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Holocene lake-level recession, permafrost aggradation and lithalsa formation in the Yellowknife area, Great Slave Lowland

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The Great Slave Lowland occupies the north shore of Great Slave Lake. After glaciation, it was inundated by Glacial Lake McConnell and ancestral Great Slave Lake. Holocene lake-level recession around Yellowknife is determined from accelerator mass spectrometer ages of peat and detrital organics. In the last 8000 years, recession occurred at about 5 mm/year, and permafrost is youngest near the modern shoreline and older at higher elevations. Silty-clay sediments are abundant, and lithalsas (ice-rich permafrost mounds within mineral soils) occurring within 40 m above the present lake level are less than 6000 years old. They are common on Yellowknife River alluvium deposited within the last 3000 years. Lithalsas on this surface are assumed to have developed as permafrost aggraded into saturated sediments, and ground ice has formed within the last 250 years.
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Holocene lake-level recession, permafrost
aggradation and lithalsa formation in the
Yellowknife area, Great Slave Lowland
S.A. Wolfe and P.D. Morse
Geological Survey of Canada, Natural Resources Canada, Ottawa,
Ontario, Canada
ABSTRACT
The Great Slave Lowland occupies the north shore of Great Slave Lake. After glaciation, it was inundated by Glacial
Lake McConnell and ancestral Great Slave Lake. Holocene lake-level recession around Yellowknife is determined from
accelerator mass spectrometer ages of peat and detrital organics. In the last 8000 years, recession occurred at about 5
mm/year, and permafrost is youngest near the modern shoreline and older at higher elevations. Silty-clay sediments are
abundant, and lithalsas (ice-rich permafrost mounds within mineral soils) occurring within 40 m above the present lake
level are less than 6000 years old. They are common on Yellowknife River alluvium deposited within the last 3000 years.
Lithalsas on this surface are assumed to have developed as permafrost aggraded into saturated sediments, and ground
ice has formed within the last 250 years.
RÉSUMÉ
L’écorégion « Great Slave Lowland » se situe sur la rive nord du Grand lac des Esclaves. Suite à la déglaciation, elle a
été inondée par le lac glaciaire McConnell, ainsi qu’un précurseur du Grand lac des Esclaves. La récession holocène du
niveau du lac près de Yellowknife a été déterminée en datant des tourbes et des débris organiques par spectrométrie de
masse par accélérateur (AMS). Pendant les 8000 dernières années, le niveau du lac a baissé d’environ 5 mm par
année, le pergélisol le plus jeune se retrouve donc près du littoral moderne, alors que le plus âgé se retrouve sur les
terres plus élevées. Les argiles limoneuses abondent et l’on retrouve des lithalses (buttes cryogènes riches en glace)
plus jeunes que 6000 ans jusqu’à 40 m au-dessus du niveau du lac moderne. Celles-ci se retrouvent souvent sur les
sédiments alluviaux de la rivière Yellowknife qui furent déposés durant les 3000 dernières années. Les lithalses sur ces
surfaces se sont probablement développées avec l’expansion du pergélisol dans des dépôts saturés, la glace de sol
s’étant formée durant les derniers 250 ans.
1 INTRODUCTION
The Great Slave Lowland is a low-relief, Precambrian
granitic bedrock plain extending about 250 km along the
north shore of Great Slave Lake (Fig. 1), and includes
Yellowknife, the capital of the Northwest Territories, which
resides near the present-day shoreline at 62.4°N latitude
and 114.4°W longitude. Following retreat of the
Laurentide Ice Sheet, inundation by Glacial Lake
McConnell and ancestral Great Slave Lake resulted in
extensive deposition of silts and clays throughout the
Lowland. In a cold-continental setting, permafrost
aggradation into these sediments has given rise to
widespread discontinuous permafrost with extensive ice-
rich permafrost mounds (i.e. lithalsas) across the region
(Wolfe et al. 2014).
The primary purpose of this paper is to establish a
preliminary estimate of the rate of Holocene lake-level
recession in the Yellowknife area, to provide a baseline
estimate of regional terrestrial emergence and permafrost
aggradation. A secondary purpose is to examine the
distribution of lithalsas in the Yellowknife area in relation
to elevation and surficial sediments in order to estimate
their age and environmental settings of formation.
Permafrost within a lithalsa exposed by thaw slumping
into the Yellowknife River permits direct assessments of
the origin of the ice, and the age and depositional
environment of the enclosing sediment.
