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Mojtahid M.,University of Angers | Mojtahid M.,French Climate and Environment Sciences Laboratory | Mojtahid M.,McGill University | Jorissen F.,University of Angers | And 3 more authors.
Journal of Foraminiferal Research | Year: 2010

The microhabitats and the composition of living benthic foraminiferal faunas in the sediments deposited off the Rhône River mouth are directly influenced by the Rhône River input. In this shallow-water environment (20-98 m water depth), the vertical distribution of the species is not well defined, probably due to the low penetration of oxygen into the sediment. We show the existence of two types of species: "predominantly superficial" taxa showing a density maximum in the topmost sediment layer, and "potentially/ predominantly infaunal taxa" that are also frequent in the topmost sediment, but which show considerable densities in anoxic deeper sediment layers as well. In the area that is strongly influenced by river input (near the river mouth and in the southwestern direction, following the river plume), the fauna is composed mostly of "potentially/predominantly infaunal" species adapted to a higher contribution of terrestrial organic matter, generally of lower quality, and to the low penetration of oxygen into the sediment. This fauna is composed of species tolerant to strong environmental stress (e.g., Nonionella turgida, Nonion scaphum, Rectuvigerina phlegeri, and Valvulineria bradyana). The stations less influenced by fluvial input (located south and east of the river mouth) are composed of species that colonize oxygenated interstices in the upper centimeter of sediment. The dominant species are mainly "predominantly superficial" taxa (e.g., Cassidulina carinata, Bulimina aculeatd), which are known to react quickly to provisions of labile organic matter. The correlation between these two types of faunas and the two prevalent environmental factors suggests that the vertical distribution of living foraminifera in front of the Rhône River mouth is primarily controlled by the quality of the organic matter and less by the quantity of the organic matter and depth of oxygen penetration into the sediment. Source

Mojtahid M.,UK National Oceanography Center | Mojtahid M.,University of Angers | Mojtahid M.,Laboratoire detude des bio indicateurs Marins LEBIM | Griveaud C.,University of Angers | And 6 more authors.
Revue de Micropaleontologie | Year: 2010

In a 10-stations bathymetrical transect in the Bay of Biscay, we observed important changes in the density, composition and microhabitats of live foraminiferal faunas from the outer continental shelf to the abyssal plain. Four zones are recognised: (1) at the upper continental shelf (140. m water depth), foraminiferal densities are very high and the superficial sediment is occupied by Bolivina subaenariensis and Valvulineria bradyana. Globobulimina spp., Chilostomella oolina and Nonion fabum dominate the infaunal niches, which are positioned close to the sediment-water interface due to a strong compaction of the vertical succession of redox zones. (2) At the upper continental slope stations (300-1000. m), foraminiferal densities are high and the superficial sediments are dominated by Uvigerina mediterranea/peregrina. Deeper in the sediment, intermediate infaunal niches are occupied by Melonis barleeanus. Due to a deeper oxygen penetration, the deep infaunal taxa Globobulimina spp. and C. oolina live at a considerable depth in the sediment. (3) At the mid and lower slope stations (1000-2000. m) in the superficial sediment Cibicidoides kullenbergi and Hoeglundina elegans progressively replace U. mediterranea. U. peregrina is still a dominant taxon, reflecting its preference for a somewhat intermediate organic flux level. Deep infaunal taxa become increasingly rare. (4) At the lower slope and abyssal plane stations (deeper than 2000. m), faunal densities are very low and the fauna is composed exclusively by shallow infaunal species, such as Nuttallides umboniferus and Melonis pompilioides. The foraminiferal data together with the pore water data in the sediment give evidence of the presence of a trophic gradient from very eutrophic settings at the upper continental shelf towards oligotrophic settings at the abyssal area. © 2010 Elsevier Masson SAS. Source

Mojtahid M.,French National Center for Scientific Research | Jorissen F.J.,French National Center for Scientific Research | Garcia J.,French National Center for Scientific Research | Garcia J.,Laboratoire detude des bio indicateurs Marins LEBIM | And 11 more authors.
Palaeogeography, Palaeoclimatology, Palaeoecology | Year: 2013

