Cryoconite hole connectivity on the Wright Lower Glacier,
McMurdo Dry Valleys, Antarctica
SHELLEY MACDONELL,1,2
* MARTIN SHARP,3
SEAN FITZSIMONS2
1
Centro de Estudios Avanzados en Zonas Áridas (CEAZA), Universidad de La Serena, Campus Andrés Bello, Raúl Bitrán
1305, La Serena, Chile
2
Department of Geography, University of Otago, PO Box 56, Dunedin, New Zealand
3
Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta T6G 2E3, Canada
*Correspondence: Shelley MacDonell <shelley.macdonell@ceaza.cl>
ABSTRACT. Cryoconite holes can be important sources and stores of water and nutrients on cold and
polythermal glaciers, and they provide a habitat for various forms of biota. Understanding the hydrological
connectivity of cryoconite holes may be the key to understanding the transport of nutrients
and biological material to the proglacial areas of such glaciers. This paper aims to characterize and
explain spatial variability in the connectivity of ice-lidded cryoconite holes on a small, piedmont
glacier in the McMurdo Dry Valleys through geochemical analysis of cryoconite hole waters. Solute concentrations
in both surface and near-surface ice and cryoconite holes, vary greatly along the glacier centerline,
and all sample types displayed similar spatial patterns of variability. Using chloride as a tracer, we
estimated variations in cryoconite hole connectivity along the glacier centerline. We found that a previously
used mass transfer method did not provide reliable estimates of the time period for which cryoconite
hole waters had been isolated from the atmosphere. We attribute this to spatial variability in both
the chloride content of the surface ice and surface ablation rates. The approach may, however, be used
to qualitatively characterize spatial variations in the hydrological connectivity of the cryoconite holes.
These results also suggest that ice-lidded cryoconite holes are never truly isolated from the nearsurface
drainage system.
KEYWORDS: cryoconite hole, glacier hydrology, glaciochemistry, supraglacial hydrology
1. INTRODUCTION
Drainage or storage of water on glacier surfaces is dictated by
the structure of the hydrological network and the temporal
variability of hydrological connections. On cold glaciers,
cryoconite holes can be important determinants of temporal
and spatial patterns of water movement due to their ability to
create and store liquid water beneath the ice surface, even
where the surface ice is not at the melting point (Wharton
and others, 1985; Fountain and others, 2004, 2008;
MacDonell and Fitzsimons, 2008).
Cryoconite holes form when differential ablation causes a
water-filled depression to form within the surface ice layer.
Depending on the air temperature, the developing hole
may either stay open to the atmosphere, or may develop an
ice cover, thereby ‘isolating’ the trapped meltwater from external
processes. In the latter case, hole formation is likely to
accelerate once sediment is incorporated into the ice. This
occurs because shortwave radiation can penetrate ice and
therefore continues to cause melting at the ice/sediment
interface, while longwave emission from the upper surface
only cools the ice at the surface and not at depth (Brandt
and Warren, 1993; Hoffman and others, 2014). The resulting
ablation will cause the hole to grow until it reaches an equilibrium
whereby the rate of hole deepening is approximately
equal to that of surface lowering around the hole (Gribbon,
1979). Cryoconite hole development is largely restricted to
cold-based and polythermal glaciers in high latitude and
high-altitude regions (Takeuchi, 2002; Fountain and others,
2004; Anesio and others, 2007), as well as some temperate
glaciers (Anesio and others, 2010), where low melt rates
generate insufficient runoff to wash sediment from the
glacier surface. In arid regions, such as the McMurdo Dry
Valleys, cryoconite holes are often ice covered. According
to previous studies using changes in chloride mass as a
proxy for isolation time, cryoconite holes in Antarctica can
remain isolated from both the atmosphere and other holes
for several years (Fountain and others, 2004; Bagshaw and
others, 2007).
Cryoconite holes have attracted a great deal of attention
recently because they are the most biologically active
habitat within glacier systems (Säwström and others, 2002;
Edwards and others, 2011) and are important nodes for nutrient
and hydrological cycling (Irvine-Fynn and others, 2012;
Bagshaw and others, 2013). The nutrient and biotic profiles
exhibited within cryoconite hole waters are determined by
initial ice concentrations and interactions between sediment,
microorganisms, water, light and carbon dioxide (Tranter and
others, 2004, 2005; Hodson and others, 2008). However, the
primary control on differences between holes is likely the
local drainage system structure and function (Edwards and
others, 2011). Therefore, in order to characterize nutrient
cycling and biological productivity on glacier surfaces, an
understanding of the hydrological connectivity of cryoconite
hole systems is required.
On several glaciers, cryoconite holes have been observed
to influence the routing of surface and near-surface meltwater
(e.g. Fountain and others, 2004; MacDonell and
Fitzsimons, 2008; Cook and others, 2016). Depending on
the configuration of the supraglacial drainage system, cryoconite
holes can be connected via a variety of processes.
Journal of Glaciology (2016), 62(234) 714–724 doi: 10.1017/jog.2016.62
© The Author(s) 2016. This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (http://creativecommons.
org/licenses/by/4.0/), which permits unrestricted re-use, distribution, and reproduction in any medium, provided the original work is properly cited.
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Firstly, if there is no ice cover on the cryoconite holes, water
can flood the cryoconite hole from the surface, and water can
evaporate from the hole. Such a situation differs little from
standard overland flow processes, as the expectation is that
cryoconite holes will then delay water flow in a manner
similar to overdeepening in a stream channel. Connections
between ice-lidded holes have been thought to occur via
fractures within the ice, or via near-surface or surface channels.
