Abstract
A morphometric study on the polycystine radiolarian species Actinomma boreale (Cleve) from ten trigger weight core-tops from the Norwegian-Iceland Seas, three piston cores taken offshore western Norway and three surface sediment samples from Lygrepollen, Sogndalsfjord and Høyangsfjord (western Norwegian fjords) shows a variation in morphology that groups A. boreale into three distinct clusters, interpreted to be related to different oceanographic settings. The largest specimens of A. boreale are found in the western Norwegian fjords, the smallest in the Iceland Sea, giving an apparent positive correlation with temperature. Down core studies in piston cores from the Norwegian Sea demonstrate a considerable size variation of the cortical shell of A. boreale. In the eastern Norwegian Sea, the climatically cold Younger Dryas had a population of A. boreale that was characterized by large cortical shells, while the climatically warm Holocene population was dominated by small sized cortical shells, showing a negative correlation with temperature. We suggest that the large sized conical shell population of A. boreale in the Younger Dryas is not reflecting precisely the sea-surface water temperature. Another factor must play the dominant role here, probably nutrients.
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