Jökull


Jökull - 01.12.2007, Page 19

Jökull - 01.12.2007, Page 19
Early Pleistocene molluscan migration to Iceland off” mode, with practically no deep water formation in the North Atlantic (Ramstorf, 2000). The carbon- ate record and IRD record from the Vøring Plateau have been used by Jansen et al. (1990) and Jansen and Sjøholm (1991) to access the record of glaciations in Scandinavia and the extent of NADW formation, and they have pointed out that the biogenetic carbonate productivity at present is related to the influx of warm Atlantic water into the Norwegian Sea. The North Atlantic Current carries warm, saline sea water past Iceland on its way to the Norwegian-Greenland Seas where it sinks and forms the NADW, which then flows back across the ocean ridges between Greenland and Scotland (Figure 1). Deep sea cores from the North Atlantic reveal a record of changes in ! 18O, !13C, and CaCO3 concentrations indicating linkage between ice volumes and shifting fronts between sea water masses and deep water formation (Ruddiman and MacIntyre, 1973; Raymo et al., 1990; Jansen and Sjøholm, 1991). According to Helmke et al. (2003), the western Ice- land Plateau shows moderate high IRD during certain late Matuyama interglaciations indicating that at this time the interglacial Polar Front was located closer to the Iceland Plateau than today and was even located over the Plateau. Then the Iceland Plateau was un- der influence of polar waters, which is in good ac- cordance with the conditions in northern Iceland dur- ing MIS 31 as described elsewhere in this paper. It has been concluded that the Polar Front between the surface water mass of the Labrador and Greenland- Norwegian Seas on the one hand and the warm At- lantic water on the other shifted northward during the Weichselian deglaciation (Ruddiman and MacIntyre, 1973). Furthermore, Eiríksson et al. (1993) have shown that the front shifted northward across Iceland into the Norwegian-Greenland Seas at 1.5 Ma during the deposition of the Svarthamar Member unit of the Breiðavík Group on the Tjörnes Peninsula, North Ice- land. The Polar Front was obviously shifted northward across Iceland into the Norwegian-Greenland Seas during deglaciation in Late Matuyama time, indicat- ing that the Greenland-Scotland Ridge did not form barriers for the North Atlantic Current at 1.1 Ma, as was the case at 1.5 Ma (cf. Eiríksson et al., 1993). It is considered that the shift was so rapid when the front passed northern Iceland that the cold water masses of the East Icelandic Current were not given the time to transport arctic species to North Iceland at the junc- tion of isotope stages 32 and 31. CONCLUSIONS During the Neogene the circulation pattern around Iceland changed repeatedly. New palaeontological data add to the sporadic record of Early Pleistocene climatic events preserved in sedimentary deposits of the Búlandshöfði Formation on the north side of the Snæfellsnes Peninsula, western Iceland and the Máná Formation of the Breiðavík Group on Tjörnes in northern Iceland. The sedimentation on Snæfellsnes took place in a sedimentary basin that was gradually filled up with sediments and volcanics from the east and southeast. The sediments of the Máná Formation were also deposited in a sedimentary basin. These two basins were contemporaneous for a while during the Early Pleistocene. Faunal assemblages in these sedi- ments, K/Ar dated to about 1.1 Ma, include serpulids, barnacles, gastropods, bivalves, and echinoids. Early Pleistocene migration of arctic molluscs to Iceland during the uppermost part of marine isotope stage 32 strongly indicates an eastward shift of the East Greenland Current to the Icelandic west coast while the Polar Front lay considerably south of Ice- land. Apparently the arctic species Tachyrhynchus erosus, Portlandia arctica, and Tridonta placenta did not reach northern Iceland at this time, probably be- cause of a rapid shift of the Polar Front across the Ice- landic north coast when the front passed the island. Therefore the cold water masses of the East Icelandic Current were not able to transport arctic species to the coasts of North Iceland at the junction of isotope stages 32 and 31. During the initial stages of the following inter- glacial (MIS 31) more thermophilic species migrated to western Iceland, and among them were the lit- toral gastropods Littorina littorea and Nucella lapil- lus. Most of these species did not reach northern Ice- land. They obviously came to the south and west coasts of Iceland during strengthening of the warm Irminger Current. However, its influence on the Ice- JÖKULL No. 57 17
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