Intermediate Nepheloid Layers Research Articles

Particle flux across the mid-European continental margin

Results are presented from particle flux studies using sediment trap and current meter moorings along a transect at the European continental margin at 49°N within the EU-funded Ocean Margin Exchange (OMEX) project. Two moorings were placed, at the mid- and outer slope in water depths of 1500 and 3660 m, with traps at 600 and 1050 m and at 580, 1440 and 3220 m, respectively. Residual currents at the mid-slope follow the slope contour, whereas seasonal off-slope flow was registered at the outer slope. At 600 m on the slope fluxes are similar to those in the abyssal North Atlantic. The flux of all components (bulk dry weight, particulate organic and inorganic carbon, lithogenic matter and opal) increased with water depth. Highest fluxes were recorded at 1440 m at the outer slope, where off-slope residual currents mediate particle export. The injection of biogenic and lithogenic particles below the depth of winter mixing results in the export of particles from shallower waters. Calculated lateral fluxes of particulate organic carbon exceed the primary flux by over a factor of 2 at 1440 m on the outer slope. Estimated lateral fluxes of suspended particulate matter in the water column and intermediate nepheloid layers at the outer slope are potentially large compared to sinking fluxes measured by sediment traps. A comparison is made of particle flux at three continental margin sites and two sites in the adjacent open North Atlantic, from which it is seen that bulk and organic matter flux increases exponentially with proximity to the shelf break. The percentage contribution of particulate organic carbon to biogenic fluxes increases from a mean of 5.7% in the abyssal N. Atlantic to 13.9% at the continental margins.

Slope transport of suspended particulate matter on the Aquitanian margin of the Bay of Biscay

Spatial and temporal characteristics of the water masses and the dispersion of the suspended particulate matter were investigated using current meter, hydrographic and nephelometric observations, gathered during the ECOFER experiment (1989–1991) in the Cap-Ferret Canyon on the Aquitanian margin of the Bay of Biscay. While characteristics of the deep water masses were stable from one year to another, large hydrographic change in the upper 500 m related to winter renewal induced by poleward advection of warm and saline water along the continental slope. The slope circulation and seasonal eddy activity appear as important dynamical mechanisms that control the entrainment and the dispersion of the suspended particulate matter from the neritic domain to the deep ocean. A predominantly northward along-slope current with occasional reversal characterizes this circulation. The nephelometric structures also showed seasonal changes in the overall suspended particulate matter content, but recurrent features, such as the presence of intermediate nepheloid layers at the shelf-break depth and various depths along the slope (∼500, 1000 and 2000 m), were observed. These nepheloid layers extended off the slope to about 10–30 km, but especially laterally along the slope. Their presence indicated that suspended particulate matter exchanges between the shelf and the slope occurred mainly in the head of the canyon and along the southern open slope. The intermediate nepheloid layers around 500 m depth detached from the slope particularly in regions where the bottom slope is close to critical for the M 2 internal tide.

