Draft 1 Syngenetic dynamic of permafrost of a polar desert solifluction lobe , Ward Hunt Island , Nunavut

Repeated freeze–thaw cycles on slopes trigger sorting and solifluction mass movements, while subsequent displacement of material modifies the geomorphology of slopes as well as permafrost dynamics. This study focuses on the geomorphology and the cryostratigraphy of a polar desert stone-banked solifluction lobe with the objective to clarify the impact of slow mass movements on ground ice aggradation. The morphology of the solifluction lobe was characterized by peripheral ridges of coarse gravel, partially surrounding a depression filled with finer sediments saturated with water and covered by organics. Cryostratigraphic analysis demonstrated that the solifluction lobe’s formation led to the development of a syngenetic layer of permafrost with an ice content that varied according to the location in the lobe. The ice-rich cryofacies formed in the central depression of the lobe should act as a buffer to potential active layer deepening, slowing down its thawing, whereas the ice-poor cryofacies formed under th...


INTRODUCTION
Repeated freeze-thaw cycles trigger various types of mass movements on periglacial slopes, with solifluction being the most widespread slope deforming process (Kinnard and Lewkowicz 2005;Ridefelt et al. 2009).Although solifluction rates are generally limited to a few centimeters per year, they are, in the long term, the dominant mechanism of slope modification in periglacial environments (Åkerman 1996), thereby contributing greatly to the evolution of periglacial landscapes (Matsuoka 2001).In cold permafrost regions, where active layer freezing occurs both downward and upward, solifluction is driven by frost creep (downslope movement due to freezethaw cycles), gelifluction (flow of unfrozen saturated soils during thawing of frozen substrate) and plug-like flow (soil sliding at the slip plane located at the boundary between the active-layer and the ice-rich permafrost) (French 2007;Harris et al. 2008;Mackay 1981;Matsuoka 2001).On Ward Hunt Island, these processes have created stone-banked solifluction lobes of about 0.1 to 1 m in height and 2 to 25 m in width (Verpaelst 2016); sizes that are similar to those reported in the literature (Matsuoka 2001).
Cryostratigraphy (e.g.shape, organisation, distribution and volume of ground ice in soil, sediment or bedrock) studies the layering of ice and sediments within permafrost, and identifies and correlates stratigraphic units (usually layers) of permafrost (French 1998;French and Shur 2010;Murton 2013;Gilbert et al. 2016).It includes the analysis of cryostructures and cryofacies.
Cryostructures analysis describes the shape, distribution and proportion of pore and segregated ice within frozen sediment that are visible to the naked eye (Shur and Jorgenson 1998;French and Shur 2010;Murton 2013).Cryofacies are bodies of frozen ground that are distinguished from adjacent bodies in term of their cryostructure pattern, sediment texture and volumetric ice content (Katasonov 1978;Murton and French 1994;French and Shur 2010;Murton 2013).The main interest of cryostratigraphy lies on its ability to reflect the environmental conditions that led to the D r a f t 4 ground ice formation, therefore contributing to our understanding of permafrost origin and dynamics, deposition environment, processes of ground ice aggradation and degradation, and thermal history (Katasonov 1978;French 1998;Shur and Jorgenson 1998;Gilbert et al. 2016).
Cryostratigraphy of slope deposits reflects the accumulation of material over the original surface of the slope.Ice build up depends largely on soil moisture, which, in the active layer, is also a key factor for the deformation and downslope displacement of a solifluction lobe.During the warm season, sediment saturation due to the melting of ground ice and snowmelt water input from upslope snowbanks raise pore water pressure and accentuate the downslope displacement of material (Tika et al. 1999;Sassa et al. 2007).The accumulation of sediments on solifluction lobes causes the rise of the permafrost table over time, as the active layer thickness must remain in equilibrium with the local climate conditions.While the bottom part of the active layer progressively reaches a perennially frozen state, water migration towards the upward progressing freezing front (i.e. the bottom of the active layer) results in the formation of an ice-rich layer in the upper permafrost (Fig. 1) (Shur 1982(Shur , 1988;;Guodong 1983;French and Shur 2010).Upward permafrost aggradation, termed syngenetic, has been extensively studied in Siberia and North America in eolian and fluvial deposits (e.g.Shur and Jorgenson 1998;Fortier and Allard 2004;Shur et al. 2004;Bray et al. 2006;Kanevskiy et al. 2011;Schirrmeister et al. 2013, Gilbert et al. 2016 and references therein).In contrast, very few studies have addressed the cryostratigraphy of slope deposits (Zhigarev 1967;Gravis 1969;Kanevskiy 2003;Abramov et al. 2008), and none of these have been conducted in polar desert conditions.

