Jökull - 01.12.2007, Síða 14

Símonarson and Leifsdóttir
Table 3. Recent Tridonta borealis from southwest Iceland compared to T. placenta from the interglacial deposits
in Stöð, Snæfellsnes, western Iceland. In both cases 15 specimens were measured. – Yfirlit yfir helstu einkenni
gimburskelja (Tridonta borealis) og legskelja (Tridonta placenta) úr hlýskeiðslögum í Stöð á Snæfellsnesi. Í
báðum tilvikum voru mæld og skoðuð 15 eintök.
Species Shape Ratio Ratio Hinge Adductor scars Periostracum
h/l * w/l **
Tridonta Triangular - 0.88 0.40 Two anterior Anterior muscle scar Rather stiff with faint
borealis ellipsoidal and laterals in the kidney-shaped growth lines or smooth
rather flat right valve and glossy, yellow
white-dark brown
Tridonta Ovate - elongated 0.68 0.31 One anterior Anterior muscle scar Rather loose with faint
placenta and very flat lateral in the banana-shaped - growth lines or frayed,
right valve elliptical dull and non-glossy,
brown-dark brown
*ratio height/length ** ratio width/length
cella lapillus, Mytilus edulis, and Semibalanus bal-
anoides. The molluscs belong to theMytilus-Littorina
epifaunal bottom assemblage indicating sea tempera-
tures similar to those of the present day in the area
(Leifsdóttir, 1999). The number of species decreases
strongly upward in the Höfði Member and in the up-
permost part we have only found three molluscan
species where trace fossils, mainly foodichnia and
domichnia, are common.
All the species found in the Búlandshöfði Forma-
tion are extant, living in Icelandic waters today, except
the prosobranch gastropod species Littorina littorea
and Tachyrhynchus erosus and the bivalve species
Portlandia arctica, Tridonta placenta, and T. mon-
tagui warhami. The faunal assemblages are North
Atlantic and 37 of the species found are living in
arctic seas, but 31 of them are also found in boreal
seas south of Iceland, and nine reach the lusitanian
region in their distribution. The arctic assemblages
in the Búland Member with P. arctica as a prominent
species, a cold water species not living in Iceland to-
day, strongly indicate arctic sea temperatures during
deposition (Leifsdóttir, 1999). On the other hand, the
assemblages in the sediments of the Höfði Member
contain thermophilic species which indicate sea tem-
peratures similar to those of the present day in the
area and increasing sea temperature while they were
deposited (Leifsdóttir, 1999). Thus, the assemblages
strongly indicate palaeotemperatures representing a
glacial-interglacial cycle. In older as well as younger
Pleistocene sediments in Iceland we have encountered
corresponding lithological cycles as described here:
from glacial to proglacial and then to shallow marine
or littoral sedimentation that coincide with faunal suc-
cessions, reflecting a change from arctic to boreal as-
semblages. Oxygen isotope analyses of marine shells
have revealed a temperature difference up to 8!C in
some of these cycles as in those in the older part of
the Breiðavík Group sequence (Vilhjálmsson, 1985).
Therefore the faunal succession described here, re-
flecting changes from arctic to boreal conditions, is
considered to depend on different water masses in the
sea around Iceland and not related to different depth
relations.
The depth range of the species indicates shallow
sublittoral environments along an open coast, except
for the assemblages in the Höfði Member that lived in
intertidal zones (Leifsdóttir, 1999). The salinity was
12 JÖKULL No. 57