Intermediate Nepheloid Layers Research Articles

Far-field effects of the Nile damming on the silica cycle in the Southeastern Mediterranean Sea

Silica plays a key role in the growth of silicifying primary producers (e.g., diatoms) and hence the ocean carbon pump. The Mediterranean Sea's eastern Levantine Basin (ELB) is a low silica (and low N and P) ultra-oligotrophic basin. Before 1965, Nile autumn floods were a major source of dissolved silica (DSi) and other nutrients to primary producers of the ELB continental shelf, also known as the Nilotic cell. The construction of the Aswan High Dam (AHD) in the mid-1960s, blocked these floods, drastically diminishing the autumn-diatom blooms offshore the Nile delta. However, the far-reaching and long-lasting effects of the Nile damming on the Si cycle in the ELB remain unclear. Here, we studied the changes in DSi in the surface water offshore Israel and the distribution of biogenic silica in deep-sea short sediment cores, collected hundreds of kilometers from the Nile outlet, at depths range of 1100–1900 m, offshore the ELB Israeli coast. We show post dam reduction and termination in flood related seasonality of DSi and a concurrent decrease (of up to 79 %) in biogenic silica (BSi) accumulation rates in surficial sediments relative to underlying sediments. These changes reflect the effects of Si (dissolved and particulate) retention by the AHD on diatoms production, export and burial in the ELB. This far-field effect was demonstrated in deep-sea areas subjected to intense lateral transport of resuspended sediments from the shelf via intermediate nepheloid layers and to coastal water intrusions, along the path of the pre-dam, flood plumes. Our core records show that the AHD worsened nutrient-diminished, exceptionally unfavorable conditions for diatoms that persisted in the deep ELB at least during the last four millennia.

Nepheloid Layer Structure and Variability Along the Highly Energetic Continental Margin of the Northern South China Sea

AbstractObservations of hydrography and suspended particles supply a large amount of information regarding material transport with respect to the changing marginal sea. To investigate the nepheloid layer structure and variability along the highly energetic continental margin of the northern South China Sea, we combined optical data and time series of acoustic backscatter data. The surface nepheloid layers and subsurface nepheloid layers were widely developed, but the bottom nepheloid layers (BNLs) and intermediate nepheloid layers (INLs) showed regional differences. On the western side of Dongsha Island, there was a large variability of strong INLs, with concentrations up to 0.94 mg L−1, and they occasionally formed and exceeded 200 m in a few hours, with concentration changes up to an order of magnitude. On the eastern side, INLs thicker than 100 m were rare. However, the BNL was larger and more stable, and the strongest BNLs with nepheloid inversion had concentrations up to 0.55 mg L−1 and unexpectedly thickened to be greater than 1,500 m, extending upward to the thermocline. Time series data found that the recurring nonlinear internal waves (NLIWs) dominated the enhancement of BNLs in the Dongsha Slope. The NLIW‐induced vertical velocity reached 0.5 m s−1, increasing the concentration of BNLs and resuspending sediment to 10 m above the bottom. The NLIWs strongly erode the sediments and provide a material source for the upward development of BNLs, while centimeter‐scale rippled scours form on the seafloor. These findings provide evidence of strong lateral transport across continental margins.

Physical and biogeochemical controls on seasonal iron, manganese, and cobalt distributions in Northeast Atlantic shelf seas

