Dense shelf water cascades and sedimentary furrow formation in the Cap de Creus Canyon,northwestern Mediterranean Sea

To investigate the processes by which sediment is transported through a submarine canyon incised in a continental margin affected by recurrent dense shelf water cascading events, several instrumented moorings were deployed in the Cap de Creus Canyon from September 2004 to September 2005. This was done as part of the EuroSTRATAFORM Program that investigated sediment transport and accumulation processes in the Gulf of Lions. Results obtained in this observational study confirm that major cascading events can effectively contribute to the rapid export of sediment from the shelf and upper slope to deeper environments, and suggest that the associated strong currents carrying coarse particles are able to erode the canyon floor and generate sedimentary furrows. During winter 2004–2005, persistent northerly winds and the absence of river floods contributed to decrease the buoyancy of coastal waters and to dramatically enhance the intensity of dense shelf water cascades in the Gulf of Lions. Under such conditions, cascading continuously affected the entire Cap de Creus upper canyon section for more than a month and sustained cold temperatures and down-canyon steady currents >60 cm/s (up to 100 cm/s), showing periodic fluctuations that lasted between 3 and 6 days. Increases in suspended sediment concentrations were associated with dense shelf water cascading outbursts, but the magnitude of the concentration peaks decreased with time, suggesting a progressive exhaustion of the resuspendable sediments from the shelf and canyon floor. Grain size analyses of the particles caught by a near-bottom sediment trap show that dense shelf water cascades are able to transport coarse sediments (up to 65% sand) in suspension (and presumably as bed load), which have the potential to abrade the seafloor and generate erosive bed forms. The orientation of a large field of “wide” (i.e., widths about 1/2 spacing indicative of erosive formation) sedimentary furrows recently observed in the Cap de Creus Canyon clearly coincides with the preferential direction of highest velocities measured by the moored current meters, indicating a causative relationship between contemporary dense shelf water cascades and furrow formation.     

Suspended sediment transport in the Gulf of Lions (NW Mediterranean): Impact of extreme storms and floods

In situ observations were combined with 3D modeling to gain understanding of and to quantify the suspended sediment transport in the Gulf of Lions (NW Mediterranean Sea). The outputs of a hydrodynamic–sediment transport coupled model were compared to near-bottom current and suspended sediment concentration measurements collected at the head of seven submarine canyons and at a shallow shelf site, over a 6-month period (November 2003–May 2004). The comparisons provide a reasonable validation of the model that reproduces the observed spatial and time variations. The study period was marked by an unusual occurrence of marine storms and high river inputs. The major water and sediment discharges were supplied by the Rhone, the largest Mediterranean river, during an exceptional flood accompanying a severe marine storm in early December 2003. A second major storm, with moderate flooding, occurred in February 2004. The estimate of river input during the studied period was 5.9 Mt. Our study reveals (i) that most of the particulate matter delivered by the Rhone was entrapped on the prodelta, and (ii) that marine storms played a crucial role on the sediment dispersal on the shelf and the off-shelf export. The marine storms occurring in early December 2003 and late February 2004 resuspended a very large amount of shelf sediment (>8 Mt). Erosion was controlled by waves on the inner shelf and by energetic currents on the outer shelf. Sediment deposition took place in the middle part of the shelf, between 50 and 100 m depth. Resuspended sediments and river-borne particles were transported to the southwestern end of the shelf by a cyclonic circulation induced by these onshore winds and exported towards the Catalan shelf and into the Cap de Creus Canyon which incises the slope close to the shore. Export taking place mostly during marine storms was estimated to reach 9.1 Mt during the study period.     

Characterization of sedimentary deposits at the confluence of two tributaries of the Pará River estuary (Guajará Bay,Amazon)

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.     