2 BACKGROUND AND STUDY AREA
The Great Slave Lowland is bordered to the south by
Great Slave Lake, and to the north by the Great Slave
Upland of higher relief. As with much of northern Canada,
the region was glaciated during latest, Wisconsinan,
glaciation. With the retreat of the Laurentide Ice Sheet,
about 13 000 cal BP, it was inundated by Glacial Lake
McConnell that formed along the retreating glacial margin
between about 13 000 and 9500 cal BP (Lemmen et al.
1994; Smith 1994). At its maximum extent at about 10 700
cal BP, the lake included the combined basins of Great
Bear, Great Slave and Athabasca lakes (Fig. 1). Lake
McConnell was largely the result of glacio-isostatic
loading (Lemmen et al. 1994). Evidence of the lake exists
in the form of raised deltas, strandlines, beaches and
wave-washed eskers and drumlins. In the Great Slave
basin, maximum lake elevation was between 320 and 350
m asl along the East Arm of Great Slave Lake (Smith
1994; Kerr et al. 2014), compared to only about 180 m asl
at Fort Smith (Vanderburgh and Smith 1988), the
elevation difference due to differential isostatic uplift.
Fig. 1. Great Slave Lowland, Northwest Territories with
approximate margin of the Laurentide Ice Sheet and
extent of Glacial Lake McConnell at about 10 700 cal BP
During recession, the lake first separated from the Great
Bear basin, and remained in existence until about 9500
cal BP, when the remaining two basins separated
(Lemmen et al. 1994). The resulting ancestral Great Slave
Lake continued to decline due to differential uplift, towards
its present elevation of 156 m asl, constrained by the
Mackenzie River outlet at Fort Providence (Vanderburgh
and Smith 1988).
Glacial Lake McConnell and ancestral Great Slave
Lake left prominent signatures on the Lowland landscape,
with silt and clay covering nearly 70% of the land area
(Wolfe et al. 2014). The Lowland area, being submerged
to depths of 100 to 150 m, was a deepwater basin for fine-
grained glaciolacustrine sediment deposition. During
recession of ancestral Great Slave Lake, waves and rivers
re-worked and re-deposited sediments in nearshore
environments (Fig. 2A). Consequently, glaciolacustrine
and lacustrine deposits are broadly distributed across the
region.
The area presently experiences a continental subarctic
climate, with an annual mean air temperature of -4.1°C,
cold winters (-25.6°C January mean), warm summers
(17.0°C July mean), and an average of 281 mm of
precipitation with 40% falling as snow (Environment
Canada, 2015). The area occupies poorly-drained low
relief terrain characterized by numerous water bodies
separated by fens, peatlands, mixed woodlands, white
birch (Betula papyrifera) and black spruce (Picea
mariana) forests and bedrock outcrops (Fig. 2A and B).
Permafrost is extensively discontinuous (Heginbottom et
al. 1995) and lithalsas, which are permafrost mounds
formed by ice segregation in fine-grained sediment, are
locally widespread with nearly 1800 identified in the region
west of Yellowknife (Wolfe et al. 2014).
Lithalsas in the Yellowknife area are typically circular
to elongate, 25 to 50 m wide and up to 300 m long and 8
m high. They are typically vegetated by birch or birch-
spruce forests and commonly occur along the margins of
small ponds, which in some cases represent thermokarst
ponds related to thawing of former lithalsa terrain (Fig.
2A). The raised topography of lithalsas, combined with the
typical deciduous (birch) forest cover makes these
features readily recognizable in the field and on stereo
aerial photographs, which were used by Wolfe and Kerr
(2014) to map the lithalsa distribution in the area.
Terrestrial emergence accompanied lake-level
recession throughout the Great Slave Lake basin. Given
the low gradient of the Lowland region, in some areas a
few metres of elevation adjustment resulted in
considerable land exposure over a short time period (Fig.
2B).
Permafrost aggradation followed terrestrial
emergence, though the regional climate has varied during
the Holocene. Prior to 6000 cal BP, exposed terrain north
of the Lowland was dominated by tundra, shrub tundra
and spruce forest tundra (Huang et al. 2004), implying
conditions colder than present. Between 6000 and 3500
cal BP treeline moved northward of its present position in
response to climate warming (Moser and MacDonald
1990; MacDonald et al. 1993), and it is uncertain if
permafrost was locally sustained within the Lowland at
that time. The regional climate cooled, but was variable,
from about 3000 Cal BP to present (Huang et al. 2004).
