Jökull - 01.12.2007, Blaðsíða 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