Bottom Nepheloid Layer Research Articles

Coccolithophore fluxes in the Norwegian-Greenland Sea: Seasonality and assemblage alterations

Coccolithophore fluxes were investigated by sediment trap studies at two well separated oceanographic sites in the Norwegian and Greenland seas from 1990 to 1992. Each trap mooring comprised traps at 500 and 1000 m water depth and 300 m above the seafloor. Both sites were characterized by a strong seasonality in coccolithophore fluxes. In the Norwegian Sea fluxes were about 10 to 20 times higher than in the Greenland Sea. Maximum fluxes (13 × 10 6 ind. m −2d −1 for the Norwegian Sea and 2.4 × 10 6 ind. m −2d −1 for the Greenland Sea) were reached during high sedimentation phases in late summer and autumn. The settling assemblages represented already highly altered remnants of the former living communities. Dominant species were Emiliania huxleyi in the Norwegian Sea and Coccolithus pelagicus in the Greenland Sea. Despite severe alterations the coccolithophore assemblages of each site were characterized by a distinct signature reflecting the local oceanography. The settling assemblages underwent only minor alterations during sinking from 500 to 1000 m water depth. By contrast, resuspension and lateral advection within an extensive bottom nepheloid layer strongly influenced the assemblages of the deep sediment traps 300 m above the seafloor, wiping out the distinct seasonality in coccolithophore fluxes and diminishing the differences in assemblage compositions between the two sites.

Interactions of mesoscale features with Texas shelf and slope waters

During April and May 1993, the typical south and southwestward directed flow over the Texas shelf interacted with current rings located just off the Texas shelf in the Gulf of Mexico. The rings, a cyclone/anticyclone eddy pair, were oriented such that water was transported onto the shelf between the rings and water was drawn from the shelf on the rings' offshore-directed limbs. A shelf convergence zone transported water to the shelf edge where some of the water became entrained between the cyclone/anticyclone pair. Active shelf-slope exchange such as that seen in the April-May 1993 episode may be a more typical example of synoptic shelf circulation than the general circulation pattern suggested by averaging multiannual observations. Nutrient and particle distributions were influenced by the interactions of the shelf current with the rings. Where the cyclonic and anticyclonic eddies directed flow off the shelf, particles were transported off the shelf. Additionally, particles in the bottom nepheloid layer were transported off the shelf in the shoreward limb of the cyclone, even though currents in this limb were directed along the isobaths. Nutrient concentrations, hence nutrient fluxes, were generally low in near-surface waters every-where in the study area. At mid-depth and in near-bottom waters, nutrient fluxes were highest near the cyclone because of higher current speeds in this feature and higher nutrient concentrations at depth.

Transfer mechanisms and biogeochemical implications in the bottom nepheloid layer. A case study of the coastal zone off the Rhone River (France)

Particles supplied to the coastal zone are involved in numerous biogeochemical processes that rapidly modify particulate composition. The Bottom Nepheloid Layer (BNL) thus plays a significant role on the budget of elements transferred to the coastal zone. Although it should not be ignored in flux studies, little is known about the quantity and the quality of particles transported within it, mainly due to sampling difficulties. From field experiments in the coastal zone off the Rhone River, the physical mechanisms occurring within the BNL were delineated. Its temporal variability was then examined, regarding the influence of wind, river discharge and current velocity, in order to investigate the temporal scales that control the BNL's character and the processes occurring within it. In the BNL off the Rhone, particulate transfer can be as important, quantitatively, as in the river plume, and larger by far than within the water column.

