Jökull - 01.12.2007, Side 18

Símonarson and Leifsdóttir
decades cold polar water from the East Greenland
Current occasionally reaches the northwest Icelandic
shelf (Stefánsson, 1994). If the conclusion is cor-
rect that these species did not reach northern Iceland
when the contemporaneous Máná Formation was de-
posited, this might indicate that the East Icelandic
Current did not transport them to the area. This cur-
rent pattern indicates a considerable strengthening
of the North Atlantic Current during marine isotope
stage 31 and more intensified and faster formation
of North Atlantic Deep Water (NADW) than during
former deglaciation periods (cf. Jansen et al., 1990,
Jansen and Sjøholm, 1991). This is reflected in a
rapid shift of the Polar Front across the north coast of
Iceland during the periodical shift of the front across
the island into the Norwegian-Greenland Seas.
When the sediments of the Höfði Member of the
Búlandshöfði Formation were deposited, during the
interglacial period that followed the glacial at 1.1 Ma,
the marine gastropod thermophilic species Littorina
littorea and Nucella lapillus reached West Iceland,
but they have not been recorded from the contem-
poraneous Máná Formation on Tjörnes, North Ice-
land. These species obviously came from the south or
southeast when the clockwise warm Irminger Current
along the south and west coast of Iceland was at least
strong enough to keep the East Greenland Current
from the coast. It was able to transport these species
to western Iceland together with several other ther-
mophilic littoral species such as the gastropodOnoba
aculeus and the cirriped Semibalanus balanoides. Ap-
parently, the current was not strong enough to carry
them to the eastern part of northern Iceland, except
for the tolerant bivalve Mytilus edulis. During the se-
vere years of 1965-1971 the Irminger Current north
of Iceland became mixed with colder water masses
with reduced salinity from the East Icelandic Current
(Stefánsson, 1994). This diminished the influence of
the Irminger Current eastward along the north coast
of Iceland and consequently it had almost completely
lost its strength off the northeast and east coasts, and
at the same time the sea temperatures decreased con-
siderably. During the deposition of the Máná Forma-
tion the conditions might have been similar to those
in 1880-1920 before N. lapillus and the bivalve Zir-
faea crispata reached northern Iceland but were liv-
ing along the west coast (cf. Áskelsson, 1935). Then
the sea surface temperatures off northern Iceland were
about 1.5!C lower than during the period 1920-1941,
when the average annual sea temperature (10 years
cycles) was 5!C (Stefánsson, 1969). In this context it
should be borne in mind that long periods of abnor-
mally high or low temperature in the atmosphere on a
global scale will cause profound changes in the ocean
and cannot be regarded merely as longer versions of
short-lived local temperature anomalies. However,
this migrational pattern indicates a further strength-
ening of the North Atlantic Current during the initial
stages of the interglacial period at 1.1 Ma (MIS 31).
GLOBAL AND SPATIAL SIGNALS
IRD record from ODP site 907 on the Iceland Plateau
shows a considerable decrease in ice-rafted mate-
rial reflecting decreasing ice volume from isotope
stage 32 to stage 31 (Jansen et al., 2000). The ben-
thic foraminiferal oxygen isotope record from East-
ern Equatorial Pacific site 849 further indicates a con-
siderably increasing sea water temperature (Mix et
al., 1995). From sedimentological and geochemical
proxy records of sediment cores from the eastern Ice-
landic Plateau, Helmke et al. (2003) concluded that
the Matuyama Chron interglaciations MIS 17, 19, 27,
and 31 reveal rather short full-interglacial episodes
lasting only about 1-2 ka. Furthermore, they expe-
rienced during certain intervals, such as MIS 11, 27,
and 31, similar-surface temperatures that were equiv-
alent to the recent surface water conditions. The
carbonate data indicated enhanced surface water bio-
productivity during these interglaciations, but in the
Nordic seas such conditions are generally attributed
to intervals with high potential heat transport via
strengthening of the Norwegian Atlantic current (Fig-
ure 1).
It has been proposed (Broecker et al., 1985) that
the oceanic circulation pattern has two stable states
corresponding to interglacial conditions, contrasting
with intensified and glacial conditions with a reduced
production of North Atlantic Deep Water (NADW).
The hypothesis of Broecker et al. has been somewhat
modifiedwith addition of the third state, the “switched
16 JÖKULL No. 57