Roselyne Buscail

Trophic control of benthic foraminiferal abundance and microhabitat in the bathyal Gulf of Lions, western Mediterranean Sea

Surface sediment was sampled at two bathyal sites in the southwestern Gulf of Lions in the western Mediterranean Sea in February and August 1997 to study the distribution and microhabitat of living (Rose Bengal stained) deep sea benthic foraminifera. Both standing stock and diversity of the faunas, and the microhabitat of distinct species mirror the trophic situation and the depth of the oxidised layer at the different sites. Our results suggest that the faunas do not comprise highly opportunistic species and are adapted to rather stable environments. In the axial channel of the Lacaze-Duthiers Canyon, organic matter fluxes are enhanced due to advective transport of organic matter resulting in elevated oxygen consumption rates in the surface sediment and a rather thin oxidised layer. The corresponding benthic foraminiferal fauna is characterised by rather high standing stock and diversity, and a well-developed deep infauna. In addition to freshly deposited phytodetritus, more degraded organic matter seems to be an important food source. In contrast, at the open slope, organic matter fluxes and oxygen consumption rates in the surface sediment are lower and the oxidised layer is much thicker than inside the canyon. The corresponding benthic foraminiferal fauna comprises mainly epifaunal and shallow-infaunal species with much lower standing stocks and clear differences between February and August. In August standing stocks are higher and the average living depths of most species shift towards the sediment surface. These differences can be attributed to patchiness or represent a seasonal trophic signal.

Particulate matter and organic carbon budgets for the Gulf of Lions (NW Mediterranean)

An analysis of sedimentological, sediment trap and hydrological data was performed to investigate the transport and fate of particulate matter in the Gulf of Lions. The sedimentological properties outline the major sedimentary units of the shelf (Rhone prodelta, mid-shelf mud belt, outer shelf) and slope. The geometry of these sedimentary units and the southwestwards increase of particulate fluxes on the slope highlight the influence of the general cyclonic circulation on the dispersion of land-derived particulate matter. Considering the known input and output terms, budgets of particulate matter and organic carbon in the Gulf of Lions are proposed. Inputs were river supply, atmospheric deposition and primary production; outputs were sediment burial and advective export on the slope. Degradation of particulate organic carbon in the water column and at the sediment-water interface was also estimated. Mass and POC budgets were balanced within uncertainties. Nevertheless, these results illustrate the difficulty in establishing budgets based on elements estimated at different time scales, for which the system is not necessarily in a steady state.

Fluxes and budget of organic matter in the benthic boundary layer over the northwestern Mediterranean margin

Sediment traps were deployed at depths of 26 and 645 m at two stations on the continental margin of the Gulf of Lions (northwestern Mediterranean). During the same period, surficial sediments were sampled by box corer. The material collected by bottom sediment traps and in corresponding surface sediments was analysed for total organic carbon, hydrolysable organic carbon, nitrogen, sugars, amino acids and lignin-derived compounds. Seasonal variations in organic inputs and the difference between particles from bottom layers and sediment were compared. For the continental shelf station, the annual averages of organic compound fluxes were found to be: 552 mg m−2 d−1 (orgC), 183 mg m−2 d−1 (N), 283 mg m−2 d−1 (hydrolysable orgC), 181 mg m−2 d−1 (Ceq. glucose) and 478 mg m−2 d−1 (amino acids). These values would have to be reduced by half if the large fluxes of autumn, due to resuspension during storm events, were excluded. For the slope, the average annual fluxes were evaluated as: 92.7 mg m−2 d−1 (orgC); 9.4 mg m−2 d−1 (N); 74.1 mg m−2 d−1 (hydrolysable orgC); 11.8 mg m−2 d−1 (Ceq.glucose); and 68.2 mg m−2 d−1 (amino acids). The values obtained for material trapped over the shelf are 4–7 times (orgC and amino acids) and 15–19 times (sugars and nitrogen) higher than for the slope. In contrast, the content in organic compounds of surficial sediments on the slope is 2–3 times higher than that of the shelf deposits. Budgets of orgC transformation at the sediment-water interface were based on calculations which include bottom orgC fluxes, sedimentation rates and orgC content for the first centimetre of deposits. For the continental shelf area, 5.3 g m−2 y−1 have accumulated and 16.7 g m−2 y−1 are mineralized. For the canyon and adjacent slope, the figures are 0.4 and 0.6 g m−2 y−1, respectively. Over the upper adjacent slope, the major part of organic matter is transported by advective processes, which contribute to the sediment interface, especially by chanelling through the canyon axis. Advective inputs of fresh organic material are maximized in late winter-early spring. Nearly 50% of the continental shelf organic matter is assumed to be exported from the shelf, to the adjacent slope.