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The objectives of this paper are: 1) to characterize the geomorphology and cryostratigraphy of a typical polar desert solifluction lobe and describe the processes of its formation; 2) to demonstrate that solifluction lobe development and morphology impact the sub-surface hydrology of slopes, the accumulation of organic matter, and the aggradation of ground ice; and 3) to demonstrate that the aggradation of ground ice affects the permafrost stability.
Air temperature records from the SILA network weather station located on the northern side of WHI at ~1 km east of the study site indicate, for the period 1995 -2015, a mean annual air temperature of -17.9 °C, with minimum monthly averages of -33.4 °C in February and maximum of +1.5 °C in July (Centre d'études nordiques (CEN) 2016).Precipitation measured during the period 1981 -2010 at Alert, 170 km to the east, averaged 158 mm yr -1 (89 % as snowfall) (Environment Canada 2016).Between 1996 and 2014, the total annual thawing index (TI, cumulative number of degree-days above 0°C during a calendar year (van Everdingen 1988)) on WHI varied between 47.6 (1996) and 156.8 degree-days (2012), with a mean of 100, and the total annual freezing index (cumulative number of degree-days below 0°C during a calendar year (van Everdingen 1988)) varied between 5992 (2010) and 7175 (2001), with a mean of 6600 (Paquette et al. 2015).For the period 2006 -2012, incoming solar radiation in summer varied between 1542 MWm -2 (2012) and 1973 MWm -2 (2009), with a mean of 1758 MWm -2 (Paquette et al. 2015).
The landscape of WHI is marked by the presence of Walker Hill (436 m above sea level (a.s.l)), composed of limestone and characterized by steep debris-covered slopes (Vincent et al. 2011).The northern, eastern and southern coasts of the island exhibit gentler slopes that are located below the D r a f t 6 Holocene marine limit (≥ 62 m a.s.l., Lemmen 1989) and which are covered by sand and gravel raised-beach deposits (England 1978).A gravelly boulder drift with clasts of local and foreign origin is present on the inner island.Cryogenic weathering on WHI has created a widespread cover of frost-shattered debris all over the hillsides, which are slowly being reworked by the downslope movement of material, forming solifluction lobes and sheets, block-streams, sorted and non-sorted stripes, cryogenic steps and bench-like features with a downslope border of rock rubble (Vincent et al. 2011).Stone-banked solifluction lobes in particular are widespread along the low elevation (below upper marine limit) of northern WHI slopes (Figs.2c and 3).They form exclusively downslope of snowbanks, which provide the meltwater essential to sediment transport and solifluction lobes formation.The morphology of solifluction lobes, which is similar throughout the island, is characterized by lateral and frontal ridges made of coarse gravel and cobbles, surrounding a longitudinal depression made of finer material where water flows at or near the surface during the snowmelt period, saturating the sediments and allowing the growth of vegetation (Fig. 4).Water also flows out of the lobes through their coarse frontal ridge.
A stone-banked solifluction lobe located at the base of Walker Hill (20 m a.s.l), on the northern edge of Ward Hunt Island, was studied between 2013 and 2015.Surficial deposit included a mixture of sand and gravel beach deposits mixed with frost-shattered colluvial rock debris.The shape, size and height of the selected lobe was of similar magnitude to other lobes in the study area, although morphometric and size variability exist across the landscape.

