Jökull - 01.12.1990, Blaðsíða 88

a.p.s.l. during the Alleröd stage (Einarsson, 1985;
Ingólfsson, 1987a,b). According to Einarsson (1985)
the highest raised beaches in Iceland are in general
about 11.000 years old i.e. of late Alleröd age. The
Reykjavík region has been covered by sea at some ear-
lier times as indicated by marine sediments and raised
beaches. The highest one is in Öskjuhlíð at 43 m above
present sea level and is thought to be of Preboreal age
(Hjartarson, 1989a).
In this paper attempt is made to connect the
observed geological record of the 'late Weichselian
glaciation in Iceland with data from elsewhere, re-
flecting the climatic oscillation within the North At-
lantic region. In particular the isotopic record from
the Greenland ice and lake sediments in Switzerland
and evidence from deep sea sediments are reviewed
for this purpose.
GREENLAND ICE CORES
Several ice-cores have been recovered from
Greenland and Antarctica for paleoenvironmental
studies. These include studies of climatic variations
during the last 120.000 years, different amount of pre-
cipitation during the same period, volcanic activity
throughout the world, dust concentration and changes
in the atmospheric concentration of CO2.
It has been demonstrated that the isotopic compo-
sition of the ice depends chiefly on the cloud conden-
sation temperature (Picciotto et al., 1960) at the time
of snow deposition on the surface of the ice sheet.
The cloud temperature is mainly determined by the
distance to the polar front or more precisely, since
the cloud cooling (neglecting cooling due to adiabatic
lifting) is mainly due to radiation to space, the travel
time for the occluded cyclons carrying snow to the
glaciers. Increasing 6-values reflect shifts of the po-
lar front to higher latitudes and warm periods whereas
low ó-values are obtained during cold periods.
ISOTOPIC DATA — <5 - NOTATION
Natural water consists mainly of 3 isotopic species
namely H2160 which is the most abundant light
component and the less abundant heavy components
H2I80 and HDlöO, the abundance ratios being ap-
proximately 106 : 2000 : 157 respectively. Isotopic
ratios of water are expressed in the <5 scale where 6 is
the relative deviation of the ratio of the heavy com-
ponent to the light one to that of the standard SMOW
(Standard Mean Ocean Water) (Craig, 1961):
ólsO
and
<5D
where D
' (18o/16o)sample
.(180/160)smow
(0/H)samp[e ^
_(D/H)smow
deuterium (2H)
* 1000
* 1000
In natural waters the range of <5180 values is today
found from 0 promille in the World oceans to —60
promille for high altitude Antarctic precipitation. For
<5D the corresponding values are 0 promille and —470
promille respectively. An interesting relationship ex-
ists between <5180 and <5D in high latitude precipitation.
It is expressed in the so called ”Meteoric Water Line“
^D = s * <5lsO + d (Craig, 1961; Dansgaard, 1964)
where the slope s equals 8 and d the deuterium excess
equals lOpromille.
The climatic significance of the <5-values in polar
precipitation has been discussed by several authors
(see for example Dansgaard et al., 1973, Merlivat and
Jouzel; 1979, Jouzel etal., 1987, Johnsen et al., 1989).
It was demonstrated, for most of the Greenland Ice
Sheet (Dansgaard et al., 1973) that a linear relation
exists between <5180 and the mean annual temperature
as measured at 10 meters depth in the fim, such that
1 promille increase in <5180 corresponds to 0.67°C
increase in temperature. This relationship is normally
assumed to hold also in the case of temporal changes
due to climatic shifts.
A more rigorous approach was taker. by Johnsen
et al. (1989), who considered the entire precipita-
tion process from evaporation in the warm oceans to
the precipitation of snow on the Greenland Ice Sheet.
They described the variations in both <5180 and <5D by
a model which accounted for kinetic effects both dur-
ing evaporation in the source area and condensation
of ice crystals. The model was able to explain the ob-
served annual variations in the deuterium excess (d)
84 JÖKULL, No. 40, 1990