Dissolved (<0.2 μm) trace metals (dTMs) including iron (Fe), manganese (Mn), and cobalt (Co) are micronutrients that (co-) limit phytoplankton growth in many ocean regions. Here, we present the spatial and seasonal distributions of dFe, dMn, and dCo on the Northeast Atlantic continental margin (Celtic Sea), along a transect across the shelf and two off-shelf transects along a canyon and a spur. Waters on the continental shelf showed much higher dTM concentrations (dFe 0.07–6.50 nmol L−1, average 1.41 ± 0.96 nmol L−1, n = 138; dMn 0.868–14.8 nmol L−1, 2.75 ± 2.37 nmol L−1, n = 148; dCo 54.8–217 pmol L−1, 109 ± 32 pmol L−1, n = 144) than on the slope (dFe 0.03–1.90 nmol L−1, 0.65 ± 0.43 nmol L−1, n = 454; dMn 0.223–1.14 nmol L−1, 0.58 ± 0.20 nmol L−1, n = 458; dCo 27.3–122 pmol L−1, 71.7 ± 11.7 pmol L−1, n = 441), attributed to strong dTM contributions from a low-salinity endmember, i.e., riverine discharge. Benthic sedimentary input via reductive dissolution (especially for dFe and dMn), delineated by short-lived radium (Ra) isotopic activities (223Raxs and 224Raxs), was only prominent at a station (Site A) characterized by fine sediments. On the continental slope, dMn levels at depth were mainly determined by the formation of insoluble Mn oxides and the intrusion of Mediterranean Outflow Waters. In contrast, dFe and dCo concentrations at depth were balanced by the regeneration from remineralization of sinking organic particles and scavenging removal. In addition, bottom and intermediate nepheloid layers along the slope illustrated both elevated dTM concentrations and Ra isotopic activities. The presence of nepheloid layers is especially significant along the canyon transect relative to the spur transect, demonstrating the importance of slope topography on the off-shelf transport of dTMs into the Northeast Atlantic Ocean.As a seasonal stratified shelf sea, dTMs and nutrients showed synchronized seasonal variations on the shelf, indicating the influence of biological processes in addition to source effects. Surface dFe and dCo were depleted in summer due to enhanced biological uptake, while sub-surface dFe and dCo were elevated in summer and autumn ascribed to the remineralization of sinking organic particles. In contrast, surface dMn levels were predominantly controlled by the seasonal variations in photoreduction, while sub-surface dMn concentrations were relatively constant throughout the year. The combined effects of fluvial and benthic sources, topographical controls, and biological processes shape the seasonal variations of dTM distributions. Such seasonal variations in dTMs and biological activities can affect the biological carbon pump on the Northeast Atlantic continental margin, and may further influence the carbon cycle in the Atlantic Ocean via the dynamic dTM exchange between continental margins and the open ocean.

Formation Mechanisms and Characteristics of the Marine Nepheloid Layer: A Review

Marine nepheloid layer is widely distributed in the oceans and marginal seas. The concentration of suspended particles in the nepheloid layer is significantly higher than that of the adjacent layers. Marine nepheloid layers include the surface nepheloid layer (SNL), intermediate nepheloid layer (INL), and bottom nepheloid layer (BNL). As a transport pathway for the particulate matter in the oceans, nepheloid layer is important to the carbon cycle and the source–sink system at the continental margin. This paper focused on the characteristics of the marine nepheloid layer and the mechanism of internal solitary waves that form INL and BNL, providing a reference for further research on the marine ecological environment dynamic process and source–sink system. BNL is formed by suspended seabed sediments with strong currents near the bottom, but the fate of BNL remains unknown. Marine nepheloid layer significantly affects the transportation of materials within the ocean. The transport of sediments by ISWs dramatically affects the formation of deep seabed sedimentary landform. However, the formation process and the transport contribution of INL and BNL are still unclear. The systematic and long-term research on the marine nepheloid layer in the world oceans is still limited.

Influence of Natural Processes and Bottom Trawling in the Nepheloid Layer Structure Off Vancouver Island (British Columbia, Canada, NE Pacific)