Local mean annual air temperatures have recently
warmed at a rate of about 0.3°C per decade since the
1940s (Riseborough et al. 2013) with an accelerated trend
of about 0.6°C per decade since 1970 (Hoeve et al.
2004), consistent with a pan-Arctic warming trend
beginning AD 1966 (IPCC 2013).
Smith (1994) reconstructed the rate of lake-level
lowering of Glacial Lake McConnell and Great Slave Lake
from radiocarbon-dated buried wood in raised deltaic
sediments on the southern side of Great Slave Lake.
Based primarily on woody debris collected along former
delta-front positions of the Slave River delta (Fig. 1),
Vanderburgh and Smith (1988) estimated the Holocene
lake-level recession of Great Slave Lake to be about 2
mm per year from 7000 14C yr BP (about 8000 cal BP) to
present. Although long-term uplift rates for the Yellowknife
area are unknown, present-day uplift is reported to be on
the order of 6.3 +/- 0.06 mm per year based on a time
series of 15 years (Mazzotti et al. 2011).
Given difference in maximum elevation of Glacial Lake
McConnell between the northern and southern sides of
the basin, we hypothesize that the rate of Holocene lake-
level recession for the north shore of Great Slave Lake
and the Yellowknife area, exceeded that for the south
shore of the lake derived from the Slave River Delta by
Smith (1994). Therefore, this study derives a preliminary
estimate of the rate of Holocene lake-level recession for
the Yellowknife area. The implications of lake-level
recession are assessed in the context of local permafrost
landscape evolution by reconstructing shorelines and
examining lithalsa distribution and cryostratigraphy in the
Fig. 2. Lithalsa and ground ice examples in the Great Slave Lowland. A) Yellowknife River extending southward into
Back Bay on Great Slave Lake, with alluvial plain and lithalsas in the foreground; B) Lithalsa terrain at Site 5 along NWT
Highway 3 near Boundary Creek at an elevation of about 10 m above the present level of Great Slave Lake; note Great
Slave Lake at a distance of about 7 km in background; C) Ground ice slump at Site 6 along the Yellowknife River; D)
Slump headwall at Site 6 exposing alluvial and lacustrine sediments with segregated ground ice at depth.
Yellowknife area in relation to elevation and surficial
sediments in order to estimate their ages and
environmental settings of formation.
3 METHODS
3.1 Peatland cores and lake-level recession
Accelerator mass spectrometer (AMS) dating of peatland
sites represents a means to establishing the timing of
lake-level recession in the Great Slave Lowland, where
terrestrial organic accumulation occurred following
emergence. Peatlands are relatively rare in the Lowland
representing less than 2% of the total surface terrain
cover (Wolfe and Kerr 2014). Unlike permafrost areas
further south, peat plateaus are uncommon in the Great
Slave Lowland, as the surrounding terrain underlain by
unconsolidated sediments typically also contains
permafrost (Morse et al. in press). Peatland sites selected
for dating were typically near, but above, ponds and did
not appear to have undergone alterations in hydrology
that, especially, would have affected peat initiation.
Cores were obtained from peatlands in the Yellowknife
area, ranging in elevation from 205 m asl near the margin
with Great Slave Upland to 165 m asl within the Lowland.
Elevations were derived from multiple sources, including
topographic base maps, GPS and Lidar data (where
available), and were conservatively estimated the nearest
5 metres (± 2 m). Elevations were derived for both the
sample sites and the nearby surface water levels. Local
surface water levels were used as the estimates of land
surface elevation at the time of terrestrial emergence, as
these discounted the effects of heave due to permafrost
aggradation and ground ice formation and accumulation
of peat above the actual mineral surface, which is variable
and can locally exceed 3 m.
In most instances, coring was undertaken with a
CRREL corer using a 7.6 cm diameter core barrel,
although at one site (Site 4) a peat auger was used to drill
multiple boreholes to confirm stratigraphic contacts. Holes
were cored/augered through the active layer and
underlying frozen peat, and into the underlying sediment.
