Showing papers by "Frans Jorissen published in 2005"

Benthic foraminiferal recovery after recent turbidite deposition in cap breton canyon, bay of biscay

Abstract: The vertical distribution of living (rose Bengal stained) benthic foraminifera from three multiple cores from Cap Breton Canyon, Bay of Biscay, France, has been investigated with the objective of monitoring the recolonization and subsequent evolution of the foraminiferal fauna following the deposition of a turbidite layer. The first samples, taken in May 2000 in the axis of Cap Breton Canyon, contained a young turbiditic sequence, most likely deposited during the heavy storm of December 1999. Four months after this sedimentological event, the composition of the living benthic foraminiferal fauna was almost monospecific in the >150 μm fraction, which contained mainly Technitella melo, a species which is otherwise very rare or absent in the Cap Breton Canyon and open slope assemblages. This species was accompanied in the 63–150 μm fraction by adult specimens of Cassidulina carinata and Fursenkoina bradyi, and exclusively juvenile specimens of Bolivina subaenariensis and Bulimina marginata. This fauna represents the first stage of foraminiferal colonization after the turbidite deposition. The samples taken one year later, in June and September 2001, at approximately the same location, contained a more variable foraminiferal assemblage strongly dominated by Bolivina subaenariensis. Foraminiferal assemblages in samples taken just below the successive turbidite sequences contained nearly the same faunal elements as the surface assemblages sampled in 2001. The benthic foraminiferal assemblages from the canyon axis sampled in 2001 show the same composition as other canyon axis faunas dominated by B. subaenariensis. We suggest that the recovery of the foraminiferal faunas in this extremely unstable environment takes about 6–9 months, and that the community structure more or less permanently stays in an early stage of ecosystem recolonization.

Living benthic foraminifera from “la grande vasière”, french atlantic continental shelf: faunal composition and microhabitats

Abstract: The French continental shelf of the Bay of Biscay is occupied by a large body of silty sediments called “la Grande Vasiere”. In the northern part of the Bay of Biscay, the living benthic foraminifera, of the 63–150 μm and >150 μm size fractions, inhabiting this outer shelf silt belt have been studied at four stations (100–130 m deep), together with some environmental parameters (grain size, carbon content, pore water oxygen and nitrate + nitrite content). The faunas of “La Grande Vasiere” are characterized by a high foraminiferal density in the 63–150 μm size fraction, which is often dominated by Nonionella iridea. Although the ecology of this species is not well known, its appearance seems to be related to pulsed organic supplies. In our study area, this species is dominant at three stations, but almost absent at a fourth one, where the organic carbon content is minimal. The foraminiferal microhabitat is, for most taxa, restricted to the top four centimeters: only Stainforthia fusiformis has a distinctly deeper microhabitat (mean average living depth = 4.5 cm). Although free oxygen is limited to the uppermost centimeter, a major part of the foraminifera occurs between 1 and 4 cm deep, in an interval with maximum pore water nitrate + nitrite values. The nitrate + nitrite profiles allow us to distinguish between stations A and B, both at a depth of about 100 m. In the days prior to sampling, the organic matter flux seems to have been less intensive at A and B than at stations C and D (depth of 130 m), where the nitrate + nitrite maxima are positioned just below the sediment-water interface. The distinction between these two pairs of stations is confirmed by the faunal composition, with much lower densities of supposedly opportunistic taxa (such as N. iridea ) at stations A and B, and by the faunal penetration depth, with much deeper penetration at stations C and D. Our results suggest a strong impact of seasonal outer-shelf phytoplankton bloom events on the composition and density of the benthic foraminiferal faunas.