Jökull - 01.01.2019, Síða 158

Society data report
A 25.6 m long firn core extracted from the Grímsvötn ice shelf
in June 1993
Magnús Tumi Gudmundsson1 and Jósef Hólmjárn2
1Institute of Earth Sciences, University of Iceland, Sturlugata 7, 101 Reykjavík, Iceland mtg@hi.is
2Rauðagerði 12, 108 Reykjavík. https://doi.org/jokull2019.69.157o
During the annual spring expedition of JÖRFÍ on June
19–20 1993, a 25.6 m deep hole was drilled into the
firn layer at 64◦25.0’N, 17◦20.1’W where the eleva-
tion was 1440 m a.s.l. (Figure 1a). A continuous core
was extracted, its density and temperature measured
on site and samples taken for further analysis (Gud-
mundsson, 1993). Conditions changed in Grímsvötn
following eruptions in the region in the years after the
core was drilled. The Gjálp eruption occurred 10 km
to the north in 1996 and during the Grímsvötn erup-
tions in 1998, 2004 and 2011 volcanic vents were ac-
tive 2–3 km from the 1993 drill site (Gudmundsson
and Larsen, 2015). Fallout of tephra in these erup-
tions has resulted in large changes in surface albedo
in summer, leading to increases in surface melting
and reduced annual mass balance within Grímsvötn
(Björnsson and Pálsson, 2008; Reynolds et al., 2018).
This was already apparent in a study of a 100 m core
drilled in Grímsvötn in 2002 (Figure 1a) that showed
that the entire annual layers of 1999–2001 had melted
away (Thorsteinsson et al., 2003). The depth-density
and temperature profiles obtained in 1993 have re-
mained unpublished until now but are presented here
for reference and comparison with other data. Figures
2 and 3 show the drilling operation on 19–20 June.
The core had a measured length of 25.3 m (Figure
1b). The uppermost 6.0 m are the 1992–1993 annual
layer. From 6 m depth the density increases broadly
linearly from 550 kg m−3 to approximately 900 kg
m−3 at the bottom (the uncertainty in density values
is 30–40 kg m−3). Considerable fluctuations occur,
with the density peaks mainly resulting from thick ice
lenses. The core reached ice density (830 kg m−3)
at 24.6 m and remained above that value to the max-
imum cored depth. The hole remained dry and did
not reach down to the the level of groundwater in the
ice shelf. The firn-ice transition observed in Gríms-
vötn at just under 25 m depth, is shallower than the 35
m observed at approximately 350 m higher elevation
of 1790 m a.s.l. on Hofsjökull (Thorsteinsson et al.,
2002). However, both holes show similar behaviour
of a roughly linearly increasing density with depth.
Temperature (Figure 1c) was measured in the core
as it was extracted with a pin-type thermometer, cali-
brated for zero degrees with readings taken in a water-
snow mix. In order to try to resolve for unperturbed
firn temperature an experimental Pt-type thermometer
was used, where a spring pressed the sensor against
the hole wall.
The resistor temperature measurements were done
one day after termination of drilling. The cold part
between 6 and 9 meters had warmed, suggesting that
interaction with air in the two days since drilling had
warmed up this upper part of the hole. The below-
freezing values from the Pt-meter in the lowermost 10
meters are somewhat surprising. We cannot rule out
that this is an artifact. However, it is possible that low
air temperatures during the night prior to measure-
ment resulted in cold air sinking into the hole and con-
vecting heat out of it. This effect would have caused
a slight, temporary cooling of the walls of the hole in
the warmer bottom part.
JÖKULL No. 69, 2019 157