Recent benthic foraminifera from the continental margin of northwest Africa: Community structure and distribution

Abstract

Sediment grab samples were collected at 107 locations along the continental margin of northwest Africa. These samples form a series of depth transects between the Straits of Gibraltar and Dakar, Senegal. The greater than 250 μm size fraction was retained for a census of both the live and dead foraminifera. After deleting trace occurrences, Jaccard and correlation coefficient-based cluster analyses were performed to decipher the community structure for this margin. The geographic distribution of the four major faunal provinces recognized is conventional in that for both the live and dead assemblages three are related to depth: upper slope and shelf, middle slope, and lower slope and continental rise. The biotopes and thanatotopes within these provinces are strongly restricted along vertical or latitudinal boundaries when the data are analysed using Jaccard coefficients. Dendrograms constructed from correlation coefficients emphasize depth-related faunal communities. Dominant species such as Cibicides lobatulus, Trifarina fornasinii, Planulina ariminensis, Uvigerina finisterrensis, and C. wuellerstorfi are generally distributed within a definite depth range along this margin and strongly influence the correlation coefficient-based dendrograms. Other distributions are clearly not depth-related but correspond to various environmental variables, for example: Cancris auriculus — coarse substrate; Bolivina subaenariensis — oxygen minimum; Cibicidoides kullenbergi and Hoeglundina elegans — low organic carbon. Uvigerina peregrina is also a notable exception to depth-dependence in that this dominant species and the province it represents are generally confined to the continental slope south of Cape Blanc. Its preference for that region may arise from the high organic carbon and fine grain-size of the sediment there, from low salinity of the bottom water, or from other unknown variables. The most important vertical faunal boundary occurs between Cape Barbas and Cape Blanc (between 22° and 21°N latitude). In contrast, the Canary Island Ridge intersects the continental margin at a saddle depth of about 1500 m, but that physiographic barrier has little effect on provincialism of the deep-water benthic foraminifera.