The oxygen minimum zone in the Arabian Sea during 1995

This paper focuses on the characteristics of the oxygen minimum zone (OMZ) as observed in the Arabian Sea over the complete monsoon cycle of 1995. Dissolved oxygen, nitrite, nitrate and density values are used to delineate the OMZ, as well as identify regions where denitrification is observed. The suboxic conditions within the northern Arabian Sea are documented, as well as biological and chemical consequences of this phenomenon. Overall, the conditions found in the suboxic portion of the water column in the Arabian Sea were not greatly different from what has been reported in the literature with respect to oxygen, nitrate and nitrite distributions. Within the main thermocline, portions of the OMZ were found that were suboxic (oxygen less than ∼4.5 μM) and contained secondary nitrite maxima with concentrations that sometimes exceeded 6.0 μM, suggesting active nitrate reduction and denitrification. Although there may have been a reduction in the degree of suboxia during the Southwest monsoon, a dramatic seasonality was not observed, as has been suggested by some previous work. In particular, there was not much evidence for the occurrence of secondary nitrite maxima in waters with oxygen concentrations greater than 4.5 μM. Waters in the northern Arabian Sea appear to accumulate larger nitrate deficits due to longer residence times even though the denitrification rate might be lower, as evident in the reduced nitrite concentrations in the northern part of the basin. Organism distributions showed string relationships to the oxygen profiles, especially in locations where the OMZ was pronounced, but the biological responses to the OMZ varied with type of organism. The regional extent of intermediate nepheloid layers in our data corresponds well with the region of the secondary nitrite maximum. This is a region of denitrification, and the presence and activities of bacteria are assumed to cause the increase in particles. ADCP acoustic backscatter measurements show diel vertical migration of plankton or nekton and movement into the OMZ. Daytime acoustic returns from depth were strong, and the dawn sinking and dusk rise of the fauna were obvious. However, at night the biomass remaining in the suboxic zone was so low that no ADCP signal was detectable at these depths. There are at least two groups of organisms, one that stays in the upper mixed layer and another that makes daily excursions. A subsurface zooplankton peak in the lower OMZ (near the lower 4.5 μM oxycline) was also typically present; these animals occurred day and night and did not vertically migrate.

The pattern of cross-slope depositional fluxes

A simple model with horizontal and vertical diffusivities and settling velocity is used to calculate expected distributions of suspended particulate matter in a section across the continental shelf and slope. Dependencies on the shelf and slope profile, diffusivities, settling velocity, cross-slope advection and boundary sources/sinks are explored. It is found that the strongest factors are relative values of diffusivities and settling velocity, and the distribution of sources and sinks – including bottom deposition or resuspension. The latter is the principal means whereby an increased concentration near the bottom is likely, and is suggested as the usual reason for increased deposition recorded by sediment traps nearer the bottom. Observed thin, near-horizontal intermediate nepheloid layers put bounds on the vertical diffusivity and settling velocity, e.g. O(10 -4 m 2 s -1, 10 -5 m s -1) over Goban Spur in OMEX.

Influence of melting icebergs on distribution, characteristics and transport of marine particles in an East Greenland fjord

A detailed study of distribution of suspended sediments in an East Greenland fjord with a high iceberg production rate reveals the existence of intermediate nepheloid layers (INLs). Observed INLs extended from the head to the mouth of the fjord at water depths between 100 and 400 m. Particulate organic carbon and nitrogen, chlorophyll a, and nutrient measurements and observation by microscope and energy dispersive X‐ray analyses were used to characterize marine particles. Particles in the INLs are composed of glacial flour including >50 μm quartz and feldspar grains with angular and sharp edges, considered to be released from melting icebergs. In situ photographs show large aggregates (>1 mm) at high concentration (>300 particles per liter) in and below INLs. These aggregates, in comparison with the dispersed particle size distribution, demonstrate that smaller size particles (e.g., clay) settle effectively along with larger single size grains. In Kangerlugssuaq Fjord, the mass transport of marine particles was governed by the subsurface iceberg melting, producing observed INLs, rather than the surface meltwater plume. This suggests that the subsurface water temperature controls release of iceberg debris and the existence of warm subsurface water, as well as the spread of cold and fresh water in the surface layer, needs to be considered to evaluate the occurrence of ice‐rafted debris layers, including Heinrich layers. This study provides the field evidence of a modern analogue on ocean conditions that could form iceberg‐rafted layers.

