Annick Vangriesheim

Direct observation of intense turbidity current activity in the Zaire submarine valley at 4000 m water depth

A large turbidity current was detected in the Zaire submarine valley at 4000 m water depth. Current meters, turbidimeter and sediment trap deployed on a mooring located in the channel axis, although they were damaged, recorded the signature of a very high energy event. An average velocity of more than 121 cm s−1 was measured 150 m above the channel floor. Coarse sand and plant debris were collected at 40 m height. The turbidity current clearly overflowed the edges of the valley as demonstrated by the large quantity of turbiditic material (464 mg organic carbon m−2 d−1) found in the sediment trap moored 13 km south from the channel axis.

Material supply to the abyssal seafloor in the northeast Atlantic

Downward particle flux was measured using sediment traps at various depths over the Porcupine Abyssal Plain (water depth ~4850 m) for prolonged periods from 1989 to 1999. A strong seasonal pattern of flux was evident reaching a maximum in mid-summer. The composition of the material changed with depth, reflecting the processes of remineralisation and dissolution as the material sank through the water column. However, there was surprisingly little seasonal variation in its composition to reflect changes in the biology of the euphotic zone. Currents at the site have a strong tidal component with speeds almost always less than 15 cm/sec. In the deeper part of the water column they tend to be northerly in direction, when averaged over periods of several months. A model of upper ocean biogeochemistry forced by meteorology was run for the decade in order to provide an estimate of flux at 3000 m depth. Agreement with measured organic carbon flux is good, both in terms of the timings of the annual peaks and in the integrated annual flux. Interannual variations in the integrated flux are of similar magnitude for both the model output and sediment trap measurements, but there is no significant relationship between these two sets of estimates. No long-term trend in flux is evident, either from the model, or from the measurements. During two spring/summer periods, the marine snow concentration in the water column was assessed by time-lapse photography and showed a strong peak at the start of the downward pulse of material at 3000 m. This emphasises the importance of large particles during periods of maximum flux and at the start of flux peaks. Time lapse photographs of the seabed show a seasonal cycle of coverage of phytodetrital material, in agreement with the model output both in terms of timing and magnitude of coverage prior to 1996. However, after a change in the structure of the benthic community in 1996 no phytodetritus was evident on the seabed. The model output shows only a single peak in flux each year, whereas the measured data usually indicated a double peak. It is concluded that the observed double peak may be a reflection of lowered sediment trap efficiency when flux is very high and is dominated by large marine snow particles. Resuspension into the trap 100 m above the seabed, when compared to the primary flux at 3000 m depth (1800 mab) was lower during periods of high primary flux probably because of a reduction in the height of resuspension when the material is fresh. At 2 mab, the picture is more complex with resuspension being enhanced during the periods of higher flux in 1997, which is consistent with this hypothesis. However there was rather little relationship to flux at 3000 m in 1998. At 3000 m depth, the Flux Stability Index (FSI), which provides a measure of the constancy of the seasonal cycle of flux, exhibited an inverse relationship with flux, such that the highest flux of organic carbon was recorded during the year with the greatest seasonal variation.

Distribution of rare earth elements and neodymium isotopes in suspended particles of the tropical Atlantic Ocean (EUMELI site)

We analyzed the REE, Mn and Al concentrations and Nd isotopic ratios in marine suspensions collected on filters (0.65 μm porosity) with in situ pumping systems in the tropical northeastern Atlantic (20°N, 18–31°W). Previously we reported the same parameters on large sinking particles collected with moored sediment traps at the sites. Shale-normalized REE patterns of the filtered suspensions are characterized by a larger light REE (LREE) to heavy REE (HREE) enrichment compared to the trapped material and a Ce anomaly that evolves positively with depth. Depth profiles of REE/Al show maximum values at 50–100 m, where the Mn/Al ratio also reaches a maximum. The profile of the Nd isotopic ratios of the filtered suspensions shows variations similar to those of the seawater. These results suggest that the filtered suspensions preferentially scavenge the LREE, especially Ce, and that the particulate Mn oxides are potential REE carriers. The relationship between the Ce anomaly and the Ce/Al ratio demonstrates that the particulate Ce anomaly is formed by (1) the LREE adsorption onto the particulate Mn oxides in the surface water, (2) Ce(III) oxidation to insoluble Ce(IV)O2 and (3) preferential desorption of strict trivalent REE from the Mn oxides in deep water. Estimated authigenic Nd contents, using Nd isotopic ratios, decrease with depth. This is consistent with the adsorption of the REE in surface water and their desorption in deep water, suggested by the Ce anomaly formation. All the results show that the suspended particles record more clearly the authigenic REE contribution than the trapped material does. The suspended matter plays a key role in the scavenging of particle-reactive elements.