However, hydrological connectivity via inter-granular
drainage within the upper 20 cm of the ice has been reported
(Wakahama and others, 1973; MacDonell and Fitzsimons,
2012; Hoffman and others, 2014; Cook and others, 2016)
and this has significant implications for understanding the
structure, function and dynamics of supraglacial drainage
systems. Depending on the hydrological system structure
on a particular glacier, an individual hole may oscillate
between being connected and isolated, with connection/isolation
times being as long as several years. To date, no studies
have monitored water flow directly, but some authors have
used variations in chloride concentrations between cryoconite
holes to infer patterns of isolation duration (Fountain
and others, 2004; Bagshaw and others, 2007).
The aim ofthispaperistocharacterize andexplain the spatial
variability of connectivity between cryoconite holes and the
near-surface drainage system on Wright Lower Glacier, a
small piedmont glacier in the McMurdo Dry Valleys,
Antarctica. We use the chloride technique to infer variations
in cryoconite hole connectivity with elevation along a glacier
centerline, and perform a sensitivity analysis of the technique
to helpevaluate theresults.Finally we discusswhetherchemical
concentration variations do in fact provide a measure of the iso-
lationdurationofcryoconiteholes,orwhethertheyprovideonly
a relative measure of degree of connectivity.
2. SITE DESCRIPTION
The Wright Lower Glacier lies at the western end of the
Wright Valley in the McMurdo Dry Valleys (77°25′S,
162°47′E), and shares an accumulation area with the
Wilson Piedmont Glacier (Fig. 1). Annual precipitation
rates at Lake Brownworth (BR-AWS; Fig. 1) are among
the lowest recorded in the McMurdo Dry Valleys
(Fountain and others, 2010). The mean annual air temperature
is −20°C but temperatures can reach +8°C in
the summer (Doran and others, 2002). The glacier
surface can be divided into two main topographic units.
On the true right side of the glacier, the surface is
deeply dissected by a series of supraglacial stream channels.
The remainder of the glacier surface consists of
near-horizontal or slightly undulating terrain (referred to
as a planar surface).
On the Wright Lower Glacier, the average percentage
area covered by cryoconite holes is 3.5%, although the distribution
is not uniform (Table 1; MacDonell and
Fitzsimons, 2008, 2012; MacDonell and others, 2013). As
the air temperature is generally below freezing year round,
cryoconite holes normally have a near-permanent ice lid
that will only disappear during extreme high air temperature
events in summer (Foreman and others, 2004). MacDonell
and Fitzsimons (2012) showed that cryoconite holes could
be hydrologically connected by surface fractures, shallow
sub-surface channels, coalescence with other cryoconite
holes, and/or intergranular drainage. Spatial patterns of connection
modes were not observed. It appeared that connections
via cracks and other near-surface channels were
largely ‘fortuitous’, such that a cryoconite hole had to form
within a channelized zone to become connected to a
channel network. It did not appear that the distribution of
cryoconite holes influenced the formation of near-surface
channels, as they tended to follow extension cracks.
Intergranular drainage was widely observed from early
December onwards during repeat mapping of cryoconite
holes in a fixed grid as well as during excavation of surface
ice across the glacier during December 2005 and January
2007, but the spatial extent and duration of such drainage
was not known.
Fig. 1. Map of the Wright Lower Glacier, McMurdo Dry Valleys, Antarctica, show sampling locations. The inset shows two cryoconite holes
that are ∼45 cm in diameter.
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3. METHODS
To assess the spatial patterns of hydrological connectivity of
cryoconite holes, we used the method employed by Fountain
and others (2004) to estimate cryoconite hole isolation at ten
sites along the glacier centerline (Fig. 1). This method
requires the following input data: (1) chloride concentrations
of surrounding ice and cryoconite hole waters, and (2) measurements
of hole characteristics such as depth, area and rate
of deepening as estimated from local ablation rates (Tables 1,
2; MacDonell and Fitzsimons, 2012).
Water samples were collected from five cryoconite holes
at each of ten locations along the centerline of the glacier
on the 29 and 30 December 2005 (locations A–J; Fig. 1),
yielding a total of 50 samples. The ten sites were located
over a 1 km distance and sited on average, 100 m apart.
The criteria for selecting the sampled holes were that they
(1) were large enough to contain at least 1000 mL of water
and (2) were not visibly connected to near-surface channels.
The second condition essentially excluded cryoconite holes
positioned in the middle of near-surface channels, as it was
difficult to differentiate the boundaries of such holes. At the
same locations, five surface ice samples were collected
from a 40 to 60 mm depth adjacent to each ablation stake.
This depth was selected to avoid sampling weathered
surface ice (Hoffman and others, 2014). The surface ice
sampled did not contain either sediment or cryoconite
holes. In addition 23 samples were collected at a depth of
750 mm below the glacier surface at 50 m intervals
between sample locations A–L (Fig. 1). These ‘deep ice’
samples were taken below the deepest cryoconite hole
depth, and so are likely not to have been influenced by
surface hydrological processes and associated chemical
modifications.
Cryoconite hole water was sampled by drilling a 50 mm
diameter hole through the ice lid using a Kovacs auger and
extracting the water with a syringe. Samples were taken
from the upper surface of the hole water and it was
assumed that the waters were relatively well mixed.
Surface ice samples were extracted using a chisel sterilized
with ethanol and flamed, and ‘deep ice’ samples were collected
with a Kovacs auger and the outer ice removed
using a sterilized and flamed chisel before analysis. All
samples were melted and filtered in the field through a 45
µm cellulose nitrate filter paper. Before the samples were filtered,
the apparatus was rinsed three times using the sample
water before the sample was collected in a 125 mL Nalgene
bottle for solute concentration analysis. After filtering, the
samples were frozen. During transportation and storage,
the samples were not exposed to temperatures higher than
−18°C.