Continental margins are transitional areas between the land and the deep ocean where large amounts of particulate matter are supplied, transported, and ultimately deposited in the deep sea. High-energy hydrodynamic processes such as storms, ocean currents, or internal waves and tides, as well as bottom trawling activities contribute to the resuspension and remobilization of these particles. These mechanisms favor their transference from the continental shelf to the continental slope and maintain high concentrations of particulate matter in the water column forming nepheloid layers. The temporal evolution of the hydrographic and nepheloid structure in the upper slope off Vancouver Island (British Columbia, Canada, NE Pacific) was assessed by analyzing 4 months (from mid-August to late-November 2018) of sensor data from Ocean Networks Canada (ONC) NEPTUNE cabled seafloor observatory. The distribution of particulate matter during the study period illustrated a well-defined turbidity structure consisting of surface nepheloid layers (SNLs) at &amp;lt;100 m depth associated to primary and secondary productivity, intermediate nepheloid layers (INLs) between 150 and 300 m depth at the shelf-break and upper slope domain, and bottom nepheloid layers (BNLs) developed at &amp;gt;400 m depth. Moderate storm events occurred during fall when the more intense INLs were recorded at shelf-break depths. However, not all the INLs recorded during this period occurred in coincidence with these storms, indicating that these INL detachments were modulated by a different sediment resuspension mechanism. Analyses of fishing vessel activity during the study period revealed that trawlers operated over the same depth range as these INLs. Our results suggest that, in combination with the regional currents, the presence of continuous fishing along the continental slope off Vancouver Island contributes to the advection of suspended sediment particles, playing a major role in their transfer as nepheloid layers.

Distribution of suspended particulate matter at the equatorial transect in the Atlantic Ocean

Abstract. A suspended particulate matter distribution against a hydrographical background was studied at the oceanographic transect across the equatorial Atlantic in the year 2000. An area of abnormally high suspended matter volume concentrations was found above the Sierra Leone Rise in the entire water column (eastern part of the transect). The suggested explanation for the anomaly is based on the ballast hypothesis whereby solid particles are incorporated as ballast into suspended biogenic aggregates, leading to increased velocities of sinking. This occurs within the Northwest African upwelling area, where the plankton exposed to the Saharan dust abundance form a significant number of aggregates, which are later transported equatorward via the Canary Current. An intermediate nepheloid layer associated with the Deep Western Boundary Current was recorded from the South American Slope at depths of 3200–3700 to 4300 m above the Para Abyssal Plain. Antarctic Bottom Water enriched in suspended matter was found mostly in the troughs at 40–41∘ W. It was detached from the bottom, coinciding with the core of the flow due to the bottom rise “dam” located up-stream. The grain size of particles along the entire transect has a polymodal distribution with 2–4 and 8–13 µm modes. The registered rise in percentage in some parts of the transect of the 7–21 µm sized particles suggests the presence of the well-known coarse mode (20–60 µm) formed by aggregation of transparent exopolymer particles (mucus).

Turbulent Mixing and the Formation of an Intermediate Nepheloid Layer Above the Siberian Continental Shelf Break

AbstractIntermediate nepheloid layers (INLs) form important pathways for the cross‐slope transport and vertical export of particulate matter, including carbon. While intermediate maxima in particle settling fluxes have been reported in the Eurasian Basin of the Arctic Ocean, direct observations of turbid INLs above the continental slope are still lacking. In this study, we provide the first direct evidence of an INL, coinciding with enhanced mid‐water turbulent dissipation rates, over the Laptev Sea continental slope in summer 2018. Current velocity data show a period of enhanced downslope flow with depressed isopcynals, suggesting that the enhanced turbulent dissipation is probably the consequence of the presence of an unsteady lee wave. Similar events occur mostly during ice free periods, suggesting an increasing frequency of episodic cross‐slope particle transport in the future. The discovery of the INL and the episodic generation mechanism provide new insights into particle transport dynamics in this rapidly changing environment.

A mixed turbidite – contourite system related to a major submarine canyon: The Marquês de Pombal Drift (south‐west Iberian margin)

