Variations in mass balance and snow and firn densities along a transect in the
percolation zone of the Greenland Ice Sheet
Vicki Parry (1), Peter Nienow (1), Douglas Mair (2), Julian Scott (3), Duncan Wingham (4)
1 – School of Geosciences, University of Edinburgh, UK, 2 – School of Geography and the Environment, University of Aberdeen, UK, 3 – British Antarctic Survey, Cambridge, UK, 4 –Centre for Polar Observation and Modelling, University College London, UK.
RESULTS
Fig 2. Fig. 2
Accurate elevation changes over large areas of the polar ice sheets can be
measured using satellite radar altimetry, from which mass balance can be derived.
650
600
In the percolation zone seasonal changes in snow pack density ensure that
changes in surface elevation cannot be directly correlated with changes in mass.
Mean
kg m-3
404
524
S.D Coeff Variation
kg m-3
%
20
56
5
11
•The average Core
2005 (post-melt) density
is 20% greater than Pit
2006 (pre-melt) (Fig. 2)
from T1 to T7.
550
Core 2005
y = -0.36x + 1200
-3
Density kg m
We aim to determine the influences of summer densification along a 57 km transect
in the percolation zone of the Greenland Ice Sheet (GIS).
2006 pit
2005 core
500
R2 = 0.79
450
T1
T7
400
FIELD SITES
Pit 2006
y = -0.03x + 470
350
R2 = 0.05
300
1600
Fig. 1
The field sites are
located along a 57 km
transect on the EGIG
line in the percolation
zone on the west of
the GIS (Fig 1.).
1650
1700
1750
1800
1850
1900
1950
2000
2050
2100
Elevation m
•Pre-melt (Pit 2006)
density shows no
change with elevation.
Post-melt (Core 2005)
density decreases by
36 kg m-3 for every
100 m elevation
increase from T1 to T7
(Fig. 2).
700
Fig. 3
Difference between gradients:
2003 and 2004 - 99% significant
2004 and 2005 - 97.5% significant
2003 and 2005 - 97.5% significant
Core 2004
y = -1.28x + 3050
650
•Density decreases with
increasing elevation
from T4 to T6 for Cores
2003, 2004 and 2005
(Fig. 3).
600
Denstiy kg m-3
INTRODUCTION
550
500
T4
Core 2003
y = -0.31x + 1100
450
•The gradients of
densification with
elevation for Cores
2003, 2004 and 2005
are all statistically
significantly different at
or above 97.5% (Fig. 3).
T5
Core 2005
y = -0.77x + 2000
400
T6
350
300
1850
1870
1890
1910
1930
1950
1970
1990
2010
2030
Elevation m
DISCUSSION: Implications for mass balance measurements derived from elevation changes measured by satellite radar altimetry:
If densification due to meltwater percolation and refreezing is not accounted for, changes in elevation may be misinterpreted as a change in mass.
The transect extends from
T1 (69o 44 N, 48o 7 W) at
1680 m elevation to T7 (69o
56 N, 46o 48 W) at 2050 m
elevation (Fig 1).
E.G. Assuming no mass change, and applying the average
densification from the transect, 120 kg m-3, there is a
surface lowering of 0.24 m in a 1 m thick snowpack.
r=404 kg m-3
1m
WE=0.4 m
r=524 kg m-3
WE=0.4 m
However, if temperatures
change so will densification
rates and elevation
changes will occur without
any change in mass.
0.76 m
Fig 4.
600
JAR 3 323 m
500
JAR 2 568 m
400
300
JAR 1 962 m
200
Fig 4. shows the number of Positive Degree Days (PDD) at 4 AWS stations (Steffen and others
1996) down glacier from T1 from 1996 to 2005.
METHODS
Density measurements were made by measuring volume and mass of samples
from visually identified stratigraphic layers in snowpits and shallow cores.
Location
Time
Retrieved from and to
Results from:
T1 – T7
Spring
2006
Spring 2006 surface to Last
Summer Surface (LSS) 2005
2005-2006 pre-melt
accumulation
2005 Core T1 – T7
Spring
2006
2005 LSS (beneath 2005
winter accumulation) 1.5 m
2005 post-melt
accumulation
2004 Core T4 – T6
Autumn LSS 2004 (current surface)
2004
2004 post-melt
accumulation
2003 Core T4 – T6
Spring
2004
2003 post-melt
accumulation
2006 Pit
LSS 2003 (beneath 2003
winter accumulation)
The shallow cores may extend through more than one years accumulation, so the
bottom part may include part of the previous years accumulation.
100
Swiss Camp 1149 m
0
There is a modest positive correlation showing that since 1996 the number of PDDs per year
have been increasing by 17 PDDs per year at 323 m, and 6 PDDs per year approximately at the
ELA (1149 m).
1996
1998
2000
2002
2004
2006
Years
Using a lapse rate of 1 oC for every 142 m (Hanna and others 2005) for Core 2005 results, there is an increase in density of 51 kg m -3 for every 1 oC warming.
With predicted increases in temperature in the Arctic of 2 to 3 oC by 2040 (ACIA 2005), for a 1 m snowpack, pre-melt density of 404 kg m-3, there will be an increase in
post-melt density of 102 – 204 kg m-3 and a decrease in elevation of 0.21 – 0.34 m, with no mass loss.
Thus, changing temperatures in the Arctic also need to be considered.
CONCLUSION
In order for accurate mass balance measurements to be derived from accurate elevation change measurements, future changes in rates of densification must be
considered. We have found that in the percolation zone of the Greenland ice sheet, elevation changes may not directly relate to mass change due to seasonal variations
in densification of the snowpack associated with processes of surface melt, percolation and refreezing. As expected, densification decreases with increasing elevation,
but this gradient changes annually. Firn density is related to annual melt and accumulation (Braithwaite 1994), both of which vary. With projected rising temperatures,
rates of densification will increase with a subsequent impact on elevation change but not necessarily on mass balance at higher elevations in the percolation zone.
ACKNOWLEDGEMENTS
REFERENCES
This work is a contribution to the validation of the ESA CryoSat. The work is
funded by NERC through grant NER/O/S/2003/00620.
Braithwaite, R., M. Laternser, W. Pfeffer. 1994. Variations of near-surface firn density in the lower accumulation area of the Greenland ice sheet, Patkitsoq, West Greenland. J. Glaciol., 40(136), 477-485.
Our thanks for help in the field go to Yenz, and for logistical support go to Kristian Keller and Rene
Forsberg of KMS and DNSC, Copenhagen, Malcolm Davidson of the European Space Agency, Robin
Abbot of VECO Polar Resources, Kate Bar Friis of Kangerlussuaq International Science Support and
the Danish Polar Centre.
Pfeffer, W., M. Meier, and T. Illangasekare. 1991. Retention of Greenland runoff by refreezing: Implications for projected future sea level change. J Geophys. Res. 96(C12), 22,117-22,124.
Steffen, K., J. Box, and W. Abdalati, 1996 Greenland LCimate Network: GC-Net, in Colbeck, S. C. Ed. CREEL 96-27 Special Report on Glaciers, Ice Sheets and Volcanoes, trib. To M. Meier, pp. 98-103.
ACIA. 2005: Arctic Climate Impact Assesment. Cambridge university press, New York, pp.27