Recent sediments, sediment accumulation and carbon burial at Goban Spur, N.W. European Continental Margin (47–50°N)

To quantify recent sediment accumulation, carbon fluxes and cycling, three N.W. European Continental Margin transects on Goban Spur and Meriadzek Terrace were extensively studied by repeated box- and multicore sampling of bottom sediments. The recent sediment distribution and characteristics appear directly related to the near-bed hydrodynamic regime on the margin, which at the upper slope break on the Goban Spur results in along-slope and periodic off-slope directed transport of particles, possibly by entrainment of particles in a detached bottom or intermediate nepheloid layer. From the shelf to the abyssal plain the surface sediments on the Goban Spur change from terrigenous sandy shelf sediments into clayey silts. 210Pb activity decreases exponentially down core, reaching a stable background value at 10 cm (shallower stations) to 5 cm (deeper stations) sediment depth. 210Pb profiles of repeatedly sampled stations indicate negligible annual variability of mixing and flux. The 210Pb xs flux to the sediment shows a decreasing trend with increasing water depth. Below about 2000 m the average 210Pb xs flux is about 0.3 dpm cm −2 y −1, a third of the fluxes measured on the shelf and upper slope stations. Sediment mixing rates (D b) correlate with macro- and meiofaunal density changes and are within the normal oceanic ranges. Lower mixing rates on the lower slope likely reflect lower organic carbon fluxes there. Mass accumulation rates on Meriadzek Terrace are at maximum 80 g m −2 y −1, almost twice as high as at Goban Spur stations of comparable depth. A minimum accumulation rate of 16.6 g m −2 y −1 is found at the Goban Spur upper slope break. Organic carbon burial rates are low compared to other margins and range from a lowest value of 0.05 g m −2 y −1 at the upper slope break to 0.11 g m −2 y −1 downslope. A maximum organic carbon burial rate of 0.41 g m −2 y −1 is found on Meriadzek Terrace. Carbonate burial rates increase along the northern transect from the shelf (13 g m −2 y −1) via a low (9.3 g m −2 y −1) on the upper slope break to the deep sea (30.7 g m −2 y −1). Carbonate burial is highest on Meriadzek Terrace (44.5 g m −2 y −1). The N.W. European Margin at Goban Spur and Meriadzek Terrace cannot be considered a major carbon depocenter.

Spatial and temporal variability of particulate matter in the benthic boundary layer at the N.W. European Continental Margin (Goban Spur)

Near bottom water samples and sediments were taken during five cruises to 6 stations forming a transect across the N.W. European Continental Margin at Goban Spur. Flow velocity spot measurements in the benthic boundary layer (BBL) always increased from the shelf to the upper slope (1470 m) from 5 to 9 cm s −1 in spring/summer and from 15 to 37 cm s −1 in autumn/winter. Decreasing values were detected at the lower slope (2000 m) and the lowest values of ca. 2 cm s −1 at the continental rise at 4500 m water depth. Long term measurements with a benthic lander at 1470 m show that currents have a tidal component and reach maximum velocities up to 20 cm s −1, sufficiently high periodically to resuspend and transport phytodetritus. During these long-term observations, currents were always weaker in spring/summer than in autumn/winter. Critical shear velocities of shelf/slope sediments increased with depth from 0.5 to 1.7 cm s −1 and major resuspension events and Intermediate Nepheloid Layers (INLs) should occur around 1000 m. Chloroplastic Pigment Equivalents (CPE) ranged from 0.0 to 0.21 μg dm −3, Particulate Organic Carbon (POC) from 12 to 141 μg dm −3 and Total Particulate Matter (TPM) from 0.2 to 10.0 mg dm −3. Aggregates in the BBL occurred with a median diameter of 152 to 468 μm. Data on suspended particulate matter in the near-bottom waters showed that hydrodynamic sorting within the particulate organic fraction occurred. Phytodetritus was packaged in relatively large aggregates and contributed little to the total organic carbon pool in nearbottom waters (CPE/POC ca.0.2%). The main organic fraction has low settling velocities and high residence times within the benthic boundary layer. As POC was not concentrated in the near bed region the degree to which carbon is accessible to the benthic community depends on aggregate formation, subsequent settling and/or biodeposition of the POC. Close to the sea bed downslope transport may dominate. Under flow conditions high enough to resuspend fresh phythodetritus from sediments at the productive shelf edge, this could be transported to 1500 m (Goban Spur) or abyssal depth (Canyon site between Meriadzek and Goban Spur) within 21 days.