In the laboratory, samples were melted and were analyzed
within 24 h. When not being used, the samples were sealed
to avoid evaporative loss and stored at 4°C. Chloride (Cl−
),
ammonium (NH4
+
), nitrate (NO3
−
) and phosphate (PO4
2−
) concentrations
were analyzed using a Foss FIAStar 5000
Flow Injection Analyzer; concentrations of sodium (Na+
), magnesium
(Mg2+
), potassium (K+
) and calcium (Ca2+
) were determined
using a Varian Spectra 240FS Atomic Absorption
Spectrometer. Repeat samples were analyzed for each analyte,
and a blank sample (class 1 water) and standard of known concentration
were analyzed every 15 samples. The blank sample
and standard results were used to assess the errors associated
with the solute concentration analysis (Table 3). At the time of
analysis it was not possible to measure SO4
2−
and HCO3
−
concentrations,
so these have been estimated for each sample
from the total charge balance errors (CBE).
Chloride concentrations were used to estimate the length
of cryoconite hole isolation. Fountain and others (2004) suggested
that the duration of hole isolation could be calculated
from the difference between the measured concentrations of
a non-reactive, non-biologically utilized solute, such as Cl−
(Gooseff and others, 2004) and the surrounding glacier ice
concentration. Chloride is a useful marker in this environment
because it is irreversibly deposited within snowpacks
(Cragin and McGilvary, 1995), and is highly soluble. The
dominant source of Cl−
in the lower valley system is the
Ross Sea, however a very small contribution from the valley
floor cannot be eliminated (Witherow and others, 2006).
The contribution from lithic sources is likely to be insignificant,
and we assume the amount of sediment contained
within the hole should not affect the Cl−
concentration or
mass (Fountain and others, 2004). Therefore, Cl−
mass will
only fluctuate due to inputs from ice/snow melt or the effects
of freeze-concentration. Fountain and others (2004) suggested
using Cl−
concentrations to calculate the length of hydrological
isolation (in years) using:
Δt ¼
MðtÞ
a × i
À h
 
dz
dt
 À1
ð1Þ
where: M(t) is the mass of Cl−
(μmol) at a particular time t (a); a
is the cross-sectional area of the hole (m2
); i is the average
Table 1. Areal distribution of cryoconite holes along the glacier centerline (after MacDonell and Fitzsimons, 2012)
Site Measurement area Distance along centrelinea
Elevation Slope Annual ablation rateb
Area coverage Number of holes
m2
m m a.s.l. ° mm w.e. a−1
%
A 39 0 306 5 −184 2.2 8
B 17 100 315 3 −99 6.6 7
C 38 200 321 4 −132 3.0 12
D 28 300 328 2 −172 3.5 10
E 56 400 332 2 −111 2.5 13
F 43 500 336 2 −81 6.3 21
G 32 600 339 2 −88 8.2 14
H 28 700 342 5 −85 8.6 9
I 50 800 359 2 −79 6.7 18
J 70 1000 366 0 −71 4.9 22
a
Distance measured from sampling location A.
b
Annual ablation rate of the ice surface between 2004 and 2007.
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concentration of Cl−
(μmol m−3
) in the surrounding ice, which
is assumed to be constant in space and time; h is the depth of
the hole (m); and dz/dt is the melt rate at the base of the hole,
which is assumed to be equal to the ablation rate of the ice
surface. Fountain and others (2004) argued that if Eqn (1)
yields a value >0, then that value represents the number of
years for which the hole has been hydrologically isolated.
Conversely, if it yields a value <0, the implication is that the
cryoconite hole is hydrologically connected.
4. MAJOR ION CONCENTRATIONS IN
CRYOCONITE HOLES AND SURFACE ICE
4.1. Major ions in ice
Major ion concentrations were evaluated in surface and
near-surface (750 mm depth) ice to determine baseline glaciochemical
signatures that would inform the analysis of
cryoconite hole waters (Tables 4, 5). The dominant major
ions in the surface ice samples were Na+
and Cl−
, followed
by Ca2+
and K+
(Table 4). The dominant major ions are
similar to those reported in other areas of the McMurdo
Dry Valleys (Lyons and others, 2003; Tranter and others,
2004, 2005), however the concentrations are lower, and
the prominence of Ca2+
is reduced. As seen throughout the
McMurdo Dry Valleys, it is likely that Na+
and Cl−
are of predominantly
marine origin as the main source of precipitation
is from the Ross Sea (Fountain and others, 2010). The relative
concentrations of the other major ions (Ca2+
, K+
) are likely to
be influenced greatly by local lithological sources (Lyons and
others, 2003) and biological activity. The CBE suggest there
are also small amounts of HCO3
−
and SO4
2−
in the samples,
accounting for ∼32% of the anion load. In order to better
understand the CBE composition, SO4
2−
concentrations
were estimated using the marine SO4
2−
: Cl−
ratio of 0.14
(Kroopnick, 1977), and HCO3
−
was estimated from the
remaining CBE. This suggests that nonanalysis of HCO3
−
accounts for the largest fraction of the CBE, which is approximately
equal to Ca2+
+Mg2+
, likely due to the production of
Ca2+
, Mg2+
and HCO3
−
by calcium carbonate weathering
(Fortner and others, 2005). This pattern held for surface ice
and deep ice samples at most locations, but differences
were observed at sites E and F for both surface and deep
ice samples, and at site D for surface ice, and site G in
deep ice (Tables 4, 5). At these sites, CBE was lower and
SO4
2−
values were slightly elevated, which suggests that
there may be slightly elevated rates of silicate weathering occurring
within these regions (Fortner and others, 2005).