Seasonal variation in sediment transport on the Russian River shelf,California

Near-bottom currents, light transmission and scattering, and bottom pressure were measured with GEOPROBE tripods and vector-averaging current meters during June 1979 to April 1980 on the central shelf 10 km west of the Russian River, California. The instruments were located on the mid-shelf mud belt composed of bimodal sandy clayey silts contributed principally by the Russian River. During the summer season of persistent northwesterly, upwelling-favorable winds, the average and maximum current speeds 5 m above the bottom were 11 and 31 cm s^?1, respectively. The mean (subtidal) flow at 5 m above bottom was poleward and slightly offshore at about 6 cm s^?1. The strongest wave-generated bottom currents were about 10 cm s^?1, but oscillatory velocities > 5 cm s^?1 were infrequent. Suspended-matter concentrations, derived from the optical data at 1.9 m above the bottom, ranged from 1 to 6 mg l^?1. The optical data show that the currents and waves were generally below threshold levels for sediment erosion through the summer. In contrast, during the autumn and, particularly, the winter months, the average and maximum concentrations of suspended matter increased substantially. The increases were primarily caused by larger waves from distant storms and short intervals of strong currents associated with local storms and, secondarily, by the large seasonal flow of the Russian River. Wind-driven and wave-generated bottom currents were as large as 37 and 45 cm s^?1, respectively, during local storms in December 1979 and February 1980. Suspended-matter concentrations averaged about 7 mg l^?1 during non-storm winter periods, but increased to nearly 150 mg l^?1 during a December storm. Estimates of suspended-matter flux near the bottom show that the local winter storms, which had a combined duration of about 12 days, could account for 30 to 50% of the total annual suspended-sediment transport at the mid-shelf site. Although intervals of large swell were at times superimposed on southward advective currents, the major sediment-transport events were caused by strong southerly winds that produced poleward bottom currents with a significant offshore component. The primary aspects of the distribution of modern sediments on this shelf are in good agreement with the observed poleward transport.     

Entrainment of fine-grained surface deposits into new ice in the southwestern Kara Sea,Siberian Arctic

The entrainment of bottom deposits (silt and clay) into newly formed ice was investigated in the Amderma/Vaygach flaw lead in the southwestern Kara Sea, Siberian Arctic. Fine-grained bottom deposits and sea ice sediments (SIS) were analyzed by granulometry, scanning electron microscopy and X-ray diffractometry. On average, SIS contain by a factor of four times more silt than the shelf deposits (66.7% vs. 16.3%), and the SIS clay percentage is more than three-fold of the bottom value (31.2% vs. 9.1%). Sand-sized particles are significantly less abundant in SIS compared to bottom sediment (2.1% vs. 74.6%). The preferred entrainment of silt into ice is underpinned by the enhanced silt-to-clay-ratio in SIS compared to bottom deposits. Though silt is preferably entrained into SIS, no evidence was found for preferential ice-entrainment of any silt sub-fraction (coarse, medium or fine). However, sub-angular- and angular-discoidal silt particles are favorably entrained into local sea ice. Clay mineral assemblages in SIS and shelf surface sediments match very well revealing that no individual clay mineral is preferably enriched in SIS or reduced at the bottom. The general textural, compositional and statistical match of fine-grained shelf surface deposits and SIS proves that bottom sediment is the principle source for ice-entrained material in the study area. We propose e.g. wave action and thermohaline convection to take sediment particles upward from the bottom nepheloid layer into the well-mixed 10–40 m deep water column of the Amderma/Vaygach flaw lead, and the turbulent process of suspension freezing to bring sediment particles and frazil crystals into contact, finally leading to the formation of sediment-laden ice. The role of SIS entrainment and export for local/regional shelf erosion and coastal retreat is of minor importance in the SW Kara Sea compared to other circum-Arctic shelf seas. However, the characteristic clay mineral assemblage of local SIS and bottom deposits can help identify the origin of SIS both on regional and Arctic-wide scales.     