Controls on the organic chemical composition of settling particles in the Northeast Atlantic Ocean

Abstract The organic matter of sinking particulate material collected in the Northeast Atlantic Ocean (ca. 49°N, 16°W) was investigated in order to determine temporal and depth-related variability in its composition. Three sediment traps were deployed at nominal depths of 1000 m (below the permanent thermocline), 3000 m (representing the deep-water fluxes) and at 4700 m, about 100 m above the seafloor (just above the benthic boundary layer). The samples span a 28-month sampling period from October 1995 until February 1998, each sample representing a period of between 7 and 28 days. Total organic carbon and total nitrogen contents decrease with depth, as did the absolute concentrations of most biochemicals measured in this study, such as intact proteins and individual lipids. However, concentrations of proteins relative to total organic carbon and total nitrogen did not show any significant change with depth, implying that they are not being rapidly degraded and so may provide an important supply of nitrogen to the benthos. Fluxes of protein, TN and TOC are significantly correlated at all depths. Lipid compositions vary temporally. During periods of high flux, particularly in the summer, the lipids are richer in ‘labile components’, namely unsaturated fatty acids and low molecular weight alcohols. During periods of low flux other compounds, such as sterols, steroidal ketones and a trisnorhopan-21-one are more abundant. One sample, taken close to the seafloor, was highly enriched in lipids, sterols and fatty acids in particular; this may represent detritus derived from bottom-dwelling invertebrates.

Rapid post-bloom resuspension in the northeastern Atlantic

The importance of particulate matter input on the seafloor following spring bloom and of resuspension processes in middle and high latitudes of the north Atlantic ocean has been recently emphasized. However the possibility of the coupling between both processes until now, has not been reported. To study the fate of the particulate matter after its arrival on the bottom, a deep-sea observatory has been launched in the NEA (Nuclear Energy Agency) area of the northeastern Atlantic abyssal hill province. Time series of botton currents, near bottom particle concentration measurements and time lapse photographs were obtained from March to July 1988, while particulate matter fluxes were recorded with a sequential sediment trap. The most interesting results are the observation of a series of events starting with the arrival on the bottom of 5–10 mm aggregates following the spring bloom, their disaggregation within 2 weeks immediately followed by a rise of the near-bottom particle concentration, concomitant with the passage of a bottom current eddy. These observations illustrate how rapid disaggregation rates may lead to resuspension following high particulate matter input.

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.

Vertical and temporal variations of particle fluxes in the deep tropical atlantic

Results from the study of particle samples collected during two years of sediment trapping experiments at three depths (2000, 200, 10 m above the bottom) in the oligotrophic Cape Verde abyssal plain (4600 m depth) are reported in this paper. Hydrodynamical and sediment data are assessed in order to ascertain their influence on the carbon budget in the deep-sea. The currents affected by tidal, inertial and longer time-scale oscillations (20–30 and 100–120 days) were sufficiently weak (maximum ⩽14 cm s-1) that trap collection was probably unbiased. The average daily flux estimated at 35.2 mg m-2 d-1 at 2000 m a.b. decreased at 28.8 mg m-2 d-1 at 200 m a.b. Organic carbon, nitrogen and carbonate carbon fluxes decreased between these two depths, suggesting organic carbon oxidation and carbonate dissolution during descent or different sources of particles. At 10 m a.b., the total particle flux (41.3 mg m-2 d-1) increased due to near bottom sediment resuspension. The current orientation explains more resuspension recorded in the first data set. A clear seasonal signal was not observed in this study. However, spectral analysis of the data showed a monthly variation, which might have been due to the reproductive cycle of several planktonic species or some hydrodynamical effect. The degree of correlation between fluxes at 2000 and 200 m a.b. suggests that the largest particles sank at rates equal to or greater than 180 m d-1. At 4600 m depth, 0.3% of the surface primary production reached the deep-sea floor. Estimation of the carbon budget in the bioturbated sediment indicated that only 2% of the organic matter carried by the downward flux was buried in the sediment.