Microtopography of solifluction lobe
To evaluate the morphology of the lobe, a 3D numerical model was created using a terrestrial laser scan (VX spatial station; Trimble ®, single 3D point accuracy: 10 mm at ≤ 250 m) from six D r a f t different base stations (all positioned between 2 to 5 m from the lobe) to remove shading effects.
The point cloud obtained (mean resolution: 0.14 m) was represented as a surface using a triangulated irregular network (TIN) method (Fig. 5a).To measure the elevation of the lobe, the VX spatial station survey was linked to a geodesic landmark recorded using a Global Navigation Satellite System (R8 GNSS; Trimble ®, precision: x-y +/-8 mm and z +/-15 mm).Elevations obtained were corrected by the Canadian Centre for Remote Sensing and orthometric heights were used.

Solifluction lobe cryostratigraphy and sediment properties
We used a cryostratigraphic approach (French 1998(French , 2007;;French and Shur 2010) to explain the dynamics of permafrost ground ice aggradation and degradation in a solifluction context.To characterize the cryostratigraphy of the lobe, permafrost coring was performed using an earth auger (BT360; Stihl ®) equipped with a 10.8 cm diamond carbide core barrel.Four boreholes were drilled: in the central depression of the lobe (borehole F1), on a lateral ridge (borehole F7) and on the slope adjacent to the lobe (boreholes F2 and F3;Figs. 5a,b).Each core had the drilling mud removed and was kept frozen during transport.The thaw depths were measured with a steel probe at each meter along a transect connecting the boreholes (Figs.5a, b).A trench was dug in the front of the lobe, the stratigraphy was characterized and the sediment was sampled.
Micro-computed tomography, a non-destructive imaging technique, was used to image the ice and sediment components of the permafrost.The cores were scanned using a tomodensitometer (SOMATOM Definition AS+128; Siemens ®) with a vertical resolution of 0.4 mm.Voxel (3D pixels) values obtained vary according to the density of the scanned material; denser materials (rock and sediments) appear in white and pale grey, while lower density material, such as ice and air appear in grey and black.

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The cryostructures of the solifluction lobe includes a description of the samples extracted from the following geomorphological subunits, central depression, peripheral ridges, slope, ice wedge, active layer, and are grouped into cryofacies having distinct volumetric ice content and ice patterns (Murton and French 1994;Shur and Jorgenson 1998;French and Shur 2010).In this study, we propose to link the surface morphology of solifluction lobes to typical cryofacies with contrasting cryostructures and ice contents.
The volumetric ice content (VIC) of the permafrost was not calculated from tomodensitometry due to significant underestimation errors associated with the technique's resolution which is generally coarser than the porosity of the material (Lapalme et al. in review).The VIC of the cores was thus determined with a liquid displacement method (immersion of vacuum-sealed samples in water) and calculated using: where V t is the total volume of the frozen sample (cm 3 ) and V i is the volume of ice (cm 3 ) estimated from weight loss after drying, using the theoretical density of ice (0.9175 g cm -3 ).Cores were weighted, dried and weighted again to provide gravimetric ice content (GIC) (ASTM-D2216 2010 for comparison with other studies not using VIC.The grain size distributions of the sediments were measured based on sieve and hydrometer (on samples > 5 % of fines (< 0.063 mm)) analyses using semi-logarithmic intervals.They were classified according to the Udden -Wentworth scale (Gravel and cobbles ≥ 2 mm; Sand from 2 mm to 0.063 mm; Silt and clay ≤ 0.063 mm).The textures of sampled sediments were presented with a ternary diagram created using the Gradistat software (Blott and Pye 2001).
The saturated hydraulic conductivities (K s , m s -1 ) of the active layer sediments (central depression, ridge and slope) were measured in laboratory using a saturated hydraulic conductivity D r a f t 9 device (KSAT; UMS ®, accuracy of the pressure sensor: 1 Pa).For sediments with high hydraulic conductivity, constant head tests were performed based on the Darcy equation (Darcy 1856): where L is the length of the sample (m), V is the percolated volume of water (m 3 ), H is the height of the water column (m) above the surface of a sample, A sample is the area of the sample (m 2 ) and t is the time (s).For sediments with low hydraulic conductivity, falling head tests were performed using: where A Bur is the area of the burette (m 2 ), ∆t i→i+1 is the difference of two reading times t i and t i+1 (s), H ୲ is the pressure head difference at time t i (cm) and H ୲ శభ is the pressure head difference at time t i+1 (cm).

