Drowned and buried valleys on the southern New England continental shelf

Bathymetry and seismic reflection profiling have revealed a sequence of seven post-Jurassic drowned or buried drainage systems on the southern New England shelf. The basement and younger stream patterns have a dominant southward trend with preferred drainage avenues from the mainland to the middle shelf indicated by superposed valley ground positions from unconformity to unconformity over time. Fluvial action under stable tectonic conditions is inferred by low valley height/width ratios with higher ratios related to ice modification of inner shelf pre-glacial river valleys.Fluvial processes responding to sea-level withdrawals have greatly influenced the shelf's later development. Periodically during post-Paleocene time, sediment from subaerial erosion has been transported to the shelf edge by streams. Deltaic deposition on a subsiding base has controlled outbuilding on the outer shelf where the frequent presence of overlying drainage networks is the result of numerous sea-level regressions. Since Eocene time, sediment has been channelled to the deep sea via Block Canyon and its progenitor.Locally structures created by erosion and glacial deposition have governed drainage direction. On the inner shelf, late Tertiary - early Pleistocene streams were diverted southeastward and southwestward by the magnitude of Long Island's Coastal Plain escarpment and by secondary cuestas between eastern Long Island and Block Island. The probable eastern reach of Dana's southern Sound River valley can be traced from northeastern Long Island across Block Island Sound. An early Woodfordian end moraine of the Wisconsin stage impounded melt waters in Block Island and Rhode Island Sounds. Where the moraine was breached near Block Island, fans were formed adjacent to the water gaps. In Rhode Island Sound the earlier and later Woodfordian end moraines deflected some mainland drainage toward the southwest.     

Hardbottom development and significance to the sediment-starved west-central Florida inner continental shelf

Hardbottoms are sequence boundaries and condensed sections that offer clues for the interpretation of the incomplete record of Tertiary continental shelf evolution. Seaward of 5 km, 50% of the inner west-central Florida shelf seafloor is flat hardbottom. These lithified surfaces are punctuated by shorefacing, scarped hardbottoms that trend shore-parallel (330°–0°) and vary in relief (up to 4 m). Scarped hardbottoms are the only natural relief on the inner shelf and support a diverse benthic community, the activities of which erode the outcrops, producing undercuts in excess of 1 m. Outcropping hardbottom strata are comprised of distinct, phosphate-rich, mixed carbonate–siliciclastic lithofacies, that range in age from Miocene to Quaternary. Miocene units are dolomite-rich and mark the upper surface of the inner shelf bedrock (Hawthorn Group). Dolomite within these beds (silt-sized, cloudy centered rhombs) fall into two age groups, correlating with highstands at 15 and 5 Ma. This lithofacies is consistent with models that indicate an increased flux of organic matter – resulting from topographically induced upwelling – promoting dolomitization during early burial diagenesis in the sulfate-reduction zone. Quaternary units are calcite-rich and perched atop the shelf bedrock. Samples of these units record a complex diagenetic history and multiple sea-level fluctuations. Based on evidence of primary marine cementation, they are interpreted to be hardground (non-deposition) surfaces, forming as a function of sediment starvation and minimal sediment movement. Decreased highstand magnitude or duration may have resulted in the absence of a significant organic component to Quaternary hardbottoms, which, in turn, may prevent subsequent dolomitization. These outcrops are a potential source for sediments to the inner shelf, not only as habitat for biological sediment production, but also through their destruction. The undercut, shorefacing, scarped hardbottom morphology displayed by west-central Florida hardbottoms is indicative of bio-erosion. Preliminary studies indicate a potential mass of 0.04 kg m⁻² yr⁻¹ of siliciclastic sediment is released to the inner shelf.     

辐射沙脊群南部浮游动物的生态特征

依据2007年5月(春)、8月(夏)辐射沙脊群南部所获得的浮游动物样品,分析了该海域浮游动物的分布、时空变化及其与生态环境因子的关系。结果表明,调查海域浮游动物66种(不含鱼卵和仔鱼),生态属性以暖温性、近岸低盐种类为主;浮游动物总丰度春季(1 558.57 ind/m~3)>夏季(305.79 ind/m~3);饵料浮游动物生物量春季(2 923.75 mg/m~3)>夏季(335.46 mg/m~3);水平分布均呈现中北部高、南部低的特点。调查海域中部(4～5断面)多样性指数(H’)、丰富度指数(d)及均匀度指数(J)值较其他断面要低。相关性分析表明,春季浮游动物丰度和饵料浮游动物生物量的分布变化与浮游植物的丰度密切相关,夏季则与海水平均盐度的变化相关性显著;春、夏季浮游动物总丰度分布分别取决于优势种短尾类大眼幼体(Brachyura megalopa)和小拟哲水蚤(Paraca/anus parvus)的分布。     

