Sediment Trap Data Research Articles

Biogenic barium fluxes to the deep sea: Implications for paleoproductivity reconstruction

Dymond et al. (1992) have recently proposed an algorithm to reconstruct paleoproductivity from biogenic Ba (bio‐Ba) accumulation rates in sediments. Their equation is based on sediment trap data which indicate that Corg/bio‐Ba ratios in settling particles are higher in the western Atlantic compared to the Pacific. From this observation they have suggested that the flux of bio‐Ba to the seafloor may depend on dissolved Ba concentrations in intermediate and deep waters which are significantly higher in the Pacific compared to the Atlantic. Accordingly, they have introduced a factor related to dissolved Ba concentration in their equation as a variable which strongly influences paleoproductivity estimates. In an attempt to confirm the proposed dependency of bio‐Ba fluxes to the seafloor on dissolved Ba concentrations in seawater we have compiled additional data on organic carbon and bio‐Ba fluxes in the deep sea. These data confirm Dymond et al.'s findings that settling particles have significantly higher Corg/bio‐Ba in the western Atlantic compared to the Pacific. However, we also found lower ratios in traps deployed in the North Atlantic, similar to those found in the Pacific, while in the Panama Basin we found ratios as high as those in the western Atlantic. From these observations we conclude that dissolved Ba concentration is not an important factor in regulating the flux of bio‐Ba to the seafloor. Instead, we propose that high Corg/bio‐Ba ratios found in the western Atlantic, the Panama Basin, the Arabian Sea, and some stations in the Nordic Seas result from the addition of refractory organic carbon from nearby continents, shelves, or slopes. If that is confirmed, the algorithm proposed by Dymond et al. (1992) could be simplified and could provide a powerful means to estimate paleoproductivity. In addition, deviations from the Corg/bio‐Ba ratios in settling particles could be used to estimate the input of continental or shelf‐derived refractory organic matter into the deep sea.

An in situ experiment to investigate the depletion of seston above an interface feeder field on the continental slope of the western Barents Sea

During 'Meteor' Cruise 17 in July 1991, bottom water and sediment samples were taken from 3 stations on a 3 7 km long transect along the continental slope of the western Barents Sea between 74 58.3' N , 14 44.8' E and 74 59.8' N, 14 40.1' E, across an area occupied by macrofauna feeding at the sediment-water interface at 1340 m water depth. Profiles of oxygen, total particulate matter, total inorganic matter, particulate organic carbon, bacterial abundance and chlorophyll equivalents were taken in the benthic boundary layer (10 to 40 cm height above the sea floor). Between the upstream and downstream stations total particulate matter and particulate organic carbon decreased by 41 and 23%. respectively. at a height of between 10 and 25 cm above the seabed. Chlorophyll equivalents in the bottom water increased by 8.5'36, in the sediments by 56%. Abundance of macrofauna showed no significant correlation to bottom water and sediment data. Compared to vertical particle fluxes, as determined from sediment trap data, particle fluxes to the sea floor were corrected by a lateral advection factor of 2 to 3.7.

Does Local Resuspension Maintain the Benthic Nepheloid Layer in Southeastern Lake Michigan?

Time-series observations of water temperature, water transparency, and current velocity at four stations in southeastern Lake Michigan show that the benthic nepheloid layer is probably not maintained by the local resuspension of bottom sediment. Local resuspension may occasionally occur in the deep parts of the lake, but it seems likely that vertical mixing and offshore advection of sediment-laden water maintain the benthic nepheloid layer during most of the year. Although sediment trap data have been interpreted as indicating that local resuspension does occur, it is more likely that the increased sediment fluxes observed near the bottom are due to vertical redistribution of material already in suspension.