In all cases, in situ organic samples were collected from
the base of the peat unit and above the mineral sediment
for AMS dating by Beta Analytic and AMS ages were
converted to calendar years using Calib©7.0.4 (2014). In
situ basal ages are considered to represent the minimum-
limiting ages of peat accumulation, and thus for terrestrial
emergence. In instances where two cores were obtained
from the same site (e.g. Sites 3 and 4), the older of the
two AMS ages was used to represent the limiting age. At
one site, detrital organics (wood charcoal) collected from
within the underlying subaqueous alluvial sediments were
also dated to bracket emergence. These detrital ages are
considered to represent maximum-limiting ages of
subaqueous alluvial deposition in relation to former Great
Slave Lake levels.
3.2 Shoreline reconstruction and landscape evolution
Relations between lake-level history and landscape
evolution were examined using topographic Canadian
Digital Elevation Data (CDED) available from GeoGratis
(http://geogratis.cgdi.gc.ca/), combined with published
surficial sediment and lithalsa distributions for the
Yellowknife area (Wolfe and Kerr 2014). Topographic data
were used to reconstruct raised shoreline levels. Timing of
sediment deposition or reworking, including beach sands
and alluvial silts, as well as the maximum-limiting ages of
lithalsas, were estimated from their elevations and
discussed in the context of reconstructed lake-level
recession rates.
A slump exposing near-surface sediments and ground
ice, situated on an island in the Yellowknife River about 1
km (Fig. 2C) north of Back Bay, was examined to
determine sediment and ground ice origin and age. Debris
along a portion of the headwall was cleared to expose at
2.2 m high by 6 m wide section (Fig. 2D). This was
measured, sketched and photo-documented, noting
stratigraphic contacts and ice-textures. Sediment samples
were collected for grain size analysis, Atterberg limits, and
moisture content performed by the Geological Survey of
Canada. Ice samples were collected from the lower
portion of the section, where visible ice content exceeded
70%, and waters were submitted to the NWT Taiga Lab
for isotopic analysis. In situ roots within unfrozen and
frozen portions of the exposure were collected for AMS
dating by Beta Analytic.
4 RESULTS AND DISCUSSION
4.1 Peatland cores and Holocene lake-level recession
Table 1 summarizes AMS ages, locations, depths,
material and depositional environments of organic
samples. Figure 3A illustrates the near-surface
stratigraphy and AMS ages derived from seven peatland
cores at five sites of varying elevation in the Lowland.
Core depths vary from about 1.5 m at Site 2 to 4.5 m at 5
and peat thicknesses range from 93 to 295 cm. Calibrated
AMS ages range from about 8000 to 1100
14
Sediments immediately underlying peat at these sites
are typically clayey-silts, with layers of sandy-silt and silty-
clay at greater depths (Fig. 3A). Note that at Site 3,
bedrock was encountered beneath a relatively thin layer
of sediment underlying peat. These fine-grained
sediments most likely represent alluvial or lacustrine
sediments pre-dating terrestrial emergence. At Site 5,
near Boundary Creek 30 km west of Yellowknife, detrital
charcoal contained within clayey-silt sediments date to
about 1710 (1810 to 1610) and 1460 (1533-1395) Cal BP,
respectively, with an apparent age reversal relative to
depth of deposition (Fig. 3A). These ages indicate
deposition within a subaqueous environment, possibly
either a shallow bay of ancestral Great Slave Lake or a
former alluvial plain of Boundary Creek. In either case, the
charcoal was most likely derived from a burned, forested
surface at a higher elevation. This is significant, as it
indicates that terrestrial emergence here must have
occurred between about 1460 (the age of the younger
detrital organics) and 1170 (the age of the basal peat) Cal
BP, at a site presently about 10 m above, and more than
6 km inland of, Great Slave Lake (Fig. 2B). Thus, given
the low elevation gradient, considerable terrestrial
emergence occurred over a relatively short time period.
C years BP.
A general relation is apparent between basal peat AMS
ages and elevations across the area (Fig. 3B).
A preliminary Holocene lake-level reconstruction may
be derived for ancestral Great Slave in the Yellowknife
region from the AMS ages obtained beneath the
peatlands in the region. Figure 3B illustrates the derived
lake-level recession rate for the Yellowknife area using
calibrated radiocarbon ages. A rate of approximately
Table 1. AMS dates used to infer late Holocene lake-level recession and terrestrial emergence in the Great Slave
Lowland.
Site Age
(
14
2σ range
(Cal BP)
C BP)
Lat.
(°N)
Long.
(°W)
Elev.
