Living small-sized (63-150 µm) foraminifera from mid-shelf to mid-slope environments in the Bay of Bisca

Live (rose Bengal stained) foraminiferal faunas of the 63–150 μm size fraction have been investigated in surficial sediment (0–1 cm) from mid-shelf to mid-slope environments in the Bay of Biscay. Eleven stations were sampled in April 2002 and March 2004 between 80 and 2000 m water depth (mwd). Earlier studies on the temporal variability of phytoplankton primary production suggest that our stations were sampled at the most eutrophic period of the year. In response to the decrease of exported organic matter flux to the seafloor along our bathymetric transect, foraminiferal standing stocks decrease from ~1400–2000 specimens per 50 cm3 on the continental shelf (100–140 mwd) and upper slope (550 mwd) to about 400 specimens per 50 cm3 at mid-slope stations (2000 mwd). At all stations, the faunas contain an important amount of small opportunistic species that are favored by seasonal phytodetritus input. On the continental shelf where phytoplankton bloom, events may be geographically restricted; the foraminiferal response is dependent on the distance to the surface-water primary-production cells. Textularia porrecta is very abundant at an 80-m-deep station that is close to the coast and characterized by a high sedimentation rate of fine-grained particles. Foraminiferal faunas are dominated by Nonionella iridea, Cassidulina carinata and Bolivina ex. gr. dilatata at the outer-shelf stations (110–140 mwd) that are under the direct influence of spring bloom phytodetritus input in the northern Bay of Biscay. A fauna dominated by Bolivina dilatata/spathulata and Bolivina subaenariensis is found in the southeastern Bay of Biscay at a 140-m-deep outer-shelf station located seaward of the Adour River estuary, where the sediment is probably enriched in terrestrial organic matter. Apparently, differences in foraminiferal composition between outer-shelf areas in the northern and southeastern Bay of Biscay are related to differences in organic matter quality. On the continental slope, a bathymetric zonation of taxa is observed from upper-slope sites (550–1000 mwd) rich in Epistominella exigua and Uvigerina peregrina to mid-slope stations (1600–2000 mwd), where Nuttallides pusillus and Gavelinopsis translucens dominate the small-sized living fauna. This bathymetric foraminiferal zonation probably reflects a trophic gradient between upper-slope eutrophic stations and mid-slope, more oligotrophic sites. Our zonal description of small-sized living foraminifera (63–150 μm) is new for the Bay of Biscay and may provide the basis to reconstruct former export production regimes in marginal paleo-environments from temperate latitude areas

Spatial and temporal distribution of benthic foraminiferal faunas in the Bay Biscay

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Live foraminiferal faunas from a 2800 m deep lower canyon station from the Bay of Biscay : Faunal response to focusing of refractory organic matter

International audienceA 2800 m deep station was sampled on three occasions, in January 1999, June 1999 and April 2000, in the lower part of Cap-Ferret Canyon (Bay of Biscay). This area is characterised by a rapid accumulation of fine-grained sediments and by important inputs of reworked organic matter in an intermediate state of decay. Diagenetic reactions within the sediment follow the well-established depth sequence resulting from the oxidation of organic deposits by different electron acceptors. At our station, live benthic foraminiferal faunas differ strongly from faunas previously collected at nearby open slope sites at a comparable water depth. Spectacularly high densities of deep infaunal species are observed in the deeper parts of the sediment for all three sampling periods. In our opinion, these high deep infaunal densities are a direct response to the massive flux of partially degraded organic matter, which is slowly introduced into the deeper parts of the sediment, where it induces a rather stable succession of redox gradients. Melonis barleeanus lives in the dysoxic part of the sediment whereas Globobulimina affinis appears preferentially close to the zero oxygen boundary. Both taxa occupy niches where the highest content of Mn (III, IV)-oxides and -oxihydroxides and Fe (III)-oxides are recorded. The fact that most of the geochemical reactions within the sediment are directly or indirectly catalysed by heterotrophic and chemolithoautotrophic bacterial consortia could suggest that deep infaunal foraminifera may be highly specialised protozoans able to feed on, or live in symbiosis with these prokaryotic communities

Sedimentary structure of inferred cyclic‐step bedforms in submarine volcaniclastic slope deposits, Cuatro Calas, south‐east Spain

Cyclic steps are widespread on submarine slopes of many modern insular volcanoes. This paper provides the first detailed description and interpretation of the sedimentary structures and depositional architecture of cyclic-step deposits of such bedforms formed on the submarine slope of an ancient volcano. The partially depositional cyclic steps are preserved in a 67 m thick coset of 1 to 12 m thick cobble-based units of middle Miocene submarine volcaniclastics, exposed along a cliff outcrop in south-east Spain. The main structure in the units is unidirectional crude low-angle cross-bedding passing upward to centimetre to decimetre-scale diffuse stratification more or less parallel to the unit bounding surfaces. The depositional architecture produced by inferred sinuous to straight-crested cyclic steps is compared with deposits of crescent-shaped cyclic steps formed in confined settings. With a novel method, a maximum cyclic step height and length of 22 m and 460 m, respectively, have been calculated. The architecture of some of the thicker cyclic-step units is complicated by structures that were formed as cyclic-step trough-fills, by superimposed cyclic steps or downstream migrating antidunes. These structures possibly reflect adaptation processes of the bedform morphology to a lower strength of the hydraulic jumps and related density flows. In the upper, less well-exposed part of the succession more steeply inclined gravel backsets that probably represent deposits of crescent-shaped cyclic steps accreted in a more energetic, confined setting proximal to the coastal source of the density flows. A facies model of straight-crested cyclic steps is presented that may aid in the identification of similar bedforms in submarine volcaniclastic environments and comparable non-volcanic settings