Densification and refreezing in the percolation zone of the Greenland Ice Sheet: implications for mass balance measurements
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Date
2009Author
Parry, Victoria
Metadata
Abstract
In order to increase coverage, mass balance changes of the world’s ice sheets are
increasingly derived from surface elevation changes measured via satellite. Across
the percolation zone of the Greenland Ice Sheet, meltwater, percolation and
refreezing cause a re-distribution of mass through densification which may result in
elevation change with no associated mass loss. Therefore, densification processes
need to be quantified, spatially and temporally, and accounted for in mass balance
measurements. This thesis investigates the relationships between patterns of
elevation change and temporally and spatially variable accumulation and
densification processes. In doing so, it provides an important contribution to the
validation of the European Space Agency’s CryoSat-2 mission by placing error bars
on the accuracy to which changes in satellite-measured ice-mass surface elevation
represent real changes in ice mass.
Temporal variability in near-surface (<10 m) snowpack and firn density and
structure was measured in snowpits, shallow cores and using a neutron probe in the
spring and autumn of 2004 at ~1945 m elevation (T05, 69o 51N, 47o 15W) in the
percolation zone of the Greenland Ice Sheet. Results show that average snowpack
density increased by 26% from spring to autumn, with a 5% (7.6 cm) increase in
elevation, and a corresponding 32% increase in mass. Spatial variability was
investigated at 11 sites along two transects at spatial scales of 1 m – 10 km. Whilst
there was little variability in small scale (1 - 100 m) density changes, ‘seasonal
densification’ increased at lower elevations, rising to 47% 10 km closer to the ice
sheet margin at 1860 m a.s.l. The spatial variability in seasonal densification was
further investigated in spring 2006 at seven sites located at ~10 km intervals along a
57 km transect spanning a 350 m elevation range. Snowpits and shallow cores reveal
no significant variation in spring (prior to melt) snowpack density but following
summer melt and refreezing cycles, seasonal densification decreased with
increasing elevation at 32 kg m-3 per 100 m. Measurements at three sites ranging in
elevation from 1860 – 2015 m and spanning three melt-seasons show inter-annual
variation in the seasonal densification gradient.
In order to obtain a longer time series of mass balance, a 17 m core retrieved in
spring 2004 was analysed for stratigraphy, density and ionic and isotopic
concentrations to identify annual layers. Unfortunately, the seasonal melt cycle
(whereby on average 10% of the snowpack undergoes melt), results in a complex
stratigraphy and density and ionic concentrations that cannot be resolved into a
seasonal signature. However, the δ18O and δ D isotopes show clear sinusoidal
fluctuations, which have been used to derive annual mass balance from 1986 to
2003. These show a mean annual accumulation of 53.7 cm w.e. (s.d. 12.9 cm w.e.)
although the accuracy of these measurements is compromised by the percolation of
meltwater through more than more year’s snowpack.
These findings confirm that estimates of mass balance cannot be calculated solely
from observed changes in surface elevation. However, predicting spatial and
temporal variations in densification is not straightforward because of the complex
inter-annual variations in the processes of accumulation, melt, percolation and
refreezing.