Spatial variability in sediment composition and evidence for resuspension in a large, deep lake

Spatial variability in the composition of sediments in Pyramid Lake, Nevada, was assessed in June 1992 by collecting surficial samples at 32 sites. For those sites, organic fractions were highest in the centre of the lake and low near the single inflow to the system. The spatial patterns for organic C, N, and P in lake sediments indicated that small organic-rich particles are preferentially deposited in the centre of the lake. This sorting of particles by water depth was supported by a corresponding decrease in the particle size distribution of sediments with station depth and by limited sediment-trap data. Further, a comparison of sediment-trap particle flux rates with the net sediment accumulation rate for the centre of the lake based on 210Pb decay indicated that the resuspension of bottom sediments in Pyramid Lake (zmax = 102 m) must occur throughout the lake.

Temporal and spatial variability of biogenic particles fluxes during the JGOFS northeast Atlantic process studies at 47°N, 20°W

Particle fluxes to 3100 m depth at 45°50′N, 19°30′W were measured using time-series sediment traps during a 17 month period encompassing 1989 and 1990 JGOFS spring bloom process studies in the northeast Atlantic. There was a marked intra-annual variability in fluxes of mass, particulate inorganic carbon (PIC), particulate organic carbon (POC) and opal, appearing as two major flux events in each year. In 1989, the first flux event represented the settlement of spring bloom-type material, whereas the second, in autumn, was heavily enriched in mucopolysaccharides. In 1990, in contrast, the two flux events comprised spring bloom-type material and arrived at depth at different times relative to the 1989 events. The intra- and interannual variability evident for all three biogenic components was most notable for POC: (i) the autumn 1989 event supplied twice as much POC to 3100 m as the earlier spring bloom settlement—a quite unexpected observation—and (ii) the annual average POC flux in 1989 was 3–4 times more than in 1990. A synthesis of process study datasets with sediment trap data enables an evaluation of the coupling of deep fluxes with surface-water events. Spatial variability of the 1989 deep flux events is assessed by comparing the sediment trap data reported here with those from a second site ∼ 100 km away ( Honjo and Manganini, Deep-Sea Research II, 40, 587–607, 1993). The timing and magnitude of the 1989 spring bloom settlement was indistinguishable in the two datasets, indicating no spatial variability in flux between these sites. In contrast, the autumn 1989 flux event was barely recorded at the second site. Given the biogeochemical importance of this latter event to deep waters, most notable in terms of its contribution to POC flux, this observation of deep-water mesoscale flux variability indicates a significant problem in determining regional carbon budgets. Construction of basin-scale budgets is a central goal of JGOFS and for this to be achieved further studies of mesoscale variability of particle flux are essential.

Sedimentation of terrigenous red soils in coastal area adjacent to an estuary and coral reefs, the Okkubi River estuary, Okinawa Island, Japan

Sedimentation process of fine-grained terrigenous red soils was observed on the seafloor by coral reefs near an estuary using a paired mooring of turbidity meter and sediment trap or a mooring of sediment trap and current meter. Silty clays contained in the bottom calcareous sands were mainly resuspended and trapped in both surface and bottom layer traps, at noncatastrophic normal sedimentation periods. In addition, silty clays were supplied to this mooring site by inflowing river. Turbidity and the flux rate determined by sediment traps show certain relationship accompanied with the coefficient, which is changed by precipitation, current, and other conditions. Resuspension process caused by a typhoon was recorded as both core sequence of sediment trap and time-series data of bottom current. Maximum velocity of 49.5 cm/s was recorded from bottom currents resulting from the passing of attyphoon at the mooring site. A graded sand layer is interbedded in dark-gray, silty clay and considered to be a resuspended sediment resulting from the passing of the typhoon. Flux of the resuspended sediments by the typhoon was estimated from this core sequence and compared to the flux observed at the normal sedimentation.

