Since 2005, a rapid increase in summer air temperature, surface energy balance and
surface melt over glaciers and ice caps in the Canadian Arctic has been observed
using a combination of field and satellite observations, climate model results and
global reanalysis data [1-3].
These changes have the potential to impact the amount of refreezing of percolating
water, which can affect the distribution of snow and ice facies, firn stratigraphy, and
ultimately the rate and distribution of surface runoff.
Although satellite data can be used to quantify the changes in the distribution of snow
and ice facies over large bodies of ice [4], results are often of low-resolution.
The use of ground-based instruments to determine the spatial variability of firn and ice
layers can provide important information to validate and complement current
knowledge of snow facies changes.
We investigated changes in the subsurface ice and firn layers on
the Devon Ice Cap since spring 2007, addressing 4 key points:
i.
Calibrate and validate GPR-inferred ice and firn stratigraphy
using firn cores.
ii. Quantify the degree of horizontal variability in the firn and ice
layers to support multi-year GPR survey analysis.
iii. Investigate changes in the stratigraphy of the accumulation
zone.
iv. Link the observed changes in ice and firn stratigraphy to
changes air temperature.
Note: GPR survey results for a given year were collected in
spring and show the over winter snowpack and the structure of
the underlying firn as it was at the end of the previous summer.
GPS
IR photogrammetry
Spatial horizontal variability is smaller in the percolation than in the
superimposed ice region, and is larger at layer boundaries and when firn is
present within ice. Standard deviation ranged between 0.020 and 0.083;
since the average GPR reflectivity is between 0.832 and 0.980, this
represents 2.0-10% of the average reflectivity.
In 2012, GPR surveys were also made
in grid patterns measuring 100 m
across the transect by 200 m along the
transect at 4 different locations along
the main transect to examine the
spatial horizontal variability in the
internal reflecting horizons (IRH)
detected by the GPR.
Density at natural
core breaks
Firn cores were drilled using a Kovacs
ice coring drill at all sites in 2006, 2011
and 2012, and every other site in 2008,
2009 and 2010. No cores were drilled
in 2007. Cores drilled in 2010 were
less than 2 m long and were
discarded. Depth of the cores analyzed
ranged from 3 to 15 m.
• During the spring 2007 survey, the upper-limit of the SI was located just below
MB 15-9 (1430 m a.s.l.), and the region between MB 12-6 (1520 m a.s.l.) and
MB 15-9 (1435 m a.s.l.) was defined as the wet snow zone. Ice layers in the
wet snow zone were formed during (or at the end of) the warm summer of
2005.
• A significant thickening of the ice layers in spring 2009 compared to spring
2007 is observed in the MB 12-6/MB 15-9 region. The upper-boundary of the
SI remains unchanged, below MB 15-9. The wet snow zone, on the other
hand, migrated 3 km up the main transect, from MB 12-6, to just below HB 91 (1600 m a.s.l.). The bottom ice layer formed during the 2005 summer acts
as an impermeable layer for percolating water, explaining why the ice layer
does not extend deeper.
• Although no GPR survey is available for spring 2008, the record of JJA mean
air temperature shows that the 2007 and 2008 summers were warm; this
suggests that changes in the subsurface stratigraphy began during the warm
summer of 2007.
Cores were logged on site between
2006 and 2011. In 2012, cores were
logged using pictures taken with two
different infrared filters.
Density measurements were taken
using the firn cores’ natural breaks,
providing data every 0.04-0.25 m;
GPR positioning was determined measurement error is 0.022 kg m3.
through concurrent real time differential
kinematic measurements with an
accuracy of < 0.5 cm. after postprocessing.
• In spring 2010, the SI upper boundary migrated to MB 15-9, as seen by the ice
layer now reaching the surface. The wet snow zone continued its upward
migration and its upper limit was at HB 9-1 (1610 m a.s.l.).
• The GPR survey from spring 2011 shows the second main shift in ice layer
distribution. The SI upper boundary migrated to MB 12-6 (1520 m a.s.l.), and a
second ice layer formed in the wet snow zone, now extending to just below
MB 8 (1640 m a.s.l.). The 2010 summer was the warmest year of the 20062011 period. High surface melt and refreezing of percolating water are
responsible for these changes.
• Changes observed in spring 2012 include the migration of the SI upper
boundary to HB 11- 5 (1550 m a.s.l.), and the thickening of the upper ice layer
in the wet snow zone.
The depth of the shallowest transition layer between ice and firn of thickness greater
than 0.10 m was used for the first GPR velocity calibration at each site. In Figure 2, this
depth is 4.5 m at Site 2.
Calibration at the 4 sites led to a mid-range velocity of 0.23 m/ns for firn and ice. Using
this velocity, depth error of the ice layers ranged from 0.10 m to 0.25 m, and depth
error of the firn layers ranged from 0.10 m to 0.35 m.
At HB 4-7, vertical structure is observed in the GPR profile.
This is characteristic of the percolation area. Although no
IRH can be identified, there is homogeneity in the profile.
Although the recent warming over the Devon Ice Cap affects the dynamics and
surface mass and energy balances of the entire ice cap, the main changes in
subsurface ice and firn stratigraphy between springs 2007 and 2012 were
observed in the accumulation area, in the region between sites HB 9-1 and MB
15-9; the focus area was extended to MB 5-9.
The evolution of the upward migration of the superimposed ice (SI) and wet
snow zones is shown in Figures 7-8 and described below.