Rippled scour depressions on continental shelf bank slopes off Nordland and Troms,Northern Norway

Multibeam bathymetry acquired under the MAREANO programme from the continental shelf off Nordland and Troms, northern Norway, show bedforms that we have interpreted as rippled scour depressions. They occur in three areas offshore on bank slopes facing southeast, more than 15 km from land. They are generally found where the slope gradient is low, in water depths of 70–160 m. Individual depressions are up to 3 km long, 1 m deep and up to 300 m wide. They occur in areas where sediments evolve quickly from glacial deposits on the banks to post-glacial muddy sediments on the glacial troughs. Multibeam backscatter and underwater video data show that depression floors are covered by rippled, gravelly, shelly sand. Ripple crests are parallel or slightly oblique to the depression axis orientation. Sand without bedforms is observed between the depressions. TOPAS seismic lines show that the uppermost seismic unit consists of the sand between the depressions. The base of this unit may be the last transgressive/tidal/wave ravinement surface. Physical oceanographic modelling indicates that maximum current velocities are up to 0.6 m/s in the rippled scour depression areas. Stronger currents appear to inhibit the building of these features. Tidal currents play an important role as they trend parallel to the southeast banks slopes and are likely responsible of the gravelly ripples formation inside the depressions as well as the persistence of these depressions which are not covered by finer sediments. On Malangsgrunnen bank, some of the rippled scour depressions are in the extension of NW–SE furrows located on the bank. Simulated bottom currents indicate currents mainly perpendicular to these furrows, as for the rippled scour depressions on the bank slopes. Nevertheless, these features could also highlight currents coming from the northwest which reach the bank margin and continue down to the areas of the rippled scour depressions. These currents could be responsible for the formation of some of the bedforms, together with tidal currents.     

Large-scale bedforms along a tideless outer shelf setting in the western Mediterranean

High-resolution multibeam swath-bathymetry and sediment samples were collected across the outer shelf region of the Columbretes Islands (southern Ebro continental shelf, western Mediterranean Sea). Bathymetric data from the submerged part of the Columbretes volcanic system revealed the presence of three main relict sand bodies along the outer shelf, at 80–116 m depth range, above which asymmetric and slightly asymmetric large and very large 2D and 3D subaqueous dunes were observed. These bed features were recognized, mapped and quantified with the aim of evaluating their potential formation mechanisms in relation to the local hydrodynamic and morphologic settings of the area. Dunes range from 150 to 760 m in wavelength and from tens of centimeters to 3 m in height, and are among the longest ever recognized in an outer shelf region. These bedforms are mostly composed of medium-sized sandy sediments, presumably coming from the degraded relict sand bodies on top of which they have developed, mixed with fine fractions from the recent draping holocenic sediments. The orientation of the dunes is SSW, progressively turning W towards the southernmost sector of the area, following the trend of the shelf-edge. Contemporary hydrodynamic measurements at the Ebro continental shelf-edge show that recorded currents are insufficient to form the observed bedforms and that stronger currents are required for sediment mobilization and dune formation. Based on their morphology and orientation, it is proposed that these bedforms are produced by the action of the southward-flowing Liguro-Provençal-Catalan (LPC) geostrophic current. The LPC probably reaches high near-bottom currents during energetic hydrodynamic events through interactions with the seafloor morphology of the study area. Subaqueous dunes are expected to be basically inactive features with respect to present-day processes, although they can be reactivated during high-energy events. The small Δh/λ ratio measured in the dune fields of the Columbretes shelf revealed that the dune heights fall below the values predicted by the Flemming (1988) global equation, as observed in other outer shelf settings also dominated by unidirectional flows. This may suggest a different morphodynamic character of large dunes formed on outer shelves in a micro-tidal regime.     

Sediment transport and resuspension due to combined motion of wave and current in the northern Adriatic Sea during a Bora event in January 2001: A numerical modelling study