Microbial carbon utilization on the continental shelf and slope during the SEEP-II experiment

Abstract Microbial processes were measured as part of the Shelf Edge Exchange Processes II (SEEP-II) experiment, which was designed to re-examine the hypothesis that a substantial fraction of primary production on the continental shelf is exported to the slope and deeper ocean. Bacterial-mediated carbon remineralization was defined as the sum of measured bacterial production plus estimated bacterial respiration. Carbon remineralization, summed over the water column plus surficial sediment, was 90–180 mg C m−2 day−1 on the shelf and 270–540 mg C m−2 day−1 on the slope. These are equivalent to 12–24% and 33–66% of daily primary production on the shelf and slope, respectively. The majority of bacterial activity occurs in sediments on the shelf, but not the slope it is attributable to bacteria in the deep water column. Concurrent sediment trap data indicate that material sinking from surface waters overlying the slope can supply only a small fraction of carbon remineralized by bacteria in the intermediate water column. Bacterial production in slope water and sediments may be subsidized by export from the shelf. Because of the greater width of the shelf than slope, this additional demand on shelf carbon would represent only 7.5–15% of shelf primary production. Mean specific growth rates of sediment bacteria were very uniform at depths between 40 and 2000 m. This suggests bacterial growth may be limited by some common factor, although environmental factors known to affect growth rates generally vary with depth. DNA synthesis rates (measured by thymidine incorporation) were comparable to rates observed for marine bacteria in nutrient-replete culture. The implication is that sediment bacteria were not limited by nutrient availability. If true, water column and sediment bacterial production are likely to be controlled by very different processes: water-column bacteria by both nutrient limitation and grazing, and sediment bacteria by grazing alone.

Thorium isotopes as indicators of particle dynamics in the upper ocean: results from the JGOFS North Atlantic Bloom experiment

Measurement of 234Th and 228Th in suspended and sinking particles made during the 1989 JGOFS North Atlantic Bloom Experiment permit estimation of the rates of particle cycling. Using a simple model of thorium-particle interactions applied to water column and floating sediment trap data at 150 and 300 m, the rate constant, β2, for aggregation of small suspended particles into large rapidly sinking (∼150 m day−1) particles increases from ∼0 to ∼30 y−1 over the course of the bloom. The rate constant for disaggregation of sinking particles, β−2, similarly increases from ∼100 to ∼300 y−1 over the same period. These suggest that small particle residence times (relative to packaging or aggregation) decreasesto ∼15 days and that large particle residence times (relative to disaggregation) decrease to ∼1 day as the bloom progresses. Late in the bloom, particles are cycled such that aggregation of suspended particles (∼2 μg 1−1 day−) is comparable to particle break-up (∼3 μg 1−1 day−). Errors on the rate constants, calculated by propagating estimated errors on the individual terms in the model, are large and arise principally from uncertainty in the gradient in activity and mass fluxes between the two trap depths. However, the values calculated independently from the two tracers (234Th and 238Th) generally agree to within 30%. The 234Th balance for the upper water column (Buesseleret al., Deep-Sea Research, 39, 1115–1137, 1992) suggests that a substantial portion of the thorium and mass flux is not recorded by the traps. If it is assumed that this flux is carried on more slowly sinking particles (∼50 m day−1) that are not trapped efficiently, and these particles directly interact with the suspended particles pool in the same fashion as the trapped sinking particles, calculation of aggregation and disaggregation rate constants late in the bloom shows a higher value for β2 but a comparable value for β−2 relative to the values determined for the trapped particles. This suggests that the slowly sinking material (e.g. marine snow) is more effective at aggregating small, suspended particles than are the rapidly sinking particles. Temporal increases in β2 and β−2 for the trapped particles are matched by increases in the rate constants for decomposition of particulate organic carbon and nitrogen (2–35 y−1 for C; 4–40 y−1 for N), suggesting that increases in microbial activity are directly reflected in rates of particle aggregation and disaggregation.