AbstractSynchronous interaction between bottom currents and turbidity currents has been reported often in channel–levée systems where the thickness of the turbidity currents exceeds that of the levées. Such interplay between along‐slope and down‐slope sedimentary processes is one of the mechanisms by which ‘mixed turbidite–contourite systems’ can originate. However, bottom currents flow over large areas of the seafloor, including continental slopes characterized by deeply incised submarine canyons rather than channel levées. In these cases, a direct interaction between along‐slope and down‐slope currents is, theoretically, unlikely to take place. In this study, oceanographic, swath bathymetry, multichannel seismic data and sediment cores are used to investigate a 25 km long, 10 km wide and up to 0.5 km thick deep‐sea late Quaternary deposit that sits adjacent to the north‐west flank of one of the major canyons in the North Atlantic, the São Vicente Canyon, in the Alentejo Basin (south‐west Iberian margin). The area receives the influence of a strong bottom current, the Mediterranean Outflow Water, which has swept the continental slope at different water depth ranges during glacial and interglacial periods. Architectural patterns and sediment characteristics suggest that this sedimentary body, named Marquês de Pombal Drift, is the result of the interaction between the Mediterranean Outflow Water (particularly during cold periods) and turbidity currents flowing along the São Vicente Canyon. Because the canyon is incised significantly deeper (ca 1.5 km) than the thickness of turbidity currents, an additional process, in comparison to earlier models, is needed to allow the interaction with the Mediterranean Outflow Water and transport sediment out of the canyon. In the São Vicente Canyon, and likely in other canyons worldwide, interaction of turbidity currents with contour currents requires intermediate nepheloid layers that export the finer‐grained fraction of turbidity currents out of the canyon at the boundary between major water masses.

Lithogenic Particle Transport Trajectories on the Northwest Atlantic Margin

AbstractThe neodymium isotopic composition of the detrital (lithogenic) fraction (εNd‐detrital) of surface sediments and sinking particles was examined to constrain transport trajectories associated with hemipelagic sedimentation on the northwest Atlantic margin. The provenance of resuspended sediments and modes of lateral transport in the water column were of particular interest given the energetic hydrodynamic regime that sustains bottom and intermediate nepheloid layers over the margin. A large across‐margin gradient of ∼5 εNd units was observed for surface sediments, implying strong contrasts in sediment provenance, with εNd‐detrital values on the lower slope similar to those of “upstream regions” (Scotian margin) under the influence of the Deep Western Boundary Current (DWBC). Sinking particles collected at three depths at a site (total water depth, ∼3,000 m) on the New England margin within the core of the DWBC exhibited a similarly large range in εNd‐detrital values. The εNd‐detrital values of particles intercepted at intermediate water depths (1,000 and 2,000 m) were similar to each other but significantly higher than those at 3,000 m (∼50 m above the seafloor). These observations suggest that lithogenic material accumulating in the upper two traps was primarily advected in intermediate nepheloid layers emanating from the adjacent shelf, while that at 3,000 m is strongly influenced by sediment resuspension and along‐margin, southward lateral transport within the bottom nepheloid layer via entrainment in the DWBC. Our results highlight the importance of both along‐ and across‐margin sediment transport as vectors for lithogenic material and associated organic carbon transport.

Particulate rare earth element behavior in the North Atlantic (GEOVIDE cruise)

Abstract. Particulate concentrations of the 14 Rare Earth Elements (PREE), yttrium, and 232-thorium were measured in 200 samples collected in the epipelagic (ca. 0–200 m) and mesopelagic (ca. 200–1500 m) zones of the North Atlantic during the GEOVIDE cruise (May/June 2014, R/V Pourquoi Pas?, GEOTRACES GA01), providing the most detailed snapshot of the PREE distribution in the North Atlantic so far. Concentrations of particulate cerium (PCe) varied between 0.2 and 16 pmol L−1, while particulate neodymium (PNd) concentrations ranged between 0.1 and 6.1 pmol L−1. Particulate ytterbium (PYb) concentrations ranged between 0.01 and 0.50 pmol L−1. In addition, this study showed that PREE distributions were also controlled by the biological production in the upper sunlit ocean and by remineralization processes in the mesopelagic area. Low surface concentrations combined with normalized PREE patterns displaying a negative Ce anomaly and HREE enrichments pointed to freshly formed biogenic particles imprinting the seawater signature. A significant relationship between biogenic silica (BSi) and PHREE was also observed in the Labrador and Irminger seas, due to the occurrence of strong diatom blooms at the sampling time. In order to identify dissolved-particulate processes independent of the ionic radius, we used PHo∕PY ratios and showed that absorption processes were predominant in the upper ocean, while adsorption processes dominated at deeper depths. This study highlighted different lithogenic fractions of PREE and dispersion depending on the shelf: off the Iberian margin, up to 100 % of the PREE were determined to have a lithogenic origin. This lithogenic input spread westward along an intermediate nepheloid layer (INL), following isopycnals up to 1700 km away from the margin. In contrast, along the Greenland and Newfoundland margins, the circulation maintained lithogenic inputs of PREE along the coasts.

