Claudia Schroder

Recent deep-water benthonic foraminifera were analysed from 25 box-cores taken on the Continental Rise of Nova Scotia, the Southern Bermuda Rise, and Nares Abyssal Plain. The samples cover a depth range from 2225m to 2779m. Approximately 265 species including 104 arenaceous taxa (related to 10 families) were identified. The arenaceous taxonomy stresses morphotype variability as a function of sediment-type. Classical deep-water collections by H. B. Brady, E. Heron-Allen, A. Earland, and J. A. Cushman served as taxonomic model and were critically reexamined. The percentage of arenaceous foraminifera is increasing progressively with depth. Contour currents strongly influence the nature of substrate which in turn influences the composition of the foraminiferal community. The region below the axis of the Western Boundary Undercurrent (WBUC, 2700-4000m) is subject to winnowing processes which results in an increased Foraminiferal Number and a high species diversity. Fragile arenaceous species such as Rhizammina algaeformis seem to prefer zones without major current activity and consequently dominate on the upper Nova Scotia Rise (2200-2500m) above the zone of the WBUC and in abyssal regions. The middle rise arenaceous assemblage, living on a coarse grained substrate, is characterized by coarse grained taxa such as Hyperammina sp.1, Rhabdammina sp. and Astrorhiza crassatina and complemented by species with robust, sphereical tests such as Cribrostomoides subglobosus, Recurvoides scitulus, and Trochammina cf. globigeriniformis. Arenaceous species such as Reophax scorpiurus, Reophax bilocularis, Psammosphaera fusca, and Lagenammina tubulata are non-selective in their choice of wall material. These species reflect the sediment substrate, resulting in a large intraspecific variability. The arenaceous assemblage on the Nares Abyssal Plain is characterized by a fragile network of delicate species of the family KOMOKIACEA. Other common abyssal species are Adercotryma glomerata, Nodellum membranceum, Ammomarginulina foliacea, and Cystammina galeata. Results suggest that arenaceous foraminifera are more likely controlled by the substrate than by watermasses. Depth ranges, particularly of non-selective taxa, are extended at the lower limit by comparing with published data.

Modern calcareous species form an important component on the Nova Scotia Rise, where 3 assemblages have been identified. The upper continental rise assemblage (2220-2500m) is dominated by Elphidium excavatum and Uvigerina peregrina associated with the occurrence of the NADW. Increased numbers of Stainforthia concava and Hoeglundina elegans seem to indicate a transitional mixed watermass of the NADW and the NSOW. The middle continental rise assemblage (2750-3600m) is dominated by Eponides pusillus and controlled by NSOW carried southward by the WBUC. With the approach of the lysocline the calcareous assemblage becomes less diverse. Remaining species such Epistominella umbonifera and Epistominella exigua are abundant and indicate the influence of AABW.

The vertical change in sediment in the upper 30 cm is compared with changes in the arenaceous fauna. The effects of selective preservation, chemical solubility of the ferruginous compound in the cement of various species and shifts through transport and reworking processes are reflected in the vertical distribution of the arenaceous assemblages. Based on test construction and depth distribution pattern arenaceous species are ranked into three major classes reflecting fossilization potential.

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Pages: 307
Supervisor:  F. Gradstein