Recent sediment accumulation, organic carbon burial and transport in the northeastern North Sea

Organic carbon and nitrogen and sedimentation rate determinations were made of boxcores from the Norwegian Channel, North Sea. The geographical distribution of recent sedimentation areas were defined by analysis of published data and 3.5 kHz penetrating echo sounder data. The annual dry bulk sediment accumulation in the northeastern North Sea is established at 74 × 10 6 tons. As the average organic carbon content for the Norwegian Channel and Skagerrak/Kattegat sediments is 0.6 and 1.8%, respectively, the organic carbon accumulation rate in the northeastern North Sea could be calculated. The total organic carbon accumulation in the Norwegian Channel and Skagerrak/northern Kattegat is 0.17 × 10 6 tons · yr −1 and 0.83 × 10 6 tons · yr −1, respectively. Less than 10% of this is accounted for by local primary production. This means that more than 90% of the organic carbon buried in the sediments of the northeastern North Sea must have been supplied from different sources. Terrigenous sources supply 20% of the organic carbon. The remainder is marine organic matter produced elsewhere in the North Sea, or imported from the Atlantic Ocean, Norwegian Sea and the Baltic Sea. Storm wave and -current induced (near) bottom nepheloid layers are the main likely mechanism to transport fine grained sediments and associated organic matter from the North Sea plateau into the Kattegat, Skagerrak and Norwegian Channel.

Cycling of high‐molecular‐weight dissolved organic matter in the Middle Atlantic Bight as revealed by carbon isotopic (13C and 14C) signatures

Carbon isotopes (13C and 14C) and elemental composition (C and N) in two fractions of colloidal organic matter (COM) were measured to study the origin and cycling of dissolved organic matter (DOM) in the Middle Atlantic Bight (MAB). COM1 (1 kDa–0.2 µm) was 59% of the bulk DOM in surface Chesapeake Bay waters and decreased to 30–35% in waters of the MAB. COM10 (10 kDa–0.2 µm), which was the high‐molecular‐weight (HMW) component of COM1, comprised 3–12% of the bulk DOM, with highest concentrations in Chesapeake Bay waters and the lowest in deep waters in the MAB. Δ14C values of COM1 decreased from nearshore (‒21 to +12‰) to offshore and from surface (‒166 to ‒85‰) to bottom waters (‒400 to ‒304‰). Although Δ14C values of surface‐water HMW COM10 were generally high (‒2 to ‒7‰), values for bottom‐water COM10 were much lower (‒129 to ‒709‰). The high Δ14C values in the surface water suggest a particulate origin of pelagic COM, consistent with the contemporary Δ14C values of particulate organic matter (POM). The very low Δ14C values of bottom‐water COM10 imply that in addition to the pelagic origin, sedimentary organic C may serve as an important source for the benthic colloids in the bottom nepheloid layer. The general flow direction of organic carbon is from POM to HMW and to LMW DOM. Three colloidal end‐members were identified in the MAB as well as in the Gulf of Mexico: estuarine colloids with high Δ14C values, high C : N ratios, and lower δ13C values; offshore surface water colloids with intermediate Δ14C values, lower C : N ratios, and higher δ13C values; and offshore deep‐water colloids with low Δ14C values, intermediate C : N ratios, and variable δ13C values.