Origin and distribution of terrestrial organic matter in the NW Mediterranean (Gulf of Lions): Exploring the newly developed BIT index

The Branched and Isoprenoid Tetraether (BIT) index is based on the relative abundance of nonisoprenoidal glycerol dialkyl glycerol tetraethers (GDGTs) derived from organisms living in terrestrial environments versus a structurally related isoprenoid GDGT ‘‘crenarchaeol’’ produced by marine Crenarchaeota. The BIT index varies between 0 and 1, representing marine and terrestrial organic matter (OM) end-members, respectively (Hopmans et al., Earth Planet. Sci. Lett., 224, 107–116, 2004). In this study, the applicability of the BIT index to trace terrestrial OM is tested in combination with other organic parameters (TOC, C/N ratio, δ13Corg, total lipid, and n-alkane) in the Gulf of Lions, a river-dominated continental margin of the western Mediterranean. We analyzed a variety of soils and riverbed sediments from the continent as well as surface sediments from the shelf and canyons. The BIT index in soils and riverbed sediments shows high values (>0.9), while it varies between 0.02 and 0.83 in marine sediments, decreasing seaward from the inner shelf to the slope. For marine surface sediments, high BIT values are associated with lower δ13Corg values as well as higher TOC contents and higher n-alkane concentrations. Our results confirm that the BIT index can be applied in coastal marine environments in order to characterize terrestrial OM as proposed by Hopmans et al. (2004). Therefore the BIT index is a useful addition to the proxies presently available for studying the origin and distribution of OM in continental margins and especially valuable in multiproxy studies.

Seasonal variability of the organic matter in a sedimentary coastal environment: sources, degradation and accumulation (continental shelf of the Gulf of Lions—northwestern Mediterranean Sea)

The temporal variations of the superficial (0–1 cm) sedimentary organic matter were studied at a depth of 26 m on the continental shelf of the Gulf of Lions (northwestern Mediterranean). The samples were analyzed for total organic carbon, coarse organic carbon (>40 μm), hydrolyzable organic carbon, nitrogen, total amino acids, total and individual sugars (HPLC), lignin-derived compounds (HPLC) and kerogens (acid-soluble, humic substances and humin). Seasonal variations of the organic compounds are related to the sedimentological, hydrodynamical and physico-chemical environmental conditions. The mean annual values of the different organic compounds analyzed show the low quantities and their evolved character at the sediment-water interface: 0.5% total org C (TOC) (d.w.), 0.049% N (d.w.), CN: 11.2, coarse org C (COQ: 62% of TOC, hydrolyzable org C: 45% TOC. The labile compounds represent a low percentage of the total organic matter (TOM), amino-acids: 12% of TOM and sugars: 5% of TOM. The relative proportions of soluble (humic) and insoluble kerogens (humin), respectively 6% and 94% of TOC are typical of a highly evolved organic matter. The large contribution of plant remains confirmed by the high proportion of COC, corresponds to a low proportion of humic substances and a high degree of condensation (HC = 1.3). The infrared spectroscopy determination of the functional groups of the humic substances permits us to confirm both autochtonous (marine) and allochtonous (terrestrial) sources of organic matter in the Tet prodeltaic accumulation area. Numerous functional groups identified reveal the fresh quality of the organic inputs at the sediment-water interface. Aliphaticity is well marked and nitrogenous compounds (1 and 2 amines) correspond to autochtonous production (in spring: phyto- and zoo-planktonic blooms in the euphotic zone; in summer: primary production under the thermocline and phytobenthic blooms). Sugars are well represented, but from two origins: phytoplanktonic, and issue from terrestrial high plants (ligno-cellulosic complexes). The aromatic functional groups are well identified (aromatic ether, phenol) as are the markers of a large contribution of terrestrial organic matter. The active hydrodynamical (resuspension) and biological, (consumption, bioturbation) events participate in the frequent reworking of the superficial deposits (muddy-silts) and, consequently, the active degradation (oxidization) of the organic matter. The more labile organic matter is rapidly consumed by the intense biological benthic activity. The organic C content is low (0.5% d.w.) except during the autumnal input (2.5% d.w.). This input (8 cm thick) disappears one month later, because of the resuspension induced by littoral currents. This same phenomenon induces during all the year the reworking of the first centimeter of the deposit. In terms of budget of the carbon cycle in the coastal area, this active hydrodynamical environment shows the supremacy of the carbon degradation over its burial.