Nepheloid structure and hydrographic control on the Barcelona continental margin, northwestern Mediterranean

Contemporary processes controlling the off-shelf transport of sediment particles on the Barcelona continental margin were identified using CTD/transmissometer profiles during three hydrographic surveys. A well defined pattern of particulate matter distribution consisting in surface, intermediate and near-bottom nepheloid layers related with topographic and hydrographic structures was observed. Suspended sediment supplied by rivers or resuspended on the shelf is transferred to the slope mainly through the bottom nepheloid layer. A detachment of this layer is produced at the shelf break generating an intermediate nepheloid layer that transports sediment from the shelf to the slope. This transport is observed along the whole shelf break, although the maximum transference takes place through the Foix submarine canyon. On the slope, an increase of particulate matter concentration has always been recorded along the 300–500 m isobaths associated with a permanent intermediate nepheloid layer. This slope nepheloid structure is controlled by the slope density front dynamics and by the associated geostrophic current. This pattern of particulate matter distribution had not been observed before in the study area, and shows some peculiarities different from any other continental margin that has been studied.

Direct measurements of carbon and carbonate export from a coral reef ecosystem (Moorea Island , French Polynesia)

The export of carbon and carbonate from coral reefs was investigated through a multidisciplinary investigation of the hydrological, geochemical, sedimentological and biological features of Tiahura reef on the northwestern coast of Moorea Island (French Polynesia). The hydrology of the fore-reef is characterised by prevailing longshore western currents and a strong thermocline. As revealed by turbidity structures (benthic and intermediate nepheloid layers) and by the amount of particles collected by near-bottom sediment traps, horizontal and downslope advections of particles dominate over offshore vertical transport. The exported material is rich in carbonate (ca. 80%) and poor in organic matter (ca. 4%). Sedimentation rates at 430 m depth, i.e. definitive export, reached 209.6 mg m-2 d-1 (dry weight). Estimates of carbon and carbonates export for Tiahura reef also reported here represent respectively 47% and 21% of the organic and inorganic carbon produced within the reef.

Benthic and Intermediate Nepheloid Layers in Lake Biwa

Benthic nepheloid layers and intermediate nepheloid layers in Lake Biwa were observed between 1991 and 1993, and the characteristics of both nepheloid layers are discussed using field data from Pre-BITEX in 1991-1993 and BITEX'93 in 1993.The benthic nepheloid layer in Lake Biwa is formed during a stratified period from May to October. The top edge of the benthic nepheloid layer is very sharp as if it is a step function, and turbidity is higher in the benthic nepheloid layer than outside. The vertical distribution of turbidity in the benthic boundary nepheloid layer does not fit to the Rouse distribution. We showed that internal waves induce the oscillating flows in the benthic boundary layer, and it makes such distributions of suspended sediments as a step function.The intermediate nepheloid layer in Lake Biwa was also discovered during Pre-BITEX and BITEX'93. The intermediate nepheloid layer observed during BITEX moved in one direction with a continuously horizontal velocity of 2.0 to 4.0 cm⋅sec-1, for almost 8 hrs. The intermediate nepheloid layer was about 800 m long and 5 m thick, and it brings suspended sediments rich in Mn.By comparing these physical data with the results of chemical analyses, we can verify that the material in nepheloid layers originated from bottom sediments.

An intermittent, intermediate nepheloid layer in Emerald Basin, Scotian Shelf

Intermittent, intermediate nepheloid layers were observed on the Scotian Shelf in late April, 1987. These 10–30 m thick nepheloid layers were found between 140 and 200 m in the main part of Emerald Basin and 120–140 m in the northern basin, well below the surface mixed layer and about 90 m above the sea bed in both cases. Both biological and physical considerations are used to assess mechanisms for the observed intermediate nepheloid layers. Depths of intermediate nepheloid layers coincide with the critical depth for possible generation and amplification of internal waves with semi-diurnal ( M 2) internal tidal frequency. Intermittent particle resuspension at the “critical” depth on the Basin slope appears to be the likely cause of the observed intermittency of the intermediate nepheloid layers. Observations of intermediate nepheloid layers at the three stations in the main part of Emerald Basin and the northern basin indicate that this process is not a rare event, but rather a basin-wide phenomenon.

