Waves and currents near the continental shelf edge

The abrupt depth increase which characterises the edge of many continental shelves determines a reduced horizontal length scale and a localised transition from shelf seas to the deep ocean. Particular forms of motion which may arise from the steep slopes include topographically guided currents along the slope, shelf-break upwelling, topographic Rossby waves and internal lee waves in the tidal current. The ocean/shelf mismatch may lead to a clear separation of water types, substantial reflection (from the shelf-edge neighbourhood) of all oceanic and shelf motions with periods greater than a few hours, and interaction between barotropic and baroclinic motions. Unstable longshelf currents, interleaving water masses, strong internal tides and internal waves, and narrow canyons enhance mixing across the shelf edge.     

Near-bed turbulence measurements,stress estimates and sediment mobility at the continental shelf edge

As a contribution to ocean-shelf exchange studies, the Sediment Transport and Boundary Layer Equipment STABLE has been deployed west of Spain near 42°40’N in 200m water depth, in December and August, and at three other continental shelf edge and slope locations off north-west Europe. Turbulence at 8Hz and acoustic back-scatter at 4Hz were measured at three levels between 0.3 and 1 m above the bed.Typically the speed profile shows little shear; the turbulence data are necessary to estimate near-bed stress. Reynolds stress and total turbulence energy approaches give consistent results; total turbulence results appear more robust but can be affected by long surface waves. Typical peak values of friction velocity u₁ per tide were 10 mm/s off Spain and 7–18 mm/s at the other locations, and they were asymmetric owing to along-slope flow. Record maxima were 30 mm/s, 11–48 mm/s respectively, sufficient to move local sediment except for a short ‘event-free’ record at Goban Spur. Acoustic backscatter (SPM) showed peaks matching peaks of u₁ or waves in the case of winter deployments off Scotland and Spain.Comparison of these STABLE measurement periods with contemporary current records of longer duration suggests that bed stresses in all these upper-slope locations are often sufficient to move the local sediment. Off Spain, shelf sediments are re-suspended and transported down-current. The sandy fraction settles soon, but the fine fraction (w_S < 0.1 mm/s) is progressively winnowed down-current to the shelf edge, suggesting export of the bound organic matter.     

Living macromolluscs from a paleo-reef region on the northeastern Venezuelan continental shelf

Drowned reefs, fossil reefs or paleo-reefs, are important ecologically as areas of high biodiversity, foraging, shelter environment, and as areas supporting the spawning aggregations of economically important reef fish species. This is particularly significant when the structures are situated in a wide soft-bottom continental shelf. The presence of limestone structures, fossil reefs and pinnacles dating from circa 8 to 9 ka, to the north of the Paria Peninsula in north-eastern Venezuela, has been known to local fishermen for decades. Using echograms obtained during acoustic fisheries evaluations and the scarce previously available information, an improved location map of hard-bottom structures was made. Benthic samples to study macromolluscs were taken at depths between 54 and 93 m using an unmodified 2-m beam trawl. Four trawl samples were located over fossil reef areas while another four were situated in soft-bottom valleys between limestone structures. Fossil reefs in the area showed a highly patchy distribution. A total of 91 species from 43 Bivalvia, Gastropoda and Scaphopoda families were found, Gastropoda being the dominant class with 49 species. Paleo-reef-covered areas showed higher species richness and only 21% of the species found were common to both substrates. Gastropods Tonna maculosa and Polystira albida were the most abundant species and occurred in both substrate types. Bivalve life habits, a mixture of organism–substrate relationships, shell fixation, mobility and feeding type, differed significantly according to bottom type. Six species are recorded for the first time for eastern Venezuelan waters. Bottom heterogeneity plays an important role in marine ecosystems, providing shelter to fish populations and may be significant as breeding and nursery areas. Its presence in a region with biogeographical interest, situated in the confluence of three major provinces and with oceanographic conditions varying seasonally from upwelling dominated to Orinoco River discharges, makes this the area of interest and it should be evaluated as a possible Marine Protected Area.     