In surface ice samples, the mean Cl−
: Na+
ratio is 1.26 ±
0.08, slightly greater than the sea-water ratio of 1.16 (Keene
and others, 1986) and also greater than the deep ice ratio of
1.16 ± 0.14, suggesting that the ratio may have been modified
post-deposition. The high ratio is not uncommon
throughout the McMurdo Dry Valleys. For example, Lyons
and others (2003) report high ratios for the Commonwealth
(1.20–1.37), Howard (1.13–1.54) and Newall (1.72) glaciers,
and Tranter and others (2005) and Bagshaw and others
(2007) report negative values for Na+
once corrected for
sea salt. Both Lyons and others (2003) and Tranter and
others (2005) suggest there may be another source of Cl−
in the region, such as volcanic gases or a far-travelled
aerosol from the stratosphere to which HCl−
has been
added. It is also possible that the relative abundance of Cl−
could be due to the incorporation of frost flowers blown in
from nearby sea ice (Wolff and others, 2003; Domine and
others, 2004) or from sublimation enhancement (Lyons and
others, 2003). Frost flowers develop on the surface of thin
Table 2. Mean cryoconite hole physical properties at each of the surveyed locations along the glacier centerline (Fig. 1)
Site Elevation Diameter Water depth Head space Ice thickness Total depth Water volumea
Maximum volumeb
m a.s.l. mm mm Mm mm mm m3
m3
A 306 385 161 5 176 342 0.021 0.022
B 315 503 146 21 169 336 0.036 0.038
C 321 429 168 6 255 429 0.026 0.026
D 328 416 165 9 212 386 0.023 0.025
E 332 475 175 41 192 408 0.031 0.040
F 336 574 155 13 192 360 0.047 0.051
G 339 644 160 10 142 312 0.056 0.058
H 342 468 74 87 121 282 0.016 0.039
I 359 653 156 23 131 310 0.050 0.060
J 366 623 155 56 57 268 0.057 0.066
Measured properties at each site included water depth, head space (i.e. the air space between water and the ice lid in the hole), ice thickness and total depth.
Mean values were calculated from the five holes surveyed at each site. The average diameter value given was calculated by averaging the diameter in the north–
south direction with that measured in the east–west direction (after MacDonell and Fitzsimons, 2012).
a
Water volume corresponds to the volume of water in the hole at the time of measurement.
b
Maximum volume corresponds to the maximum space available to hold water (i.e. the combination of head space and current water depth).
Table 3. Summary table for the precision, accuracy and detection
limit of solute analyses (unless otherwise stated, values are given
as μeq L−1
)
Solute Precisiona
Accuracyb
Detection limitc
Na+
8 3 25
Mg2+
3 1 9
K+
6 2 17
Ca2+
13 4 41
Cl−
1 2 4
NH4
+
3 2 9
NO3
2−
1 0 0
PO4
2−
1 0 4
Values were calculated as outlined in Fitzsimons and others (2008).
a
Precision was calculated as the standard deviation of repeated assays of
blank standards.
b
Accuracy was calculated as the mean of the differences between the measured
and expected concentration.
c
Detection limit was calculated as the standard deviation of repeated assays
of blank samples multiplied by three.
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sections of sea ice and their Cl−
concentrations are higher
than those of sea ice brines and sea water (Douglas and
others, 2012). Frost flowers are readily removed from sea
ice surfaces, and in conjunction with other processes can
alter the chemistry of local air masses (Domine and others,
2004). Once these air masses are forced onshore, aerosols
contained within them can be deposited. As Cl−
is irreversibly
deposited in snow and ice, it will not evaporate or sublimate
from the surface unless the feature is removed.
Therefore, sublimation, which is the prevalent ablation
process on the surface of the Wright Lower Glacier
(MacDonell and others, 2013), should result in further enrichment
of immobile chemical species such as Cl−
at the
surface (Ginot and others, 2001).
Solute concentrations vary spatially along the glacier centerline
(Fig. 2; Table 4). Figure 2 and Table 4 show that the
concentration of Cl−
in the surface ice varies across the
glacier surface. Essentially, surface ice closer to the terminus
has a lower Cl−
concentration (at a given depth) than ice
further from the terminus, but the variation with distance is
not linear. Between sites A and E, mean chloride concentration
varies between 10 and 14 µeq L−1
, whereas between F
and J the mean concentrations lie between 16 and 31 µeq
L−1
. There is a step change between sites E and F (13–31
µeq L−1
, respectively), which coincides with a marked decrease
in surface ablation rates (111–81 mm w.e. a−1
, respectively).
While it is possible that Cl−
concentrations
vary with distance from the source, in this case the Ross
Sea, this effect is likely to explain only a small percentage
of the variation over the 1 km length of the study transect.
The implication may be that higher melt rates near the terminus
result in enhanced flushing of Cl−
from the surface
ice. Therefore, any analysis of residence times using a Cl−
signature should take into consideration the underlying
spatial signature seen in both surface and deep ice samples.
