellicoe Channel reached 48 cm s−1. Furthermore, the disturbance generated large waves that stirred bottom sediments down to over 100 m water depth. Such events are probably the major agent of sediment redistribution in the Hauraki Gulf. The net effect of storm and calm weather currents is to move sediment across the outer to middle shelf where, in the western and central Gulf it accumulates, and in the eastern Gulf it escapes eastward via Colville Channel.  相似文献   

15.
The continental margin of northern Sinai and Israel, up to Haifa Bay, is the northeastern limb of the submarine Nile Delta Cone. It is made up predominantly of clastics from the Nile and its predecessors. The continental shelf and coastal plain of Israel are built of a series of shore-parallel ridges composed of carbonate-cemented quartz sandstone (locally named kurkar), a lithification product of windblown sands that were piled up into dunes during the Pleistocene. The drop in global sea level and regression during the last glacial period exposed the continental shelf to subaerial erosion and created a widespread regional erosional unconformity which is expressed as a prominent seismic reflector at the top of the kurkar layers. The subsequent Holocene transgression abraded much of the westernmost kurkar ridges, drowned their cores, and covered the previous lowstand deposits with marine sands, which were in turn covered by a sequence of sub-Recent clayey silts. The Mediterranean coasts of Sinai and Israel are part of the Nile littoral cell. Since the building of the Aswan dams the sand supplied to Israel's coastal system is derived mainly from erosion of the Nile Delta and from sands offshore Egypt that are stirred up by storm waves. The sands are transported by longshore and offshore currents along the coasts of northern Sinai and Israel. Their volume gradually declines northward with distance from their Nile source. The longshore transport terminates in Haifa Bay where some sand is trapped, and the test escapes to deeper water by bottom currents and through submarine canyons, thus denying Nile-derived sand supply to the 40-km-long 'Akko-Rosh Haniqra shelf. The sand balance along Israel's coastal zone is a product of natural processes and human intervention. Losses due to the outgoing longshore transport, seaward escape, and landward wind transport exceed the natural gains from the incoming longshore transport and the abrasion of the coastal cliffs. The deficit is aggravated by the construction of (1) seaward-projecting structures that trap sands on the upstream side and (2) offshore detached breakwaters that trap sands between themselves and the coast. The negative sand balance is manifested by the removal of sand from the seabed and the consequent exposure of archaeological remains that were hitherto protected by it. The sediments that escape seaward from the longshore transport system form a 2.5- to 4-km-wide sandy apron adjacent to the shore that extends to where the water is 30 - 40 m deep. The apron's slope (0.5 - 0.8) is steeper than the theoretical equilibrium slope for the median grain-size diameter in this zone (0.1 - 0.3 mm). The beach sands and the apron's surficial sands are well sorted. Their grain size decreases with distance from shore, from 0.2 - 0.3 mm nearshore to 0.11 - 0.16 mm by the drowned ridge. The coarse-grained fraction consists of skeletal debris (commonly 5 - 12% carbonate matter) and wave-milled kurkar grains (locally named zifzif). In deeper water, the basal sands underlying the fine-grained sediment cover consist of 1- to 30-cm layers whose composition ranges from silty sands to various types of sands (fine, medium, coarse, and gravelly) to zifzif. For the most part, they contain large amounts of skeletal debris (20 - 60%) and small fragments of kurkar. Two types of kurkar rock were encountered offshore: a well-sorted, fine- to medium-grained (0.074 - 0.300 mm) lithified dune sand with variable amounts of carbonate cement, ranging from hard rock of low permeability to loose sand; and a porous sandstone made up predominantly of algal grains and skeletal debris (calcarenite).  相似文献   

16.
基于2020年7月特大洪水期间长江口浑浊带南槽、北槽和北港多站位同步实测水沙动力数据,研究了河口浑浊带分粒级输沙时空特征及其对泥沙来源响应的指示意义,结果表明：1）北港和北槽是流域泥沙净向口外输移的主要输沙通道,南槽是海域泥沙净向口内输移的主要输沙通道,主槽内粉砂是主要输沙组分,占比63.2%,口外粉砂和黏土是主要输沙组分,分别占43.2%、40.9%;2）大潮粉砂输运占比高于小潮,黏土输运占比低于小潮,口外测站砂组分在大小潮期间在横沙浅滩和九段沙间沿岸输移,横沙浅滩附近大、小潮离岸输沙分别是北港口外的1.7倍和8倍,不利于横沙浅滩淤涨;3）当前流域减沙高达70%,此次特大洪水期间黏土、粉砂和砂三组分近底净向口内输移为减沙背景下的口外供沙提供了有力的佐证。  相似文献   

