ABSTRACT: We investigated benthic foraminiferal densities, omposition, and microhabitats at three sampling stations on the Portuguese margin, at 980, 1860 and 3125m water depth. For each site, we studied two sets of three replicate cores, sampled during two successive multi-corer deployments. Our results show an overall trend of decreasing foraminiferal densities from the shallowest to the deepest site, in response to a decreasing labile organic matter flux to the sea floor. Faunas at 980m are strongly dominated by Uvigerina mediterranea, which shows a conspicuous faunal density maximum close to the sediment surface. At 1860m, the surface faunas are much poorer, and no longer dominated by a single taxon. Cibicidoides obertsonianus, C. kullenbergi, Uvigerina peregrina, Gavelinopsis translucens and Hoeglundina elegans are present in about equal quantities. The deep infaunal community at these two stations is dominated by Globobulimina affinis, and has comparable densities. At 3125m, the faunas are very poor in 5 of the 6 cores, and mainly consist of agglutinant taxa. At all three stations, important differences are observed between the 6 studied cores. At 980m, the faunal density is about 4 times higher in one of the cores, mainly due to high numbers of Uvigerina mediterranea. This could be explained by the position of this core in a topographical depression, where organic matter concentrates. At 1860m, two sets of three cores, taken during two successive multicorer deployments, contain different amounts of deep infaunal taxa. Finally, at 3125m, one of the 6 cores contains an exceptionally rich deep infaunal community (up to 450 individuals per 50cm), dominated by the calcareous taxa Fursenkoina bradyi, Globobulimina affinis and Chilostomella oolina. The fact that the subrecent fossil faunas of three studied cores from this site are all enriched in these taxa indicates that the observed high infaunal standing stocks are a recurrent phenomenon, in response to a currently unidentified process that strongly concentrates metabolisable organic matter in deeper sediment layers.
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