The Adriatic Sea general circulation model coupled to a third generation wave model SWAN and a sediment transport model was implemented in the Adriatic Sea to study the dynamics of the sediment transport and resuspension in the northern Adriatic Sea (NAS) during the Bora event in January 2001. The bottom boundary layer (BBL) was resolved by the coupled model with high vertical resolution, and the mechanism of the wave–current interaction in the BBL was also represented in the model. The study found that, during the Bora event of 13–17 January 2001, large waves with significant wave height 2 m and period of 5 s were generated by strong winds in the northwestern shelf of the Adriatic where the direction of wave propagation was orthogonal to the current. The combined motion of the wave and current in the BBL increased the bottom stress over the western Adriatic shelf, resulting in stronger sediment resuspension there. Combining stronger bottom resuspension and strong upward vertical flux of resuspended sediments due to turbulent mixing, the model predicted that sediment concentration near the Po River was much higher than that predicted by the model run without wave forcing. The study also shows that wave–current interaction in the BBL reduced the western Adriatic Coastal Currents (WACCs) in the shallower north. It is concluded that wave forcing significantly changed the sediment distributions and increased the total horizontal fluxes over the western shelf. These results signified wave effect on sediment flux and distribution in the NAS, and suggested that waves cannot be neglected in the study of dynamics of sediment transport and resuspension in the shallow coastal seas. By including the tidal forcing in the coupled model, we also examined the effect of tides on the sediment transport dynamics in the NAS.     

Benthic oxygen fluxes and sediment properties on the northeastern New Zealand continental shelf

We examined spatial variations in benthic remineralisation (measured as sediment oxygen consumption (SOC)) and sediment properties on the northeastern New Zealand continental shelf and slope to assess the importance of benthic mineralisation in this ecosystem and to provide data for more complete global carbon budgets. SOC measured in dark incubations conducted in early summer ranged from 128 μmol m⁻² h⁻¹ at the deepest (360 m) to 1222 μmol m⁻² h⁻¹ at the shallowest (4.2 m) site and decreased significantly with water depth (p<0.001, r²=0.78, SOC=1222.8−456.3×log₁₀[water depth], n=14 sites). These rates were in the range found on continental shelves elsewhere (64–1750 μmol m⁻² h⁻¹, n=30 studies) and had a very similar distribution with water depth. SOC was also measured in light incubations at seven sites (4.2–35 m water depth) to examine the effects of microphytobenthos and accounted for 42–106% of rates measured in the dark. Measurements of near-bed light intensities suggested that microphytobenthos production was not solely regulated by light intensity but evidently influenced by other factors. A two-dimensional PCA ordination of surface sediment properties accounted for 83.3% of the total variance in the data and divided the study area into three clusters that corresponded well to its spatial division into the shallow (<30 m) Firth of Thames, the Hauraki Gulf (30–50 m) and the northern shelf-slope region. In the Firth of Thames sediments were very fine-grained with low CaCO₃ and high total organic matter and pigment content, and low C:N ratios. The northern shelf-slope sediments showed the opposite trends to the Firth of Thames and those in the Hauraki Gulf had mostly intermediate values. Dark SOC was significantly correlated with sediment organic matter, carbon, nitrogen, pigments and silt/clay content (p<0.05, r=0.55–0.85) but a multiple linear regression revealed that water depth was the only significant predictor. Calculations suggest that approximately 13%, 10% and 34% of primary production is remineralised in the sediments of the northern shelf-slope region, Hauraki Gulf and Firth of Thames, respectively, indicating a strong benthic–pelagic coupling on the northeastern New Zealand continental shelf that was particularly pronounced in the Firth of Thames due to its shallow depth and significant terrestrial and riverine inputs.     

Wind-driven diversion of summer river runoff preconditions the Laptev Sea coastal polynya hydrography: Evidence from summer-to-winter hydrographic records of 2007–2009

This paper examines the role of atmospheric forcing in modifying the pathways of riverine water on the Laptev Sea shelf, using summer-to-winter hydrographic surveys from 2007 to 2009. Over the two consecutive winter seasons of 2007–2008 and 2008–2009 in the area of the winter coastal polynya, our data clearly link winter surface salinity fields to the previous summer conditions, with substantially different winter salinity patterns preconditioned by summer atmospheric forcing. In the summer of 2007, dominant along-shore westerly winds in the cyclonic regime force the Lena River runoff to flow eastward. In contrast, in the summer of 2008, dominant along-shore easterly winds over the East Siberian Sea and on-shore northerly winds over the Laptev Sea in the anticyclonic regime lock the riverine water in the vicinity of the Lena Delta. Over the coastal polynya area in the southeastern Laptev Sea these patterns precondition a surface salinity difference of 8–16 psu between the winters of 2008 and 2009. Overall, this indicates a residence time of at least half a year for riverine water on the Laptev Sea shelf. Future climate change associated with an enhanced summer cyclonicity over the eastern Arctic may turn more riverine water eastward along the eastern Siberian coast, resulting in weaker vertical density stratification over the Laptev Sea shelf, with possible impact on the efficiency of vertical mixing and polynya dense water production.     

