Jökull - 01.12.2007, Síða 3

Reviewed research article
Early Pleistocene molluscan migration to Iceland –
Palaeoceanographic implication
Leifur A. Símonarson and Ólöf E. Leifsdóttir
Institute of Earth Sciences, University of Iceland,
Askja, Sturlugata 7, 101 Reykjavík, Iceland
email: leifuras@hi.is
Abstract – Four marine molluscan species that migrated to Iceland during the deposition of sedimentary se-
quences on the north side of Snæfellsnes, western Iceland, at about 1.1 Ma, are not living in Iceland today.
Three of these species are arctic and reached the area during an Early Pleistocene deglaciation. The fourth is
thermophilic and arrived during the following interglacial, together with several littoral species now living in
Iceland. The arctic species probably migrated to Iceland from the west or northwest due to a southward shift
of the cold and euhaline East Greenland Current to the Icelandic west coast. At that time the Polar Front was
lying considerably south of Iceland, but then followed a periodic northward shift of the front. The fact that the
arctic species did not reach northern Iceland at this time may indicate a rapid shift of the Polar Front across
the north coast of Iceland that minimized the influence of the East Icelandic Current during the deglaciation.
Several thermophilic littoral species that migrated to western Iceland during the following interglacial did not
reach northern Iceland. They came from the south or southeast during strengthening of the warm Irminger
Current. However, the current’s influence on the Icelandic north and northeast coasts was probably limited
because of mixing with colder water masses with reduced salinity from the East Icelandic Current.
Key words: Iceland, Early Pleistocene, molluscs, migration, ocean currents.
INTRODUCTION
Several marine molluscs living in shallow water have
changed their area of distribution, especially during
the Neogene. The bivalve taxodont species Portlandia
arctica is a well-known example as its fossil occur-
rence demonstrates repeated southward expansion of
cold Polar Water along the North Atlantic and Pacific
coasts during glaciations, as much as 2000 km south
of its present southern boundary (Jessen et al., 1910;
Símonarson et al., 1998). The migration of marine
molluscs mainly takes place during their pelagic lar-
val stages as the larvae are transported to new areas
by ocean currents. Arctic species usually have a very
short, or entirely lack, a pelagic larval stage of devel-
opment, which probably slows their migration (Thor-
son, 1936). However, it obviously does not prevent
their migration, as demonstrated by P. arctica.
The main oceanic circulation pattern around Ice-
land (Figure 1) was probably established when the
closing of the Central American Seaway caused a flow
of surface water from the Pacific through the Bering
Strait into the Arctic Ocean and induced a major mi-
gration of Pacific molluscs to the North Atlantic and
Iceland at 3.6 Ma (Backman, 1979; Haug and Tiede-
mann 1998; Marincovich, 2000). Furthermore, the
closing is supposed to have considerably increased the
JÖKULL No. 57 1