Jökull - 31.12.2001, Blaðsíða 29

Ice core drilling on Hofsjökull
The summit area of Hofsjökull was first visited
in 1911 (Wunder, 1912) and expeditions were made
there in 1930, 1951, 1956 and 1976 (Sigurðsson,
200 la). The surface of the ice cap and the subglacial
bedrock topography were mapped in 1983 (Björns-
son, 1988). This survey confirmed the presence of
a subglacial mountain massif of volcanic origin un-
der the central part of the ice cap, including a large
volcanic caldera. The ice thickness reaches a maxi-
mum of 750 m in the centre of the ice-filled caldera,
which has an area of ~40km2. The average thickness
of the ice cap is 215 m (Björnsson, 1988). The annual
mass-balance of Hofsjökull has been monitored con-
tinuously since 1988 (Sigurðsson, 2001b). Net bal-
ance at the summit site has varied between 531 cm wa-
ter equivalent (w.eq.) in 1991/1992 and 239cmw.eq.
in 1996/1997. The average net balance in the period
1987/2001 was 327cmw.eq. The net balance of the
entire ice cap was positive in 1988/89 and during the
period 1991 to 1994, but negative in other years. On
average, the cumulative specific mass balance of the
ice cap has been negative by about 3-4 m during the
14 years of mass-balance measurements.
FIELDWORK
A team of four scientists and three technicians car-
ried out the expedition in the period 30.7.-11.8. 2001.
All gear was transported to the northern edge of the
ice cap by trailers and 4WD vehicles. A Pisten Bully
snow vehicle was used to transport a ski-equipped liv-
ing hut, the drill and accessories, and other equipment
to the summit of the ice cap, where a camp was estab-
lished at the position 64°48.54'N, 18°52.14'W, eleva-
tion 1790m (Figure 1). The ice cap summit is located
above the eastern rim of the subglacial caldera, where
the local ice thickness is approximately 300 m. The
main reason for selecting this site for the drilling was
the possibility of a direct comparison of data derived
from the ice core with the 14 year record of snow ac-
cumulation obtained at the same location.
Transport from Reykjavík and setting up of camp
and drilling equipment took 2.5 days. Drilling took
place inside a tent placed within a 1 m deep trench
(Figure 2). The ice cores were logged, photographed
and packed in plastic bags inside the tent and stored
on site in a small freezer. The cores were then packed
in 1 m sections into polystyrene boxes, which were
transported a distance of 20 km down to the north-
ern margin of the ice cap, where they were stored
at -25 °C in a 20 ft freezing container powered by a
diesel engine. The container was transported by truck
to Reykjavík in one day at the completion of the field-
work, and the ice core was shipped to the Alfred We-
gener Institute for scientific analysis.
DRILLING OPERATIONS
Drill design
The ice-core drill used was built at the Alfred We-
gener Institute. It has been used with success to re-
trieve shallow cores of 100-320 m length in Green-
land and Antarctica. In 1997 it was first tested in
temperate ice, when a 70 m core was drilled on the
northern dome of Langjökull, Iceland (Thorsteinsson,
1997). Figure 3 shows pictures of the drill along with
a description of its individual parts.
Drilling
The core length drilled in each run is shown in Figure
4 and additional information on the drilling progress is
presented in Table 1. No problems were encountered
while penetrating the fim layer, and the water table in
the ice cap at a depth of 35 m was reached in less than
5 hrs. Drilling then became more difficult; in the in-
terval 30—40 m the average core length dropped from
>1 m to <0.4 m. No more than 6 hours could be used
for drilling on each of the first four days, because of
the time spent on maintenance and on attempts to seal
water-sensitive components of the drill. Drilling took
place during daytime only and between 26 and 45 mns
were performed each day. The time used for each mn
is shown in Figure 5. Runtime increases steadily with
depth, from <10min near the surface to >20 min to-
wards the end of the drilling. The average mntime was
14.3 min. Occasional runtime peaks were almost al-
ways due to extended surface time. Most core breaks
(Figure 5) were between 200 and 400 kg and the av-
erage was 290 kg. A slow downward increase is ob-
served. Occasional core breaks exceed 600 kg and the
highest recorded break was 700 kg. To calculate the
JÖKULLNo. 51 27