(m asl)
1
Depth
Material Depositional
environment
1
7130 ± 40
8020-7865
62.5537
114.0172
210 / 205
Peat
terrestrial
2
6730 ± 30
7670-7515
62.4796
114.4013
210 / 205
Wood
terrestrial
3
6580 ± 40
7565-7430
62.4564
114.5311
195 / 195
Wood
terrestrial
3
5880 ± 30
6780-6640
62.4564
114.5311
195 / 195
Peat
terrestrial
4
1870 ± 30
1880-1725
62.5061
114.2759
185 / 175
Wood
terrestrial
4
1590 ± 30
1545-1410
62.5061
114.2759
185 / 175
Peat
terrestrial
5
1240 ± 30
1270-1075
62.5280
114.9606
170 / 165
basal peat
terrestrial
5
1770 ± 30
1810-1610
62.5280
114.9606
170 / 165
burned wood
alluvial
5
1570 ± 30
1533-1395
62.5280
114.9606
170 / 165
burned wood
alluvial
6
modern
na
62.5275
114.3128
165 / 156
in situ root
terrestrial
6
80 ± 30
260-25
62.5275
114.3128
165 / 156
in situ root
alluvial
1
Elevations include collection elevation and local surface water (lake/pond/or stream) elevation. Local water level
elevations, rather than collection elevations, are used in lake-level reconstructions.
Fig. 3. Elevation-age relations in the Great Slave Lowland.
A) Near-surface stratigraphy and AMS ages beneath
terrestrial peatlands at varying elevations. B) Calibrated
AMS ages in relation to elevation. Dashed line represents
approximation of lake-level recession and terrestrial
emergence.
5 mm/year is inferred. This is roughly two and half times
that of the southern side of Great Slave Lake (Smith and
Vanderburgh 1988). This may be attributed to either a true
difference in the rates of Holocene post-glacial isostatic
adjustment as hypothesized, or to differences in
methodology (i.e. in situ terrestrial peatlands dated in this
study versus allochthonous buried logs used on the Slave
River Delta). Although Smith (1994) also estimated the
rate prior to 8000 Cal BP (18 mm/year) no estimate is
made for Yellowknife, though it may be significantly
greater.
4.2 Shoreline reconstruction and landscape evolution
Figure 4 depicts the present-day (Fig. 4A), and + 40 m
reconstructed (Fig. 4B), shorelines in the Yellowknife
area. At present, the Yellowknife River flows along a 6 km
channel from Tartan Rapids (TR in Fig. 4A), with broad
alluvial plains and two main islands along the river, into
Fig. 4. Shoreline reconstructions based on elevation in the
Yellowknife area, with surficial sediment and lithalsa
distributions. A) Present-day shoreline, also showing + 20,
+ 40 and >50 m elevations. B) Shoreline configuration at
+ 40 m, and showing > 50 m elevation relative to present
day lake level. TR = Tartan Rapids
Back Bay on Great Slave Lake where alluvial plains also
occur at the north end of the bay (Fig. 4A). In contrast,
with lake levels at + 40 m relative to present-day elevation
this was part of the larger Yellowknife Bay (Fig. 4B), and
the river entered Great Slave Lake several kilometres
tothe north (not shown in figure). At that time, lake levels
were locally near the dividing limit between the Great
Slave Upland and Lowland, and the surface of sandy
outwash deposits in the Yellowknife area were reworked
as beaches (Fig. 4B). With lake-level recession, the
Yellowknife River became confined to the present-day
channel, with alluvium deposited in shallows along the
margins of the river. Based on the local relief, this
occurred when the lake was less than + 20 m relative to
present-day level (Fig. 4A).
Utilizing the Holocene lake-level recession of about 5
mm per year over the last 8000 years, the + 40 m
shoreline relative to present-day is estimated at about
8000 Cal BP (Fig. 4B). Thus, most of the Great Slave
Lowland was terrestrially exposed within the last 8000
years. More specifically, in the local Yellowknife area,
Back Bay formed about 4000 years ago when lake levels
were about + 20 m relative to present. The Yellowknife
River was not confined to its present channel below
Tartan Rapids until about 3000 years ago, when lake level
was about + 15 m. Tartan Rapids itself is relatively new,
as Prosperous Lake above the rapids is less than + 2 m
relative to Great Slave Lake. Similarly, most of the
alluvium along the Yellowknife River was deposited within
the last 3000 years, and probably more recently.