The GPR velocity was calibrated using 2012 firn core stratigraphy at the 3 HB sites and
Site 2 using a built-in routine for hyperbola velocity calibration in the EKKO_View
Deluxe software from Sensors and Software. Analysis was extended to 8-m deep
since only firn is present below this depth.
Figure 2: Example of GPR velocity calibration for Site 2 using ice core
stratigraphy. White represents ice layers, and red and blue, firn of
different densities. SEC2 gain and a Hilbert transform envelope were
applied to the GPR data. 0-5 ns is a mixture of fresh snow and noise.
Ice layers show little horizontal spatial variability
compared to the 0.8-1.0 firn and 5.7-5.9 m
firn/ice layers. This is due to the uneven
refreezing of percolating meltwater.
The
presence of residual firn within a thick
homogeneous ice layer also increases spatial
horizontal variability (1-4.75 m, not shown).
Fig. 1 shows that the distance between GPR tracks does not exceed 65 m.
Based on results shown in Fig. 4, differences in GPR profile derived from the
comparison of multi-year GPR transects will not be caused by the horizontal
spatial variability associated with the different GPR tracks.
Figure 6: Example of the GPR profile of the surveyed transect from 0 km (Site 1) to 39.2 km (Site 3) for spring 2010. Dewow filter,
background subtraction, GPS topography correction and a SEC2 gain were applied to the data. White layers represent ice, and
purple, firn. The different firn facies are identified. The area within the 2 vertical dotted-lines is zoomed-in below. V.E. = 200X
GPR data were collected along the
main transect each spring between
2007 and 2012 using a Noggin 500
MHz instrument from Sensors and
Software. Data collected in 2008 were
spring
discarded
due to the malfunction of the
instrument.
Figure 5: Example of slice images at 4 different
depths for site HB 13-7. A Hilbert transform envelope,
amplitude equalization and migration were applied.
Red and white represents firn, and blue represents
ice. Greater spatial variability is associated with firn.
Coring
GPR
Figure 1: Devon Ice Cap fieldwork sites. The thick line represents the main
GPR transect. The bottom right pictures shows an example of the distance
separating GPR tracks; distance ranged between 5 m and 65 m. GPR grid
surveys were made at the three HB sites and Site 2 in 2012. Firn cores have
been drilled at all the labeled sites along the transect. 2-m air temperature
was measured for the 2006-2011 period with a Beta Therm 100K6A probe
mounted on an automated weather station (AWS) at Site 2. Contour intervals
are 50 m.
Figure 4: Standard deviation of the GPR reflectivity of the 100 m X 200 m grids
between 0.04 m and 8 m depth at a 0.2 m-interval for Site 2, HB 13-7, HB 9-1 and
HB 4-7. GPR reflectivity were given a value from 0 to 1 based on their color (black =
0, white=1).
Figure 7: (Top) Elevation profile between MB 5-8 and MB 15-9 from corrected differential GPS survey. (Bottom) Spring 2007-2012
GPR profiles between MB5-8 and MB15-9. Wave-like patterns are associated with rolling topography. Data processed as in Fig. 6.
VE = 200X. (Right) JJA mean 2-m air temperature between 2006 and 2011 at Site 2 from AWS measurements.
Figure 8: Spring 2006-2012 3-m bulk density from firn core analysis at each site.
An increase in 3-m bulk density is observed at all sites between MB 5-8 and MB
15-9. Relative to spring 2006, the bulk density in spring 2012 increased from
17% at MB 8 to 35% at MB 12-6. The increases are larger at sites HB 9-1 and
below, the region where most of the stratigraphic changes took place.
By spring 2009, the 3-m bulk density already reached levels comparable to those
found in springs 2011 and 2012. Summers of 2007-2008 were at the beginning
of the accelerated warming period [2]. The sharp increase in JJA mean air
temperature compared to previous years is responsible for the increase surface
melt energy, surface melt and refreezing of meltwater, which led to an increase in
ice content in the first 3 m below the ice cap surface.
HB 9-1 is in the wet snow zone. An ice layer is present at 23 m depth.
HB 13-7 is situated in the superimposed ice (SI) region. A 4m thick ice layer is well identified above a 2-m firn layer (5-7
m deep) and a 0.75-m ice layer (7.25-8 m deep). Some
horizontal spatial variability is observed in the thickest ice
associated with the recent transition from the wet snow to
the SI zone.
Figure 3: Example of a 100-m GPR line across the main transect line at each of the 4 grid sites
showing the GPR characteristics of 3 different firn facies in the accumulation area of the Devon Ice
Cap in spring 2012. Dewow filter, background subtraction, topography correction and a SEC2 gain
were applied to the data. V.E = 4.1X
GPR characteristics at Site 2 are similar to HB 13-7. Greater
horizontal spatial variability is observed associated with the
non-continuous firn layers between 2 and 4 m deep.
References:
Spatial variability of firn and ice layers:
• Grids in the SI area showed more horizontal variability due to the uneven distribution of firn layers within the thicker ice.
• Overall, little small-scale horizontal variability is present in the accumulation area of the Devon Ice Cap.
• Changes in GPR profile from multi-year comparisons are not associated with spatial horizontal variability of firn and ice layers.
Changes in the subsurface firn and ice layers between spring 2007 and spring 2012:
• The SI area migrated from 1430 m a.s.l. in spring 2007 to 1550 m a.s.l. in spring 2012.
• The wet snow zone migrated from 1520 m a.s.l. in 2007 to 1640 m a.s.l. in spring 2012.
• A 17-35% increase in 3-m bulk density was observed at all sites between MB 5-8 and MB 15-9.
• Observed changes are linked to an increase in percolation and refreezing of melt water at higher elevations.
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