Observation of edge waves trapped on the continental shelf in the vicinity of Makurazaki Harbor, Kyushu, Japan

Sea-bottom pressure gauges were used to measure sea levels at two points on the shelf off the southern coast of Satsuma Peninsula, Kyushu, Japan. Spectral analysis of the observed records and the tide-gauge record of Makurazaki Harbor revealed several predominant common peaks. At the same time, the eigenmodes for the trapped waves on the shelf and inside Makurazaki Bay were obtained numerically using a two-dimensional model, and the periods and the spatial distribution of amplitudes of the proper modes were obtained. A comparison of the calculated modes with the periods and phase patterns of the observed peaks clarified that peaks with periods of 19.5, 16, 13.3, and 12.2 minutes in the shelf region were the modes of standing-edge waves, and the peak with the period of 16 minutes in Makurazaki Harbor was the fundamental mode of the harbor. Among the modes of standing-edge waves, the mode of the period 16 minutes on the shelf had nearly the same period as that of the fundamental mode of Makurazaki Harbor. An analysis of changes of spectral densities of these two modes confirmed that the fundamental mode of the Makurazaki Harbor was induced by this standing-wave mode.     

High-resolution analysis of the deposition pattern on the Amazon sub-aquatic delta and outer continental shelf

Continental shelves off major river deltas or estuaries act as an interface between terrestrial environments and marine depo centers like the Amazon Fan. In order to understand sedimentary processes on the Amazon sub-aquatic delta and outer shelf, Late-Quaternary erosional and depositional structures have been investigated with an ultra high-resolution seismic survey system (4 kHz). The wavy morphology of the outer shelf implies the presence of tidal sand ridges or meandering channels. In the latter case, the local presence of channel fill deposits suggests several cut-off loops (ox bows). The inter-channel areas are consequently interpreted as levees, which reach their maximum height of 10 m in the area between the Amazon river mouth and the Amazon canyon. The morphology of terraces in front of the sub-aquatic delta reflect erosional processes which presumably occurred during accelerated sea level rise at the beginning and the end of the Younger Dryas. A carbonate platform is present in the northwestern survey area close to the shelf break. Its water depth of 120–130 m implies an evolution during the last glacial maximum. The asymmetric shape of the Cabo Norte Shoal on the topset of the sub-aquatic delta is typical of bottom current produced shoals. The steeper northwestern flank lies in the lee position of the northwest flowing coastal current. Southeast of the shoal the <10 m thick uppermost sequence of presumably Holocene age consists of lobes that dip with the same apparent angle as the foreset towards the offlap break. Within the area 20 km northwest of the shoal the uppermost sequence forms a smooth depression and may be erosionally truncated at the seafloor. Here, the prograding direction includes a northwest component. A shale diapir, which rises from a transparent underlying sequence, marks the transition to an area where the upper sequence is not resolved in the data.     

Movement of sediment on the Gulf of Mexico,continental slope and upper continental shelf

Abstract

Grain size, coarse fraction analyses, and depositional environment as interpreted from microfauna are related to the character of sparker reflections at the location of core holes drilled by Exxon, Chevron, Gulf, and Mobil on the continental slope of the northern Gulf of Mexico. Continuous sparker reflections are correlated with slowly deposited, evenly bedded sediments containing bathyal faunas. The coarse fraction is dominated by the tests of foraminifera. Discontinuous, discordant reflections and diffractions are correlated with sediments more rapidly emplaced in the bathyal environment of the continental slope by slumping and sliding from the continental shelf. Their coarse fraction is dominated by terrigenous sand grains. A large portion of the volume of continental slope sediments appears to consist of these “displaced”; sediments, including an area 3–24 km wide and 80 km long, southeast of Corpus Christi, Texas. Comparable processes of movement of sediments are interpreted on the continental shelf south of the Southwest Pass of the Mississippi River. Bathymetry in this area is characterized by a series of subaqueous “gullies”; radiating from the river mouth and leading to terraces at their southern extremities. Side‐scan sonar and PDR surveys show a rough bottom in these “gullies”; and terraces, as contrasted with a relatively smooth bottom elsewhere. The rough bottom is interpreted as indicative of slump and creep of the sediments from shallower water. Some foundation soil borings in this area south of Southwest Pass find a low‐strength material gradually increasing in strength with depth. Other borings find a “crust”; of anomalously strong material 8–15 m below the mudline. The microfauna recovered from the “crust”; has moved to its present position by slump or creep from shallower water along a pattern comparable to the gullies shown in the present‐day bathymetry.     