Lithalsa distribution in relation to elevation and alluvial
deposits in the Yellowknife area provides further insight
into potential timing of permafrost aggradation. Figure 5
shows the frequency distribution of lithalsas mapped in
Figure 4, relative to their elevation and, based on lake-
level recession rates, their maximum age. Whereas most
lithalsas reside within + 30 m of present lake level, and
are thus no older than 6000 years, more than half reside
with + 15 m, and thus have formed potentially within the
last 3000 years. Notably, nearly all lithalsas formed within
+ 10 m of present lake level elevation (the last 2000
years) have developed on alluvium of the Yellowknife
River and Back Bay. This suggests that these saturated
clayey-silt deposits, in close proximity to a water source,
are conducive to lithalsa formation during permafrost
aggradation. Noting the location of other lithalsas in
Figure 4, it is likely that these have similarly formed where
alluvium or nearshore lacustrine sediments were
deposited.
Fig. 5. Lithalsa occurrence by elevation in the Yellowknife
area also showing occurrence of lithalsas on Yellowknife
River alluvium and maximum-limiting age approximations
based on Holocene lake-level recession rates.
4.3 Recent ground ice formation
Regarding the abundance of lithalsas along the
Yellowknife River, a retrogressive thaw slump (Site 6) on
an island in the river, 1 km north of Back Bay (Fig. 4A),
Fig. 6. Cryofacies description, analytical results (including
AMS age and δ18O values) and environmental
interpretation of ground ice exposure in slump at Site 6 on
the Yellowknife River.
was examined to assess the origin of the ice, and the age
and depositional environment of the enclosing sediment.
The slump has regressed landward from the river by
about 40 m (Fig. 2 C) and the elevation of the ground
surface at the headwall (Fig. 2D) is about 8 m above river
elevation. Figure 6 shows the stratigraphic and analytical
results, and interpretation of the section and Table 1
includes results from two AMS ages at this site. An
organic cover (Unit 1), comprised primarily of un-
decomposed moss, extended to 20 cm depth, below
which an unfrozen clayey-silt (Unit 2) with a blocky texture
extended to a maximum depth of about 90 cm, with
abundant woody roots in the upper 20 to 45 cm. This unit
contained nearly equal amounts of clay (< 4 µm) and silt
(< 63 µm to 4 µm), and a small fraction of fine-grained
sand, and was a low-plasticity clay in which the existing
water content of 28% dry weight was slightly above the
plastic limit of 23%. A thawed, undulating clayey silt and
sandy silt unit (3A and 3B) extended across the section
from about 83 to 100 cm depth. The clayey-silt sub-unit
contained 70% silt, 27% clay and minor sand, and was a
low plasticity clay with a water content of 19% at about the
plastic limit. An AMS age from a root contained within the
sediments was modern. The sandy-silt sub-unit contained
71% silt, 10% clay and 19% fine sand, with a moisture
content of 16%. A silty-clay (Unit 4), with an erosional
upper contact with Unit 3, extended to the base of the
exposure. This was divided into two sub-units based on
the thaw depth at time of observation. The upper thawed
sub-unit (4a) extended from 100 to 120 cm depth, and
contained 51% clay, 46% silt and 3% sand, and was an
intermediate plasticity clay with a moisture content of
30%, which was well above the plastic limit of 20%, and
approaching the liquid limit of 38%. The lower frozen sub-
unit (4b) extended from 120 cm to the base of the
exposure at 220 cm. It contained 58 to 64% clay, 35 to
40% silt and a small fraction of fine-grained sand, and an
intermediate plasticity clay. Ice content within the frozen
sub-unit increased substantively from 40 to 70% visible
ice by volume, and ranged from wavy lenticular to layered
sub-horizontal to inclined lenses to reticulate ice with
suspended ataxtic soil clasts up to 20 cm in diameter.