Core KS10b (North East Atlantic, 43°50'N, 2°03'W, water depth 550. m) was recovered at a location of high sediment deposition, and hence includes a high-resolution palaeoclimatic record for the southeastern Bay of Biscay over the last 9000 cal yr BP. The effect of global and regional climatic forcing factors (e.g., orbital forcing, North Atlantic Oscillation: NAO) and their subsequent control on water temperature, sea-level, hydrology and continental influence were investigated by studying planktic and benthic foraminiferal faunas (> 150 μm), sedimentology, and stable isotopes. Results indicate probable episodic long-term incursions of the warm and salty Iberian Poleward Current (IPC) into the Bay of Biscay during the Holocene by the intermittent presence of subtropical Globigerinoides ruber in the otherwise temperate fauna. These incursions seem to be triggered by negative NAO-like conditions. Our data show five main climatic periods, which are in general agreement with the literature data on the climatic variability in the North Atlantic and the Iberian Peninsula: The early Holocene (~ 9000-7400 cal yr BP) is characterized by low sea-level, significant downslope transport, eutrophic benthic settings and high surface water productivity. These environmental characteristics are probably due to the general wet and warm climate under a prevailing negative NAO, increasing precipitation and river runoff and favouring the incursion of the IPC into the Bay of Biscay. The climatic optimum appears between ~ 7400 and 6000 cal yr BP under a generally positive NAO index. In the following (~ 6000-3500 cal yr BP), surface water cooling is matching a steep decline of the Northern Hemisphere summer insolation. Meanwhile, benthic settings are changing progressively towards present day trophic conditions. The time interval from ~ 3500 to 1800 cal yr BP encompasses the Iberian-Roman Humid period characterized by warm and humid conditions, and increasing primary production under a prevailing negative NAO index. The last 1800. years were characterized by stable cool conditions, reflecting a change towards a positive NAO state. © 2013 Elsevier B.V. Source

Goineau A.,CNRS Nantes Laboratory of Planetology and Geodynamics | Goineau A.,Laboratoire detude des bio indicateurs Marins LEBIM | Fontanier C.,CNRS Nantes Laboratory of Planetology and Geodynamics | Fontanier C.,Laboratoire detude des bio indicateurs Marins LEBIM | And 16 more authors.
Biogeosciences | Year: 2012

In the context of the French research project CHACCRA (Climate and Human-induced Alterations in Carbon Cycling at the River-seA connection), living (rose Bengal-stained) benthic foraminifera were investigated at two stations (24 and 67m depth) in the Rhône prodelta (NW Mediterranean, Gulf of Lions). The aim of this study was to precise the response of benthic foraminiferal faunas to temporal changes of the Rhône River inputs (e.g. organic and terrigeneous material). Each site was sampled in April 2007, September 2007, May 2008 and December 2008, permitting to observe foraminiferal faunas of the 63-150 and >150 μm size fractions under a wide range of environmental conditions. Obvious variations in foraminiferal faunal composition were observed during the four investigated periods at the shallowest Station A located in the close vicinity of the Rhône River mouth. After major Rhône River flood events, different colonisation stages were observed with foraminiferal faunas responding with an opportunistic strategy few days to weeks after the creation of a peculiar sedimentary environment (Leptohalysis scottii, May 2008) or high organic matter supplies (Ammonia tepida, December 2008). Under more stable conditions, relatively diverse and equilibrated faunas grew in the sediments. Species benefited from noticeable input of riverine phytodetritus to the sediment during spring bloom conditions (April 2007; e.g. Bolivina dilatata, Nonionella stella, Stainforthia fusiformis), or high amounts of still bio-available organic matter under more oligotrophic conditions (September 2007; e.g. Ammonia tepida, Psammosphaera fusca). The reduced influence of the Rhône River input at the farther Station N led to less contrasted environmental conditions during the four sampling periods, and so to less obvious variations in foraminiferal faunal composition. During reduced riverine influence (i.e. low Rhône discharge), species able to feed on fresh phytodetritus (e.g. Clavulina cylindrica, Hopkinsina atlantica, Nonionella iridea and Nonionella turgida) benefited from eutrophic conditions of the spring bloom (April 2007, May 2008). Conversely, the occurrence of Nouria polymorphinoides under oligotrophic conditions (September 2007, December 2008) was indicative of a benthic environment potentially disturbed by bottom currents. This study put into evidence the extremely rapid response of benthic foraminiferal faunas to strong variations in environmental conditions mostly induced by the Rhône dynamics. © Author(s) 2012. Source

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