Yangtze offshore,China: highly laminated sediments from the transition zone between subaqueous delta and the continental shelf

This study examines the deposition of highly laminated muddy sediment in vibrocores recovered from a depth of 40–55 m of water and located 120 km east of the Yangtze River mouth. X-radiographs show numerous sharp-based sedimentary rhythmics with nearly parallel and undulated laminations, interbedded with silty lenses and interbeds. The laminated sediment varies from clayey silt to silty clay. AMS radiocarbon dates on well-preserved bivalves are primarily younger than 5500 y BP, indicating the formation of the laminated sediment under the present sea-level conditions. Macro- and microfossils in the sediments are consistent with the modern offshore sedimentary setting. The laminated sediment originated from the Yangtze estuary, but the sedimentation processes were not deltaic. The highly laminated sediment that lies below the normal wave base of 10 m was deposited on the seaward margin of the Yangtze subaqueous delta, closely associated with submarine tidal and storm-generated currents.     

Effects of longshore shelf variations on barotropic continental shelf waves,slope currents and ocean modes

Effects of continental shelf bends, converging depth contours and changing depth profiles are discussed. Some analysis is carried out for previously unstudied cases. Separate oceanic interior and shelf flow problems are formulated for a sufficiently narrow shelf. The ocean interior ‘sees’ only an integrated shelf effect, typically increasing shelf-edge amplitudes, retarding longshore Kelvin-wave propagation and increasing natural mode periods by 0 (10%). On the local shelf, the flow matches to the ocean interior and is nondivergent. Effects on shelf waves and slope currents depend subtly on the nature of the longshore variations. Curvature and contour convergence do not per se imply scaterring or generation of shelf waves. Indeed, any depth h(ξ) where ▽² ξ(x,y) = 0 (a condition approximating longshelf uniformity in the topography's convexity) supports essentially the same shelf waves as do straight depth contours (DAVIS, 1983), and slope currents follow depth contours. Scattering results rather from breaks in analyticity of the depth profile. Hence calculations for small isolated features (necessarily highly convex or concave) may overestimate scattering, and superposition for realistic topography may lead to much self-cancellation among scattered waves. Otherwise, examples show a strong preference for scattering into adjacent mode numbers and into any shelf wave mode near to its maximum frequency. A shelf sector, where the maximum shelf wave frequency maxω is less than the frequency ω of an incident shelf wave, causes substantial scattering unless maxω and ω are very close. Adjustment of slope currents to changed conditions takes place through (and over the decay distance of) scattered shelf waves.     