4.2. Cryoconite hole water chemistry
Analysis of the cryoconite hole water chemistry along the
glacier centerline shows that Cl−
, Ca2+
, Na+
and Mg2+
are
the dominant ions and that concentrations of Cl−
, Ca2+
and Mg2+
are up to an order of magnitude higher than in
clean glacier ice (Tables 4–6). The cryoconite hole water is
also enriched in NO3
−
and depleted in NH4
+
compared with
surface ice, however values of both species are very close
to the detection limits. The mean CBE (69 µeq L−1
) is larger
in cryoconite hole samples than in the surface and deep
ice samples, which suggests that concentrations of HCO3
−
and/or SO4
2−
are much higher in the cryoconite hole
samples than in the ice samples. This may indicate that
there is a larger contribution from weathering to the cryoconite
hole waters. As Ca2+
+Mg2+
is almost always higher
than the CBE in cryoconite hole samples, HCO3
−
is likely
not fully balancing Ca2+
+Mg2+
. As the SO4
2−
amount
Table 4. Mean solute concentrations in surface ice samples along the glacier centerline (μeq L−1
)
Site n Σ+
K+
Na+
Ca2+
Mg2+
NH4
+
Σ−
NO3
−
PO4
−
Cl−
SO4
2−
HCO3
−
A 5 16 2 9 1 1 3 11 1 0 10 1 4
B 5 27 5 11 6 1 4 20 1 5 14 2 5
C 5 21 6 8 2 1 4 16 1 4 11 2 3
D 5 20 5 9 3 1 2 19 1 6 12 2 0
E 5 20 4 10 4 1 1 18 1 4 13 2 0
F 5 48 7 25 9 5 2 41 1 9 31 4 3
G 5 38 7 19 7 3 2 28 1 3 24 3 7
H 5 39 6 21 7 3 2 26 1 0 25 4 10
I 5 24 4 12 5 2 1 21 1 4 16 2 1
J 5 40 9 25 3 2 1 30 1 0 29 4 6
Mean 50 29 5 15 5 2 2 24 1 4 19
Standard deviation 50 15 2 7 3 1 2 17 0 8 9
NB: HCO3
−
+ SO4
2−
= 5.
Table 5. Mean solute concentrations in deep ice samples along the glacier centerline (μeq L−1
)
Site n Σ+
K+
Na+
Ca2+
Mg2+
NH4
+
Σ−
NO3
−
PO4
−
Cl−
SO4
2−
HCO3
−
A 2 53 13 26 9 5 0 32 2 1 29 4 17
B 3 22 2 8 9 3 0 12 1 0 11 2 8
C 3 18 2 7 7 2 0 10 1 0 9 1 7
D 3 15 2 10 2 1 0 10 1 0 9 1 4
E 3 24 4 15 2 3 0 20 2 0 18 3 1
F 3 44 5 26 8 5 0 36 2 0 34 5 3
G 3 43 3 27 9 4 0 39 3 0 36 5 0
H 3 44 4 19 18 3 0 25 2 0 23 3 16
I 3 46 5 21 16 4 0 23 1 0 22 3 20
J 3 25 4 15 1 5 0 17 2 0 15 2 6
K 3 24 3 16 0 5 0 18 1 0 17 2 4
L 2 27 4 16 0 7 0 19 1 0 18 3 5
Mean 23 31 4 17 6 4 0 21 2 0 19
Standard deviation 23 29 4 12 11 2 0 15 1 0 14
NB: HCO3
−
+ SO4
2−
= 10.
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derived from seawater does not account for the difference, it
is likely there is an additional source of SO4
2−
, such as soils
from the valley floor (Lyons and others, 2003). However, as
SO4
2−
was not measured this cannot be confirmed.
Table 7 shows the results of a series of two-tailed Pearson
correlation analyses relating the concentrations of Na+
,
Ca2+
, Mg2+
and Cl−
to cryoconite hole and site properties.
Distance from the terminus and annual ablation rate provided
the strongest controls over solute concentrations, indicating
that concentrations increased up-glacier and with
decreasing ablation rate (for all variables, r is significantly different
from zero at >99%; Table 7). As ablation rate, distance
from the terminus, elevation and surface slope all vary spatially
together, it is not surprising that they relate to solute
concentrations in similar ways. Cryoconite hole depth is
the only physical property of cryoconite holes that is correlated
concentrations of all solute species. These results
suggest that site properties play a more important role in determining
cryoconite hole chemistry than individual hole
characteristics.
When the concentrations of each solute for each sample
type (i.e. surface ice, deep ice and cryoconite hole water)
were compared with sample locations, it was found that for
all sample types: Cl−
had a distinctive peak at site G; Na+
and Mg2+
peaked at sites F and G and Ca2+
was relatively
high between sites F and I (Tables 4–6). For the cryoconite
holes, the high concentrations of solutes between sites F
and I are probably related to weathering of sediment as the
charge imbalances are highest at these locations. However
Cl−
also peaks in this location. These sites have the highest
cryoconite hole cover (Table 1), which is most probably
related to sediment accumulation in this area of the glacier
(Fountain and others, 2004). Here cryoconite holes cover
6–8% of the glacier surface, which is twice the average
(Table 1). The reasons for greater sediment accumulations
in this region are currently unknown. It is possible that
there is a maximum possible distance for sediment transport
by westerly winds, or that at lower elevations more sediment
has been removed by water flow. In addition, there is an
abrupt up-glacier decrease in ablation rate between sites E
and F, and so it is likely that there is less meltwater available
to wash sediment from the surface, enabling cryoconite hole
development (MacDonell and others, 2013).
5. CRYOCONITE HOLE CONNECTIVITY
5.1. Model results
In order to assess the ratio of stored to transported water,
an understanding of potential and real cryoconite hole connectivity
is required. As a first attempt at understanding connectivity
between holes, the ‘cryoconite hole isolation
Fig. 2. Mean values of chloride concentrations in surface ice and cryoconite hole water at sites A–J along the glacier centerline.
