Jökull - 01.12.1977, Blaðsíða 25

The best fit is obtained for H = 580 m and
X = 2.18 m/yr. Clearly, when data points are
close to each other, an error is introduced due
to uncertainties in timing within each year. To
reduce this inaccuracy data points were deleted
that were within five years from the next one
above. Repeating the above calculations with
the reduced array of data yielded H = 562 m
and X — 2.26 m/yr. The conclusion is thus that
the correct thickness is probably nearer to the
range 560—590 m than to 520 m.
We could proceed and try to relax some of
the remaining simplifying assumptions leading
to (a). However, awaiting the results of work
now in progress (Helgi Björnsson, pers. comm.)
on the ice core itself, we defer further discus-
sion on the effects of melting and ice dynamics
on the profile, and let it suffice to point out
the following: If we have melting at the bottom
of the glacier amounting to x meters of ice a
year, the results above remain essentially valid
except the true thickness will be (1—x/X)
times the estimated thickness. On the other
hand, if the horizontal velocity component of
the ice only remains constant to a depth of D
nieters, and then drops linearly, say, to zero at
the bottom, the above results again remain
essentially valid as long as dj < D, except the
true thickness will now be (1 + (H—D)/H)
times the estimated thickness, H.
X~t variation
It should be pointed out here, that the cal-
culated X’s represent the balance for ice (p —
0.9) except in the first calculation, wliere no
correction was made for tlie transformation of
snow to ice. Hence the difference in X (2.56 vs.
2.53 m/yr) for the two calculations.
In Fig. 8 are plotted the X/s for the various
time intervals, with all the data points included
(a), and with the reduced data array (b). Intui-
tively, there must be some correlation between
annual balance and climate, for when the
climate deteriorates the glaciers advance, and
vice versa. Tliis relationship may, however, be
complex, for the annual balance is the result
of the difference between two numbers, the
winter precpitation and the summer ablation
(or more precisely, tlie sum of the winter bal-
ance and the summer balance). In a separate
Fig. 8. Mean annual balance (m per year) for
the intervals between the teplira layers in the
Bárdarbunga core, plotted against time. (a) AIl
the data points included, (b) data points de-
leted so as to eliminate all time-intervals 5 years
or shorter. X — the mean value for the entire
period. The dashed curve is tlie Fourier-spect-
rum for the 018/010 measurements in the Camp
Century ice core from Greenland (Johnsen et
al. 1970). The dashed lines in (a) are for the
tephra sequence 1892—1889—1883, the whole
lines for 1892—1887—1883 (see the text).
Mynd 8. Reiknuð meðalafkoma timabilanna
milli gjóskulaga i Bárðarbungulijarna. (a) Allir
gagnapunktar teknir með, (b) gögnin grisjuð
þannig að bil milli laga sé aldrei styttra en 6 ár.
Brotni ferillinn eru hitastigsgögn frá Grœnlandi
(Johnsen o. fl. 1970). Brotnu línurnar sýna af-
komu fyrir tímabilin 1892—1889—1883, en lieil-
dregnu linurnar fyrir 1892—1887—1883. X sýnir
meðalafkomu alls timabilsins 1972—1650.
publication we hope to compare the X—t pro-
file to other independently obtained climatic
parameters, such as the D/H profile of the core
itself.
The smoothness of the X—t profile may be
an indication of the correctness of the depth-
time profile itself. Large cleviations (spikes)
miglit indicate an incorrect data point. How-
ever, as mentioned earlier, especially for closely
spaced points the interval will be closely de-
pendent on the time of year the eruption took
JÖKULL27. ÁR 23