Variability of baroclinic tidal currents on the Mackenzie Shelf,the Southeastern Beaufort Sea

Semidiurnal tidal currents on the outer shelf of the Mackenzie Shelf in the Beaufort Sea were found to be strongly influenced by the locally generated baroclinic tide. Two primary factors are involved in this process: (1) the sharp shelf break along the northeastern Mackenzie Shelf, promoting the generation of vigorous internal tidal waves; and (2) the proximity to critical latitudes for M₂ and N₂ motions locking these waves and preventing them from leaving the source region. As a result, internal tides are resonantly trapped between the shelf and critical latitudes. The physical properties and temporal variations of tidal motions were examined using current meter measurements obtained from 1987–1988 at four sites (SS1, SS2, SS3, and SS4) offshore of the shelf break at depths of ∼200 m. Each mooring had Aanderaa RCM4s positioned at ∼35 m below the surface and ∼50 m above the bottom. Complex demodulation was used to compute the envelopes (amplitude modulation) of these components. A striking difference in the variability of clockwise (CW) and counterclockwise (CCW) tidal currents was found. The CW tides are highly variable, have greater amplitude, exhibit a burst-like character associated with wind events and contain about 80% of the total energy of the semidiurnal tidal currents. In contrast, the CCW components have a more regular temporal regime with distinct monthly, fortnightly and 10-day modulation at astronomical periodicities associated with frequency differences M₂–N₂ (0.03629 cpd), S₂–M₂ (0.06773 cpd), and S₂–N₂ (0.10402 cpd). Significant horizontal correlation of the CW current envelopes was found only between stations near the northeast Mackenzie Shelf, indicating this to be the main area of baroclinic internal wave generation.     

Origin and distribution of surface sediments and human impacts on recent sedimentary processes. The case of the Amvrakikos Gulf (NE Ionian Sea)

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.     

Near-inertial currents in the DeSoto Canyon region

Near-inertial currents in the DeSoto Canyon region are described using current and wind observations taken between April 1997 and March 1998 for the “DeSoto Canyon Eddy Intrusion Study”. Distinct energy peaks are present at near-inertial frequencies for the clockwise spectrum and there is little energy at the same frequencies for the counterclockwise current spectrum. In this region, amplitudes of the near-inertial currents can be as high as 40 cm s⁻¹. These currents are surface-intensified and display an increase in amplitude from the shelf break to offshore. Between November 1997 and March 1998, they were effectively generated by shifting winds accompanying passages of cold fronts. For this time period, near-inertial currents are reasonably well-simulated by a mixed-layer model forced by observed winds. During summer 1997, however, enhanced near-inertial motions often resulted from resonance between winds and existing currents.     

Spatial and temporal variability of seawater properties,current velocity and SPM concentration off Cassino Beach-Rio Grande-Southern Brazil