Organic matter distribution in burrows of the thalassinid crustacean Callichirus laurae, Gulf of Aqaba (Red Sea)

AbstractAnalysis of sediments in and around the large and deep burrows made by the callianassid shrimp Callichirus laurae, Jordan Gulf of Aqaba (Red Sea), showed that organic carbon (OC), humic matter (HM), humic (HA) and fulvic acids (FA) are redistributed both quantitatively and qualitatively by sediment reworking activities: 1.OC and humic matter (HM) concentrations are ca. 30 times higher in the stomach content than in surface sand, proving a selective food uptake.2.a 11 to 17 times increase in OC and HM is observed in the mucus-rich burrow wall when compared to ambient sediment. Among humic substances, FA seem preferentially absorbed by the shrimp, while HA are concentrated in the fecal pellets.The spatial distribution of organic matter in the form of a 3D network is discussed in relation with sampling procedures. Due to inadequate sampling gears, organic rich networks such as those of thalassinid shrimps are rarely considered in conventional sampling strategies, leading to an underestimation of the sediment organic content.

210Pb, manganese and carbon: indicators of focusing processes on the northwestern Mediterranean continental margin

Abstract Organic carbon contents, 210 Pb data, and organic and oxidised forms of manganese in sediments are used for reconstructing the shelf and deep-sea transport of particulate matter and for evaluating early diagenesis conditions. The latter are used as indicators of focusing processes on the northwestern Mediterranean continental margin. Twelve different physiographical sites were sampled between 400 and 2000 m in the Pyrenean and Golfe du Lion canyon axes and on their open-slopes. In the canyon axes, which indent the Pyrenean slope, 210 Pb activities show a regular exponential decrease with sediment depth: computed sedimentation rates range from 170 to 198 cm 10 −3 yr. For an identical regular exponential decrease in the Golfe du Lion canyon, the sedimentation rate is lower (119 cm 10 −3 yr). On the lower open slopes and deep-sea fans, rapid deposition (turbidites and surficial sliding) occurs onto sediments that were deposited regularly at a rate of about 175 cm 10 −3 yr. A deposit enriched in organic carbon is identified at mid-slope between 500 and 1300 m. In this area, surface sediments of the canyon axes are richer in organic carbon than at the adjacent open slope sites (0.9% against 0.6% dry weight). Canyon axes are more active than open-slopes as channels for organic matter transfer. In the axial channel of the Pyrenean canyon, organic carbon decreases exponentially in the uppermost 12 cm. In the Golfe du Lion canyons, only the upper 2 cm of sediment are enriched in organic carbon. This is related to lower sedimentation rates in the Golfe du Lion area as compared to the Pyrenean area. On the upper and lower open slopes, the organic carbon contents decrease rapidly within 1–2 cm of sediment. There is no exponential evolution profile and the residence times at the sediment-water interface are longer than in the canyons. The highest surface Mn concentrations are also found in the middle of the continental slope. The average is 64 μg g −1 for Mn associated with the organic phase (Mn(org)) and 1400 μg g −1 for (Mn(Ox)). As for organic carbon contents, canyons are areas of high concentrations. The lowest values (4 μg g −1 for Mn(org) and 495 μg g −1 for Mn(Ox)) are found for the lower open-slope (1200–1700 m) where there is active surficial remobilisation and gravity mass movements. Moreover, the vertical distribution of Mn has allowed us to identify sites located at depths of about 1000 m where the redox discontinuity is located in the first few centimetres and where the (Mn(Ox)) and the (Mn(org)) profiles display a positive gradient from the deepest layers to the sediment-water interface. The distribution of the various parameters was caused by three different mechanisms: (1) bottom advection of particulate matter transferred from the continental shelf deposits to the slope along the canyon axes; (2) scavenging of the dissolved phases of metals and (3) sedimentary focusing of the particles in a “favoured” area of the continental slope—a mid-slope depocenter. These data contribute to our knowledge of the role of continental margins in the global recycling of carbon and other materials.

