Bottom Nepheloid Layer Research Articles

Near-bottom suspended sediment variability caused by storms and near-inertial internal waves on the Ebro mid continental shelf (NW Mediterranean)

Near-bottom currents and suspended sediment concentrations (SSC) recorded on the Ebro continental shelf at 60 m depth during the transition between fair and stormy weather have revealed some of the major processes controlling the sediment re-suspension and transport at mid shelf depths in this NW Mediterranean area. Storm waves were the main contributor to the transport of sediment, although they only resuspended surface sediments during extreme wave conditions ( H s >4.5 m and T s >11 s ). During the recording period, the net suspended sediment flux along the shelf was one order of magnitude higher than the net suspended sediment flux across the shelf, and the resultant transport was towards the SW and seaward. Fluctuations of the near-bottom SSC during fair weather showed a periodicity around 17 h, and were related to the occurrence of near-inertial internal waves caused by wind pulses during storm events. The energy propagation of these internal waves reached critical angles at the mid and outer shelf, which could favor the development and maintenance of the bottom nepheloid layer on the Ebro continental shelf, as well as, on other Mediterranean continental shelves.

Optics, particles, stratification, and storms on the New England continental shelf

In situ beam attenuation and chlorophyll fluorescence were correlated with concentration and bulk composition of particles in shelf waters during summer and spring under different physical forcing conditions to determine if optical parameters could be used as an additional tracer in examining the process of mixing in shelf waters. Time series measurements were made for two 18 day periods during high stratification (late summer 1996, Δσt = ∼3.0 kg m−3 surface to bottom) and low but rapidly developing stratification (spring 1997, Δσt = 0.05 to 0.5 kg m−3) in 70 m of water in a midshelf environment south of Martha's Vineyard, Massachusetts. When defined by hydrography and optical profiles, four layers were identified during the summer: the surface mixed layer, the particle/chlorophyll maximum, the midwater particle minimum, and the bottom nepheloid layer. Fast moving solitons perturbed the water column briefly, but no storms perturbed the system until large surface swells from Hurricane Edouard intensified and thickened the nepheloid layer. Bulk composition and optics of particles in and above the nepheloid layer were distinctly different after the passage of Hurricane Edouard. The hurricane passage demonstrated that intense atmospheric forcing greatly influences both hydrographic and optical properties in the entire water column, even when highly stratified (Δσt = ∼3.0 kg m−3, decreasing to 0.8 kg m−3 post hurricane), and causes massive resuspension, due initially to wave shear stress that was later dominated by current shear. Restratification progressed rapidly after the hurricane passed. During spring the water column started as a weakly stratified two‐layer system hydrographically and optically but evolved into three layers as stratification developed. Strong spring storms affected both surface and bottom layers but with decreasing impact as the water column stratified.

Do bottom mixed layers influence 234Th dynamics in the abyssal near-bottom water column?

Dynamics of the natural radioactive particle tracer 234Th (half-life: 24.1 days) within the abyssal water column up to 1000 m above bottom and within surface sediments of the northeast Atlantic (Porcupine Abyssal Plain; depth: ≈4845 m) were investigated. Distributions of transmissometer voltages and potential temperature indicated a subdivision of the near-bottom water column into a benthic mixed layer (BML; thickness: ≈10–65 m) and the layer above the BML up to the upper boundary of the bottom nepheloid layer (BNL; thickness: ≈1000 m). Comparison of 234Th fluxes (dpm m −2d −1) in sediment traps, vertical fluxes derived from 234Th/ 238U-disequilibrium in the near-bottom water column and excess 234Th inventories in the surface sediment provided evidence for lateral advection of 234Th-depleted water and a ‘missing sink’ for 234Th. A simple one-dimensional steady-state box-model approach was applied to investigate 234Th dynamics. Estimated residence times suggest the BML and the resuspension zone of the surface sediment to be highly dynamic systems with respect to particle cycling and sorptive reactions on a time scale of weeks. Model results indicate that, through the chemical forcing of changing particle concentration, a thickening BML results in (1) increasing residence times of particulate 234Th in the BML with respect to the net fluxes across the upper boundary of the BML and into the surface sediment; (2) declining adsorption rate constants in the BML; (3) increasing desorptive fluxes in the BML resulting in (4) enhanced 234Th decay in the BML; (5) decreasing net fluxes of particulate 234Th from the BML to the upper BNL above the BML and to the sediment. Potential consequences for carbon cycling in the water column of the deep ocean are discussed.