Slope and solifluction lobe morphology
Measurements and observations of the studied solifluction lobe allowed us to evaluate the implications of the lobe's morphology on organic matter accumulation and sub-surface hydrology.
The studied solifluction lobe was located at an elevation of 19.7 m a.s.l. on a north facing 12° slope.The lobe was 27 m long and 13.5 m wide and was delimited by a frontal ridge up to 11.7 m wide and 1.2 m high.On the sides of the lobe, lateral ridges, up to 4 m wide and 0.5 m high, confined a longitudinal central depression.This depression was 15 m long, 6 m wide and 0.2 m deep (in comparison with the lateral ridges).Compared to the slope surface, which was mostly made of washed-out gravelly beach deposits and coarse frost shattered debris, the ridges material essentially consisted of reworked and sorted coarse gravel (Fig. 5c, see Fig. 7a for material texture D r a f t 10 and Fig. 7b for grain size).The cryofacies, cryostructures, moisture content, grain size and bubble descriptions of the permafrost cores are presented in Table 1.In the central longitudinal depression, finer reworked silty sand sediments, transported from upslope, were deposited and mixed with gravel.Melt water originating from uphill snowbanks flowed at or near the surface in the central depression, saturating the sediments.These moist conditions favoured the growth of mosses, black biological crusts, bacterial mats and a few plants, which strongly contrasted with the rest of the slope where only very sparse vegetation grew (Fig. 4).This water then partially flowed out of the lobe through the ridges, which were in turn almost entirely devoid of organic matter.A washed-out gravel conduit acting as a subsurface preferential flow path was observed in the frontal ridge (Fig. 6).

Sediment composition, active layer depth and hydraulic conductivity
Analysis of the solifluction lobe material indicates a correlation between the lobe's morphology and the sediment sorting and grain sizes.Sorting processes concentrate coarse material in ridges, which in turn controls sub-surface hydrology and soil moisture prior to syngenetic bottom-up permafrost aggradation.Fig. 7 shows the textural group of the material for the different sections of the lobe.The sediments of the central depression (borehole F1) were mainly composed of muddysandy gravel, while the material of the slope (boreholes F2 and F3) and of the peripheral ridge (borehole F7) was mostly gravel.Gravelly sediments were found at the surface of the slope in boreholes F2 (at depth of 0 -27 cm), F3 (at depth of 0 -10 cm) and at the surface of the lateral ridge in borehole F7 (at depth of 0 -60 cm).Muddy sandy gravel sediments were found at depth in all boreholes.
The thaw depths measured on the slope on June 18 2013 varied from 0 cm where there was still snow on the ground, to 23 cm on bare ground.On the lobe itself, maximum thaw depths were twice as deep in the peripheral ridges (31 cm) than in the central depression (14 cm).On the same date, The saturated hydraulic conductivity (K s ) of the active layer measured in the central depression, in the peripheral ridge and in the slope are presented in Fig. 8. Results show high values for the gravel material on the surface of the slope and ridges (from 5.5 * 10 -5 m s -1 to 7.1 * 10 -1 m s -1 ), whereas low values were recorded in the muddy-sandy gravel material of the ridges at depth and in the central depression (from 6.3 * 10 -8 m s -1 to 1.8 * 10 -6 m s -1 ).Through the frontal ridge, K s of the washed-out gravel conduit was considered to be at least 1 * 10 -2 m s -1 .Considering that the horizontal distance between the gravel conduit and the slope was about 12 m and that the elevation difference was about 3.8 m, the residence time of water in the ridge was estimated to be less than 30 minutes.