Table 6. Mean solute concentrations in cryoconite hole samples along the glacier centerline (μeq L−1
)
Site n Σ+
K+
Na+
Ca2+
Mg2+
NH4
+
Σ−
NO3
−
PO4
−
Cl−
SO4
2−
HCO3
−
A 5 78 6 28 28 15 1 37 1 0 36 5 36
B 5 127 6 40 56 24 1 65 7 1 57 8 54
C 5 141 10 42 58 29 2 77 6 0 71 10 54
D 5 197 14 64 79 40 0 129 5 6 118 17 51
E 5 207 13 75 78 41 0 138 5 0 133 19 50
F 5 292 14 94 121 63 0 199 13 0 186 26 67
G 5 296 14 95 124 63 0 214 18 0 196 27 55
H 5 268 13 72 123 60 0 160 13 0 147 21 87
I 5 242 12 71 107 52 0 144 15 0 129 18 80
J 5 169 14 66 62 27 0 168 9 0 159 22 0
Mean 50 202 12 65 84 41 0 133 9 1 123
Standard deviation 50 102 5 28 45 23 1 85 7 4 74
NB: HCO3
−
+ SO4
2−
= 69.
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duration’ calculation was performed (Eqn (1); Fountain and
others, 2004). This analysis was based on the 50 holes
sampled in this study. The sampled cryoconite holes did
not include holes visibly connected to near-surface channels,
as it was difficult to assess the boundaries of these features.
Therefore, it is possible that the level of connectivity is
higher than estimated from the model results. It should be
emphasized that the fraction of holes visibly connected to
near-surface channels was very low.
At least 25% of the measured holes on the planar surface
(n = 50) were connected to other holes, inter-granular passageways,
and/or near-surface conduits (Fig. 3a). As holes
that had obvious conduits and cracks were avoided during
sampling, the connections are likely intergranular in
nature. The computed isolation durations suggest that the
average age of water in the holes was 3.5 a, and that 37 of
the 50 holes sampled were hydrologically isolated, with a
maximum age of 18 a at sampling location E. The hole
with the highest calculated isolation age had much higher
concentrations of both Na+
and Cl−
than surrounding
holes, and also had higher water and total depth than surrounding
holes. The percentage of cryoconite holes in this
study that were connected was considerably lower than
was reported by Fountain and others (2004) for Canada
Glacier, McMurdo Dry Valleys, where 56% of 16 holes
sampled were judged to be connected (via surface and
near-surface channels; Fountain and others, 2004).
However, all holes sampled in this study were expected to
be ‘isolated’, as holes that were obviously connected via conduits
and cracks were avoided during sampling. Most of the
holes that were found to be connected were located in the
terminus regions near sites A and B (Fig. 3a), although three
holes at site H were subsequently judged to be ‘connected’.
Two-tailed Pearson correlation analyses showed that the
calculated cryoconite hole isolation time was significantly
positively correlated with water depth (p < 0.01) and total
depth (p < 0.05), but with no other site characteristics
(Table 8). Logistic regression showed that the probability of
a hole being connected was positively related to elevation
(p < 0.01), hole diameter (p < 0.05) and water depth in the
Table 7. Two-tailed Pearson correlation coefficient results of comparison between cryoconite hole solute concentrations and site properties
Diameter Water
depth
Head
space
Ice
thickness
Total
depth
Water
volume
Distance along
center
Elevation Slope Annual ablation
rate
Na+
0.154 −0.324 0.059 −0.088 −0.287 −0.045 0.480 0.542 −0.387 −0.459
Ca2+
0.121 −0.401 −0.093 0.027 −0.326 −0.129 0.409 0.370 −0.126 −0.439
Mg2+
0.012 −0.468 0.092 0.023 −0.283 −0.220 0.378 0.338 −0.112 −0.406
Cl−
0.083 −0.424 0.208 −0.170 −0.354 −0.152 0.525 0.491 −0.394 −0.459
NB: Italics show significant correlation with 95% confidence, bold shows significant correlation with 99% confidence.
Fig. 3. Length of isolation of cryoconite holes between sites A and J along the glacier centerline using (a) glacier averaged surface ice chloride
concentrations and ablation rate (hollow circles in all figures); (b) site specific chloride concentrations (black triangles); (c) site specific
ablation rates (black triangles); and (d) site specific chloride concentrations and ablation rates (black triangles). NB: Each flag site has five
measurements and repeated values are plotted overlying one another.
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hole (p < 0.05), and negatively related to local slope angle
(p < 0.01) and distance from the terminus (p < 0.05).
The increased connectivity of cryoconite holes near the
glacier terminus may be due to higher air temperatures enabling
higher rates of melt and inter-granular flow, and to inputs
of water from upstream. Holes near the terminus are likely to
be more transient, and hence more shortlived, due to the
higher likelihood of surface stripping melt events at lower
elevations caused by relatively high air temperatures
(Fountain and others, 2004; MacDonell and Fitzsimons,
2012). However, as inter-granular drainage was observed
across the glacier surface, it is difficult to determine what
causes a cryoconite hole to be identified as ‘isolated’ or ‘connected’,
as any hole experiencing inter-granular drainage is
by definition ‘connected’. It seems likely that Eqn (1) does
not give a binary ‘connected’ or ‘isolated’ signal, but rather
a relative measure of connectedness to the drainage
system. To confirm these results, the use of Eqn (1) is tested
for model sensitivity to ice chloride concentrations, and the
spatial variability of surface ablation rates.