Seasonal survey of metazoan meiofauna and surface sediment organics in a non-tidal turbulent sublittoral prodelta (northwestern Mediterranean)

As part of the French research program ECOMARGE, metazoan meiofauna abundances along with local physical, chemical and microbial characteristics were monitored generally monthly over a 14-month period in a silty-muddy prodelta located at 26 m depth off the Teˆt River outlet in the northwestern (NW) Mediterranean. Meiofauna densities ranged from 1266 to 5586 individuals 10 cm−2, and were dominated by nematodes (91–98%), followed by harpacticoid copepods (1.37–6.96%) and kinorhynchs (up to 1.40%). Bottom water temperature varied between 9.5°C in winter to over 20°C in early autumn. All parameters analysed in the surface sediment also exhibited high temporal variability: fine particle content (7.6–18.4% < 40 μm), alcium carbonate (5–8.6%), total organic carbon (4.1–7.3 mg g−1), organic nitrogen (0.35–0.95 mg g−1), C:N ratios (5.6–19.7), total amino acids (159–2358 μg g−1) and sugars (315–1553 μg g−1), lignin by-products (6–61 μg g−1), chlorophyll- a (0.30–1.20 μg g−1), phaeopigments (1.12–3.62 μg g−1), total chloroplastic pigments (1.42–4.58 μg−1), viable bacterial numbers (1.9 x 105–3.9 x 107 Colony Forming Units ml−1), microheterotrophic total uptake (18.3–74.2% after 5 h) and respiration (5.2–28.8%) of14C-glucose. Seasonal trends were not always obvious, and some variables were significantly correlated with each other. Because the survey was carried out in a complex system affected by strong frequent winds and irregular discharges of the river, different expressions of these climatic factors were included in the data set in an attempt to understand the relationships between variables and ultimately highlight the forces driving the observed abundance patterns. Further analysis showed that: (1) most of the parameters were strongly influenced by the climatic events that occurred prior to sampling, and (2) most of the correlations were largely explained by the concomittant influence of these events on the corresponding variables. Although some parameters were enhanced, many were affected negatively due to physical disturbance (mixing, resuspension, erosion). The observed patterns and relationships appeared to result primarily from the high physical variabilily of the system combined with a low sampling frequency.

Deep-sea foraminifera from the Cassidaigne Canyon (NW Mediterranean): Assessing the environmental impact of bauxite red mud disposal

Benthic foraminiferal assemblages were investigated from two sites along the axis of the Cassidaigne Canyon (NW Mediterranean Sea). Both areas are contaminated by bauxite red mud enriched in iron, titanium, vanadium and chromium. These elemental enrichments are related to bauxite-derived minerals and various amorphous phases. At the shallowest station located very close to the pipe outlet, the benthic living foraminiferal community is characterised by a very low diversity and by an unusual dominance of Gyroidina umbonata and Bulimina marginata. The mechanical stress related to downslope transport of red mud is a likely source of hydro-sedimentary pollution precluding the settlement of diverse fauna. The living and dead foraminiferal faunas from the deepest site are typical of oligo-mesotrophic conditions prevailing in natural environments. There, bauxite residues have obviously no environmental impact on foraminiferal faunas. The bioavailability of trace metals is likely low as elemental enrichments were not observed in foraminiferal test chemistry.

Unexpected biotic resilience on the Japanese seafloor caused by the 2011 Tohoku-Oki tsunami

On March 11th, 2011 the Mw 9.0 2011 Tōhoku-Oki earthquake resulted in a tsunami which caused major devastation in coastal areas. Along the Japanese NE coast, tsunami waves reached maximum run-ups of 40 m, and travelled kilometers inland. Whereas devastation was clearly visible on land, underwater impact is much more difficult to assess. Here, we report unexpected results obtained during a research cruise targeting the seafloor off Shimokita (NE Japan), shortly (five months) after the disaster. The geography of the studied area is characterized by smooth coastline and a gradually descending shelf slope. Although high-energy tsunami waves caused major sediment reworking in shallow-water environments, investigated shelf ecosystems were characterized by surprisingly high benthic diversity and showed no evidence of mass mortality. Conversely, just beyond the shelf break, the benthic ecosystem was dominated by a low-diversity, opportunistic fauna indicating ongoing colonization of massive sand-bed deposits.