Moisture content of the combined soil and ice in the upper
portion of the sub-unit was 45%, equivalent to the liquid
limit. In contrast, the moisture content of the soil
component (without excess ice) obtained from a clay clast
in the lower portion of the sub-unit was 36%, still
exceeding the plastic limit of 23%. Oxygen isotopic values
of the excess ice from between 160 and 200 cm depth
ranged from -16.0 to -17.5‰ and the δ18O value of a
single ice crystal was -16.6. Co-isotopic ratios of δ18O/
δD fall within the range of natural lakes in the area
(Gaanderse 2015), confirming a modern water source for
the ice. An AMS age of an in situ root along a sediment-
ice margin in the section was 80 ± 30
14
Underlying silty-clay sediments from Unit 4 are
interpreted as lacustrine deposits, based on their high
combined silt and clay content. The clay content within
this unit is lower, however, than that typical of Glacial
Lake McConnell sediments, which usually exceed 70%
(Gaanderse 2015), suggesting that these are not
glaciolacustrine sediments. More likely, they represent
ancestral Great Slave Lake deposits, likely in relation to
the expanded Yellowknife Bay as depicted in Fig. 4B or
are more recent. The overlying sandy-silt and clayey-silt
Unit 3, with an erosional unconformity at its base, is
interpreted as fluvial channel deposits, with the sandy-silt
representing Yellowknife River channel deposits and the
clayey-silt representing side deposits in shallower water.
This would correspond to a time when the Yellowknife
River was confined below Tartan Rapids and entering into
the north end of Back Bay as it does today. Unit 2
sediments are interpreted as alluvial plain deposits,
representing a shallow-water or seasonally-flooded
environment as presently observed along the Yellowknife
River shoreline.
C BP.
The nature of the ground ice from 120 cm to the base
of the exposure is indicative of segregated ice that formed
epigenetically as permafrost aggraded into alluvial
sediments following terrestrial exposure. The significant
increase in volume of ice below 160 cm depth suggests
freezing rates were slow with unlimited available water.
Significantly, the in situ roots within these the sediments
indicate the ground was unfrozen until about 80 ± 30
14
Atterberg limits indicate clay-like properties of these
fine-grained lacustrine and alluvial sediments, being
conducive to both ice segregation and sensitivity to thaw.
Field water contents, exceeding the plastic limit at the
base of the active layer and within underlying frozen
sediments, enable ongoing thaw slumping by promoting
sliding and flowing of liquefied sediments. It is possible
that slumping is further promoted by the increase in slope
from surface heave through ice segregation in permafrost
at depth, facilitating soil failure at the base of the active
layer.
C
BP. This age equates to between 253 and 25 calendar
years ago (Calib.©7.0.4, 2014), suggesting that
permafrost aggradation into the sediment likely occurred
within the last two hundred years at most. Furthermore, as
the ground surface of the headwall is presently 8 m above
river level, this suggests that as much as 8 m of ground
ice has formed within this time period, heaving the ground
surface.
The ground ice exposure on the Yellowknife River
(Site 6 shown in Fig. 4B) provides an example of
comparatively recent historical permafrost aggradation
into alluvial sediments, accompanied by epigenetic
ground ice formation, with ongoing lake-level recession.
This suggests that the processes of permafrost
aggradation and ground ice (i.e. lithalsa) formation have
continued under historically modern climate conditions.
5 CONCLUSIONS
The Great Slave Lowland was initially inundated by
Glacial Lake McConnell, and subsequently by ancestral
Great Slave Lake at about 8000 cal BP. Concerning
regional terrestrial emergence and permafrost
aggradation, the main conclusions of this study are:
1. In the Yellowknife area, Holocene lake-level
recession of ancestral Great Slave Lake occurred at a
rate of about 5 mm/year over the last 8000 years.
2. As lake-level recession exposed land within the
Great Slave Lowland, permafrost aggraded into fine-
grained lacustrine, alluvial and underlying glaciolacustrine
sediments following terrestrial emergence. Ice-rich
permafrost mounds (lithalsas) formed in some areas as
permafrost aggraded into saturated sediments.
3. In the local Yellowknife area, most lithalsas
formed within the last 6000 years, and many are locally
associated with alluvial deposits along the Yellowknife
River, which were deposited within the last 3000 years as
the river became confined to a channel below Tartan
Rapids.
4. The process of permafrost aggradation and
ground ice formation has continued in recent historical
times along alluvial floodplains the present-day shoreline
of the Yellowknife River.
ACKNOWLEDGEMENTS
This paper is a contribution to the Climate Change
Geoscience Program of Natural Resource Canada. We
gratefully acknowledge the field assistance of Adrian
Gaanderse and Mike Palmer and review of a draft version
of the paper by Phil Bonnaventure.
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Full-text available
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Geomorphic origin of a lithalsa in the Great Slave Lowlands
  • A J Gaanderse
Gaanderse, A.J. 2015. Geomorphic origin of a lithalsa in the Great Slave Lowlands, Northwest Territories, Canada. MSc. Thesis. Department of Geography and Environmental Studies. Carleton University. 155 pp.