5.2. Sensitivity analysis of the cryoconite hole
isolation duration calculation
Two assumptions that have been used in previous calculations
of cryoconite hole isolation duration on McMurdo
Dry Valleys glaciers are (1) that the chloride concentration
of the ice surrounding the holes is uniform across the
glacier surface, and (2) that the surface lowering rate is spatially
uniform (Fountain and others, 2004; Bagshaw and
others, 2007).
To test whether incorporating the spatial variability of
chloride concentrations in surface ice into the isolation duration
calculation significantly alters the final results, cryoconite
hole isolation calculations were performed using (1) an
area-average chloride concentration (19.7 µeq L−1
), and (2)
the chloride concentration from the nearest surface ice
sample (Fig. 3b; Table 4). The choice of which surface ice
Cl−
concentration to use in the isolation duration calculation
results in a variation of up to 10 a in the isolation age of an
individual cryoconite hole in the most extreme case.
Incorporating the spatial variation of surface ice chloride
concentrations generally yielded lower isolation durations
for sites site F, and higher isolation durations below F. From
this calculation, it is clear that the natural variability in Cl−
is sufficiently large that the choice of Cl−
value has a significant
influence on the results.
The second set of assumptions used in previous studies is
that the surface lowering rate is constant across the glacier
surface, and that the ablation rate at the base of the hole is
equal to that of the ice surface surrounding the hole (i.e.
that all holes are in a steady state). To test the impact of violating
these assumptions, the isolation duration calculation
was repeated using both the mean ablation rate (0.11 m
a−1
) and a site-specific rate calculated from the closest ablation
stake (Fig. 3c). This comparison showed that while using
different ablation rates did not significantly affect the calculated
connectivity status, it did affect the calculated isolation
time. The calculation is extremely sensitive to the ablation
rate used, such that the lower the ablation rate, the lower
the corresponding hole isolation time. This relationship
holds because the isolation time is determined from the
mass of solute accumulated in the water volume in
the hole. This is largely a function of the rate of melting at the
base of the hole which, in turn, is assumed to match the
surface ablation rate. This has serious implications for calculating
the spatial distribution of cryoconite hole ages.
Finally, when both site-specific chloride concentrations
and ablation rates are used, the spatial pattern observed in
the other results is lost, as holes closer to the terminus
appear to be isolated for a longer period than is suggested
by the original calculation, while holes away from the terminus
have a lower isolation time than in the original calculation
(Fig. 3).
6. DISCUSSION AND CONCLUSIONS
Observations of both discrete channel connections and intergranular
drainage across the glacier surface show that while
cryoconite holes on this glacier appear to be mechanically
isolated due to the presence of an ice lid, it is likely that in
some way all cryoconite holes are hydraulically connected.
The level of connectivity is temporally unstable, in so far as
holes may disconnect and reconnect seasonally or multiseasonally.
The mechanisms of connection, and associated temporal
variability, have implications for biogeochemical
cycling, and hence the transport of nutrients and biological
material across the glacier surface and to proglacial areas.
It is likely that patterns of solute concentrations measured
across the glacier surface are indicative not only of source
material, but also of hydraulic connectivity.
Several studies investigating the role of cryoconite holes in
the surface hydrology of glaciers in the McMurdo Dry Valleys
have used Cl−
concentrations in cryoconite hole waters to
determine the level of connectivity between cryoconite
holes and the wider glacier drainage system (e.g. Fountain
and others, 2004; Bagshaw and others, 2007). While the
chloride isolation aging technique is novel and relatively
easy to apply, the method has limitations that remain
largely unaddressed in the literature. Two main issues are
that (1) the method has not been independently validated,
so there is no real basis for confidence in the results it produces
and (2) there is significant spatial variability in the
values of the key parameters used in the calculation, which
tends to suggest a more general variability in the rate of
solute elution from the ice that is related to both local melt
Table 8. (a) Two-tailed Pearson correlation coefficient results of comparison between isolation duration with hole and site properties. (b)
p-value results of logistical regression analyses comparing cryoconite hole age and connectivity with hole and site properties
Diameter Water
depth
Head
space
Ice
thickness
Total
depth
Water
volume
Distance along
center
Elevation Slope Annual ablation
rate
(a) 0.042 0.369 −0.132 0.117 0.308 0.110 0.081 0.121 −0.269 −0.128
(b) 0.032 0.040 0.554 0.611 0.398 0.063 0.023 0.010 0.002 0.351
NB: Italics show significant correlation with 95% confidence, bold shows significant correlation with 99% confidence.
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rates and distance downstream within the ablation area. If
cryoconite holes persist for long enough that they are
advected down-glacier over time and are thus surrounded
by ice with time-varying solute content and subjected to
time-varying local surface ablation rates, then the simplistic
assumptions on which the isolation calculation is based are
highly likely to be violated. The assumption of steady-state
holes requires not only that ablation rates at a given location
remain constant through time, but also that the incident radiation
receipt, the optical and physical properties of the
surface ice, and the cryoconite itself, must remain constant
(Gribbon, 1979; Fountain and others, 2008). Under this scenario,
as holes are multi-year features, single year melt rates
may not be a good approximation for the rates cryoconite
holes experience in their lifetime.
One of the primary assumptions of using the solute mass
accumulated in a cryoconite hole as a proxy for the isolation
time of the hole is that solute accumulates in the hole over
time as a result of release from melting of the base of the
hole. However, in all studies using Eqn (1) to estimate isolation
duration (including the current study), an average of
surface ice concentrations has been used instead of the concentration
at the base of the hole. As shown in Tables 4 and
5, while there is general agreement between surface and
deep ice Cl−
values, there is some degree of variation. By inference,
it is likely there is some form of concentration profile
with depth that is likely to vary across the glacier surface.
