Stable isotope profiles compared w i t h temperature
profiles in f i r n w i t h historical temperature records
S. J. Johnsen
Abstract.
Shallow temperature profiles (50-100 m) from polar ice caps contain information
about recent climatic changes. Correlation with 8(I80)-records and temperature records is
possible by using a method described in this paper. The method helps to understand the
climatic information of S(180)-records, and allows the climatic regime of ice cap stations to be
established.
Profils des teneurs en isotopes stables en comparaison de profils des températures dans le névé et
d'enregistrements historiques des températures
Résumé.
Les profils de températures jusqu'à 50 ou 100 m de profondeur obtenus dans les
calottes polaires renferment des informations sur les changements climatiques récents. Une
corrélation entre les enregistrements des teneurs en isotopes (SlsO) et des températures est
possible en utilisant la méthode décrite dans ce travail. Cette méthode aide à la compréhension
des informations climatiques fournies par les profils isotopiques et permet d'établir le régime
climatique des stations sur la calotte glaciaire.
S(180)-profiles along firn and ice cores from the Greenland ice sheet contain
long-term climatic information, which has been shown to correlate with
variations known from other parts of the North Atlantic region. The purpose
of this work is to show that the S(180)-profiles also reveal to some extent local
short-term climatic changes. This is demonstrated by comparison with
measured temperature profiles along the holes left after drilling the cores.
Temperature profiles from suitable sites on stable, high polar ice sheets are
essentially determined by the past temperature history of the ice sheet. The
actual shape of temperature profiles depends, furthermore, on how the thermal
properties of firn change with depth. Long-term climatic variations are reflected
in the temperature profile at great depths, whereas recent short-term climatic
variations determine the upper part of the temperature profiles considered here.
The climatic information contained in a measured temperature profile is
derived by comparison with profiles calculated by assuming a temperature
history for the surface and adjusting the assumption to the best obtainable fit.
Below 10 m depth, the vertical temperature gradients are often small and to a
certain degree influenced by the annual temperature cycle, at least down to 20 m
depth, which may be corrected for.
The calculated temperature profiles are obtained by numerical integration of
the differential equation for heat conduction in moving firn and ice, which also
takes into account how the thermal parameters of firn and ice are dependent on
temperature and density :
dT
dK K
—
= — ill ^llYf - 1^ _
I
dt ~ Kdy* + \{p + dpjdy Vv]Ty ~VxV +\df+'c
x
A dp G
+ or——
(1)
PA ay c
where
T = temperature [°C],
t = time [years],
vy = the vertical velocity component [m/year],
388
df
ÊlXl^lY
j\dy)
Stable isotope profiles compared with temperature profiles
389
y = depth below the surface [m],
p = density of firn at depth y [kg/m3],
a — overlying load at depth y [kg/m2],
A = annual rate of accumulation [kg m _ 2 year - 1 ] ,
c = heat capacity of ice at temperature T [J kg _ 1 °C _ 1 ] ,
K = thermal diffusivity of firn at density p and temperature T [m2/year],
G = acceleration of gravity [m/s2].
The vertical velocity component vy is calculated from accumulation rate and
density profile following a vertical strain model, whereas the horizontal velocity
components, in the vicinity of Crête, may be neglected. The thermal diffusivity
data for firn used in equation (1) are from Weller and Schwerdtfeger (1970).
10h
20H-
30H
40h
Temperature -*•
50
-30.5
-3CX3
- 30.0
"C
FIGURE 1.
Measured temperature profile from Crête (1974) compared with a
calculated profile based on the Crête 1974 8(lsO)-record.
390 S.J.Johnsen
Equation (1) contains two terms normally not used in temperature profile
calculations: the (dT/dy)2 term is due to the dependence of K and c on T. The
term has the effect of lowering the calculated temperatures whenever
temperature gradients are present in the profile. This is the case with the annual
temperature wave, which results in 10 m temperature 0.3°C colder than the
mean annual surface temperature. Including this term when calculating
steady-state deep ice profiles may reduce the surface-bottom temperature
difference by several degrees. The last term in equation (1) accounts for the
heat generated when the firn is compacted under load. This results in a small
but measureable temperature gradient in the firn (~0.1°C over the top 50 m).
Depth
m
-
•
-O- Measured
Dye-3
1
Angmagssalik
2
Godthaab
1975
- Calculated
Temperature
^
_L
FIGURE 2.
Measured temperature profile from Dye 3 (1975) compared with
calculated profiles based on 1. east Greenland and 2. west Greenland temperature records
as shown in Fig. 3. The initial temperature used was — 2°C for both calculations.
The Crête 1974 S(180)-profile (Dansgaard et al, 1974) has been used to
calculate a temperature profile to compare with a 50-m measured profile from
Crête (Fig. 1). The upper part of the measured profile is affected by the previous
summer temperature wave. The profiles show maxima at the same depth but the
Stable isotope profiles compared with temperature profiles
391
1970
1960
1950
1940
1930
1920
1910
1900
1B90
1880
-3
-2
-1
0 °C
FIGURE 3.
Measured temperature records from Greenland smoothed by a symmetrical
10-year low pass filter.
gradients in the lower part are different. This is probably due to too high
temperatures in the second half of the last century, as shown by the S(180)profile.
In calculating the temperature variations from the S(180)-variations the
formula
AT=
KA8(lsO)
was used, where K = 1.62°C ("/OQ)"1.
Figure 2 shows calculated temperature profiles based on temperature records
for more than 100 years from Angmagssalik on the Greenland east coast and
Godthaab on the west coast (Fig. 3). In Fig. 2 is also shown a measured
temperature profile from Dye 3 (south Greenland). All stations are on similar
latitudes, Dye 3 is close to the main north-south ice divide.
A conclusion to be drawn from Fig. 2 is that the temperatures on the south
Greenland dome have in the past decades been more similar to the temperatures
at the Greenland west coast then to the east coast temperatures.
Acknowledgements.
The isotope and temperature data were collected under the Greenland
Ice Sheet Program, which is supported by the US National Science Foundation, Office of
Polar Programs, and the Ministry of Greenland, Copenhagen.
392
S.J.Johnsen
REFERENCES
Dansgaard, W., Johnsen, S. J., Reeh, N., Gundestrup, N., Clausen, H. B. and Hammer, C. U.
(1974) Climatic changes, Norsemen and modem man. Nature 255, no. 5503, 24-28.
Weller, G. E. and Schwerdtfeger, P. (1970) Thermal properties and heat transfer processes of
the snow of the central Antarctic plateau. In Symposium on Antarctic Glaciological
Exploration (ISAGE) (Proceedings of the Symposium at Hanover, New Hampshire, USA,
September 1968), pp. 284-298: IAHS Publ. no. 86.