Sediment distribution patterns on the Galicia-Minho continental shelf

A sedimentological and bathymetric study of the Minho-Galicia Shelf shows a strong contrast between a southern shelf region with a thin partially relict cover of sands and gravel, and a northern region where fine-grained sediments predominate. This contrast is explained through differences in the sediment supply, the oceanographic environment (storms and ocean currents) and the morphology of the shelf which results from its underlying tectonic framework.Most sediment is supplied to the Galicia-Minho Shelf by river discharges onto the Minho Shelf, particularly that from the Douro River. In the northern part of the shelf the Galician Rías act as sediment traps rather than sediment suppliers. The bulk of the sediment washes out of the rivers during episodic storm events. While most of the coarse sediments remain deposited close to the coast, the fine-grained material is exported to the outer areas of the shelf. Subsequently, coarse sediments close to the coast are transported southwards by the littoral drift. Whereas the fine-grained material is frequently resuspended through the action of the large swells who influence reaches deep into the water. This frequent resuspension has a long-term sorting effect on the sediments. Furthermore, resuspended sediments on the middle and outer regions of the shelf are transported northwards by a poleward flowing bottom current.As a consequence of the differential transport of coarse sediments to the south, and of the fine-grained sediments to the north, the outer reaches of the Minho Shelf are relatively poor in recent sediments. In many areas relict sediments as well as features associated with ancient coastlines and river mouths, still appear as seabed features. In contrast, the northern regions of the shelf are covered by a thin veneer of fine-grained material that smooth other most of these fossil features.The fine-grained sediment fractions (mostly very fine sands to coarse silts) are deposited in two large mud patches, the Douro and the Galicia Mud Patches, which are situated at water depths of around 100–120 m. These two mud patches are both controlled by the local hydrodynamics and morphology. The Beiral de Viana, to the west of the Douro Mud Patch is a plateau, up to 20 m high lying parallel to the shelf-break and is a morphological expression of an underlying horst system. This plateau acts as a barrier that prevents the drift of some of the fine-grained material to the west, out over the shelf-break and the continental slope. The Galicia Mud Patch is situated on the eastern part of the Galician Shelf to the north of the Douro Mud Patch. It is situated near the extension of the Porto–Tomar fault, which results in the shelf being usually steep in this region, down to a depth of about 100 m. West of this area the slope is much more gentle. Northward transport of the sediment is strongly reduced by the E–W trending outcrops of plutonic and metamorphic rocks.     

Sediment dynamics on the narrow,canyon-incised and current-swept shelf of the northern KwaZulu-Natal continental shelf,South Africa

Side-scan sonar, multibeam bathymetry, Shipek^TM grab, and high- to moderate-resolution sub-bottom data for the northern KwaZulu-Natal continental shelf reveal further insights into the interactions between sediment dynamics, strong western boundary currents and submarine canyon topography. Unlike previously recognised mechanisms for bedload parting on current-swept shelves, bedload partings here are the result of complex interactions between the western boundary poleward-flowing Agulhas Current and submarine canyon topography. This has resulted in bedforms orientated orthogonally to the canyon axis, with sediments entrained equator-wards into the canyon heads before resuming their dominant southerly migration. It is in these zones of parting where the most prominent bedforms occur; these bedform fields are formed by positive feedback in the boundary layer between an increasingly undulatory Agulhas Current and a seafloor incised by regularly spaced submarine canyons. Bedform morphometrics such as wavelength–height, depth–height and distance from thalweg–height relationships show no distinct patterns, indicating that the bedforms are heavily reworked and appear to be out of equilibrium with the inherent oceanographic conditions.     

Estimates of winter currents on the Israeli continental shelf

A simplified analytical model for continental shelf wind-driven currents is adopted. The calculated results compare favorably with extensive field measurements from two separate sources. The model is used to hindcast the current climatology on the Israeli continental shelf. The maximum northward/southward alongshore currents at 10-m water depth, with a return period of 100 years, are found to be 1.28 and 0.53 m/s, respectively.     

Sediment texture,distribution and transport on the Ayeyarwady continental shelf,Andaman Sea

The Ayeyarwady continental shelf is a complex sedimentary system characterized by large sediment influx (> 360 million ton/yr), a wide shelf (> 170 km), a strong tidal regime (7 m maximum tidal range), and incised by the Martaban Canyon. Grain size distribution on the Ayeyarwady shelf reveals three distinct areas in terms of sediment texture (i) a near-shore mud belt in the Gulf of Martaban and adjacent inner shelf (ii) outer shelf relict sands and (iii) mixed sediments with varying proportions of relict sand and modern mud in the Martaban Canyon. The bulk of the terrigenous sediment discharged by the Ayeyarwady River is displaced eastwards by a combination of tidal currents and clockwise flowing SW monsoon current and deposited in the Gulf of Martaban resulting in shoaling of its water depths. Part of the sediment discharge reaches the deep Andaman Sea via the Martaban Canyon and the rest is transported westward into the Bay of Bengal by the counter-clockwise flowing NE monsoon currents.     