Reactive trace metals in the stratified central North Pacific

Vertical concentration profiles of the dissolved and suspended particulate phases were determined for a suite of reactive trace metals, Al, Fe, Mn, Zn, and Cd, during summertime at a station in the center of the North Pacific gyre. During summer the euphotic zone becomes stratified, forming a shallow (0–25 m), oligotrophic, mixed layer overlying a subsurface (25–140 m), strongly-stratified region. The physical, biological, and chemical structure within the euphotic zone during this period enhanced the effect of atmospheric inputs of Al, Fe, and Mn on mixed layer concentrations. For example, the concentration of dissolved Fe in the surface mixed layer was eighteen times that observed at a depth of 100 m. The observed aeolian signature of these metals matched that predicted from estimates of atmospheric input during the period between the onset of stratification and sampling. The distributions of suspended particulate Al, Fe, and Mn all exhibited minima in the euphotic zone and increased with depth into the main thermocline. Particulate Al and Fe were then uniform with depth below 1000 m before increasing in the near bottom nepheloid layer. Average particulate phase concentrations in intermediate and deep waters of the central North Pacific were 1.0, 0.31, and 0.055 nmol · kg −1 for Al, Fe, and Mn, respectively. The distribution of particulate Cd exhibited a maximum within the subsurface euphotic zone. Particulate zinc also exhibited a surface maximum, albeit a smaller one. Concentrations of particulate Zn and Cd in intermediate and deep waters were 17 and 0.2 pmol·kg −1. Substantial interbasin differences in particulate trace metals occur. Concentrations of suspended particulate Al, Fe, and Mn were three to four times lower in the central North Pacific than recently reported for the central North Atlantic gyre, consistent with differences in atmospheric input to these two regions. Concentrations of suspended particulate Cd and Zn were enriched in the North Pacific relative to the North Atlantic, an observation consistent with their assimilation by plankton. Reactive trace metals exhibit a range of biogeochemical behaviors that can be characterized by two endmembers, nutrient-type and scavenged-type. Nutrient-type metals, best exemplified by Zn and Cd, are primarily removed from surface waters by biogenic particles and then remineralized at depth. Internal biogeochemical cycles together with physical mixing and circulation patterns control the distributions of nutrient-type metals. Scavenged-type metals, best exemplified by Al, continue to be removed onto particles in intermediate and deep waters as well as at the surface. External inputs, such as the deposition of aeolian dust, control the concentrations and distributions of scavenged-type metals. Other metals, such as Fe, exhibit a mixture of the characteristic behaviors of these two endmembers.

Characteristics of the near-bottom suspended sediment field over the continental shelf off northern California based on optical attenuation measurements during STRESS and SMILE

Time-series measurements of current velocity, optical attenuation and surface wave intensity obtained during the Sediment Transport Events on Shelves and Slopes (STRESS) experiments, combined with shipboard measurements of conductivity, temperature and optical attenuation obtained during the Shelf Mixed Layer Experiment (SMILE), provide a description of the sediment concentration field over the central and outer shelf off northern California. The questions addressed are: (1) existence and characteristics of bottom nepheloid layers and their relationship to bottom mixed layers; (2) characteristics of temporal fluctuations in sediment concentration and their relationship to waves and currents; (3) spatial scales over which suspended sediment concentrations vary horizontally; and (4) vertical distribution of suspended sediment.

Time series of the vertical distribution of particles during and after a spring phytoplankton bloom in a coastal basin

Bottom nepheloid layer formation was observed through a time series study of the vertical distribution of suspended particles conducted during and after a phytoplankton bloom in a small coastal basin, Bedford Basin, Nova Scotia, Canada. In this study, we measured the suspended particle distribution over a 6 month period using a backward-recording (30°) light-scattering meter. Although the profiles of the particle distribution reflected episodic events such as fresh water river input and storm mixing, through the period of the study three general stages were observed. During the peak of the spring bloom in March, the water column was characterized by a high particle load in the surface mixed layer (0–8 m) with a smaller but uniform particle load in the middle and bottom water. The second stage (mid-April–July) featured bottom nepheloid layer formation (45–65 m) and a high particle load in the surface water with minimum light-scatter at mid-depth. In late July–August and bottom nephloid layer thinned and the mid-water became progressively clearer. The light-scatter measurements revealed a patchiness in the vertical particle distribution. We collected water samples from some of the depths corresponding to light-scatter peaks for measurements of particulate organic carbon (POC) concentration and mass-volume concentration of suspended matter (SPM), and for microscope observation to identify particle characteristics. These observations demonstrated a change of particle characteristics with depth as well as with time. As a bottom nepheloid layer developed, particles in this layer changed from predominantly intact diatom chain assemblages with high POC concentrations to small and degraded aggregates with low POC concentration.