Near-bottom particle concentration and flux : temporal variations observed with sediment traps and nepholometer on the Meriadzek Terrace, Bay of Biscay

Abstract The Meriadzek Terrace (a 2100m deep plateau on the North-East Atlantic continental slope) was chosen as the experimental site for a multidisciplinary programme to observe the parameters needed for a better understanding of biological processes in the benthic environment. Two approaches were used to study the input of particulate matter to the bathyal seabed: sediment traps and indirect particle concentration measurements with nephelometry. These two technologies do not measure particles of the same size range, but as we are interested in the fluctuations of the particle supply, their results are complementary. Vertical profiles of nephelometry show that over the Meriadzek Terrace there is 125m thick nepheloid layer immediately above the bottom. The dynamics in the deep layer has been determined by measurements made with a Module Autonome Pluridisciplinaire (MAP), an in situ monitoring device developed at IFREMER which measures currents, nephelometry, temperature vertical profile near the bottom. Throughout six months of measurements in 1984, the currents at 0.5m and 120m above the bottom were subject to semi-diurnal tidal oscillations. The intensity of light scattering recorded with the nephelometer on the MAP was highly correlated with current velocities especially with semidiurnal tidal oscillations which seem to induce local resuspension. There are also longer term fluctuations, notably a very strong event which lasted several days during August. This event lagged behind a period of high intensity of internal waves correlated with a reversal in current direction. The sediment trap (Pieges a Particules “PAP”) observations showed that the particle fluxes on the Meriadzek Terrace have a cycle of variation similar to primary production which is characterized by a maximum in May during the phytoplankton bloom and a minimum during January. There was also interannual fluctuation. These two kinds of results point out the different time scales (from some hours to several months) of the large temporal fluctuations which affect the near-bottom particle behaviour.

Temporal variability of near-bottom particle resuspension and dynamics at the Porcupine Abyssal Plain, Northeast Atlantic

Abstract In order to study particle behaviour and its time-variability in the near-bottom layer on the Porcupine Abyssal Plain (48°50′N, 16°30′W, 4850 m), long-term measurements were made of currents, and nephelometry and particle samples were collected using an autonomous lander between mid-1996 and mid-1998. Water samples, collected in the Bottom Nepheloid Layer within 1000 m of the bottom, were filtered for suspended particles whose contents of organic carbon, nitrogen and pigments were determined. This study was co-ordinated with a water column flux study and a detailed programme of benthic studies to understand how the abyssal boundary layer responds to and modifies inputs of organic matter from the water column (MAST3/BENGAL programme). There were strong seasonal fluctuations in the near-bottom (2 m above the bottom, mab) particle flux, whose variation were correlated in time with the water column fluxes. During the periods of peak flux, the near-bottom flux was sometimes higher than that recorded higher up in the water column, but not always at other times. These excesses were attributed to the resuspension events, since we observed a correlation between current speed and nephelometry. However, in summer the peak in the particle resuspension flux could not be explained by the variations in the tidal amplitude. Instead we attribute it to the large quantities of fresh large particles (aggregations) that had just arrived on the bottom; it was probably linked to the feeding activity and sediment reworking by the rich and varied benthic and benthopelagic megafauna. In both 1997 and 1998, the nephelometry signal (directly related to fine particle concentration) and its variability increased after the peaks in large particle flux with a time-lag of 2–3 months. We assume that this time lag corresponds to the time it takes for the large fresh particles, once they have settled on the bottom, to be disaggregated into smaller particles, and hence become subject to resuspension in the quiet current conditions then prevailing in the BENGAL area. The suspended particle analyses confirm the vertical structure of the Bottom Nepheloid Layer, the lower part of which corresponds to the Bottom Mixed Layer (BML) where resuspension and mixing are higher.

Near-bottom biological and mineral particle flux in the Lucky Strike hydrothermal vent area (Mid-Atlantic Ridge)

Abstract To provide information about the export and the distribution of hydrothermal material to the deep ocean, two sediment traps with current-meters were moored on the Lucky Strike segment of the Azores Triple Junction (Mid-Atlantic Ridge). The results of a 25-day experiment with a single trap deployed 1.5 m from a chimney (2 m above the bottom) are used as a reference for the composition of particles produced by the vent. A 392-day experiment with a time-series sediment trap positioned 500 m from the Lucky Strike vents (17 m a.b.) showed seasonal variations in the particulate flux and in its composition. Particles sampled by the trap close to the chimney (264.3 mg m −2 day −1 ) were characterised by a high concentration of sulphur (10.8%) and barium (3.19%), and a low C/N ratio (5.5). From the current data recorded at the same time, we estimate that this small trap was under vent influence for about 10% of the experiment duration. The calculated mean particle flux obtained with the multisample sediment trap 500 m away (7.7 mg m −2 day −1 ) was among the lowest recorded in the Atlantic Ocean. This situation may be explained by a very low primary production in the Azores region. No significant influence of the Lucky Strike vent particles was recorded in this trap although near bottom currents were regularly favourable to transport particles towards it. No major effect was observed on the trap efficiency by variations of the hydrodynamic conditions during this experiment. Particles in the trap included bivalve larval shells. A comparison between their hinge structure and that of postlarvae of the mytilid species found on these vents suggests a non-hydrothermal origin for the larvae in the trap.