Jökull - 31.12.2001, Side 27

Reviewed research article
Ice core drilling on the Hofsjökull ice cap
Thorsteinn Thorsteinsson1, Oddur Sigurðsson2, Tómas Jóhannesson3,
Guðrún Larsen1, Cord Drucker4 and Frank Wilhelms4
1. Science Institute, University oflceland, Dunhaga 3, IS-107Reykjavík, Iceland; thor@os.is, glare@raunvis.hi.is
2. National Energy Authority, Grensásvegi 9, IS-108 Reykjavík, Iceland; osig@os.is
3. Icelandic Meteorological Office, Bústaðavegi 9, IS-150 Reykjavík, Iceland; tj@vedur.is
4. Alfred Wegener Institutefor Polar- and Marine Research, Columbusstrasse, D-27568 Bremerhaven, Germany
Abstract - In August 2001, an ice coring program was carried out on Hofsjökull, a temperate ice cap in
Central Iceland. A shallow drill designed and built at the Alfred Wegener Institute in Bremerhaven, Germany,
and intended for use in dry holes on polar ice sheets, was used in this effort. The drilling took place near
the summit of the ice cap, at an altitude of 1790 m, above the eastern rim of a subglacial volcanic caldera.
The local ice thickness at the drill site is close to 300 m. The drilling was smooth and problem-free down to
a depth of 35 m, where the water table in the ice cap was encountered. Beyond this depth, the average core
length dropped rapidly from 1 m to 0.3 m. The main problem encountered in drilling through water-saturated
ice seemed to be related to transport of ice chips, since the chips produced during cutting did not move easily
into a collection chamber in the top part of the core barrel. Compaction of chips occurred when the spiral
section on the outside ofthe barrel had been filled. Freezing onto the cutters did not seem to be a significant
problem during drilling. Below the water table, water tended to leak into the drill motor, which nevertheless
continued to operate. ln total, a 100.2 m long ice core was drilled in 263 runs, in 9 days. The core was logged
on-site and a preliminary study ofvisual stratigraphy was carried out, revealing an abundance ofbubble-free
layers, up to 30 cm in thickness. Density measurements reveal that glacier ice (p = 830kg/m3) hasformed at
a depth of 35 m. Tephra layers from the Hekla eruptions in 1991 and 1980 werefound at 40.8 m and 69.7 m
depths, respectively. Experience gained during the drilling may be used to improve the drill so that it becomes
better suitedfor drilling in temperate ice.
INTRODUCTION
During the past few decades, ice core studies in the
polar and high mountain regions of the world have
provided a wealth of information on past climate, at-
mospheric composition and volcanic activity. Deep
cores from Greenland and Antarctica typically cover
timescales of 100-500,000 years, and shallow coring
programs are being carried out in most glaciated re-
gions of the world to study the Holocene (0-11,500
years BP), the Little Ice Age, and recent anthro-
pogenic influence on the atmosphere (Bradley, 1999).
Relatively little work of this kind has been carried
out in Iceland. Pioneering efforts were conducted in
the years 1968-1972, culminating in core drilling to a
depth of 415 m at 1800nr elevation on Bárðarbunga,
western Vatnajökull (Árnason et al., 1974). Borehole
measurements showed that the Vatnajökull ice cap is
temperate throughout. Results from these early stud-
ies indicated that seasonal variations in the concen-
tration of hydrogen isotopes become obliterated be-
cause of the exchange of isotopes between summer-
time meltwater and winter snow (Árnason, 1969).
More recent studies on snow and firn cores from the
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