Jökull - 01.12.1990, Blaðsíða 93

50" 40' 30" 20" 10’ 0"
Figure 4. Retreat positions of the North Atlantic polar
front from the glacial maximum position 18.000 yr ago
t° the modem interglacial location after 6000 yr B.R
(from Ruddiman and Mclntyre, 1981). —Breytingar
a stöðu heimskautaskilanna í Norður Atlantshafi frá
mestu ísöld fyrir um 18.000 árum til núverandi stöðu
skilanna (Ruddiman og Mclntyre, 1981).
deep sea sediments
The Glacial-Holocene transition in the Northern
Atlantic has been studied in deep sea cores (Ruddi-
man and Mclntyre, 1981). These studies demonstrate
major shifts in the oceanic polar front in concordance
with the Late Weichselian oscillation (Fig. 4). It
is suggested from this data that the NA current has
crossed the NA ocean, heading for Portugal, from the
glacial maximum until the Bölling period and during
the Younger Dryas period. Possible causes for such
excursions of the NA current have been discussed by
Fuddiman and Glover (1975). They claim from hy-
drodynamic arguments that this can only be explained
by heavy wind stress from westerly winds. In the
Dye-3 core greatly increased dust load during the cold
sPells in later half of the Glacial period provides ample
evidence for increased storms and suggests that sim-
har excursions of the NA current has happened some
14 times during the latter half of the last glaciation.
Carbon isotope measurements on benthic
foraminifera from North Atlantic sediment cores (Du-
plessy et al., 1988), faunal evidences (Schnitker, 1979)
and geochemical studies (Boyle and Keigwin, 1987;
Boyle, 1988) provide evidence for different ocean cir-
culation during glacial times, and therefore support the
hypothesis that the cold climate during glacial time is
due to lack of the NA current, which generally warms
up the North Atlantic region during the interglacial pe-
riods. It has been demonstrated that the concentration
of cadmium in foraminifera shells is proportional to
its abundance in the sea-water. As the distribution of
cadmium in the ocean matches that of phosphate and
nitrate its concentration in shells from sediments re-
veals the distribution of these chemicals in the glacial
ocean. Analyses of cadmium in foraminifera shells
show that the characteristics of the Atlantic present-
day circulation were missing during glacial time, until
about 14.000 years ago and also during the Younger
Dryas cold spell about 3.000 years later (Boyle, 1988;
Boyle and Keigwin, 1987).
Accordingly these studies further support the hy-
pothesis that the explanation for the climatic events
must involve reorganization of the ocean-atmospheric
system.
ICELAND
Iceland is situated in the North Atlantic ocean and
therefore, if the NA-current hypothesis is correct, it
should have suffered the same climatic oscillations
during the late glacial time as discussed above and are
reflected in the Greenland ice cores, lake sediments
from Switzerland and from North Atlantic deep sea
sediments.
Climatic changes are frequently observed from ge-
ological evidences. Glacial sediments are suggestive
of cold climate and advancing glaciers. Furthermore,
extent of glaciation and glacial movements can be
studied by observing glacial striations. Raised beaches
and shoreline features are on the other hand indicative
of warm climate in particular when associated with
strong silt formation due to retreating glaciers.
Many examples of changing climate can be ob-
served within the Icelandic geology. However, in the
present account we will limit the discussion on the
Late Weichselian time and concentrate on 14C-datings
of marine sediments and sea-temperature calculations
JÖKULL, No. 40,1990 89