Ultrafiltration of seawater with a zirconium and aluminum oxide tubular membrane: application to the study of colloidal organic carbon distribution in an estuarine bottom nepheloid layer

For separation of organic colloids in fresh and saline waters, a cross-flow ultrafiltration system was built with a new type of membrane made of zirconium and aluminum oxides. This membrane had a tubular design (16 cm long, 4 cm wide) and a porosity of 10 nm. The suitability of the system for organic carbon studies was first tested by laboratory experiments. In the conditions determined, we obtained acceptable values of dissolved organic carbon (DOC) blank (less than 4 μmol Cl−1) and minimal material adsorption on the membrane. Our special unit was applied to seawater samples collected from the bottom nepheloid layers (BNL) off the Rhone Delta (France). Material defined as colloidal matter in the size range of 0.01–0.7 μm was isolated by filtration on GF/F filters, followed by ultrafiltration. Organic carbon was measured in the so-called dissolved (DOC 0.7 μm) fractions. COC displayed a wide range of concentrations, from 7 μmol Cl−1 (COC/DOC = 7%) offshore, up to 88 μmol Cl−1 (COC/DOC = 49%) near the mouth, whereas truly dissolved carbon remains constant (tDOC = 89 ± 10 μmol Cl−1). In fact, a strong negative correlation was found between COC concentrations near the sediment and the distance from the mouth, indicating that in the area investigated, the major part of the colloidal material does not reach the open ocean. Furthermore, COC concentrations were positively correlated with POC concentrations, suggesting a particulate origin of colloids in the Rhone BNL.

Molecular characterization of the organic fraction of suspended matter in the surface waters and bottom nepheloid layer of the Rhone delta using analytical pyrolysis

Curie Point-pyrolysis-gas chromatography (CuPy-GC) and Curie Point-pyrolusis-gas chromatography-mass spectrometry (CuPy-GC-MS) were applied to characterize the macromolecular content of the suspended particles in the surface waters and benthic nepheloid layer of the Rhone delta. The chromatogram of the pyrolysate of the Rhone River particles revealed a low pyrolysis yield from the riverine material in which polysaccharides and lipid-derived substances prevailed. The absence of levoglucosan and other pyrolysis products related to cellulose suggested that no intact polysaccharides were present. Lignin-derived products were virtually absent. In the salinity gradient, a wide variety of products, including saturated and monounsaturated acids, phytadienes, n-alkylnitriles and pyrolysis products from proteins were determined, indicating a major contribution from freshly produced autochthonous material. A suite of dipeptides of bacterial origin was also identified. Lignin-derived products from terrigenous sources were minor. Further offshore qualitative differences, with respect to the previous samples were apparent. Polysaccharides were less pronounced, possibly due to the dilution of the suspended load of the waters, and/or the microbial consumption of these readily degradable compounds. In contrast, the relative abundances of autochthonously derived compounds increased as a result of nutrient inputs from the Rhone River which fertilize coastal waters. The occurrence of 1,1,3,3,5,5, hexamethylcyclotrioxane as well as styrene provided indications of anthropogenic inputs to the site. The macromolecular constituents of suspended solids in the benthic nepheloid layer strikingly resembled those of the riverine material. Polysaccharides together with phytadienes and C 14, C 16 and C 18 acids accounted for the major pyrolysis products. The persistence of this fingerprint in the benthic layer was observed from the mouth to stations ZD1 and ZA7. Beyond this point, due to the influence of the Liguro-Provencal current flowing westwards, the composition of the pyrolysates changed towards a marine signature. Flocculation of suspended matter in which polysaccharides would make particles stick together or salt flocculation were proposed as an alternative scenarios to explain the formation of the nepheloid layer.

Chemical Characterization of Suspended Particulate Organic Matter by Pyrolysis-Gas Chromatography Coupled with Mass Spectrometry and High Performance Liquid Chromatography in the Bottom Nepheloid Layer of the Rhône Delta