Water column profiles and near-bed time series of pressure, current velocity, suspended-particulate matter (SPM) concentration and seawater temperature and salinity were collected during three short cruises carried out in May 2005 in the shoreface and inner shelf area adjacent to Cassino Beach, southern Brazil. The measurements were part of the Cassino Experiment, a project conducted at an open, sandy coastal area known for the occurrence of patches of fairly large amounts of muddy sediments that are sporadically fluidized, transported onshore and eventually stranded on the beach. The study area is close to the Patos Lagoon mouth, being influenced by its water and suspended-sediment discharge. The presence of the Patos Lagoon outflow on the inner shelf was detected in one of the cruises (May 13) through measurements of near-surface salinity: while close to shore salinity was 29.4, a minimum value of 13.8 was measured at ∼10 km from the coast. Four days later, no trace of the plume was detected in the area. Regarding seawater temperature, no large temporal or spatial variability was documented with measured values ranging from 19.3 to 20 °C. Water column currents were prominently to N and NE, except at the outermost station, located ∼42 km from the coast, where NW-directed flows were observed at surface and mid-depth. Maximum near-bed current velocity oscillated between 18 and 42 cm s⁻¹ in the east–west direction and between 14 and 42 cm s⁻¹ in the north–south direction. Near-surface concentration of SPM oscillated between 11 and 99 mg L⁻¹, in general one order of magnitude lower than near-bed values. However, near-bed concentration of SPM showed large spatial variability: the highest value (2200 mg L⁻¹) was yielded by a water sample collected at ∼8 m water depth, at a station located ∼2 km away from the shoreline; two water samples collected 500 m, apart from this station, yielded SPM concentrations of 148 and 205 mg L⁻¹, one order of magnitude lower. Spectral analyses of near-bed current speed and SPM concentration indicate the relevance of oscillations in the low-frequency (<0.05 Hz) range. Detailed sampling of bottom sediment indicated that in May 2005 the mud patch was centered at ∼8.5 m water depth.     

Rare earth elements in sediments of the Vigo Ria,NW Iberian Peninsula

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.     

Dispersal of Mississippi and Atchafalaya sediment on the Texas-Louisiana shelf: Model estimates for the year 1993

A three-dimensional coupled hydrodynamic-sediment transport model for the Texas-Louisiana continental shelf was developed using the Regional Ocean Modeling System (ROMS) and used to represent fluvial sediment transport and deposition for the year 1993. The model included water and sediment discharge from the Mississippi River and Atchafalaya Bay, seabed resuspension, and suspended transport by currents. Input wave properties were provided by the Simulating WAves Nearshore (SWAN) model so that ROMS could estimate wave-driven bed stresses, critical to shallow-water sediment suspension. The model used temporally variable but spatially uniform winds, spatially variable seabed grain size distributions, and six sediment tracers from rivers and seabed.At the end of the year 1993, much of the modeled fluvial sediment accumulation was localized with deposition focused near sediment sources. Mississippi sediment remained within 20-40 km of the Mississippi Delta. Most Atchafalaya sediment remained landward of the 10-m isobath in the inner-most shelf south of Atchafalaya Bay. Atchafalaya sediment displayed an elongated westward dispersal pattern toward the Chenier Plain, reflecting the importance of wave resuspension and perennially westward depth-averaged currents in the shallow waters (<10 m). Due to relatively high settling velocities assumed for sediment from the Mississippi River as well as the shallowness of the shelf south of Atchafalaya Bay, most sediment traveled only a short distance before initial deposition. Little fluvial sediment could be transported into the vicinity of the “Dead Zone” (low-oxygen area) within a seasonal-annual timeframe. Near the Mississippi Delta and Atchafalaya Bay, alongshore sediment-transport fluxes always exceeded cross-shore fluxes. Estimated cumulative sediment fluxes next to Atchafalaya Bay were episodic and “stepwise-like” compared to the relatively gradual transport around the Mississippi Delta. During a large storm in March 1993, strong winds helped vertically mix the water column over the entire shelf (up to 100-m isobath), and wave shear stress dominated total bed stress. During fair-weather conditions in May 1993, however, the freshwater plumes spread onto a stratified water column, and combined wave-current shear stress only exceeded the threshold for suspending sediment in the inner-most part of the shelf.     

