Cassidulina teretis Tappan and C. laevigata d'Orbigny; their modern and late Quaternary distribution in northern seas

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Abstract
The two morphologically close benthic foraminiferal species, Cassidulina laevigata and C. teretis, were studied in 146 surface-sediment samples in the Norwegian Sea and adjacent shelf areas and in 36 stratigraphic samples from the northern Norwegian continental shelf. We differentiated these two species by their umbilical regions, pore distribution patterns and apertures. The characteristic boss-like umbilical structure of C. teretis is clearly visible in specimens stained with rose Bengal. Q-mode principal component analysis of both the "living" data and the "dead" data reveals distinct distribution patterns of C. laevigata and C. teretis, each of which dominates a characteristic assemblage of foraminiferal species. Cassidulina teretis dominates the benthic foraminiferal assemblage on the continental slope of the Norwegian Sea where temperatures of the bottom water masses are about -1.degree. C and salinities are 34.92.permill.. It is also an important constituent of the benthic foraminiferal fauna in the Barents Sea north of 72.degree. N. Cassidulina laevigata strongly dominates the benthic foraminiferal assemblage on the northern North Sea plateau and is a minor constituent of the benthic foraminiferal assemblage on the Norwegian continental shelf as far north as 72.degree. N. This distribution coincides with the sea floor covered by warm and relatively saline bottom water of Atlantic origin. In upper Quaternary sediments on the continental shelf off northern Norway, C. teretis was present in various types of glaciomarine paleoenvironments. Cassidulina teretis declined markedly in the early Holocene, whereas C. laevigata increased correspondingly. At 7,800 yr BP, C. laevigata increased even more strongly. The amelioration of the Holocene climate has favored C. laevigata on the continental shelf off Norway and forced C. teretis to retreat down the continental slope and into the northern Barents Sea.