Dynamics of sedimentation in a tide-dominated backbarrier salt marsh, Norfolk, UK

Progressive burial of artificial markers over a 5-year period is used to determine the rate and pattern of vertical accretion within a large backbarrier salt marsh on the UK east coast. Over this period, annual accretion varies spatially from 1 to 8 mm yr −1. The arithmetic mean rate for the whole marsh is 3.9 mm yr −1. Spatial variability in accretion is a joint function of (1) elevation-dependent inundation frequency and (2) progressive sediment removal from water masses advected across channel margins. Accretion is, therefore, inadequately represented by simple averaging of point measurements. Numerical integration of the ‘accretion surface’ results in a spatial average rate of around 3 mm yr −1, well below the arithmetic mean rate. Short-term sediment trap deployments show that local and long-range meteorological effects, and remobilisation of sediment deposited within tidal creeks, often mask the expected link between tidal height and sedimentation rate. Retention of sediment on plant surfaces is minimal, with direct settling accounting for approximately 95% of total deposition. Time-extrapolation of weekly sediment trap data, and comparison with the 5-year marker horizon burial, shows that processes associated with ordinary tides can account for long-term accretion over most of the marsh. However, the highest surfaces receive appreciable sediment input only during aperiodic storm events.

Particulate flux of A1, a component of land origin, in the western North Pacific

The particulate fluxes of A1 are generally greater in the western North Pacific than in the central and eastern North Pacific, Atlantic and Antarctic oceans. For instance, sediment trap data reported in this paper show the Al flux in the northern part of the Japan Trench is 12.7 mg m −2 day −1 at 5.2 km depth, 130 times greater than that in the deep Antarctic, even though total particulate fluxes are similar. The particulate fluxes of A1 extrapolated to the ocean surface layer roughly equals the observed A1 flux occurring at the ocean-atmosphere interface, suggesting that particulate A1 is atmospheric in origin. Excess A1 fluxes in the subsurface water probably indicate horizontal transport from the continental margin. This is indicated by the different Mg/K ratios of settling particles between the western and eastern North Pacific.

Paleoproductivity and carbon burial across the California Current: The multitracers transect, 42°N

The Multitracers Experiment studied a transect of water column, sediment trap, and sediment data taken across the California Current to develop quantitative methods for hindcasting paleoproductivity. The experiment used three sediment trap moorings located 120 km, 270 km, and 630 km from shore at the Oregon/California border in North America. We report here about the sedimentation and burial of particulate organic carbon (Corg) and CaCO3. In order to observe how the integrated CaCO3 and Corg burial across the transect has changed since the last glacial maximum, we have correlated core from the three sites using time scales constrained by both radiocarbon and oxygen isotopes. By comparing surface sediments to a two‐and‐a‐half year sediment trap record, we have also defined the modern preservation rates for many of the labile sedimentary materials. Our analysis of the Corg data indicates that significant amounts (20–40%) of the total Corg being buried today in surface sediments is terrestrial. At the last glacial maximum, the terrestrial Corg fraction within 300 km of the coast was about twice as large. Such large fluxes of terrestrial Corg obscure the marine Corg record, which can be interpreted as productivity. When we corrected for the terrestrial organic matter, we found that the mass accumulation rate of marine Corg roughly doubled from the glacial maximum to the present. Because preservation rates of organic carbon are high in the high sedimentation rate cores, corrections for degradation are straightforward and we can be confident that organic carbon rain rate (new productivity) also doubled. As confirmation, the highest burial fluxes of other biogenic components (opal and Ba) also occur in the Holocene. Productivity off Oregon has thus increased dramatically since the last glacial maximum. CaCO3 fluxes also changed radically through the deglaciation; however, they are linked not to CaCO3 production but rather to changes in deepwater carbonate chemistry between 18 Ka and now.