During the DYPOL-6 Mission in the Rhône delta the use of a metallic frame provided water samples at three levels of the underlying water column, 2, 1, and 0·5 m above the sea bottom. These sampling levels allowed determination of the characteristics of suspended organic material at the deeper layers of the bottom nepheloid layer and, eventually, their relations with the overlying suspended particulate matter.The results obtained by the measurement of the suspended particulate organic matter, the elemental analysis, the analysis of the main classes of organic compounds by pyrolysis-gas chromatography coupled with mass spectrometry and the determination of phenolic compounds by high performance liquid chromatography (HPLC) emphasized the functional characteristics of the Rhône deltaic system.Near the bottom, the lower water levels of the benthic nepheloid layer showed some obvious biogeochemical gradients related to the location of the sampling station: the amount of pyrolysis-derived phenolic compounds, aromatic hydrocarbons and carbohydrates in the suspended material was high at the stations directly exposed to input of the Rhône River. The stations from the river mouth to offshore areas showed the influence of the Rhône inflow. But sometimes, according to the river regime and the current orientation, the influence of the Liguro-provencal current oriented East-West was noticeable even near the river mouth. In the eastern region of the surveyed area, the stations submitted to this current indicated some specific characteristics: the nepheloid layer, concentrated 1 m above the bottom, had a high particulate organic carbon content and a relatively high amount of phenolic compounds which indicated a possible seagrass (Posidonia oceanica) origin. At the more seaward stations, although the influence of the river input was always detectable, the suspended particulate matter was less abundant and essentially supplied by the local biogenic input: the lower water levels were enriched in compounds yielding acetonitrile upon pyrolysis, i.e. nitrogen-containing compounds.If the proximity of the river source and regime influenced the suspended particulate matter distribution and the geochemical characteristics of organic matter, the evolution through time of the bottom turbid flow from one station to another also had a great influence.Near the bottom, the terrestrial character is shown in the shoreward stations by high contents of pyrolysis-derived aromatic hydrocarbons and phenols as well as by lignin-derived phenolic compounds analysed by HPLC. In contrast, a marine character appeared in the seaward stations, emphasized by high values of pyrolysis-derived compounds indicative of carbohydrates and nitrogenous compounds as well as by phenolic compounds representative of phytoplankton production. However the upper water layers of the bottom nepheloid were generally more influenced by terrestrial input.Some trends emphasizing a microstratification of the water layers in the deeper levels of the bottom nepheloid, related to the existence of very thin bottom currents, or, maybe, to resuspension processes resulting from the bottom proximity, were shown by the geochemical characteristics of the three water levels analysed.

Sediment transport, resuspension and accumulation rates in the northeastern Skagerrak

Results of watercolumn light transmission measurements in the Skagerrak and of 210Pb excess and 137Cs determinations in boxcore sediments of the eastern Skagerrak are combined to study regional sediment transport, resuspension and sedimentation patterns and rates. In the upper water layer a 20 m thick surface nepheloid layer formed by concentration of particles at the thermocline and pycnocline is best developed in the eastern Skagerrak and less well in the southwestern Skagerrak off the Danish Coast. Bottom nepheloid layers are extensively developed in the southeastern Skagerrak and indicate additional sediment input via resuspension in shallow water areas and near bottom sediment transport in suspension following the prevailing current regime. Locally the transmission in the BNL is reduced by more than 50%, compared to clear water transmission values between 85 and 90%. 210Pb and 137Cs derived sedimentation rates are highest in the eastern and northeastern Skagerrak, and least in the central deep basin. There is generally a good agreement between 137Cs and 210Pb data, indicating that sediment accumulation on the short (∼ 30 year) and longer (∼ 100 year) time scale is of comparable magnitude.

Sediment budget and transfer of suspended sediment from the Gironde estuary to Cap Ferret Canyon

A study of the particulate organic carbon content, the grain size of suspended sediment and turbidity in a transect between the Gironde estuary and Cap Ferret Canyon indicates that particles from the Gironde river are transferred to near the bottom of the canyon. The separation of turbid estuarine water into bottom and surface nepheloid layers takes place in the Gironde itself and in the nearshore zone. The development of these two separate turbid layers is due to vertical (a turbid surface plume) and horizontal (bottom layer) salinity gradients. The bottom nepheloid layer is caused primary by a horizontal supply of suspended sediment from the Gironde rather than from resuspension or vertical sedimentation as suggested by other workers. On the median and outer shelf the distribution of particles in the water column is influenced by thermal stratification. The thickness of the bottom nepheloid layer increases from the estuary towards Cap Ferret Canyon as a function of the thickness of the bottom waters. Detached nepheloid layers are observed at the shelf break coinciding with isopycnal surfaces due to the presence of cold waters in the Cap Ferret Canyon. Superimposed advective inputs from the northern part of the shelf and resuspension of particles from mud patches contribute to the supply of particles towards the canyon. Suspended sediment in transit over the shelf break represents 2–3% of the total discharge from the Gironde estuary. This is a considerable amount as 50% of the suspended sediment is trapped in the west Gironde mud patch and another undefined portion of the Gironde sediment load is transported along the coast by tidal currents.