Distribution, composition and flux of particulate material over the European margin at 47°–50°N

In the framework of the Ocean Margin Exchange project, a multi-disciplinary study has been conducted at the shelf edge and slope of the Goban Spur in order to determine the spatial distribution, quantity and quality of particle flux, and delineate the transport mechanisms of the major organic and inorganic components. We present here a synthesis view of the major transport modes of both biogenic and lithogenic material being delivered to the open slope of the Goban Spur. We attempt to differentiate between the direct biogenic flux from the surface mixed layer and the advective component, both biogenic and lithogenic. Long-term moorings, instrumented with sediment traps, current meters and transmissometers have yielded samples and near-continuous recordings of hydrographic variables (current direction and speed, temperature and salinity) and light transmission for a period of 2.5 years. Numerous stations have been occupied for CTD casts with light transmission and collection of water samples. The sedimenting material has been analysed for a variety of marker compounds including phytoplankton pigments, isotopic, biomineral and trace metal composition and microscopical analyses. These samples are augmented by seasonal information on the distribution and composition of fine particles and marine snow in the water column. The slope shows well-developed bottom nepheloid layers always present and intermediate nepheloid layers intermittently present. Concentrations are mainly in the range 50–130 mg m−3 in nepheloid layers and 6–25 mg m−3 in clear water. A seasonal variability in the concentration at the clear water minimum is argued to be related to seasonal variations in vertical flux and aggregate break-up in transit during summer months. It is suggested that the winter sink for this seasonal change in particulate matter involves some re-aggregation and scavenging, and some conversion of particulate to dissolved organic matter. This may provide a slow seasonal pump of dissolved organic carbon to the deep ocean interior. Differences in trapped quantities at different water depths are interpreted as due to lateral flux from the continental margin. There is a major lateral input between 600 and 1050 m at an inner station and between 600 and 1440 m at an outer one. The transport is thought to be related to intermediate nepheloid layers, but those measured are too dilute to be able to supply the flux. Observed bottom nepheloid layers are highly concentrated very close to the bed (up to 5 g m−3), with a population of large aggregates. Some of these are capable of delivering the flux seen offshore during intermittent detachment of nepheloid layers into mid-water. Concentrated bottom nepheloid layers are also able to deliver large particles with unstable phytoplankton pigments to the deep sea floor in a few tens of days. Calculated CaCO3 fluxes are adjusted for dissolution, which is inferred from Ca/Al ratios to be occurring in the CaCO3-saturated upper water column where up to 80% of the CaCO3 resulting from primary production is dissolved.

Benthic dynamics and carbon fluxes on the NW European continental margin

Across the Goban Spur on the NW European continental margin, laterally directed, intermittent, off-slope transport of particulate matter takes place by intermediate and bottom nepheloid layers (BNLs). These are generated by semidiurnal tidal currents, which on the upper slope reach maximum near-bed speeds of up to 20 cm s−1, and which are directed predominantly off-slope (during 15–20% of the tidal cycle). BNLs are semi-permanently present, increasing in thickness above the seabed in downslope direction but decreasing in particle density. Near-bed currents measured on the upper slope are stronger in autumn than during summer, and both long- and short-term records suggest interannual variability. Aggregate formation in the benthic boundary layer (BBL) is considered the dominant process controlling particle accumulation. The organic fraction has low settling velocities and high residence times within the BBL. The flux of lithogenic material into the sediment on Goban Spur decreases from >44 g m−2 a−1 on the shelf edge to 6.9 and 4.9 g m−2 a−1 on the upper slope, then increases to a maximum of 19.1 g m−2 a−1 on the continental rise. CaCO3 flux increases with depth from about 13 g m−2 a−1 on the shelf edge to a maximum of 30.7 g m−2 a−1 on the continental rise, with minima on the upper slope. Flux values at comparable depths on Meriadzek Terrace are considerably higher. Mineralization of organic carbon on Goban Spur, representing more than 97.7% of the deposition flux, decreases with depth from 19.13 g C m−2 a−1 on the shelf edge, to 4.39 g C m−2 a−1 on the continental rise, and 1.10 g C m−2 a−1 on Porcupine Abyssal Plain. Organic carbon burial fluxes range between 0.05 and >0.16 gC m−2 a−1 on Goban Spur, and up to 0.41 gC m−2 a−1 on Meriadzek Terrace. Over 90% of the organic carbon mineralization at the sediment–water interface and directly below the seabed is driven by oxygen, as shown by pore water modelling and in situ oxygen measurements. Denitrification is of only minor (<5%) importance for the organic carbon mineralization; anoxic mineralization plays a (minor) role on shallow stations. Inventories and fluxes of 210Pbxs in surface sediments on Goban Spur indicate that the slope below 1500 m receives only about half of the amount of relatively young sedimentary material compared to the upper slope and shelf. Yet total sediment fluxes increase from the upper slope downward, indicating a significant contribution of reworked sediment in lower-slope sediments. 210Pb-derived mixing coefficients correlate with macro- and meiofaunal density and biomass, decreasing with increasing depth downslope. Fluxes of lithogenic material, CaCO3 and 210Pbxs on the lower slope agree reasonably well with fluxes recorded in deep-water sediment traps, suggesting that the bulk of the sediment may be supplied via vertical settling through midwater depth. Benthic fluxes of organic carbon, however, are three times higher than deep-water trap fluxes, emphasizing the importance of lateral transport of organic matter over the slope. At present, the NW European continental margin at Goban Spur is not a major carbon depocenter.