Sources and transport of lipids on Amazon continental shelf

Dissolved material and recent sediment from the Amazon continental shelf have been analyzed for hydrocarbons to study the sources and potential fate of the transported organic matter. Dissolvedn-alkanes are present at low concentrations (ppb level) and are dominated by lipids from marine phytoplankton with carbon number maxima (C_max) at C₁₈/C₂₂ and an even-to-odd carbon predominance < C₃₀ (CPI_17–27 from 0.18 to 0.54). In the sediments, bimodal distributions ofn-alkane chain length suggest a mixed input of terrestrial (C_max at C₂₇/C₂₉/C₃₁ and CPI_25–33 from 0.75 to 1.82) and phytoplanktonic/microbial (C_max at C₂₀ and CPI_15–25 from 0.38 to 0.62) organic matter. Sesquiterpenes were the most significant cyclic compounds in all the dissolved samples analyzed reflecting a contribution from resinous trees to the terrestrial organic pool. On the other hand, enhanced concentrations of these compounds in the dissolved phase on the northwest portion of the Amazon shelf, contrasting with decreased concentrations in the sediment samples, suggest that dissolved lipids are released from solid phase in the intensely stirred seabed. Structured organic matter in the sediment has been characterized as being composed of, on average: 19% plant cuticles, 25% woody tissue, 13% pollens and spores, 24% amorphous material, 7% bituminite and 12% altered organic material.     

An overview of sedimentation on the amazon continental shelf

Sedimentological, geochemical, and physical-oceanographic studies of the Brazilian continental shelf near the Amazon River help provide a broad understanding of this major sediment dispersal system. Amazon River sediment accumulates as a subaqueous delta, with the most rapid accumulation (10 cm/yr) occurring near the seaward edge of the topset beds and in the foreset beds. Amazon River sediment is dispersed northwestward along the shelf and is transported beyond the Brazilian border. Radiographic studies of sediment cores delineate three sedimentary environments: interbedded mud and sand, faintly laminated mud, and bioturbated mud. The distribution of these environments is a function of proximity to the river mouth and of sediment accumulation rate.     

Preferential settling of smectite on the Amazon continental shelf

Seventy-five surface and subsurface sediment samples collected on the Amazon continental shelf were analyzed by X-ray diffraction (XRD) in the grain-size fractions smaller than 2 μm. The groups of clay minerals thus identified were smectite, kaolinite, illite, and mixed-layer illite/smectite. Calculation of relative abundance shows a predominance of smectite (37%) in the whole area, followed by kaolinite (27%), illite (23%), and a small percentage of mixed-layer illite/smectite (12%). The distal, northwest region of the continental shelf is characterized by a high abundance of smectite (41%), high salinity values, and high sediment accumulation rates (10 cm year⁻¹), whereas the proximal region is characterized by a comparatively low abundance of smectite (30%), low salinity values, and low sediment accumulation rates (<1 cm year⁻¹). This trend of increasing abundance northwestwards documents a preferential settling of smectite in the distal and more saline regions of this estuarine system. Received: 17 September 1999 / Revision accepted: 17 January 2000     

东南极普里兹湾陆架区大型底栖动物群落结构

To explore the spatial pattern of macrobenthic communities and their response to environmental factors in the Prydz Bay,samples were collected using a 0.25-m2 box corer at 10 stations from November 2012 to April 2013.A total of 50 species of macrobenthos belonging to 8 phyla and 33 families were identified,of which polychaetes(e.g.,Maldane sarsi)and sponges(e.g.,Halichondria sp.and Leucosolenia sp.)were the most prominent groups.The macrobenthos in study area were categorized into five functional groups based on the feeding type,and the detritivorous group represented by polychaetes showed the highest average abundance,while the planktophagous group represented by sponges showed the highest average biomass.Macrobenthos abundance(0–592 ind./m2)and biomass(0–1155.5 g/m2)in the Prydz Bay were relatively lower than those of other Antarctic shelf soft-bottom waters,although the compositions of the dominant species and functional feeding groups were similar.The results of the Spearman rank correlation analysis indicated that the average biomass of the macrobenthos and the biomass of the planktophagous group in the study area were negatively correlated with the water depth,sediment grain size and silt percentage.However,these variables were clearly not strong determinants of macrobenthos assemblage structure.Many factors not measured in the study,e.g.,sediment organic matter and iceberg interference,have probably influenced the spatial distribution of macrobenthic community structure in the Prydz Bay.     