Baroclinic Residual Circulation and Mass Transport Due to Internal Tides

AbstractBaroclinic (BC) tidal residual circulation due to internal tides is investigated around islands over a shallow ridge using a numerical ocean model. Internal tides enhance vertical mixing over shallow slopes, leading to horizontal density gradients that drive BC residual circulation along the main thermocline. For a strongly stratified summer case, the vertical diffusivity estimated by the Mellor and Yamada turbulence closure model exceeds 1 × 10−2 m2 s−1, and the velocity of BC residual circulations reaches 0.2 m s−1. The magnitude of BC residual circulation is larger than that of barotropic residual tidal circulation, implying that BC residual circulation due to internal tides plays an important role in coastal ocean circulation. Furthermore, BC residual flow accounts for an equal percentage (5%) of the total tidal kinetic energy as the barotropic residual flow under summer stratification conditions. Results from a coupled sediment resuspension and transport model show the growth of intermediate nepheloid layers formed by strong bottom shear stress and BC residual flow. The residual component contributes largely (23% of the total in summer) to the total suspended sediment flux. Seasonal variability is explored, with weaker winter stratification leading to reduced mixing, and thus weaker BC residual circulation and sediment flux. The magnitude of the BC residual circulation is also shown to be proportional to the square of the tidal amplitude.

The first deep-sea mooring station in the eastern Levantine basin (DeepLev), outline and insights into regional sedimentological processes

The eastern Levantine basin (ELB) is an ultra-oligotrophic, warm and saline marine system, significantly affected by climate change during the past several decades. The deployment of the first moored station at the ELB (DeepLev), at 1500 m water depth, ~50 km offshore Haifa, Israel, aimed at enhancing synchronized measurements of physical and biogeochemical dynamics. The station carries an array of sediment traps and sensors that measure physical, chemical, and biological attributes along the water column. Here we describe the station and present sediment-flux data from the first 1.5 years of operation (since November 2016). We show that, despite DeepLev's offshore location and the open-sea characteristics of the water column, lateral transport from the continental margin contributes the greatest fraction of particulate flux to the seafloor at this site. This mostly occurs via intermediate nepheloid layer plumes entering the study area between depths of 180 and 800 m. Bottom currents are weak (average ± SD = 3.5 ± 2 cm s−1) and are unlikely to cause significant resuspension. Based on sedimentation rates obtained from nearby sediment cores and sediment particle fluxes sampled in the traps, calculated accumulation rates of total particulate matter and of particulate organic carbon at the seafloor were 95–108 g m−2 y−1 and 0.5–0.6 g m−2 y−1, respectively. Offshore changes in sedimentation rates and in surface sediment elemental ratios indicate that sediment originating from the continental margins is transported at least 100 km offshore.

Bottom and Intermediate Nepheloid Layer Induced by Shoaling Internal Solitary Waves: Impacts of the Angle of the Wave Group Velocity Vector and Slope Gradients

AbstractThe widely recognized global phenomena of bottom nepheloid layer (BNL) and intermediate nepheloid layer (INL) are ubiquitous in the ocean. These phenomena are induced by shoaling internal solitary waves (ISWs), as observed in many studies. In this study, we analyzed the BNLs and INLs induced by shoaling ISWs and their detailed processes using flume experiments and field observations. ISWs suspended seabed sediment by the horizontal velocity in the vortex, and the near‐bottom vertical velocity lifted sediment into the water column to create a BNL, which detached from the slope and diffused along the isopycnals, forming more than one INL. Considering the results of previous researchers, we found that the numbers of BNLs and INLs were principally determined by the relationship between the angle of the ISW group velocity vector (α) relative to horizontal and the slope gradients (γ). In transmissive regions (γ/α &lt; 1), one BNL and more than one INL were formed. In critical regions (γ/α~1), only one BNL and less than one INL were observed. In reflective regions (γ/α &gt; 1), less than one BNL was formed and no INL. The BNL in the critical regions should be the thickest where the sediment resuspension was the greatest. The concentrations and thicknesses of BNLs and INLs were related to the energy and amplitude of ISW and the sediment condition. The results in the transmissive regions were proved by field observation. Our research will help to predict the number and magnitude of transport channels formed by shoaling ISWs from the ocean margin to the ocean interior.