Freshwater chlorophycean algae in recent marine sediments of the Beaufort, Laptev and Kara Seas (Arctic Ocean) as indicators of river runoff

Freshwater chlorophycean algae are characteristic organic-walled microfossils in recent coastal and shelf sediments from the Beaufort, Laptev and Kara seas (Arctic Ocean). The persistent occurrence of the chlorophycean algae Pediastrum spp. and Botryococcus cf. braunii in marine palynomorph assemblages is related to the discharge of freshwater and suspended matter from the large Siberian and North American rivers into the Arctic shelf seas. The distribution patterns of these algae in the marine environments reflect the predominant deposition of riverine sediments and organic matter along the salinity gradient from the outer estuaries and prodeltas to the shelf break. Sedimentary processes overprint the primary distribution of these algae. Resuspension of sediments by waves and bottom currents may transport sediments in the bottom nepheloid layer along the submarine channels to the shelf break. Bottom sediments and microfossils may be incorporated into sea ice during freeze-up in autumn and winter leading to an export from the shelves into the deep sea. The presence of these freshwater algae in sea-ice and bottom sediments in the central Arctic Ocean confirm that transport in sea ice is an important process which leads to a redistribution of shallow water microfossils.

Vertical Patterns of Suspended Matter Characteristics Along a Coastal-basin Transect in the Western Baltic Sea

Suspended particulate matter samples were collected from the water column, the bottom nepheloid layer and the ‘fluffy layer’ from four stations along a coastal-basin transect in the Pomeranian Bight, western Baltic Sea. Sampling was performed nine times between October 1996 and December 1998 for various analyses, including electron probe x-ray micro analysis for detailed mineralogical investigations.Specific vertical patterns of clay mineral distributions were found. Suspended particulate matter (SPM) in the bottom nepheloid layer and the ‘fluffy layer’ overlying sediments was enriched in organic carbon and hydrated three layer clay minerals, whereas the non-aggregated SPM was dominated by quartz and biogenic opal. It appears that separation effects operate during aggregation of mineral particles and organic matter in repeated cycles of resuspension and settling. No clear seasonal variations in the composition of the SPM were found, in spite of high spatial and temporal variability of biological and physical variables. The results suggest that preferential incorporation, possibly aided by microbiological colonization, of hydrated three layer silicates into the organic flocs is a process that occurs under a wide range of conditions. Because aggregates sink faster than individual particles, aggregate formation led to a relative enrichment of illite and smectite in the near-bottom layers. Considering the affinity of organic contaminants and heavy metals to organic matter, the selective removal of aggregated organic matter and hydrated three-layer clay minerals from the water column and enhanced transport in the near-bottom fluffy layer may be a natural cleansing mechanism operating in the shallow waters of the bight.

