Nordic Seas

Definition of Area
The Nordic Seas (Greenland, Iceland and Norwegian Seas) are located west of Norway and east of Greenland, and are bounded by the Barents Sea, Spitsbergen and the Fram Strait in the north, and by Iceland, Faeroe- and Shetland Islands in the south. The area is characterized by great lateral variation in oceanographic setting and bathymetry.

Hydrography
According to Swift and Aagaard (1981) there are three major surface water masses present in the Nordic Seas today:
1) The North Atlantic Water, which is relatively warm (>3°C) and saline (>34.9 psu), entering the area from the south as the Norwegian Current, continuing through the eastern part of Norway- and Lofoten Basins before it branches into the Barents Sea and through the Fram Strait into the Arctic Ocean;
2) The Polar Water, with a temperature below 0°C and a salinity of <34.4 psu, which originates in the Arctic Ocean, and is transported by the East Greenland Current into the Nordic Seas along the east coast of Greenland. The Polar Water masses dominate the surface waters in the Greenland Basin and the in the western part of the Iceland Plateau;
3) The Arctic Sea Water, which is a mixture of the Atlantic- and the Polar Water masses, having a salinity between 34.4 and 34.9 psu and a temperature generally below 0°C. This water masse can be found in the eastern Greenland Sea and on the eastern part of the Iceland Plateau.

While the Norwegian Current is moving northward into colder areas, it releases heat to the atmosphere and cools off. This decrease in temperature together with the high salinity content cause the dense water to sink and form a southward flowing body, which later becomes part of the North Atlantic Deep Water.

Fauna
There are large differences in radiolarian faunas within the Nordic Seas probably due to the different water masses present, and the variations in temperature and nutrition supply. Cortese et al. (2003) studied the distribution of radiolarian species in 160 surface sediment samples collected in the area and recognized (by using Q-mode factor analysis and cluster analysis) four assemblages/factors, each related to different oceanographic conditions. A short description of the results follows:

Factor 1 and Station Cluster A, both located on the Iceland Plateau, are believed to represent cold Polar and Arctic water masses. Amphimelissa setosa dominates this radiolarian assemblage, having the highest varimax factor score (4.732). Other important species are the Pseudodictyophimus gracilipes Group (2.034) and Lithocampe platycephala (1.731).

Factor 2 and Station Cluster C are mainly located in the Greenland Sea, and are associated with the gyre formed by the Jan Mayen Current and the Norwegian Current, where cold and warm waters mix. The most important radiolarian species in this assemblage are the Actinimma leptoderma/boreale Group (4.588), Actinomma leptoderma longispina (1.861) and Larcospira minor (1.798). Bjørklund et al. (1998) observed that the relative abundances of Actinomma leptoderma increased towards the ice-edge off Greenland. This observation is also in accordance with that of Swanberg and Eide (1992), who found Actinomma leptoderma to be one of the dominant species of living plankton in this area.

Factor 3 and Station Cluster D are particularly well represented in the North Atlantic, SW of Iceland. This area is influenced by warm water masses originating further south in the Atlantic Ocean and transported north by the Gulf Stream. The most important radiolarian species in this assemblage are Larcospira minor (3.421), Spongotrochus Group (S. glacialis, Spongopyle osculosa and S. resurgens) (3.294), and Phorticium clevei (1.956).

Factor 4 and Station Cluster B are mainly located off the west coast of Norway. This area is underlying the warm Norwegian Current, which sends off gyres to the north-west. These gyres are visible in the factor loadings distribution as lobate shapes (overlying the Norway Basin). Lithomelissa setosa (3.683), Artobotrys boreale (2.388) and Pseudodictyophimus gracilipes Group (1.577) are the most important species in this assemblage.

For more information about the radiolarian distributions in the Nordic Sea, including plates, tables, species distribution maps, Q-mode factor analysis- and cluster analysis results, see Cortese et al. 2003.

Radiolarians in surface sediments
The map below (modified from Cortese et al. 2003) shows the abundance of polycystine radiolarians/g dry sediment in the Nordic Seas. The highest numbers are found on the Iceland Plateau and in the north- and south-western part of Norway Basin. Low numbers do persist throughout the Lofoten Basin and the Greenland and Boreas Basins.

The Lofoten Basin is characterized by its high content of minerogenic (terrestrial derived) material, and several of the sediment cores in the area have high contents of turbiditic material originating from the Norwegian continental slope (Kellogg 1976). This suggest that the siliceous microorganism (e.g. radiolarians) produced in the overlying water masses of the Lofoten Basin have been masked by large amount of non-biogenic when reaching the sea bottom. The reason for the low radiolarian content in bottom sediments of the Greenland Sea is due to the fact that the sea for large parts of the year is covered with sea ice, giving little room for an extensive biological production, resulting in a low radiolarian flux into the sediments.

References
Bjørklund, K.R., Cortese, G., Swanberg, N. and Schrader, H.J., 1998. Radiolarian faunal provinces in surface sediments of the Greenland, Iceland and Norwegian (GIN) Seas. Marine Micropaleontology, 35, 105-140.

Cortese, G, Bjørklund, K.R. and Dolven, J.K., 2003. Polycystine radiolarians in the Greenland-Iceland-Norwegian Seas: species and assemblage distribution.

Swanberg, N.R. and Eide, L.K., 1992. The radiolarian fauna at the ice edge in the Greenland Sea during summer, 1988. Journal of Marine Research 50: 297-320.

Swift, J.H. and Aagaard, K., 1981. Seasonal transitions and water mass formation in the Iceland and Greenland seas. Deep-Sea Research, 28A(10): 1107-1129.

Kellogg, T.B., 1976. Late Quaternary climatic changes: evidence from deep-sea cores of Norwegian and Greenland Seas. In: Cline, R.M. and Hays, J.D (editors). CLIMAP Investigation of Late Quaternary paleoceanography and paleoclimatology. The Geological Society of America, Memoir 145: 77-110.