Sediment waves on the Moroccan continental rise

A nearly continuous zone of sediment waves is present on the lower continental rise off western Morocco which parallels the regional bathymetric trends. The individual sediment waves within the zone migrate upslope with time and, in general, also trend parallel to the regional bathymetric contours. These observations suggest that geostrophic contour currents are responsible for the formation of sediment waves. Physical oceanographic measurements and sea-floor photographs indicate only a very weak bottom circulation in this region. This suggests either that strong bottom currents are not essential for the formation of sediment waves or that relatively stronger bottom currents flowed along the continental margin of Morocco in the recent past. Turbidity flows may also influence the distribution of these sediment waves.     

The amplifying effect of southern continental shelf of the East China Sea on semidiurnal tides

The coast of Zhejiang Province is a zone of maximum tidal ranges in China. One of the causes is the influence of the adjacent continental shelf. A model is presented in this paper, whose theory suggests that the amplitudes of semidiurnal tides should be amplified on the southern continental shelf of the East China Sea, while those of diurnal tides should not. The values computed from the equations are basically in agreement with observations.     

Large and very large subaqueous dunes on the continental shelf off southern Vietnam, South China Sea

In the course of two regional side-scan sonar surveys on the continental shelf off southern Vietnam after the winter monsoon seasons of 2003 and 2004, and covering a total distance of over 1,000 km, the widespread occurrence of large and very large subaqueous dunes was discovered. On the basis of their size, shape, depth of occurrence and orientation, the dunes were grouped into five spatially distinct regions. In each case, a different height/wavelength relationship is observed. With the exception of region no. 3 where dune dimensions follow the mean global trend, the dimensions in the other regions lie below the mean global trend. The most plausible explanation for this is sediment starvation and/or insufficient time for the larger dunes to fully adjust to changing flow conditions. A good correlation is observed between average dune height in each region and water depth, although this is not the case for dune length. The orientation of the dunes corresponds to the direction of the current pattern induced by the regional winter monsoon winds (NE to SW and S). The generally well-developed asymmetrical shapes and the large size of the dunes suggest that the wind-induced currents are strong enough to reactivate most of the dunes during the winter monsoon season, a conclusion supported also by theoretical calculations of critical current velocities. The largest dunes, which seem to have reached their maximum sizes for the local water depths, may not be reactivated regularly but rather only by exceptionally strong episodic flows.     

Long-range sonar mapping of the continental shelf

Recent experiments in the shallow waters of the continental shelf off the west coast of Scotland have shown that it is possible to obtain geologically useful sonographs from a towed low-frequency side-scan sonar (GLORIA project) out to long ranges. Frequency-modulated pulses of 2-sec duration, centred at 6.4 kHz, with an acoustic power of 10 kW were transmitted, and the received energy was processed by a linear correlator. A maximum range of 13 km was achieved under both isothermal and stratified water conditions; however the most even irradiation of the sea floor was obtained when the water was isothermal, and under stratified conditions the maximum range could be reduced to 5 km or less if the sonar array was towed at a depth which lay within any strong temperature gradient in the water column.Examples of isometric records are shown, and it is suggested that mosaics could be built up from such records, obtained at a coverage of 170 km²/hour.     

Sources of polycyclic aromatic hydrocarbons in marine sediments from southern and northern areas of the Norwegian continental shelf

Variability in levels and sources of polycyclic aromatic hydrocarbons (PAH) in sediments from one large sea area off the coast of northern Norway (“North area”, NA) have been compared to similar data from another large area off the coast of southern Norway (“South area”, SA). Samples from NA were collected at the Norwegian continental shelf in south-western Barents Sea and north-eastern Norwegian Sea. Samples from SA were from the Norwegian Trench and the Skagerrak. Sediment cores have been dated, characterised by grain size distribution (GS) and organic carbon content (TOC), and the composition of PAH and geochemical biomarkers (alkanes and triterpanes) studied to provide an insight into the different sources of PAH. Generally, PAH levels are higher in sediments from SA compared to NA. A mixture of pyrogenic and petrogenic sources contribute to PAH levels in SA, while the contents of petrogenic PAH is negligible in surface sediments in NA. At some locations in NA, petrogenic PAH levels are elevated in the deepest sediment layers from pre-industrial times, indicating a natural input of petroleum through seepage. Occurrence of elevated levels of microbial hopanoids (hopenes) in the deepest sediment layers at some locations both in the north and the south indicate the presence of petroleum.     

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.     