Diatom fluxes in surface sediments of the Goban Spur continental margin, NE Atlantic Ocean

Abstract. Continental slopes are presumed key areas for deposition of organic carbon exported from the shelf. Analysis of across-slope differences in diatom and silicoflagellate fluxes recorded in bottom sediments of the Goban Spur margin, a typical North Atlantic slope environment, was carried out to test if they can provide information on the magnitude of advection of material from the shelf into deeper waters. Total diatom and silicoflagellate accumulation rates showed strong across-slope differences. Minimum values are recorded at the shelf break where maximum surface productivity conditions occur while the deeper sampling stations record fluxes as high as 183×106 valves cm–2 ka–1. While high diatom fluxes show a clear correspondence with the activity of a permanent bottom nepheloid layer operating in the region, they do not correlate with productivity patterns observed in the water column. Diatom assemblages are mainly composed of Chaetoceros resting spores and Thalassionema nitzschioides (Grunow) Grunow ex Hustedt, typical indicators of spring bloom conditions in the area. The absence of clear across-slope trends in the diatom assemblages is interpreted as the effect of random mixing driven by the strong hydrodynamic regime provoked by the activity of the bottom nepheloid layer. The dominance of Chaetoceros resting spores across the slope is related to important exportation of shelf-derived production. However, due to the broad ecological tolerances of the main taxa composing the diatom assemblages, they do not allow precise estimations on the magnitude of the primary vertical flux vs. the secondary lateral flux in this slope environment. Use of the tychoplanktonic and benthic diatoms, which are restricted to the neritic realm, allows only the estimation of the minimum amount of shelf-derived diatoms reaching the slope sediments (at least 13% of the total diatom assemblage for the upper slope area of the Goban Spur). This study shows that major limitations exist for the use of diatoms preserved in surface sediments of this area as tracers of shelf-derived production transported to the continental slope.

Aggregation processes in the benthic boundary layer at the Celtic Sea continental margin

Small particle scavenging rates in the benthic boundary layer (BBL) are calculated for the interaction between measured large particles and single particles at five stations (204, 667, 1445, 2285, 3660 m) at the Celtic Sea continental margin. In summer particle scavenging of small particles ⩽10 μm at the study site takes place under typical flow conditions of u ∗ >0.2 cm s −1. Scavenging of 10 μm particles can take place only at high particle concentrations, which result from sediment erosion. It is expected that the BBL aggregates found in the Celtic Sea at the mid- and lower-slope areas originate in sediment resuspension events. During these times of enhanced flow conditions, aggregates are formed and compacted by shear, which accounts for the fact that they do not disaggregate when they enter the viscous sublayer. Total clearance of bottom nepheloid layers occurs only during times of transport of large aggregates >1000 μm after the arrival of phytodetrital aggregates on the sea floor with number concentrations >200 dm −3 and flow conditions with u ∗ >1 cm s −1.

Palaeocurrent reconstruction, sediment and thorium focussing on the Iberian margin over the last 140 ka

Sedimentological data, in particular sediment grain size, from core MD95-2040 (2465 m water depth) and a nearby core (OMII-9K; 1833 m water depth), presently under the influence of Lower North Atlantic Deep Water and Deep Intermediate Water, respectively, have been used to reconstruct changes in the vigour of near-bottom water flow on the Iberian margin. The pattern of the palaeocurrent index indicates faster flow during the warm periods and slower in cold. This also holds in the Younger Dryas but there is an added effect of brief collapse of the circulation in Termination 1B due to meltwater input. The cold periods are generally characterised by elevated clay percentage (terrigenous material <2 μm diameter) and overall sediment accumulation fluxes. In comparison, warm periods of higher flow have lower depositional flux and a tendency to silt (terrigenous material 2–63 μm) dominance consistent with the higher flow speed and suppression of clay deposition. This is in contrast with a recent 230Th xs°-based study of two cores (MD95-2040 and -2039) [J. Thomson et al., Earth Planet. Sci. Lett. 165 (1999) 255–270], which concluded that increased glacial sediment and Th focussing was caused by stronger currents. The crucial factor is that the controlling speed is that of the flow at the depositional site rather than that of the erosional or transporting flow to the site. High glacial maximum fluxes of carbonate (∼6 g cm −2 ka −1) are largely composed of fine (<63 μm) non-coccolith material, suggesting a significant detrital component. It is proposed that the sediment and Th arrive on the lower slope via different routes, sediment down-slope in bottom nepheloid layers and turbidity currents, Th scavenged from the water column by organic matter, and are rapidly mixed on the bed. Removal of this material from a large area and its deposition in a small one adjacent to Oporto Seamount, together with a small amount of direct down-slope supply, accounts for the properties of the deposits. Allowance for erroneous fluxes due to possible coring expansion ‘stretching’ of the upper part of MD95-2040 does not invalidate these conclusions.