Comparison of Phytoplankton in Sediment Trap Time Series and Surface Sediments Along a Productivity Gradient

Three years of sediment trap data collected by the Multitracers project are compared with surface sediments for sites off the coast of southern Oregon, using phytoplankton microfossils. Sites were located to monitor a region frequently affected by plumes from coastal upwelling (Nearshore), one occasionally affected by the largest plumes (Midway) and an area outside the zone of coastal influence (Gyre). Diatoms overwhelmingly dominate the microfossils, with silicoflagellates an order of magnitude fewer and calcareous nanofossils absent. This absence suggests that the rarity of the group in the sediments may be a function of low production, rather than of deepwater dissolution. The Midway and Nearshore sites were generally similar in taxonomic composition and timing of flux maxima, while the Gyre site differed in both regards. Interannual variability in taxon composition appears to correlate with productivity; the most productive site (Nearshore) has the most consistent interannual trend and the least productive (Gyre) exhibits the greatest variability. The sediment assemblage at the Nearshore site is most similar to trap samples from the late fall and winter seasons, rather than the more productive spring and summer. This is partly a function of differential dissolution, but the relationship persists even when only resistant taxa are used. Two possible explanations are discussed: addition of material from the continental shelf, and inadequate representation of long‐term trends by the trapping experiment. At the Midway site the data are few and scattered, but once the dissolution effect is removed it appears that the sediments contain material from several seasons of production. At the Gyre site preservation bias results in a sediment assemblage most similar to late spring and summer, only partly corresponding to maximum production. Comparison between sites indicates that the Nearshore and Gyre sediments have characteristic assemblages which are similar to the corresponding trap collections. The Midway sediments are similar to trap samples from all 3 sites, implying that the assemblage lies on a gradient between the other two and lacks a unique character of its own.

Reconciling aggregation theory with observed vertical fluxes following phytoplankton blooms

Sediment trap data show that rapidly sinking pulses of phytodetritus form after phytoplankton blooms, even when bloom intensity is low. A numerical model of physical aggregation and sedimentation in the surface ocean was used to gauge whether predicted aggregation rates were high enough to generate postbloom sediment pulses. Initial models behaved inaccurately without a full range of particle sizes, abundant nonphytoplankton particles, and explicit hydrodynamic retardation of particle contact. Provision for background particles while tracking phytoplankton required implementation of a novel bookkeeping scheme. To address the degree of retardation for contact between particles, an expression for contact efficiency for collision by turbulent shear was developed. The most realistic way to produce model results that mimicked field data was to include background particles, to invoke particle stickiness in the range 0.1–1.0, and to make modest upward adjustments to contact efficiencies calculated for impermeable spheres. For all but the highest background particle concentrations, the magnitude of a postbloom sediment pulse scaled nonlinearly with vertically integrated cell number in the surface layer of our two layer model. The existence of a nonlinear relationship between pulse size and bloom intensity makes initial cell number per unit of area in the surface layer, and not productivity, the proximate determinant of carbon export flux. This result emphasizes the need for caution when applying established scalings between export flux and productivity. Further, it provides a mechanistic explanation both for tight pelagic‐benthic coupling under waters prone to intense blooms and for interannual variability in export flux in polar regions.

Nitrogen circulation in the water column in the Oyashio region with reference to its high productivity

In order to determine quantitatively the reason for the high productivity in the Oyashio Region, which is the southwest part of the Pacific Subarctic Region, the annual-mean vertical circulation of nitrogen in the region was estimated from the vertical profiles of nitrate, dissolved oxygen and salinity, and sediment-trap data by adapting them to the balance equations. Estimates of the upwelling velocity (1.7×10−5cm sec−1) and the vertical diffusivity (2.1 cm2 sec−1) in the abyssal zone and the primary and secondary productivities (44 and 4 mgN m−2day−1, respectively) in the euphotic zone were close to those of previous works. The estimated vertical circulation of nitrogen strongly suggested that, since the divergence (5 mgN m−2day−1) is caused by the abyssal convergence (6 mgN m−2day−1) and the positive precipitation, the local new production (22 mgN m−2day−1) necessarily exceeds not only the sinking flux (10 mgN m−2day−1) itself but also the sum of the sinking flux and the downward diffusion of dissolved and particulate organic matter (7 mgN m−2day−1) produced probably in the euphotic zone. The important roles of the abyssal circulation, the winter convection, and the metabolic activity in the bathyal zone to support the high productivity in the euphotic zone were clarified quantitatively.