Three-dimensional coupling between size-fractionated chlorophyll-a, POC and physical processes in the Taiwan Strait in summer

Coastal water masses are inherently diverse, often with multiple water sources of different origins. Knowledge of characteristic salinity and temperature fingerprints can be very useful to differentiate between component water masses. For example, summertime river plumes carry relatively warm and less saline water into coastal waters, along with nutrients, clastic and organic sediments, and terrestrial-sourced biogenic particles. In an upwelling regime, relatively colder water carries marine-sourced nutrients and particles in water masses from deeper depths offshore. These two regimes are subject to biological-physical coupling that affects their size-differentiated particulate chlorophyll-a and organic carbon contents, and we show here how both regimes can be identified in these terms in the Taiwan Strait in summer. A close examination shows that microphytoplankton is the major contributor to chlorophyll-a concentrations in both regimes at all depths, and picophytoplankton is a minor contributor in the subsurface cold-water regime. The contribution from nanophytoplankton is relatively insignificant. Shipboard hydrographic profiling and water sampling was conducted over 28 h at a fixed location off the mouth of the Minjiang River in the northern Taiwan Strait, where the river plume and upwelling regimes overlap. Water samples were filtered onboard using a nested filtration system to separate suspended particles into >153, 63–153, 10–63, and 0.7–10 μm size classes. Our findings show that in the surface nepheloid layer, the river plume accounted for the physical process affecting large (>10 μm) terrestrial-sourced non-phytoplankton POC (organic debris), with an average C/N ratio of 92.2. The Upwelling regime accounted for the physical process affecting marine-sourced POC <10 μm containing fluorescence. In the intermediate nepheloid layer, which coincided with the subsurface chlorophyll maximum layer, most large (>10 μm) POC that contained fluorescence were either upwelling- or offshore-sourced, with an average C/N ratio of 8.5. In the benthic nepheloid layer, terrestrial- and marine-sourced POC containing fluorescence of all sizes were associated with river plume and upwelling regimes. Large (>63 μm) non-phytoplankton organic particles were also present, with an average C/N ratio of 10.2. Both spatial and temporal datasets point to strong three-dimensional coupling between biogenic particles and physical processes. The river plume and upwelling regimes further differentiate size-fractionated chlorophyll-a and POC.

Experimental investigation of sediment dynamics in response to breaking high-frequency internal solitary wave packets over a steep slope

Abstract Previous studies have found that internal solitary waves (ISWs) can erode, resuspend and transport seabed sediments and form intermediate nepheloid layers (INLs) and bottom nepheloid layers (BNLs). In our recent flume experiments, the function of the BNLs and the fate of suspended sediment were studied by breaking high-frequency ISW packets over a steep slope. We found that a considerable amount of sediment was resuspended down the slope with the vortex, and it reached across the entire water column at almost the same time due to the vertical ejection (pump) of flow at the head of the bore by the breaking ISW. The maximum height of the suspended sediment reached the pycnocline. The suspended sediment may be transported slightly up to the top of the slope via the BNL. The downslope transporting distance was very short and the suspended sediment did not reach the foot of the slope via the BNL. The suspended sediment was mainly redeposited around the middle of the slope, and some fine-grained sediment suspended offshore was spread along the pycnocline via the INL to greater distances. The changes in slope elevation might be influenced by successive scouring, the redeposition of sediment, and the migration of sand waves. Our study was performed under low Iribarren number conditions, which can facilitate the development of new field experiments designed to obtain direct measurements and insights on the interactions between ISWs and seabed sediments.