Optical properties of the Kara Sea

This study was motivated by the need to understand dispersion processes which affect the redistribution of nuclear wastes in the Arctic from dump sites in the Kara Sea and in the rivers which flow into the Kara Sea. We focus on vertical profiles of light beam transmission and fluorometry made over the delta region fronting the Ob and Yenisey Rivers and over the East Novaya Zemlya Trough (ENZT). The delta region fronting the Ob River Estuary contains a large repository of particles in a dense bottom nepheloid layer with a maximum centered ∼100 km in front of the estuary entrance and covering an area of roughly 200 km diameter. This suspended particle mass repository appears to contain both sediments and detritus and lends credence to the Lisitsyn [1995] concept of the marginal filter zone. In the deep water of the ENZT we found a strong increase of beam attenuation with depth, indicating a relatively large increase of particle mass concentration from ∼50 m to the bottom (depths in excess of 300 m). The strongest concentration was adjacent to the southeast coast of Novaya Zemlya. We suggest that a type of hyperpycnical flow occurs from accumulation of sediments in the bottom waters of Novaya Zemlya fjords which then cascades down the steep slopes adjacent to the island, producing the particle mass distribution as observed by the transmissometer. The accumulation of these repositories of high particle mass concentrations in suspension would suggest that the residence time is high but that storm‐driven events could act to disperse the material.

Hydrography and sedimentation under the deep western boundary current on Björn and Gardar Drifts, Iceland Basin

Abstract The interaction of Northern Component Waters overflowing the Iceland–Scotland Ridge with sediments further south in the Iceland Basin has been responsible for highly variable sedimentation patterns since at least the late Miocene (∼5 million years ago), and has formed both Bjorn and Gardar Drifts. Presently this process is maintained by a flux of approximately 3 Sv (measured along the south Icelandic slope) of Iceland–Scotland Overflow Water (ISOW) with a mean regional flow direction to the SW, which affects sediment transport on Bjorn Drift below ∼1350 m depth and over most of the Gardar Drift. Coriolis deflection of ISOW flow causes it to move as a Deep Western Boundary Current (DWBC) in the Iceland Basin, which redistributes sediments under the influence of seabed topography. Here, hydrographic and sedimentological data are used to illustrate the effects and extent of the vigorous DWBC activity. The study presents a baseline against which inferred changes in the past can be assessed. North of ∼58°N bottom nepheloid layers with heavy suspended sediment loads are often encountered, particularly under the denser and faster-flowing core of ISOW on the eastern slope of Gardar Drift. In this area the presence of migrating, regular sedimentary waves is seen on 3.5 kHz profiles. This is consistent with both the near-bottom flow velocity probably in excess of ∼10 cm s −1 estimated by the geostrophic method and the presence of well-sorted foraminiferal sand on the seabed. South of ∼58°N dilute bottom nepheloid layers, associated with more tranquil conditions near the seabed (inferred from photographic and sedimentological evidence), show that currents on Gardar Drift are weak. Although most of the south Iceland Basin is mantled by both regular and irregular, non-migratory mudwaves, the absence of migrating structures (with the exception of current intensification associated with discrete basement highs) indicates that more sluggish currents characterise this region. Throughout the region highly variable mean Holocene sediment accumulation rates are inferred (2–40 cm ka −1 ) as a result of the DWBC interacting with the topography.

Particulate matter distribution and [formula omitted] disequilibrium along the Northern Iberian Margin: implications for particulate organic carbon export

