Jökull - 01.12.1977, Blaðsíða 4

Tephra Layers in a Drill Core from the Vatnajökull Ice Cap
SIGURDUR STEINTHÓRSSON,
SCIENCE INSTITUTE, UNIVERSITY OF ÍCELAND
REYKJAVIK, ICELAND
ABSTRACT
In the summer of 1972 a 415 m long ice core
was recovered from Bárdarbunga in AW Vatna-
jökull, at the elevation of about 1800 m. The
core contained 30 tephra layers, which have
been used for dating it. The article describes
the results of this tephrochronological effort.
Volcanic history, microprobe analysis of the
glasses, and grain size analysis, was used in the
dating. A depth-age model is presented, accord-
ing to which the core represents the years from
1972 to 1650. Twenty two of the tephras are
attributed, with various degrees of certainty, to
known eruptions in Grímsvötn (10), Kverkfjöll
(2), other volcanoes beneath Vatnajökull (5),
Katla (3), and Askja (3), leaving six layers com-
pletely unaccounted for. From the profile it
seems probable that the Mývatn Fires of 1724—
29 zuere preceded and accompanied by an erup-
tive episode (a period of rifting) on the Dyngju-
háls dike swarm.
Finally, the dated profile is used to estimate
the mean annual balance for the periods be-
tween the tephra layers, which ranges between
about 3 m/yr (recalculated into snow of density
0.4 g/cm3) for the period 1889—1892 and 7 m/yr
for the period 1883—1889, the mean annual
balance for the entire period since 1650 (4.95 m
per year), and the thickness of the glacier, about
580 m.
INTRODUCTION
During the summer of 1972 members of the
Science Institute of the University of Iceland,
and the Icelandic Glaciological Society, drilled
a 415 m hole into the ice cap of Bárdarbunga
in north-western Vatnajökull, at 1800 m eleva-
tion. The hole failed to reach the bottom of
the glacier by an unknown distance. A special
rotary drill was designed and built at the
Science Institute for the task of drilling through
a temperate glacier, yielding a core 9 cm in
diameter. Core recovery was better than 99%.
The drill, and the drilling operation, has been
described hy Björnsson (1973) and Arnason et
al. (1974).
The purposes of the drilling were (1) to mea-
sure the thickness of the glacier at this point —
seismic measurements obtained by two French-
Icelandic expeditions in 1951 and 1955 indicat-
ed a thickness in the order of 500—600 m
(Holtzscherer 1954, Thorarinsson 1955, Sigurðs-
son 1970); (2) to obtain a deuterium profile
through tlie Vatnajökull ice cap, considered
essential for the interpretation of deuterium in
geothermal waters in Iceland (Arnason 1976);
(3) to look for climatic fluctuations in the deu-
terium profile (Dansgaard 1961, Dansgaard et
al. 1969, Árnason 1976); (4) to obtain a record
of the tritium contents of precipitation in Ice-
land during the last decades (Theodórsson
1977); (5) to study variations in global pollution
of the atmosphere; (6) to yield material for
purely glaciological studies — the structure and
recrystallization of the ice, and (7) add to the
knowledge of the volcanology of this region
through the analysis of the teplira layers (vol-
canic ash) in the core.
To date the various levels in the core the
time-honoured method of tephrochronology was
intended, a metliod well developed in Iceland
and much used for the dating of soils and arti-
facts (Thorarinsson 1970). 30 layers of tephra
were retrieved from the core, ranging in bulk
frorn a few grains to 277 grams, and in thick-
ness up to 9 cm. On the basis of the tephra
layers a profile lias been constructed of age vs.
depth (Fig. 3) in the glacier. The present article
2 JÖKULL 27. ÁR