Denitrification processes in the Arabian Sea

Recent information on some consequences of the acute mid-water oxygen deficiency in the Arabian Sea, especially on carbon-nitrogen cycling, is reviewed. An evaluation of published estimates of water column denitrification rate suggests an overall rate in the vicinity of 30Tg Ny-1, but the extent of benthic contribution remains unknown. A decoupling of denitrification from primary production, unique to the Arabian Sea, is revealed by nitrite, electron transport system (ETS) activity and bacterial production data. Results of both enzymatic and microbiological investigations strongly point to a major role of organic carbon other than that sinking from surface layers in supporting denitrification. Although denitrification is associated with an intermediate nepheloid layer, it seems unlikely that the excess carbon comes with particles re-suspended along the continental margins and transported quasi-horizontally into the ocean interior; instead, the particle maximum may directly reflect a higher bacterial abundance. It is proposed that denitrification may be predominantly fuelled by the dissolved organic matter.

An intermediate nepheloid layer associated with high microbial metabolic rates and denitrification in the northwest Indian Ocean

Extensive optical, physical, chemical, and biochemical measurements made simultaneously in the northwest Indian Ocean reveal the occurrence of an intermediate nepheloid layer (INL) invariably associated with the secondary nitrite maximum. Maxima in particulate protein and the activity of the respiratory electron transport system (ETS) are also found within the INL. Since the INL persists long distances from the continental margin with an offshore intensification, it may not be related to the transport of material resuspended along the continental margin. An apparent correlation of the INL with the previously reported subsurface maximum in bacterial abundance suggests that a local increase in the abundance of bacteria may be responsible for the increased turbidity. Positive correlations of the beam attenuation anomaly with nitrite and nitrate deficit suggest that most of these bacteria may be denitrifiers. The organic carbon demand within the denitrifying layer, computed from the observed ETS activity, appears to be severalfold higher than the sinking carbon fluxes to the denitrifying layer, requiring additional modes of supply of the biodegradable organic matter. It is proposed that a bacterial maximum could be maintained with efficient utilization of the dissolved organic matter within the denitrifying waters.

A deep-ocean nephelometer to detect botton and intermediate nepheloid layers

A deep nephelometer has been developed to obtain vertical profiles of scattered light in the oceans. It was designed to study Bottom and Intermediate Nepheloid layers encountered in deep ocean. It is an adaptation of a shallow water device previously built for euphotic layer studies. The instrument and its calibration are described; results from different applications are given.

Seasonality and composition of particulate fluxes during ECOMARGE—I, western Gulf of Lions

