Jökull - 01.12.1990, Blaðsíða 89

measured in Greenland snow, by assuming the source
to be at 30-35°N, i.e. the warmtemperate belt. It also
demonstrated that changes in the á-values due to cli-
ntatic shifts are most likely due to shifts in the position
°f the North Atlantic polar front in accordance with
Ruddiman and Mclntyre (1981).
Furthermore the model predicts that the slope of
the Meteoric Line is mainly dependent on the abso-
lute water vapor pressure or mixing ratio in the source
area and that the d-values are mainly dependent on
the sea surface temperature in the same area. This
understanding of the behaviour of the é-values in high
latitude ice-cap precipitation clearly makes possible
tefined interpretation of S records in terms of paleo-
climatic and paleoenvironmental changes.
THEICE CORES
Several ice-cores have been recovered from the
Greenland Ice Sheet in order to access the various pa-
leoenvironmental parameters found in the ice. Fig. 1
shows 6-variations with depth in the deep Camp Cen-
tury ice-core, NW Greenland (77.2°N; 61.2°W). The
figure shows the annual variations in ó180 where the
summer snow is higher in <51S0 than the winter snow
und demonstrates one of the ice core dating methods,
which is based on counting of individual annual lay-
ers. The figure demonstrates that the amplitude of the
^-seasonal variations decreases rapidly from about 10
promille in the uppermost strata (a and b) to about
2 promille at greater depths (c and d). The ampli-
tude then remains about 1.5 promille for thousands of
years as the mass exchange at depths takes place only
by slow molecular diffusion in the solid ice (Johnsen
et al-, 1972; Johnsen, 1977). Therefore the dating
method based on counting of annual 6-cycles can be
aPplied successfully and with an accuracy compara-
ble to that of dendrochronology on ice as old as 8000
FP. However, the accumulation rate must exceed some
0-20 m of ice per year for the annual cycles to survive
Ihe fimification process. In deeper strata, ice layer
thinning and diffusion of the isotopes tend to oblit-
erate the seasonal pattern (Johnsen, 1977; Hammer
et al., 1986) and other dating methods must be used,
such as:
1. Annual layers in other parameters like dust, ni-
trates, etc. (Hammer et al., 1978).
2. Radioactive isotopes, especially 14C (Hammer
et al., 1986).
3. Correlation with other dated records, like deep
sea cores and pollen records (Dansgaard et al.,
1982).
4. Ice flow consideration (Reeh et al., 1985).
In Fig. 2 the 180-measurements on two Green-
land ice-cores; Dye-3 situated in Southeast Greenland
(65.2°N;43.8°W) and Camp Century in Northwest
Greenland (77.2°N;61.2°W) are compared. The deep-
est 300 m of the ó-profiles are shown but the former
core is thought to be continuous to about 90.000 yrs BP
and the latter to more than 125.000 yrs BP. The deep
parts of the profiles are indirectly dated by tentative
correlation with a deep sea foraminifera record. It
appears that all the major (5-oscillations are observed
in both cores despite the 1400 km distance between
them and the different ice flow conditions at the drill
sites. It has therefore been concluded that the violent
oscillations in the <5I80 are to be attributed to climatic
changes at high latitudes (Dansgaard et al., 1982).
In order to investigate further climatic and other
environmental variations in the high latitude North At-
lantic region, a 325 m long ice core was drilled by a
Nordic group in the Renland ice cap in the Scoresby-
sund Fjord, East Greenland, in the summer of 1988.
The preliminary results show that the ice core contains
continuous information on climatic variations during
the last 130.000 years and good correlation is observed
with the former deep Greenland ice cores Dye-3 and
Camp Century. In the Renland core the well known
climatic optimum during the Holocene is for the first
time clearly observed in an ice core from Greenland.
The studies mentioned above on deep Greenland
ice-cores have demonstrated that the late Weichselian
glaciation in the North Atlantic region was character-
ized by a long series of climatic oscillations (Dans-
gaard et al., 1984) and that the last glacial cold pe-
riod ended abruptly 10.700 years ago (Hammer et al.,
1986). In a more recent study Dansgaard et al. (1989)
focus on the very end of the Younger Dryas cold phase
(Fig. 3). On the basis of detailed heavy-isotope and
JÖKULL, No. 40, 1990 85