Light beam attenuation and scattering profiles were obtained from a total of 94 stations from the shelf edge and slope of the Iberian Margin in the early summer of 1997 and winter of 1998. Hydrographic and satellite data suggest that the summer cruise took place just prior to the seasonal upwelling period present along this margin. Our results from the slope show well-developed, high-concentration, intermediate nepheloid layers (INL) present in North Atlantic Central Water, underlain by relatively clear Mediterranean Water, which, at its salinity maximum, shows an off-shelf decrease in particle load to clear water at the outer edge of the water mass. Only low-concentration INL and weak bottom nepheloid layer (BNL) activity is observed in North Atlantic Deep Water. The presence of low BNL activity suggests weak off-shelf transport of material and weak bottom current resuspension. Good agreement between particulate matter concentration estimated from the empirical calibration of beam attenuation and scattering is found in clear water, while differences in instrumental response within the BNL and surface layer are most likely related to the differing nature of the particles present. In the upper ocean particle load calculated from beam attenuation shows a considerable excess over that calculated by scattering. Offshore spatial and temporal differences in the relationship between both measurements are consistent with a response to large particle sized biological material present in the surface layer. The upper ocean Th 234 activity was measured at a total of 23 stations and particulate organic carbon (POC) and particulate organic nitrogen (PON) at each station occupied during the summer cruise. The broadly uniform distribution of total Th 234 , and more variable particulate Th 234 , suggests that during pre-upwelling conditions on the Iberian Margin, Th 234 distribution is strongly influenced by particle type. As a consequence, particle residence times derived from Th 234 / U 238 disequilibria show a considerable range between 4 and 193 d. Estimates of export flux of POC and PON from the upper 50 m were made using the Th 234 , POC and PON results. POC and PON export ranged from 1–56 mmol C m −2 d −1 and 0.15–7.7 mmol N m −2 d −1 respectively, showing a broad offshore decrease. Comparison of the POC export with estimates of carbon fixation measured during the same period suggests that an average of 19% of the primary production is lost as a sinking particulate flux. Overall, the pre-upwelling situation observed during CD-105 was characterised by low phytoplankton biomass, primary production and low POC export fluxes.

Variability and downward fluxes of particulate matter in the Otranto Strait area

The space-time distribution and vertical fluxes of particulate matter in the Otranto channel area have been studied. A three-layered structure was observed: (i) a surface layer with abundance of suspended matter, characterised by the presence of two water masses (the Adriatic Surface Water and the Ionian Surface Water); (ii) an intermediate layer (Levantine Intermediate Water) with very low particle content; and (iii) a bottom nepheloid layer. In winter, the particulate matter concentration at the surface revealed an east–west gradient that was not evident in summer, when the water exchange through the strait was reduced and local recirculation phenomena prevailed. Sediment traps and current meters were moored on the Italian shelf and on the shelf slope. Total vertical flux values of particulate matter, which were by one-tenth lower in the slope area with respect to the shelf, were in good agreement with the distribution of suspended matter in the water column observed during the seasonal cruises. Flux values measured at the shelf station appear to be related both to biological and advective processes, whereas the main source in the slope area appears to be the vertical transfer of biogenic material.

Single particle analysis of suspended matter in the Makasar Strait and Flores Sea with particular reference to tin-bearing particles

Suspended matter samples filtered from surface waters and two depth profiles from the Flores Sea and Makasar Strait were investigated by electron probe X-ray microanalysis (EPXMA) and laser microprobe mass analysis (LAMMA). EPXMA yielded discrete morphological and chemical analysis of the major particle types of suspended matter. Cluster analysis revealed that thirteen main particle types described the composition of suspended matter of the Flores Sea and Makasar Strait. Silicates, aluminosilicates and Fe-oxyhydroxides were the predominant particle types. Suspended matter of the basins studied contained high levels of tin-bearing particles. On the basis of their composition, tin particles can be divided into three groups: (1) tin oxide/hydroxides (cassiterite, romarchite, hydroromarchite); (2) iron-oxyhydroxides with adsorbed tin; and (3) mixed oxidation state tin hydroxysulphates. Only ultra-fine cassiterite particles enter the seawater in suspended state. Dissolved tin species entering the sea have three alternatives: (1) to be scavenged by Fe-oxyhydroxides; (2) to precipitate as tin oxide/hydroxides (romarchite, hydroromarchite); (3) to precipitate as tin hydroxysulphates. The conclusion is that dissolved and suspended tin originate from local sources in the land frame of the basins as well as from remote sources in the Indonesian Archipelago. Four different sectors of the waters studied have suspended matter with different composition: (1) the Mahakam River–Delta zone; (2) the open Flores Sea; (3) the landlocked Saleh Bay; (4) the Makasar Strait proper. The depth distribution of suspended particle types is mainly influenced by: (1) the bottom nepheloid layer and calcite lysocline in the Flores Sea; (2) the high bioproduction in the surface water layer and the vertical distribution of organic matter in the Makasar Strait.