Principal component analysis (PCA) and mineral associations of litoraneous facies of continental shelf carbonates from northeastern Brazil

The distribution of mineral phases according to the provenance of carbonate and terrigenous facies of carbonate sediments from a large area of the continental shelf of northeast Brazil was investigated using a major element multivariate analysis approach. Heavy minerals such as ilmenite are restricted to the litoraneous facies of the continental shelf of the states of Paraíba and Pernambuco, and clay minerals are found in distal facies of the continental shelf of the State of Ceará. In the carbonate fraction, composed essentially by Mg-calcite and aragonite, there is co-variation between CaO/MgO and bathimetry in part of the studied continental shelf from depths between 15 and 20 m, apparently due to influence of the seawater temperature, degree of oxygenation and luminosity. The terrigenous facies are mainly composed of quartz, clay minerals, K-feldspars and micro-micaceous minerals, having Fe and Ti oxide and hydroxide minerals as major accessory phases. Major element behavior attests to the presence of arenaceous quartz-rich relict sediments in the 35, 60 and 80 m isobaths of the continental shelf of the state of Ceará which is here interpreted as a proxy of ancient coast lines during the Flandrian transgression.     

Assessing the importance of tropical cyclones on continental margin sedimentation in the Mississippi delta region

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/²³⁴Th_xs 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.     

Storm-driven sediment transport in Massachusetts Bay

Massachusetts Bay is a semi-enclosed embayment in the western Gulf of Maine about 50 km wide and 100 km long. Bottom sediment resuspension is controlled predominately by storm-induced surface waves and transport by the tidal- and wind-driven circulation. Because the Bay is open to the northeast, winds from the northeast (‘Northeasters’) generate the largest surface waves and are thus the most effective in resuspending sediments. The three-dimensional oceanographic circulation model Regional Ocean Modeling System (ROMS) is used to explore the resuspension, transport, and deposition of sediment caused by Northeasters. The model transports multiple sediment classes and tracks the evolution of a multilevel sediment bed. The surficial sediment characteristics of the bed are coupled to one of several bottom-boundary layer modules that calculate enhanced bottom roughness due to wave–current interaction. The wave field is calculated from the model Simulating WAves Nearshore (SWAN). Two idealized simulations were carried out to explore the effects of Northeasters on the transport and fate of sediments. In one simulation, an initially spatially uniform bed of mixed sediments exposed to a series of Northeasters evolved to a pattern similar to the existing surficial sediment distribution. A second set of simulations explored sediment-transport pathways caused by storms with winds from the northeast quadrant by simulating release of sediment at selected locations. Storms with winds from the north cause transport southward along the western shore of Massachusetts Bay, while storms with winds from the east and southeast drive northerly nearshore flow. The simulations show that Northeasters can effectively transport sediments from Boston Harbor and the area offshore of the harbor to the southeast into Cape Cod Bay and offshore into Stellwagen Basin. This transport pattern is consistent with Boston Harbor as the source of silver found in the surficial sediments of Cape Cod Bay and Stellwagen Basin.     

The role of remote wind forcing in the subinertial current variability in the central and northern parts of the South Brazil Bight

Data analysis of continental shelf currents and coastal sea level, together with the application of a semi-analytical model, are used to estimate the importance of remote wind forcing on the subinertial variability of the current in the central and northern areas of the South Brazil Bight. Results from both the data analysis and from the semi-analytical model are robust in showing subinertial variability that propagates along-shelf leaving the coast to the left in accordance with theoretical studies of Continental Shelf Waves (CSW). Both the subinertial variability observed in along-shelf currents and sea level oscillations present different propagation speeds for the narrow northern part of the SBB (~?6–7 m/s) and the wide central SBB region (~?11 m/s), those estimates being in agreement with the modeled CSW propagation speed. On the inner and middle shelf, observed along-shelf subinertial currents show higher correlation coefficients with the winds located southward and earlier in time than with the local wind at the current meter mooring position and at the time of measurement. The inclusion of the remote (located southwestward) wind forcing improves the prediction of the subinertial currents when compared to the currents forced only by the local wind, since the along-shelf-modeled currents present correlation coefficients with observed along-shelf currents up to 20% higher on the inner and middle shelf when the remote wind is included. For most of the outer shelf, on the other hand, this is not observed since usually, the correlation between the currents and the synoptic winds is not statistically significant.