Annual primary production and export flux in the Southern Ocean from sediment trap data

Since 1983 time-series traps have been deployed in the Atlantic sector of the Southern Ocean to measure the flux of organic carbon, biogenic silica and carbonate. The organic carbon flux data are used to calculate primary production rates and organic carbon fluxes at 100 m water depth. From these calculations, annual primary production rates range from about 170 g C m −2 in the coastal area (Bransfield Strait) to almost zero in the Permanent Sea-Ice Zone. High rates (of about 80 g C m −2 year −1 ) were calculated for the Polar Front Zone and rather low values (about 20 g C m −2 year −1 ) characterize the Maud Rise area. The estimated primary production for the entire Southern Ocean (south of 50°S), using various subsystems with characteristic carbon fluxes, is in the order of 1 × 10 9tons year −1; the organic carbon flux out of the photic layer is 0.17 × 10 9tons year −1. Our calculation of the Southern Ocean total annual primary production is substantially lower than previously reported values.

Modeling the calcite lysocline

A numerical model of calcite dissolution in contact with sediment pore water is used to predict the depth and shape of the calcite lysocline in the deep sea. Model results are compared with lysocline data from 13 regions in the Atlantic, Pacific, and Indian Oceans. The model lysocline shape is sensitive to the calcite dissolution rate constant, the calcite, organic carbon, and refractory material rain rates, and the rates of oxic versus anoxic organic carbon degradation in the sediment. The model is able to reproduce the observed lysocline, within the constraints of the sediment trap and calcite accumulation data, using a calcite dissolution rate constant of 30–100%d−1, a molar ratio of organic carbon to calcite rain rates of 0.5–1.0, and an initial CaCO3 fraction of 90% (excluding organic carbon). This rate constant is consistent with microelectrode results presented by Archer et al. [1989]. The model predicts that 30–50% of the calcite rain to the sea floor at the saturation horizon dissolves in response to organic carbon respiration, consistent with previous modeling studies. The range in the best fit value of the organic‐inorganic carbon rain rates arises from model sensitivity to uncertainty in the rate of anoxic carbon degradation in the sediment and in the rain rate of refractory material, rather than from scatter in the data. Lysocline data from the western equatorial Atlantic are anomalous to the rest of the data; this anomaly may be explained by high rates of refractory material sedimentation from the Amazon River.

Importance of continental margins in the marine biogeochemical cycling of carbon and nitrogen

THE continental margins occupy less than 20% of the surface area of the world ocean, and it is widely assumed that they do not play a significant part in the oceanic biogeochemical cycles of carbon and nitrogen. Data from 32 sediment-trap moorings, 16 in the deep sea and 16 on the continental slope1, suggest that at an average depth of 2,650 m on the slope, the combined rain of surviving shelf and slope particles yields a mean carbon flux of 6.9 g C m−2 yr−1—about ten times that at the same average depth in the deep sea (0.8 g C m−2 yr−1). Because the area of the deep sea is about ten times greater than that of the continental slopes, using the sediment-trap data and assuming a carbon/nitrogen ratio of 5:1, the equivalent total particulate offshore nitrogen loss is 0.5 × 1014 g N yr−1 at 2,650 m. If these trap observations are generally representative of the oceans and continental margins, then the supply of dissolved nitrate to the overlying euphotic zones should also be similar. Here I provide an independent estimate of the annual supply of onwelling nitrate from the deep sea to the shelves and find that it may balance the offshore flux of carbon, suggesting that the continental margins and deep sea are equally important in the carbon and nitrogen biogeochemical cycles.