As part of the ECOMARGE program (ECOsystèmes de MARGE continentale), sequential sediment traps were deployed on the continental margin of the Gulf of Lions, in the northwestern Mediterranean Sea. Two sites located in the southwestern part of this region were selected for the first phase of the experiment: inner shelf (bottom depth: 27 m) and continental slope (Lacaze-Duthiers Canyon, bottom depth: 650 m). The canyon was selected as representative of the continental slope, because canyons comprise more than 50% of the slope area. Total mass, organic matter, opal, carbonate and siliciclastic residue fluxes were measured biweekly at 50, 100, 300 and 600 m in the upper part of the Lacaze-Duthiers Canyon (from July 1985 to April 1986) and for longer periods (2 weeks to 2 months) at 10 and 25 m on the shelf (from May 1985 to June 1986). Mass fluxes increased generally with depth, reaching values as high as 20,000 mg m −2 d −1 in the 600 m slope (canyon) trap; this indicated lateral transport of biogenic and abiogenic particulate matter, from local (adjacent shelf and upper slope waters) and distant origin (Rhoˆne River). Shelf-slope particulate transfer attested by the paracontemporaneity of high flux events on the shelf and the slope and by the continuity of the shelf benthic nepheloid layer and intermediate nepheloid layers over the canyon: this rapid and took generally less than 16 days (trap sample resolution time). Particle fluxes and compositions defined a layered system, in which the surface layers< 100m) were essentially characterized by biogenic material (organic matter and opal), and deep layers chiefly characterized by the collection of elastic material (carbonate and siliciclastic material). Significant temporal increases in total mass and constituent fluxes showed two frequencies: a low seasonal frequency, which was related to seasonal variations of the Liguro-Provençal Current and to the winter increase of the Rhoˆne River and other coastal river discharges; variations at higher frequencies, which were related to various impulse events such as summer internal waves, autumn and winter storms and spring nutrient enrichment. The role of the Lacaze-Duthiers Canyon in the particulate transfer across the continental margin is discussed within the particular context of this advective system. Although the importance of local, intra-canyon resuspension of bottom sediments could not be estimated, it seems that the high suspended particulate matter concentrations and particle fluxes observed are essentially related to the fact that this canyon acts as a natural trap which collects particles from the entire Gulf of Lions. Comparison of the flux necessary to sustain the unsupported 210Pb inventory in the canyon sediments, with the mean 210Pb flux measured by traps and of 210Pb-determined mass accumulation rates with total mass fluxes indicate that the canyon partly acts as a modern sediment depocenter of the particles swept into it from the shelf and the slope; also partly as a modern conduit of particles to the deep basin. Several biological mechanisms are probably responsible for the rapid settling of particles onto the slope (within the canyon); these may, thereby, reduce lateral transfer from the shelf to the open ocean and constitute a kind of “biological barrier”.

Sediment dispersal and accumulation on the continental margin of the Gulf of Lions: sedimentary budget

An integrated study of particle transport, deposition and storage was carried out over the continental margin of the Gulf of Lions (northwestern Mediterranean Sea). The dispersal system of fine-grained sediment is discussed and a sedimentary budget of sediment transfer and accumulation is established. Sediment cores were collected from the continental shelf and slope, along the ECOMARGE—I transect. Accumulation rates were measured using radiochemical techniques. Sediment traps were deployed, from 1985 to 1986, at two sites: on the continental shelf (27 m) and in the Lacaze-Duthiers Canyon (645 m). This study revealed that the most important sedimentary processes on this margin are high seasonal variability of lithogenic particle fluxes and lateral advection. The major fluctuations are related to storm events, to an increase in river discharge and the intensity of the regional current. Flux gradients increase exponentially toward the bottom, in shallow and deep water. However, in the deeper traps at the two sites annual lithogenic fluxes decrease seaward, from 25 to 3.5 g m −2 day −1. Across this margin, sediment accumulation rates ranged from 2.9 g m −2 day −1, on the middle of the prodeltaic area, to 0.16 g m −2 day −1 at 645 m in the canyon. Smectite was found to be a natural tracer for characterizing an annual cycle of sediment input, deposition, resuspension and shelf-slope exchange processes. At the shelf site, two intra-annual cycles of sedimentation and erosion of the surface sediments were recognized. Comparison between trap deposition rates and bottom sediment accumulation rates implies that the prodelta area acts as a zone of storage (11%) for the silty material and as a temporary sink for fine-grained sediment, that is eroded from the area in winter. In the canyon, variations in deposition rates, smectite content, and grain size are related to morphological features and the various modes of transport that control sedimentation. The discontinuity in the flux gradient between surface and deep layers, and the discrepancy between long-term accumulation rates based on 14C dating and trap deposition rates suggest that fluxes in the deeper layer in the canyon are in fact essentially controlled by settling from the intermediate nepheloid layers detached from the shelf break and by along-bottom transport; also that resuspension has a longterm impact on the entire canyon maintenance. On the other hand, comparing near-bottom trap 210Pb fluxes to the fluxes required to support the excess 210Pb inventories in the sediments leads to the conclusion that much (70%) of the particulate flux passes through the canyon to the deep sea on this short time scale (< 100 years).

