Jökull - 01.12.1984, Síða 65

comprehensive description of volcanic activity
during the Holocene. More refined techniques
and new ice core data may solve the problem of
identification, but for the moment only sugges-
tions are possible.
The H IV and H V eruptions
These eruptions are C 14 dated to 4600 B.P.
and 7400 B.P! (Larsen and Thorarinsson 1977;
H. Tauber personal communication, 1983). They
produced 9 and 3 km3 of tephra, estimated as
freshly fallen. The H IV eruption is a good candi-
date, with respect to an acid fallout signal on
Greenland. A thorough discussion of these erup-
tions and traces of them in ice cores must await
the final dating of the Dye 3 core, but they have
previously been proposed as the cause of two of
the high acidity signals in the Camp Century
record (Hammer et al. 1980 a).
The latter record is problematic, especially for
dates before ca. 4000 B.P., and the H IV and H V
acid productions are best treated as part of a
discussion on the validity of the Camp Century
acidity record.
THE CAMP CENTURY ACIDITY
RECORD - A REVIEW
AND SOME REVISIONS
While the acidity of the Dye 3 core was mea-
sured in the field and under almost ideal condi-
tions, this was not the case for the Camp Century
core. When the strongest acid signals detected in
the Camp Century core were published (Hammer
et al. 1980 a), we were quite confident of the
validity of the highest signals and felt, that the
moderately high acidity peaks could be trusted.
Today, experience and more refined techniques
have taught us to be a little more careful, when
dealing with ice cores stored over long time
periods.
In 1982 we had the opportunity to remeasure
some of the important segments, of what re-
mained, of the Camp Century core. The techni-
que we used was a more refined one and our
cleaning of the core, prior to measuring, proved
to be sufficient. Not all important segments could
be remeasured, as too little core remained. The
results were informative, though somewhat dis-
turbing: Out of the 18 highest acidity signals
detected in the core (before 40 A.D.) 3 mode-
rately high signals had to be deleted, 4 signals had
to be lowered somewhat and 7 signals could not
be remeasured correctly due to bad core quality.
Four out of the 5 biggest signals were still among
the biggest. One moderately high signal was
higher than previously measured. Unfortunately
one of the signals, which had to be deleted was
the previously suggested traces of the H V 5470
B.C. eruption.
The revised Camp Century findings are shown
in Fig. 9. Note, that the figure only show those
high acidity signals, which are now safely identi-
fied in the Camp Century core; it is not a compre-
hensive figure and all doubtful measurements
have been left out. The remeasurement of the
core was not entirely concentrated on revisions.
The detection of traces of the Lakagígar —, un-
known 1259 — and Eldgjá-eruptions and the
important confirmation of the highest acidities
were reassuring findings.
Perhaps only those who have worked with the
remains of the now 16 year old Camp Century
core understand the problems involved in work-
ing with it. As only one deep ice core from
Greenland existed a few years ago, we tried to
“squeeze” as much information out of it as possi-
ble; evidently we made some mistakes, when
using a technique which was in its infancy at the
time of measuring.
The revision will be important, when the dat-
ing of the acid signals in the new Dye 3 core is
completed, because with only two existing deep
cores from Greenland, it will be a prerequisite for
a comparison of the high acid signals in the two
records, that the compared data are correct.
Fig. 9 offers a Camp Century record which can
be used for such a comparison — even though it is
not exhaustive.
VOLCANOLOGY AND POLAR
ICE CORES
In this article I have concentrated on Icelandic
eruptions, even though they have been seen on a
“general background” of volcanic traces in the
Greenland Ice Sheet. The ice sheets are unique
with respect to the possibilities of estimating the
long-range transport of volcanic debris, but there
are many aspects of their analysis and potential
use in volcanology, which I have only touched
upon.
In the near future there is a need to obtain data
from more locations, including Antarctica.
JÖKULL 34. ÁR 63