Alexis Khripounoff

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.

Benthic chamber and profiling landers in oceanography - A review of design, technical solutions and functioning

We review and evaluate the design and operation of twenty-seven known autonomous benthic chamber and profiling lander instruments. We have made a detailed comparison of the different existing lander designs and discuss the relative strengths and weaknesses of each. Every aspect of a lander deployment, from preparation and launch to recovery and sample treatment is presented and compared. It is our intention that this publication will make it easier for future lander builders to choose a design suitable for their needs and to avoid unnecessary mistakes.

Analysis of total and organic carbon and total nitrogen in settling oceanic particles and a marine sediment : an interlaboratory comparison

Abstract 10 laboratories, using their routine methods, collaborated in a comparison of analyses of total carbon, nitrogen and organic carbon in a single sample of marine sediment and a pooled sample of settling particulate material collected by sediment trap. The standard error of the means from all individual laboratories was ±3% of the mean for all total carbon results, and for total nitrogen in the marine sediment, but was ±7% of the mean for total nitrogen in the sediment trap material. Larger errors (±8% of the mean) were found for organic carbon results from both the marine sediment and the sediment trap material, with a range of results (expressed as a percentage of the untreated dry weight) for the sediment trap sample analyzed here of 5.60–8.03%. When this range is compared with that of particulate organic carbon concentration in sediment trap material obtained by different workers at various deep ocean sites (2.4–10.2%) [Wefer, G., 1989. Particle flux in the ocean: Effects of episodic production, in: W.H. Berger, V.S. Smetacek, G. Wefer (Eds.), Productivity of the Ocean: Present and Past, John Wiley & Sons, pp. 139–154.] it is apparent that a large proportion of the observed oceanic variability could be accounted for by difference in analytical technique. We suggest that the problem stems from the difficulty of accurate separation of the organic and inorganic phases and discuss the inaccuracies involved in the separation. We present evidence that the range of results is partly due to either volatilisation of organic carbon during acid treatment, or incomplete removal of organic matter during thermal treatment. If results from different laboratories are to be compared, as is presently required for basin scale and global studies of ocean carbon cycling and budgets, there is clearly a need for suitable reference materials, rigorous intercalibration and technique development.

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.

La nutrition d'echinodermes abyssaux I. Alimentation des holothuries

In deep waters, deposit-feeding holothurians represent a high percentage of the total abyssal biomass and play an important ecological role in sediment modification. The feeding of these organisms, which inhabit a nutritively poor environment, has been studied by means of analyses of intestinal contents. Four abundant species: Psychropotes longicauda Theel, Paroriza pallens (Koehler), Benthogone rosea Koehler and Molpadia blakei (Theel), collected between 2000 and 4500 m in the Bay of Biscay during three cruises organized by CNEXO-COB, were chosen for this study. The morphological characteristics of the ingested alimentary particles are described and the results of analyses of the organic matter in the guts reported. The species studied do not exhibit a strict alimentary diet; 16 types of presumably nutritive particles were distinguished in the foregut. The nutritional sources for these holothurians mainly consist of organo-mineral aggregates, faecal matter and organic incrustations on mineral particles. Selection is for those detritus particles which are richest in bio-available compounds; a negative selection for living organisms is apparent. The finest fraction of the sediment (which is also the richest in organic matter) is also ingested. The concentrations of organic carbon and nitrogen in the sediment found in the foreguts are about 4 times and 6 times greater, respectively, than the concentrations in the environmental sediment. During passage through the intestines, assimilation of organic carbon and nitrogen is 15 and 22%, respectively; assimilation is maximal in the ascending intestine loop.

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.

Organic carbon turnover time in deep-sea benthos

Abstract The ratio of organic carbon supply to the deep-sea floor estimated with sediment traps to total benthic biomass (in terms of organic carbon concentration) was used to calculate “average” turnover or residence times for the benthic community over a broad depth interval in the western North Atlantic. Ratios indicate that the biomass carbon turns over on average time scales of months, with little predictable variation over the depth interval (70m to 5300m). Detrital organic carbon in sediments at the same stations turned over on scales of years to centuries. The shortest detrital carbon turnover was just over 8 years in a sandy continental shelf environment versus 609 years at 5.3km depth on the Hatteras Abyssal Plain. The turnover time estimates calculated with POC fluxes were assessed by comparing POC fluxes with estimates of total community respiration. At each location the POC flux rates were greater than the sediment oxygen demand flux rates (in mg Carbon m−2d−1), and the disparity was much greater in the deep sea than on the continental margin. We suggest that this difference is primarily the result of a decline in the quality of the organic compounds in the rain of sinking POC, rather than physiological responses to cold or pressure. This implies that caution must be used when interpreting the relationships between the total organic carbon fluxes measured with sediment traps at great depths and rates of metabolic processes.

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.