Jökull - 01.12.2007, Síða 17

Early Pleistocene molluscan migration to Iceland
(Table 1). However, it should be kept in mind that
many arctic species that live high on the shoreface in
the arctic extend their distribution to offshore envi-
ronments in the boreal zone. The depth relations of
the species are based on observations made by Thor-
son (1941, 1944), Ockelmann (1958), Macpherson
(1971), Bernard ( 1979), Lubinsky (1980), and Gra-
ham (1988).
In Table 1 and 2 the species are grouped according
to their tolerance to reduced salinity into euhaline (>
30‰), polyhaline (> 25‰), b-mesohaline (> 15‰),
a-mesohaline (> 5‰), and oligohaline (< 5‰) condi-
tions (cf. Funder et al., 2002). The salinity relations
of the species are based on information from Thor-
son (1941), Ockelmann (1958), Lubinsky (1980), and
Reid (1996). The species have different tolerances to
reduced salinity, from polyhaline to oligohaline, but
most low salinity species can thrive in oceanic water.
Littorina littorea has a pelagic larval stage last-
ing 4-5 weeks, whereas the other species either have
no pelagic larval stage or it is very short (Thorson,
1936, 1941, 1944; Ockelmann, 1958; Fretter and Gra-
ham, 1980). Arctic species usually have a very short
pelagic larval development or it is entirely lacking,
a developmental condition that probably slows down
their migration as it mainly occurs while living in
ocean currents during the pelagic larval stage.
EARLY PLEISTOCENE MOLLUSCAN
MIGRATION TO ICELAND
Three arctic molluscan species, Tachyrhynchus
erosus, Portlandia arctica, and Tridonta placenta,
reached western Iceland during the uppermost part of
marine isotope stage 32 at 1.1 Ma. They have been
found in the BúlandMember of the Búlandshöfði For-
mation on the Snæfellsnes Peninsula, western Iceland.
Only one of them, P. arctica, has been found else-
where in Icelandic sediments. The oldest occurrences
are in sediments from deglaciation periods in the
Breiðavík Group; first in the 2.15 Ma old sediments
of the Hörgi Formation, then in the Fossgil Member
dated to 2.05 Ma, and finally in the Svarthamar Mem-
ber dated to 1.5 Ma (Eiríksson, 1981; Vilhjálmsson,
1985; Símonarson and Leifsdóttir, 2002). However, it
has not been encountered in the youngest formation of
the Breiðavík Group, the Máná Formation about 1.1
Ma (Vilhjálmsson, 1985). The fossiliferous lower ma-
rine sediments of the Máná Formation were deposited
during the uppermost part of MIS 32, also a final stage
of a glacial period. Taking into consideration the envi-
ronmental conditions prevailing during the deposition
of the lowermost marine part of the Máná Formation
it was probably possible for P. arctica to thrive there
for a while, as further indicated by the faunal assem-
blages in the lowermost siltstone (Figure 3).
Why is P. arctica not found in the marine sedi-
ments of the Máná Formation as it occurs in the three
previous marine units of the Breiðavík Group? Cer-
tainly it is rather difficult to base a conclusion on neg-
ative data. However, in this case we consider that
P. arctica, T. erosus, and T. placenta did not reach
the area during the deposition of the Máná Forma-
tion. Therefore the absence of these three species is
not considered a question of preservation. All three
species are arctic in their distribution (Table 1). It is
therefore highly probable that they migrated to Ice-
land from the west or northwest, i.e. from East Green-
land where they all thrive today and have lived since
the Upper Pliocene or Pleistocene. It is less prob-
able that they migrated from populations that lived
far to the south of Iceland during periods of glacia-
tion. Firstly the ocean currents were not favourable
for larval transport from the south during periods of
deglaciation and secondly it is not likely that arctic
species were transported northward with the warmer
North AtlanticWater when it intensified. These south-
ern populations were more or less doomed to die dur-
ing deglaciation.
The occurrences of Portlandia arctica,
Tachyrhynchus erosus, and Tridonta placenta in the
Búland Member of the Búlandshöfði Formation indi-
cate an eastward shift of the East Greenland Current
toward the Icelandic west coast while the Polar Front
was lying considerably south of Iceland during iso-
tope stage 32 at 1.1 Ma. A weak Irminger Current
branch of the warm North Atlantic Current could
not keep the cold, euhaline East Greenland Current
from the Icelandic west coast at that time (Figure 1).
It has been observed that in severe years in recent
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