17.
The narrow shelf along the coast of central Vietnam is seasonally supplied by large amounts of sediment from the adjacent mountainous hinterland following monsoonal precipitation. This study examines the fate of these sediments, and their accumulation rates along two transects across the shelf, based on analyses of radionuclides (210Pb, 137Cs), sediment texture and structure, as well as carbonate content. The inner shelf is covered by sands, and probably serves as bypass zone for fine sediments transported offshore. Sediment characteristics suggest that the transport to the mid and outer shelf is related to flood events. Averaged over the last century, the 210Pb-based mud mass accumulation rates on the mid and outer shelf vary between 0.25 g cm −2 and 0.56 g cm −2 year −1 (corresponding to linear sediment accumulation rates of 0.20–0.47 cm year −1). Along with high excess 210Pb inventories, these high accumulation rates suggest a significant sediment depocentre on the mid shelf. The 210Pb-derived sediment accumulation rates were found to be several times higher than 14C-derived rates previously reported for the Holocene, at the same location on the outer shelf. This is probably due to the incompleteness of the Holocene record, and an overestimation of the modern rate. Another explanation would be increased erosion within the rivers’ drainage basins, due to 20th century deforestation. This hypothesis is supported by the difference between recent (less sand, more lithic grains in the sand fraction) and older sediments. In terms of modern sedimentation processes and rates, the central Vietnam shelf, although being a part of a narrow passive continental margin, is similar to active flood-dominated continental margins.  相似文献   

18.
Resuspension, transport, and deposition of sediments over the continental shelf and slope are complex processes and there is still a need to understand the underlying spatial and temporal dynamical scales. As a step towards this goal, a two-dimensional slice model (zero gradients in the alongshore direction) based on the primitive flow equations and a range of sediment classes has been developed. The circulation is forced from rest by upwelling or downwelling winds, which are spatially uniform. Results are presented for a range of wind speeds and sediment settling speeds. Upwelling flows carry fine sediments (low settling speeds) far offshore within the surface Ekman layer, and significant deposition eventually occurs beyond the shelf break. However, coarser sediments quickly settle out of the deeper onshore component of the circulation, which can lead to accumulation of bottom sediments within the coastal zone. Downwelling flows are more effective at transporting coarse sediments off the shelf. However, strong vertical mixing at the shelf break ensures that some material is also carried into the surface Ekman layer and returned onshore. The concentrations and settling fluxes of coarse sediments decrease offshore and increase with depth under both upwelling and downwelling conditions, consistent with trends observed in sediment trap data. However, finer sediments decrease with depth (upwelling) or reach a maximum around the depth of the shelf break (downwelling). It is shown that under uniform wind conditions, suspended sediment concentrations and settling fluxes decay offshore over a length scale of order τs/ρf|ws|, where τs is the wind stress, ρ the water density, f the Coriolis parameter, and ws is the sediment settling velocity. This scaling applies to both upwelling and downwelling conditions, provided offshore transport is dominated by wind-driven advection, rather than horizontal diffusion.  相似文献   

19.
Cores collected from Mississippi Sound and the inner shelf of the northeast Gulf of Mexico have been examined using 210Pb and 137Cs geochronology, X-radiography, granulometry, and a multi-sensor core logger. The results indicate that widespread event layers were probably produced by an unnamed hurricane in 1947 and by Hurricane Camille in 1969. Physical and biological post-depositional processes have reworked the event layers, producing regional discontinuities and localized truncation, and resulting in an imperfect and biased record of sedimentary processes during the storms. The oceanographic and sedimentological processes that produced these event beds have been simulated using a suite of numerical models: (1) a parametric cyclone wind model; (2) the SWAN third-generation wave model; (3) the ADCIRC 2D finite-element hydrodynamic model; (4) the Princeton Ocean Model; (5) a coupled wave–current bottom boundary layer-sedimentation model; and (6) a model for bed preservation potential as a function of burial rate and bioturbation rate. Simulated cores from the Mississippi Sound region are consistent with the observed stratigraphy and geochronology on both the landward and seaward sides of the barrier islands.  相似文献   

20.
A sediment trap was deployed 3 m from the bottom at a water depth of 62 m on the southern flank of Georges Bank (41°02·2′N, 67°33·5′W) from 30 September 1978 to 10 March 1979 to qualitatively determine the size of sediments resuspended from the bottom by winter storms and to determine if seasonal changes in the phytoplankton could be observed in the trapped sediment.Bulk X-ray analyses of the trapped sediment showed layers of distinctly different textures preserved in the collection vessel. The median grain size of sampled layers ranged from 2·7 to 6·5 φ (fine sand to silt), but all layers contained a pronounced mode in the 3 φ (fine sand) range. Nine layers containing relatively large amounts of sand were present. The sand content was 75% in the coarest layers and about 32% in the fine layers. The median grain size of bottom sediments at the deployment site was considerably coarser than the trap samples, although the dominant grain size was also 3 φ.Average bottom-current speeds during the deployment period were about 30 cm s?1 with a range of 10 to 50 cm s?1. Bottom stress, computed from the observed currents and waves, suggest that 11 storms caused sufficient stress to resuspend 3 φ-sized sediments, in good agreement with the nine layers of relatively coarse sediments collected in the trap. Surface waves had to be included in the calculation of bottom stress because the bottom currents alone were insufficient to cause the resuspension of 3 φ-sized sediment.The trapped sediments contain numerous diatoms and coccoliths that are typical of late fall and winter assemblages. No clear seasonal difference in the flora was noted among sampled layers, probably due to the large influx of resuspended material and a reduced primary flux during this period. An undescribed species of Thalassiosira (G. Fryxell, personal communication), and siliceous scales of unknown systematic position were observed at all levels.  相似文献   

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