Pliocene to Recent shallow-water contourite deposits on the shelf and shelf edge off south-western Mallorca, Spain

This paper presents evidence for the presence of shallow-water contourite drifts on the south-western shelf and shelf edge off Mallorca in water depths between 150 and 275?m. These are called the Mallorca contourite depositional system (CDS). The elongate-mounded shallow-water CDS in this area is ascribed to an offshoot of the Balearic Current, which flows north to south through the Mallorca Channel as part of the overall thermohaline circulation in the Mediterranean Sea. Drift geometry suggests that the north?Csouth current is deflected into an east?Cwest flow pattern by interaction with a marked seafloor bathymetry, associated with major fault displacement. Four seismic units separated by three prominent discontinuities can be identified. The three internal discontinuities are correlated to large-scale basin-wide events: the lower Pliocene revolution (4.2?Ma), the upper Pliocene revolution (2.4?Ma) and the mid-Pleistocene revolution (0.9?Ma). The Plio-Quaternary succession has been deposited on top of a Miocene reef, which serves as an acoustic basement and is affected by a large fault, offsetting the basement on average by 150?m. Marked erosional features throughout and further incision of the Sant Jordi Channel along the basement fault in the Pleistocene deposits indicate stronger currents in this period. The Pleistocene deposits also show a pronounced cyclicity, which is tentatively ascribed to climatic variations and the effects of eustatic sea-level fluctuation over the south-western Mallorca shelf at that time.     

Submarine faults and slides on the continental shelf,northern Gulf of Alaska

Abstract

Submarine faults and slides or slumps of Quaternary age are potential environmental hazards on the outer continental shelf (OCS) of the northern Gulf of Alaska. Most faults that approach or reach the seafloor cut strata that may be equivalent in age to the upper Yakataga Formation (Pliocene‐Pleistocene). Along several faults, the seafloor is vertically offset from 5 to 20 m. A few faults appear to cut Holocene sediments, but none of these shows displacement at the seafloor. Submarine slides or slumps have been found in two places in the OCS region: (1) seaward of the Malaspina Glacier and Icy Bay, an area of 1200 km² with a slope of less than 0.5°, and (2) across the entire span of the Copper river prodelta, an area of 1730 km², having a slope of about 0.5°. Seismic profiles across these areas show disrupted reflectors and irregular topography commonly associated with submarine slides or slumps. Potential slide or slump areas have been delineated in areas of thick sediment accumulation and relatively steep slopes. These areas include (1) Kayak Trough, (2) parts of Hinchinbrook Entrance and Sea Valley, (3) parts of the outer shelf and upper slope between Kayak Island and Yakutat Bay, and (4) Bering Trough.     

Distribution of hornblende on the Atlantic continental shelf off Georgia, United States

Hornblende is the least stable, and most diagnostic, mineral of the sediment blanketing the continental shelf off Georgia, U.S.A. Recent work by others has shown that the probable sources of hornblende-rich sands on the shelf are the Savannah and Altamaha Rivers, both of which originate in the southern Appalachian piedmont. Rivers with drainage basins confined to the coastal plain carry stable, low-hornblende heavy-mineral assemblages and contribute sediment to the shelf only during episodes of regression or transgression. Distribution of hornblende on the continental shelf reveals the importance of the Altamaha and Savannah Rivers, especially the Savannah, as sources of sediment. It is postulated that original point concentrations of hornblende-rich sand associated with Late Pleistocene deltas or estuaries of the Savannah River have been modified by southwest currents, possibly during winter storms. The resulting configuration is a series of northeast-trending, linear high-hornblende anomalies. The origin of the hornblende-distribution anomalies probably is identical to the origin of linear shoals that also trend northeast across the continental shelf.