Circulation and distribution of suspended matter in the Sporades basin (northwestern Aegean sea)

A hydrographic survey, comprising 85 stations with measurements of temperature, salinity and turbidity, was performed in the semi-enclosed margin of the Sporades Basin (northwestern Aegean Sea) in June 1987. The data have been used to investigate the shelf-slope suspended matter exchanges in relation to the hydrology. The circulation, derived from hydrographic data, produces complex eddy patterns. The suspended matter distribution, determined from light scattering measurements, indicate a relationship between the suspended matter and the density-related flow field. Transport of suspended matter, supplied by rivers, is traced through the bottom nepheloid layer, mostly over the inner shelf and along the western coastline. There is however some evidence of transfer of suspended matter from the shelf towards the upper slope. The mid-depth nepheloid layer along the slope is thought to relate to advection of terrigenous material by topographically-controlled currents, and its seaward extension is inhibited by a strong anticyclonic circulation in the deep basin.

The trace metal composition of suspended particles in the oceanic water column near Bermuda

Trace metal partitioning between seawater and particles, both suspended and sinking, must be determined to understand and model oceanic trace metal scavenging processes. In order to improve the oceanic particulate trace metal data base, a new in-situ pump was developed and deployed to collect suspended particles from 0–4000 m in the Sargasso Sea near Bermuda. These particles were analyzed for Al, Fe, Mn, Co, Zn, Cu, Ni, Cd and Pb; major particulate carrier phases were estimated from Ca, opaline Si, and P. Cd is the only trace element of this group with a particulate maximum in near-surface waters. The other metals have low particulate concentrations (mol/l) in near-surface waters, and increase with depth into the upper thermocline. Particulate Al and Fe are then uniform with depth below about 1000 m until increasing in the bottom nepheloid layer. The remaining elements (Mn, Co, Zn, Cu, Ni and Pb) decrease from the mid-thermocline values to lower and relatively uniform concentrations in deep waters. The similarities among the vertical profiles for these metals suggest that authigenic Mn oxides influence their uptake from the dissolved pool, although an important or dominant role for other host phases, specifically particulate organic matter, is not ruled out by the data. Mean intermediate and deep water concentrations are: Al, 3600; Fe, 1200; Mn, 146; Co, 1.5; Zn, 7.4; Cu, 15; Ni, 6.0; Cd, 0.06; and Pb, 2.1 pmol/l. These particulate Al and Mn values agree with previous measurements in this region, but the other elements are lower by a factor of 3 (Fe, Co) to 30 (Zn), suggesting that earlier data were compromised by contamination. The suspended metal data are used in a simple steady-state two-☐ flux model to evaluate the role of suspended particles in determining the vertical flux of metals in the Sargasso Sea. The model calculations indicate that removal of suspended particles in the deep (> 500 m) water column contributes less than half of the total deep flux for all of the elements investigated except for Cd. This result implies that uptake of these metals onto particles in the deep central ocean is a minor factor in their oceanic mass balance. If this finding is generally true for oligotrophic ocean environments, whole-ocean residence times for these elements must be governed largely by processes occurring either in near-surface waters or at ocean margins.