Evidence of trawling-induced resuspension events in the generation of nepheloid layers in the Foix submarine canyon (NW Mediterranean)

Abstract The temporal evolution of water column turbidity was studied on a submarine canyon on the Barcelona continental margin. From April to June 2014, an instrumented mooring array equipped with an autonomous hydrographic profiler with a CTD and a turbidimeter was deployed in the Foix canyon axis at 870 m depth. The instruments were programmed to collect hydrographic profiles once per day to provide a view of the temporal evolution of water column characteristics from 200 to 800 m water depth. The results illustrate a well-defined water turbidity structure of particulate matter distributed in intermediate nepheloid layers (INLs) developed between 300 and 500 m water depth and above the canyon rims, and INLs and near-bottom nepheloid layers (BNLs) confined inside the canyon between 650 and 800 m water depth. Data from fishing vessels activity at the time of the deployment was obtained from Vessel Monitoring System (VMS). The presence and location of the fishing vessels in the study area suggested a relationship between trawling activity and the generation of such layers. Nepheloid layers were absent during the first part of the deployment, when there was no fishing activity within the Foix Canyon axis or at the adjacent continental slope. Later, with the beginning of trawling activity on the fishing grounds close to the canyon, both INLs and BNLs were observed in the profiling casts, suggesting a causative relationship with fishing activities. Additionally, the hydrodynamic conditions within the canyon also seem to favour particle retention and to increase water turbidity in thick BNLs when water circulation is directed up-canyon. Bottom trawling appears to act as a main sediment resuspension mechanism in the Barcelona continental slope regions, increasing suspended sediment concentration at specific water depths where fishing grounds are located. Suspended particles are then advected and propagate along and across-margin by ambient currents via nepheloid layers.

Transport of organic-walled dinoflagellate cysts in nepheloid layers off Cape Blanc (N-W Africa)

Many aspects of to the vertical and lateral transport of organic matter particles produced in the upper ocean are still not clear. This study documents the production, vertical and lateral displacement of the particulate organic matter fraction formed by dinoflagellate cysts in nepheloid layers present off Cape Blanc in November 2015. The position and extent of cyst production in the upper water column has been assessed by collecting the export rain of cysts with floating sediment traps. This has been compared to cyst associations in different intermediate nepheloid layers in the water column along two onshore-offshore transects.We document that highest cyst export production took place at the rim of a newly formed upwelling eddy/filament. We observed lateral transport up to 130 km off the shelf break into the deeper ocean in the bottom nepheloid layers (BNL) and in intermediate nepheloid layer (INL). The depth of the INL gradually increased from 600 m closest to the shelf break to 1300 m at the most offshore station. Transport in the BNL was observed in the more onshore part between 750 and 2300 m water depth. Cyst associations in both layers were comparable, indicating a common source. The large abundance of living cysts in these layers suggest that this source had a recent origin rather than being the result of resuspension of older sediment. The cyst associations of these samples differed from those of a second INL deeper in the water column as well as from samples from the more offshore part of the BNL. This indicates that the particles in this deeper INL and the offshore part of the BNL had a different origin. This origin could not be identified therefore no conclusions were drawn with respect to the extent to which these particles have been transported vertically or laterally.The cyst associations in the surface sediments differed from those of the nepholoid layers and the upper water column and did not reflect the effects of lateral transport as registered from the water column. The surface sediments contained a higher proportion of cysts that are highly resistant to aerobic degradation than the samples from the water column. This indicates that the former were modified predominantly by species-specific post depositional degradation. Selective degradation altering the cyst associations was not observed for the nepheloid layers.

Sedimentary processes and cold-water coral mini-mounds at the Ferrol canyon head, NW Iberian margin

