Jökull


Jökull - 01.12.2007, Page 17

Jökull - 01.12.2007, Page 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 JÖKULL No. 57 15
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