Vertical structure and biological activity in the bottom nepheloid layer of the Gulf of Maine

The bottom nepheloid layer (BNL) was investigated at a number of hydrographically different sites in the Gulf of Maine during August 1987. Observations were based on hydrographic measurements made from a surface ship and closely-spaced, near-bottom samples collected using a submersible. The BNL generally occurred as a turbid layer which extended 15–30 m above the bottom (m.a.b.), as indicated by in situ light transmission and increased concentrations of total suspended particulate matter (SPM). Phytoplankton pigments, electron transport activity (ETS), extracellular proteolytic enzyme activity (EPA), concentrations of particulate organic carbon and nitrogen (POC and PON), and protein were generally elevated in the BNL. They also displayed vertical distribution patterns in relation to near-bottom depth zones of increased abundances of zooplankton, bacteria and autotrophic and heterotrophic nanoplankton. We describe two zones of biological significance in the BNL. The first, at about 20 m.a.b. at most stations, was associated with greater zooplankton biomass (80 μm) and copepod abundances than those depth strata either above or below, and appeared to be related to a higher quality of food particles near the top of the BNL. A second zone was seen 1–3 m.a.b. at most stations in association with the greatest levels of SPM. This deeper zone was generally of a poorer food quality, as reflected by ratios of protein-N to total-N and showed increases in cell-specific EPA. We discuss the areal variability of the BNL in the Gulf of Maine as well as the biological enhancement and vertical structure as likely influenced by both physical and biological processes.

Sedimentation and climate-induced sediments on Feni Ridge, Northeast Atlantic Ocean

Feni Ridge has been studied in detail in the area between 54 and 56°N and 12 and 16°W by using high-resolution profiling, water column sampling with transmissometer and CTD, and by piston and box core sampling. The crest of Feni Ridge is absent in the north of the study area, but on passing towards the south it begins to take shape and to increase in height; its distance from the Rockall slope also increases towards the south. On the lower southeastern flank a secondary ridge crest has locally developed, and sediment waves along the flanks have amplitudes of up to 45 m. The highest and steepest waves are found in the south, the waves in the north being somewhat lower. Bottom nepheloid layers of 175 m near the foot of Rockall Plateau and 125 m near the crest are attributed to a Norwegian Sea Overflow Water current with a core at 2100 m. Bottom nepheloid layers of 75–100 m on the lower flanks of the ridge are thought to have been caused by Northeast Atlantic Deep Water. Piston cores reflect alternations of glacial and interglacial periods. Interglacials have a high carbonate and C org content, a positive Eu anomaly and negative Ce anomaly, and a low Rb La ratio. They are dominated by a subpolar and transitional foraminiferal assemblage. Glacials show a low carbonate and C org content, a reduced Eu anomaly and a high Rb La ratio, suggesting increasingly continental sediment sources. These deposits are dominated by a polar planktonic foraminiferal assemblage characterized by N. pachyderma (s) and G. quinqueloba. Sedimentation rates reflect differential deposition. At crestal station NA 87-25 rates are 17.1 cm/k.y. for stage 1, 14.6 cm/k.y. for stage 2, 3.4 cm/k.y. for stage 3 and 5.4 cm/k.y. for stage 4. Stage 5a–e has a mean sedimentation rate of 6.1 cm/k.y. Over the last 125 k.y. the development of Feni Ridge has taken place under a relatively low energy current regime.

Mass-physical properties of surficial sediments on the Rhoˆne continental margin: implications for the nepheloid benthic layer

Mass-physical properties of the surficial (upper 5 m) sediments on the Gulf of Lions continental margin were analysed, from more than 100 short (1 m) and longer (5 m) cores obtained during several cruises. Data include water content, unit weight, Atterberg limits (liquid limit, plastic limit, plasticity index), shear strength and compression index, and are used to determine: first, the mass property distribution, according to the main parameters influencing mass-physical properties; the relationships between these properties and the nepheloid layer on the shelf. The shoreline (lagoons) and inner shelf are characterized by low density and shear strength and high water content deposits, due to electrochemical flocculation of the sediment. The outer shelf is blanketed by higher density and shear strength and lower water content deposits generated by normal settling of suspended particles. On the inner shelf, during river peak discharges, a short-term thin bottom layer of “yogurt-like” [ Fass (1985) Geomarine Letters, 4, 147–152; Fass (1986) Continental Shelf Research, 6, 189–208] fluid-mud (unit weight lower than 1.3 mg m −3) is supplied, by a bottom nepheloid layer. During stormy periods, this “yogurt-like” layer (about 10 cm thick) partly disappears by resuspension of suspended particulate matter; this is advected, in the bottom nepheloid layer, over the shelf and the canyons within the upper slope.