The western Iberian margin has a complex morphology controlled by both geological and oceanographic processes. Compared to the submarine canyons in the southern part of this margin (e.g. Nazaré and Setubal), the canyons in the northern part (e.g. Ferrol and A Coruña) have received little attention. This study maps the geomorphological features around the Ferrol canyon head and combines them with oceanographic observations to infer the sedimentary and oceanographic processes active in this area. Furthermore, the occurrence of a cold-water coral (CWC) mini-mound province near the canyon head and the Ortegal Spur pockmark field is investigated with regard to seepage processes, oceanographic conditions and anthropogenic impact.The Ferrol canyon head and outer Ortegal Spur are characterized by erosional (erosional and abraded surfaces, contourite channels and furrows), depositional (contourite drifts and sediment waves) and mixed (contourite terrace) features, indicating a dominant control of bottom currents on the sedimentary processes. Bottom currents are related to the interaction of the canyon head topography with both the Mediterranean Outflow Water (MOW) slope current and with M2 internal tides associated to the interface (pycnocline) between Eastern North Atlantic Central Water (ENACW) and MOW. Due to this interaction, the Ferrol canyon head is subject to active sediment resuspension and forms a potential source area for the intermediate nepheloid layers along the NW Iberian upper slope.The CWC mini-mounds (1.9–2.7 m high and 75–141 m in diameter) occur between 400 and 560 m depth. Seismic facies signatures indicative of fluid-flow and morphometric analysis implies that larger CWC mini-mounds adjacent to the pockmark field initiated by colonization of pre-existing pockmarks, while smaller more clustered mounds developed independently. The observed mini-mounds are relict features likely related to their position outside of the contemporary ENACW-MOW interface which favours CWC growth through (1) increased bottom currents, enhancing food supply and (2) the presence of a potential density envelope σθ = 27.35–27.65 kg/m3, proposed to control coral larvae transport along the NE Atlantic margin. Alternatively, the lack of contemporary coral growth might be related to habitat destruction by bottom trawling. However, a preliminary age constraint reveals CWC growth occurred during the early Holocene, coeval to relict CWC mini-mounds near canyon heads on the Armorican margin. Their existence suggests a regional shift in the NE Atlantic density profile where the ENACW-MOW interface occurred up to 200 m above its contemporary position.

Sediment resuspension and the generation of intermediate nepheloid layers by shoaling internal bores

Abstract Numerical simulations of sediment resuspension due to shoaling internal bores are performed with a nonhydrostatic model, SUNTANS. Results show strong sediment resuspension induced by internal bores and a turbidity layer intruding offshore in the interior of the water column, known as an intermediate nepheloid layer (INL). Sediment resuspension processes and INLs reproduced in the model are in good agreement with observational data from Otuschi Bay, Japan. The formation of INLs by shoaling internal waves is explained as follows: (1) strong receding currents due to a previous run-up bore cause sediment resuspension along the slope, (2) the interaction of the receding currents and a subsequent run-up bore induce horizontal flow convergence and strong upward currents, (3) the upward currents lift the suspended sediments into the interior of the water column, (4) the run-up bore intrudes below the suspended sediments and forms an INL within the pycnocline. Sediment resuspension processes are also investigated for various internal Iribarren number conditions. The internal Iribarren number represents the ratio of the topographic slope to the internal wave steepness. In low internal Iribarren number conditions, strong currents near the bottom result in sediment resuspension and INL formation. In high internal Iribarren number conditions, sediment resuspension is weak, and INLs do not form.

Transport by breaking internal gravity waves on slopes

We use the results of a direct numerical simulation (DNS) with a particle-tracking model to investigate three-dimensional transport by breaking internal gravity waves on slopes. Onshore transport occurs within an upslope surge of dense fluid after breaking. Offshore transport occurs due to an intrusion of mixed fluid that propagates offshore and resembles an intermediate nepheloid layer (INL). Entrainment of particles into the INL is related to irreversible mixing of the density field during wave breaking. Maximum onshore and offshore transport are calculated as a function of initial particle position, and can be of the order of the initial wave length scale for particles initialized within the breaking region. An effective cross-shore dispersion coefficient is also calculated, and is roughly three orders of magnitude larger than the molecular diffusivity within the breaking region. Particles are transported laterally due to turbulence that develops during wave breaking, and this lateral spreading is quantified with a lateral turbulent diffusivity. Lateral turbulent diffusivity values calculated using particles are elevated by more than one order of magnitude above the molecular diffusivity, and are shown to agree well with turbulent diffusivities estimated using a generic length scale turbulence closure model. Based on a favourable comparison of DNS results with those of a similar two-dimensional case, we use two-dimensional simulations to extend our cross-shore transport results to additional wave amplitude and bathymetric slope conditions.