Therefore, it is likely that the contribution to the hole from
surrounding ice actually changes with time. As we currently
do not have fine scale measurements of Cl−
concentrations it
is difficult to estimate what impact this variability may have
on the calculated concentration, but it serves to highlight
the current unknowns in solute mobility in this environment.
In addition, as shown in Table 4, Cl−
concentrations in
surface ice clearly vary over relatively large distances, but
we are unsure as to how variable they are over relatively
short distances.
Spatial variations in surface ice Cl−
concentrations will
affect not only the probable initial Cl−
concentration of cryoconite
hole waters, but also the Cl−
concentrations in any
water that might travel through the near-surface drainage
network. To illustrate the point, Table 9 shows the concentration
of Cl−
expected in cryoconite holes at sites A and J, respectively,
assuming the hole area (0.5 m2
) and ablation
rate (0.1 m a−1
) are the same at both locations and constant
in time. At A, the surface ice Cl−
concentration is 0.01 µmol
m−3
, which represents the lowest concentration of all measured
sites (Table 4). By comparison, the surface ice concentration
at J (the highest measured stake) (0.03 µmol m−3
) is
the second highest measured. Table 9 shows that the
higher surface ice concentration at J is associated with cryoconite
water concentrations almost three times higher than
observed at A. If this water is flushed downstream, it will increase
concentrations in downstream holes, which will
cause an apparent increase in calculated residence time.
The systematic increase in solute concentration in ice with
distance from terminus in the ablation area may not reflect
differences in distance from the precipitation source, but
may reflect a trend to increasing water flow through the ice
at lower elevations that results in solute elution from the
near-surface ice in which cryoconite holes form. Under this
hypothesis, even if the model used to compute the isolation
time of a single hole is basically correct, holes of identical
ages formed in ice with different initial composition will
appear to have different ages if the variations in ice composition
are ignored or averaged out.
In the isolation duration calculation, it is assumed that any
interaction between the cryoconite hole system and the
wider hydrological system resets the hole to age ‘zero’. The
equation implies that the solute accumulation rate depends
on the basal melt rate of the hole and the solute concentration
in the ice, and that basal melting occurs at a rate that
matches the rate of freezing at the top of the hole and adds
solute to the water column. As refreezing at the top of the
hole does not remove much solute, because solute is rejected
from ice that forms during refreezing, in a steady state, isolated,
cryoconite hole, water mass remains constant over
time while solute mass (and thus solute concentration)
increases. Therefore, the solute concentration in the cryoconite
hole waters will generally be higher than in the surrounding
ice and in meltwaters derived from melting of that ice. In
theory, any flushing of a cryoconite hole that is connected to
the near-surface meltwater drainage system will be done by
water with a lower solute concentration than the water
stored in the hole. This will result in dilution of the hole
waters and, since hole volume is assumed not to change
over time, which is unlikely, a reduction in the stored solute
mass and hence the computed isolation time. However, the
addition of water from outside the hole does not necessarily
mean that the hole waters are diluted to the point that they
no longer contain solute. Thus, at best, the ‘isolation age’ is
really an inverse measure of the rate at which the hole is
flushed, and not a true measure of isolation time.
Leading from observations of cryoconite hole connectivity
and the development of intergranular drainage on the Wright
Lower Glacier (MacDonell and Fitzsimons, 2012), it seems
likely that cryoconite holes are never truly isolated from the
near-surface drainage system. The nature of the relationship
between isolation duration and flushing rate will depend
on whether or not the holes really evolve to a steady-state
water volume as the model assumes. Both the spatial
pattern of Cl−
in surface ice, and the computed isolation
durations, suggest more flushing of near-surface ice at
lower elevations, which is consistent with evidence of a
shallow englacial drainage system in this glacier. While
this paper has raised some questions about the interpretation
of results produced by the isolation model proposed by
Fountain and others (2004), it is likely that after independent
evaluation of the model assumptions and the extent to which
they are consistent with reality, the model may still be useful
as a way of identifying large scale spatial patterns in the structure
and connectivity of the shallow near-surface drainage
system on predominantly cold glaciers.
Table 9. Quantity of Cl−
expected in a theoretical cryoconite hole
at sampling locations A and J calculated from a constant ablation
rate of 0.1 m a−1
and a surface area of 0.5 m2
X Quantity of Cl−
from x years melt
a μmol
Site A Site J
1 0.0006 0.0015
5 0.0028 0.0076
10 0.0055 0.0152
20 0.0111 0.0303
30 0.0166 0.0455
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ACKNOWLEDGEMENTS
Financial assistance for S. M. was provided by a University of
Otago Postgraduate Scholarship, the Sir Robin Irvine
Antarctica New Zealand Doctoral Scholarship and the
University of Otago Postgraduate Publishing Bursary. M. S.
acknowledges support from the Canadian Arctic-Antarctic
Exchange Program (Natural Resources Canada) and an
NSERC Discovery Grant. Financial support for S. F. was provided
by a University of Otago Research Grant. Logistical
support was provided by Antarctica New Zealand. We are
grateful for field assistance provided by Lawrence Kees,
Laurel Morrison, Dorothea Stumm and Ben Varkalis and to
Tracy Connolly for drawing Fig. 1. Fantastic helicopter
support was provided by Helicopters NZ pilot Rob
McPhail. We are grateful for insightful comments from the
two reviewers.
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