Tracers and constituents indicating the nature of organic fluxes, their origin and the effect of environmental conditions

The nature of particulate organic matter was studied in suspended material sampled by bottles, particles collected by sediment traps and deposits from deltaic and open sea ecosystems of the northwestern Mediterranean. Elemental analyses were combined with pyrolysis-gas chromatography-mass spectrometry and with analysis of individual compounds such as phenols separated by high-performance liquid chromatography. In the Rhoˆne delta, a multilayered system was observed. The surficial turbid layer was enriched with fresh material of river origin. Organic matter was more altered and richer in phenols in the bottom nepheloid layer. The river regime determined the nature and quantity of suspended material: when in spate, degraded organic matter previously deposited in the river bed was transported to the sea, thereby inducing an increase of pyrolysis derived aromatic hydrocarbons. On the other hand, phenolic aldehydes increased in the bottom nepheloid layer. When water level was low, organic matter seemed to be of more local origin. The content of phenols and nitrogen-containing compounds increased. The influence of the Rhoˆne River decreased off the mouth, when terrestrial markers were diluted by products deriving from phytoplanktonic activity. Compared with suspended material, trapped organic matter was coarser, more degraded and contained more aromatic hydrocarbons. It was different in nature and size, indicating that it was trapped over a longer period. Deposits contained altered organic matter resulting from degrading processes in the water column. Sediments showed a double trend off the mouth of the river; an increase in nitrogen-containing compounds, indicating a more marine character, and a decrease in phenols and carbohydrates deriving from the terrestrial ligno-cellulosic complex. In the southwestern part of the Gulf of Lions, in the Teˆt prodelta, organic matter from sediment traps was fresher than in the Rhoˆne delta. Phenols and some carbohydrates rapidly decreased from the prodelta due to a lower runoff. During the spate, suspended material was rapidly deposited and sediments were enriched in terrestrial phenols. In the open sea environment, in the Lacaze-Duthiers Canyon at 645 m, in summer, the euphotic zone was rich in amino-sugars and contained a large diversity of phenols deriving from fish fecal pellets. In winter/spring, the development of phytoplanktonic blooms in surficial layers led to high contents of nitrogenous compounds. In deeper layers, the nature of organic matter was different from surficial layers in summer and more homogeneous in winter, although a flux of degraded material rich in pyrolysis-derived aromatic hydrocarbons and poor in nitrogenous compounds progressively sank towards the bottom. The two marine prodeltas were compared: organic matter was more degraded in suspended material and sediments collected in the submarine delta of the Rhoˆne River. Organic material originating from the river was transported further offshore, as indicated by a higher content of aromatic hydrocarbons and phenols. In the southwestern part of the Gulf of Lions, the prodelta reflected the influence of local rivers, with lower discharges.

Removal of thorium-234 by scavenging in the bottom nepheloid layer of the ocean

Concentrations of 234Th were measured in deep-sea bottom waters to assess the extent of chemical scavenging near the ocean floor. At five stations in the eastern tropical Pacific, where no appreciable bottom nepheloid layer (BNL) was observed, total 234Th was in secular equilibrium with its parent 238U, confirming earlier published results. In contrast, samples from a well-developed BNL in the western North Atlantic showed a significant depletion of 234Th due to its removal by scavenging. The largest depletions were observed in the two samples closest to the bottom (25 and 64 m above bottom) and amounted to 19% of the equilibrium value. The results can be used to test models that have been proposed to explain the existence of a BNL. It is concluded that only a model which includes local or nearby resuspension of sediment can account for the observations. From a steady-state model that assumes local resuspension, the average residence time of resuspended particles in the BNL is estimated to be 25 days.