Distribution of macroaggregates and fine-grained particles across a continental margin and their potential role in fluxes

The distribution and size of large marine aggregates (;0.5 mm) photographed in situ were compared with the small-particle distribution determined from beam attenuation (660 mm) in a section across the shelf and slope of the northern Gulf of Mexico. On the shelf changes in beam attenuation matched the trends in large particle concentration. Over the slope, beam attenuation profiles resembled the aggregates profiles in the upper 400 m with a small surface mixed layer maximum, a decrease through the upper thermocline to a minimum, and a small intermediate nepheloid layer (INL) in both large and small particles between 300 and 400 m. At the two deepest stations beam attenuation remained low from the bottom of the INL to the seafloor, while concentrations of large particles increased through the lower water column to concentrations greater than in the surface water. The distributions suggest that both large and small particles are resuspended from the upper slope and advected seaward, but on the mid slope this mode of transport occurs mostly as large aggregates. From calculations of settling speeds based on estimates of particle density, it appears that these aggregates are important in both the horizontal and vertical flux of particles along continental margins.

Suspended particle and bacterial maxima in Peruvian coastal waters during a cold water anomaly

Simultaneous optical, biological and chemical analyses of coastal waters off Peru were made during a period characterized by anomalously cold surface waters and weak wind-driven coastal upwelling. Particle size distributions and the microbial and chemical nature of the intermediate nepheloid layers provide strong evidence that bacteriol growth, settling and offshore transport of particles are major processes controlling the particulate structure of the nearshore waters. The data also support previous suggestions that mid-depth maxima in suspended particles associated with nitrite maxima have a large bacterial component. Further, these results demonstrate the effectiveness of in situ optical methods for detection and quantification of the bacterial component of particle size distributions. While some features were similar to the particulate structures observed previously in Peruvian coastal waters, the data show the region to have significant temporal and spatial variability.

A deep intermediate nepheloid layer

A weak, but very large, intermediate nepheloid layer has been observed on two occasions 8 months apart at a depth of about 2550 m adjacent to the continental slope off the Porcupine Bank. It extended along the slope for over 100 km with no trend in its intensity, and offslope to about 16 km or about 1.4 times the internal Rossby radius. Further offslope, maximum light scattering was found near the depths at which the M 2 internal tide is “critical” on the slope. It is suggested that whilst transient nepheloid layers near the slope may temporarily dominate the nephel “signal”, the pattern of light scattering far from the slope is determined by more persistent processes such as the result of sediment erosion by the larger bottom currents associated with “critical” internal waves.

Transport of mud on continental shelves: Evidence from the Texas Shelf

Transmissivity and suspended-sediment data from three cruises on the East Texas continental shelf confirm that a bottom nepheloid layer (BNL) exists across the shelf; this layer is most turbid in the nearshore and outer shelf regions. In addition, surface nepheloid layers exist close to shore due to river and bay outflows, and on the outer shelf intermediate nepheloid layers are present as a result of quasi-horizontal advection from the BNL. The character of the BNL is not directly linked to local hydrography or substrate, being instead a function of the shelf dynamics. In the nearshore regime high-energy coastal processes and high sediment input from bays and rivers maintain the BNL. At the shelf edge, the BNL is formed by near-inertial internal waves and shelf waves which resuspend and transport fine sediment. Present-day sediment deposition on the East Texas Shelf can be directly related to the above processes. Mud is accumulating in the nearshore environment, due to river input, and on the inner and midshelf regions as a result of deposition from BNLs and SNLs. On the outer shelf, BNLs and INLs transport fine sediment further offshore, so that in effect suspended sediment bypasses the outer shelf.