Basalt from Louisiana continental shelf

A basalt outcrop was discovered on Alderdice Bank on the outer Louisiana continental shelf. The basalt shows an age of 76.8 ± 3.3 × 10⁶ years. Textural, mineralogical, and chemical characteristics indicate that it is an alkali basalt of shallow intrusive origin. It was probably brought to the seafloor by salt tectonics and exposed due to salt dissolution.An accurate account of the Mesozoic geologic history of the Gulf of Mexico must consider the apparent consanguinity of all magmatic rocks of the region, including the Alderdice Bank basalt, and the apparent basinward decrease in age of magmatic activities.     

Topographically trapped waves over the continental shelf and slope

General characteristics of topographically trapped subinertia waves are discussed from the viewpoint of an eigenvalue problem and ray theory. Special attention is paid to the slope parameterS(x) (=(dh/dx)/h, wherex denotes the coordinate perpendicular to the shoreline, increasing seaward, andh(x) is the depth) which is a measure of the strength of the restoring force of the waves. Three cases for theS distribution are considered, in whichS is assumed to be positive at the coast and to tend to zero far from the coast. The first is whereS(x) decreases monotonically towards the open ocean. It is found in this case that waves are trapped near the coast. The second is whereS(x) does not decrease monotonically, but has a maximum. It is concluded that this case may contain two types of waves, i.e., those trapped near the coast and those trapped near the maximum, and the dispersion curves corresponding to different types may nearly intersect, namely, result in “kissing”. The third is whereS(x) has a negative region (corresponding to the presence of a trench). It is found in this case that an infinite sequence of waves is trapped in the negativeS region which propagate with the coast to their left (right) in the northern (southern) hemisphere besides the waves trapped near the coast. The topography in the second case corresponds to a typical continental shelf and a typical continental slope. It is shown by model calculation that trapped waves are present over the continental slope as well as over the continental shelf. Almost the same results are obtained for superinertia waves ifS is replaced by 1/h which is a measure of the restoring force of superinertia waves.     

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.     

A sea floor magnetometer for the continental shelf

Measurement of temporal magnetic variations on the sea floor is desirable in order to extend the technique of geomagnetic depth sounding into the oceans. This paper describes a recording three-component sea floor magnetometer and its use in continental shelf depths. The orientation and tilt of the instrument on the sea floor are recorded using gelatine solutions to freeze a compass card and a ball-bearing, respectively. A backing-off procedure initially nulls the magnetic field components along each of the three mutually orthogonal fluxgate sensors. Magnetic variations along each sensor axis are then recorded within a range of ± 300 nT of these nulled positions. The resolution is ± 1 nT, and with a power drain of 800 mW the magnetometer can record continuously for 30 days. The instrument capsule is moored to surface buoys for recovery in continental shelf applications. The buoys may have marker flags, radar reflectors or radio beacons attached to them to aid in relocation.     

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.     

Macrobenthic composition of the southeast continental shelf of India

Macrobenthic faunal composition was studied at six different depth ranges (30–50, 51–75, 76–100, 101–150, 151–175 and >176 m) in five transects (off Karaikkal, Parangipettai, Cuddalore‐SIPCOT, Cheyyur and Chennai) in the continental shelf of southeast coast of India. Eleven diverse taxa were found, comprising 113 species of polychaetes, 14 species of bivalves, 10 species of amphipods and ‘others’ (five tanaids, five crabs, four isopods, three echinoderms, two shrimps, two cnidarians, two fishes and one cephalochordate). Polychaetes were the dominant taxa, constituting 88.5% of the total abundance and 30.7% of the total biomass. The number of species (seven per 0.2 m² at >176 m depth range in Chennai to 46 per 0.2 m² at 30–50 m in Cheyyur), abundance (216 per 0.2 m² at >176 m in Karaikkal to 353 per 0.2 m² at 30–50 m in Cheyyur) and biomass (0.09 g per 0.2 m² at 151–175 m in Karaikkal and 4.6 g per 0.2 m² at 30–50 m in Cheyyur) of macrobenthos decreased with increase in depth. DO decreased gradually from 30 m depth; beyond 150 m, the decrease was pronounced due to the presence of the oxygen minimum zone. Using the distance based linear model (DISTLM), it was found that the environmental variables explained about 73.3% of the total variability in macrofaunal distribution. The heavy metals cobalt and mercury, as well as water pressure (proxy for depth), showed a significant relationship with macrofauna, explaining respectively 9, 7.3 and 7% of the total variability. The contribution of other variables was smaller.     

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.     

Infaunal benthic communities of the New Zealand continental shelf

Seventeen infaunal benthic communities are recognised from sampling on the New Zealand continental shelf and upper continental slope. These are characterised in terms of geographic distribution, substrate and bathymetric range, and commoner infaunal bivalves and echinoderms.     

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.