210Pb and Caesium Accumulation in the Rhône Delta Sediments

210Pb,137Cs and134Cs profiles were determined on 13 box-cores from the Rhône submarine delta (north-western Mediterranean Sea). Maximum accumulation rates were estimated using the210Pb dating method or the evolution of the137Cs/134Cs activity ratio with depth in the sediment. Rates ranged from more than 20cmyear−1near the river mouth to 0·2cmyear−1on the shelf, decreasing rapidly seawards with a preferential S-SW direction from the mouth in relation to the spreading of surface and bottom nepheloid layers in this area. Mixing and diffusion processes were evidenced by137Cs that penetrated to greater depths than expected from the210Pb derived accumulation rates. The delta area is an important sink for the riverine inputs. The137Cs inventory over the studied area (480km2) was estimated to be 19·6TBq in 1990, of which more than 40% was found in the prodelta (30km2), in the vicinity of the river mouth. This inventory appears to be well in excess (50%) of both direct and indirect137Cs inputs arising from Chernobyl and weapons test fallout in this region. If it is assumed that this excess is due only to liquid discharges by nuclear installations on the Rhône river, it represents 20% of the137Cs cumulative discharges by the various nuclear power plants and the Marcoule reprocessing plant released between 1961 to 1990. The210Pb budget shows that the sediments are also efficient to scavenge elements transported by advection or diffusion on the shelf. The Rhône delta, and more particularly the prodelta area, is thus an effective sink for particle-reactive elements and pollutants.

Dispersal and deposition of suspended sediment on the shelf off the Tagus and Sado estuaries, S.W. Portugal

The Portuguese margin in front of the Tagus and Sado rivers is characterized by a narrow shelf incised by numerous canyons and by a large mud deposit. The two estuaries that feed this continental margin have distinct impact. The suspended particulate matter concentration values in the mouth of the Tagus are four times higher than in the Sado. During the summer the surface nepheloid layer is always larger than during the winter when it is restricted near the mouth of the estuary. This nepheloid layer may reach 30 km in length extending westward. The bottom nepheloid layer usually shows higher nephelometer values, and has a typical distribution: it is usually diverted southward in the direction of the Lisbon Submarine Canyon. We estimate the amount of suspended matter being discharged annually from the Tagus estuary to be between 0.4 and 1×10 6 t. The area covered by fine deposits is about 560 km 2. Hence the thickness of sediments deposited annually should be between 0.07 and 0.18 cm. The sedimentation rates calculated from the 210Pb excess vary between 0.16 and 2.13 cm y −1 which correspond to the maximum rate. For a layer of 1 cm thick, 81,000 t of particulate organic carbon (POC) should be trapped. That would represent, with a minimum sedimentation rate between 0.07 and 0.18 cm y −1, an entrapment of 6000–15,000 t POC y −1. The trace metals content of box core samples clearly shows the anthropogenic impact in the uppermost level (5 cm thick) in the Tagus estuary and in all the sedimentary deposits (15 cm thick) on the shelf muddy area. Despite the narrowness of the shelf, a significant part of continental fluxes fails to reach the deep ocean.

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.

Sedimentation on young ocean floor at the Mid-Atlantic Ridge, 29 °N

Measurements of the sediment cover over mid-ocean ridges are potentially useful for tectonic and volcanic problems because sediment thickness variations may reflect variations in seafloor age. In profiler records recently collected with the deeply towed TOBI system, however, we find that sediments on 0–2.5 Ma seafloor do not thicken systematically with distance from the spreading axis as would be expected from increasing seafloor age. Average sedimentation rates calculated from the thicknesses show some tendency to increase with depth as found elsewhere on the Mid-Atlantic Ridge. Many of the deposits drape basement topography and occur on outlying promontories, so sediment is probably deposited from suspension in the bottom nepheloid layer rather than from turbidity currents or other gravity flows. Enhanced sedimentation rates in deeper areas, therefore, occur because of enhanced suspended load within the bottom nepheloid layer or reduced bottom movement which allow particles to settle from suspension. The profiler records reveal a variation in the style of sedimentation from draping and localised ponding on young seafloor, to accumulation in large turbidite ponds farther off-axis, which probably reflects the progressive accumulation in source areas and, due to sedimentation, progressively smoother pathways favourable to turbidity currents.

Some evidences of northward fine sediment transport in the northern portuguese continental shelf

On the basis of sedimentological and oceanographical data, a transport scheme for fine-grained material is proposed in order to explain the location of a muddy deposit present in the northern Portuguese continental shelf, north-west off the Douro River mouth. This mid-shelf deposit is recent, active and its main source is apparently the Douro River. The sediment supply can be related with the oceanographic conditions in winter regime, characterised by a poleward along-shelf current with a bottom westward component, as a consequence of a south-west wind. The resulting fine sediment transport, essentially made in the bottom nepheloid layer, is believed to have a north-west direction.