234Th cycling in the upper water column of the northwestern Mediterranean Sea

As part of the DYFAMED program, a record of 234Th particulate fluxes in the water column has been obtained with sediment traps moored in the Mediterranean Sea near Corsica. During the same period, profiles of dissolved and particulate 234Th were determined at this site. The sediment-trap data show that 234Th particulate fluxes are closely correlated with biogenic particulate fluxes. The highest fluxes are observed during late winter and early spring, when primary productivity generally increases in these waters. Water-column data indicate that dissolved 234Th is always depleted relative to its present, 238U. This depletion is generally more pronounced near the surface. Particulate 234Th represents 5 to 50% of total thorium, which indicates a significant scavenging of dissolved thorium onto suspended particles. Furthermore, some situations display an important deficit of total thorium relative to its parent. This allowed us to calculate 234Th particulate fluxes, using a steady-state scavenging model in surface waters. Calculated fluxes agreed with sediment-trap data below the euphotic zone.

234Th cycling in the upper water column of the northwestern Mediterranean Sea

As part of the DYFAMED program, a record of 234Th particulate fluxes in the water column has been obtained with sediment traps moored in the Mediterranean Sea near Corsica. During the same period, profiles of dissolved and particulate 234Th were determined at this site. The sediment-trap data show that 234Th particulate fluxes are closely correlated with biogenic particulate fluxes. The highest fluxes are observed during late winter and early spring, when primary productivity generally increases in these waters. Water-column data indicate that dissolved 234Th is always depleted relative to its present, 238U. This depletion is generally more pronounced near the surface. Particulate 234Th represents 5 to 50% of total thorium, which indicates a significant scavenging of dissolved thorium onto suspended particles. Furthermore, some situations display an important deficit of total thorium relative to its parent. This allowed us to calculate 234Th particulate fluxes, using a steady-state scavenging model in surface waters. Calculated fluxes agreed with sediment-trap data below the euphotic zone.

Flux of transuranium nuclides and chlorinated hydrocarbons in the northwestern Mediterranean

The transuranium nuclides, plutonium and americium, and selected chlorinated hydrocarbon compounds have been measured in sediment trap samples, bottom sediments and waters. These measurements have been used to quantify vertical fluxes and elucidate particulate biogeochemical cycles, of these man-made contaminants in the northwestern Mediterranean. Sediment trap experiments in the Gulf of Lions (Lacaze-Duthiers Canyon) have shown that transuranic fluxes are considerably higher in the Gulf of Lions, than those which have been reported for the northeast Pacific; principally this is due to the relatively high mass fluxes in this region of the Mediterranean. Water, surface sediment and sediment trap data indicate that 241Am is being transported downward (via particle settling) more rapidly than 239+240 Pu. From direct flux measurement, residence times for 239+240 Pu and 241Am in the upper 300 m were computed to be 2.5 and 0.14 years, respectively. Comparison of 239+240 Pu concentrations in the water column, in 1986, with similar measurements made 5–11 years earlier suggests that fallout input to the Mediterranean has decreased by nearly a factor of 2 during the period 1975–1986. Fluxes of PCB, HCB, γ-HCH and DDT residues varied by as much as an order of magnitude, or more, both temporally and with depth. The average fluxes of most of the compounds in the Lacaze-Duthiers Canyon, during 1985–1986, were quite similar to those measured off the coast of Monaco several years earlier. Generally, the increased chlorinated hydrocarbon fluxes observed with depth were a direct result of the much higher mass fluxes which occurred at depth. Demonstrated differences in PCB concentrations in the settling particles, deposited surface sediments and overlying sediment floc indicate that simple bottom sediment resuspension is not a major contributor to the enhanced PCB flux noted at depth. The PCB concentration in sedimenting particulate material collected from deeper waters, is similar to previously analysed benthic flocs; it is likely to have been advected into the canyon with the nepheloid layer, which is a common oceanographic feature in this region.