Cryostratigraphy
The drilling logs presented in Fig. 9 show the cryostratigraphy of the slopes (F2, F3) adjacent to the lobe, of the lobe's central depression (F1) and of a ridge (F7).The cryostratigraphic analysis indicated a clear distinction between the cryofacies of the different sections of the lobe, with the formation of ice-rich (central depression) and ice-poor (ridges) cryofacies.

Cryostratigraphy of the slope outside of the lobe
The permafrost of the slope outside of the solifluction lobe (boreholes F2, F3) comprised an icerich sediment whose debris content decreased with depth.In borehole F3 (55 -122 cm), the D r a f t 12 cryofacies presented suspended cryostructures and had a VIC between 48 and 67 % (Fig. 10c, f).At this depth interval, several small rounded air bubbles (< 1 mm) along with vertical elongated bubbles (≤ 1 mm) and one set of vertical longer elongated bubbles (20 mm long) were observed in the ice, together with stretched fine-grained sediments.At depth greater than 122 cm, the profile still showed suspended cryostructures, but the VIC increased to values between 72 and 93 % (Fig. 10d) and the ice had small rounded bubbles (< 1 mm), as well as bubbles shaped as hexagonal flat disks at the junction of crystals, and gravel inclusions.This type of ice meets the definition of massive ice (van Everdingen 1988) and the cryofacies of borehole F3, at depths greater than 55 cm, constitutes the general cryofacies of the slope at depth.
In F2, at depths of 35 -83 cm, an ice wedge with vertical foliation and vertical trains of bubbles was observed (Fig. 10a).Its presence in the slope could be inferred by a narrow longitudinal depression with frost cracks, parallel to the lobe.The maximum vertical extent and width of the ice wedge are unknown but presumably extends 2 to 4 m below the base of the active layer.At depths of 51 -83 cm, suspended cryostructures similar to those in borehole F3 (55 -122 cm depth) were observed as the coring crossed the edge of the ice wedge (Fig. 9a).

Cryostratigraphy of the central depression of the solifluction lobe
The cryostratigraphy of the central depression of the lobe (borehole F1) differs from the cryostructure of the slope sediment unaffected by solifluction (boreholes F2, F3).The permafrost of the depression exhibited a succession of ice-rich cryofacies underlain by ice-poor cryofacies.The assemblage of layered, lenticular, and crustal cryostructures observed at depths of 62 to 129 cm (Fig. 10e) formed the cryofacies of the central depression.The ice coatings of coarse fragments forming crustal cryostructures were mostly found on the upper surface of gravels.The ice of thicker horizontal ice lenses (up to 3 cm thick) contained elongated vertically oriented air bubbles.The VIC of this ice-rich cryofacies ranged from 40 to 59 %.Deeper, between 129 -156 cm, the VIC D r a f t 13 decreased to between 23 and 26 % forming an ice-poor gravelly cryofacies similar to the cryofacies of the ridge.

Cryostratigraphy of the peripheral ridge of the solifluction lobe
The ridges are mostly underlain by permafrost with low ice content (borehole F7).Sediments with invisible pore ice observed at depths of 74 -106 cm had a VIC varying between 20 and 25 % and formed the ice-poor gravelly cryofacies of the ridge.Between 74 and 87 cm, ice crusts had no preferential position on gravel inclusions.At depths of 87 -106 cm, crustal (preferentially observed on the upper surface of the gravels) and sparse lenticular cryostructures were observed (Fig. 10b).
These features made this cryofacies much similar to the cryofacies observed near the base of the borehole in the central depression (borehole F1, 129 -140 cm depth).Between 158 and 170 cm, the permafrost had a VIC of 52 % and exhibited suspended cryostructures.It was then underlain at 170 -177 cm by massive ice (VIC 86 %).This gradation from suspended cryostructures toward massive ice at greater depths corresponds to the general cryofacies of the slope mentioned earlier (borehole F2, 51 -83 cm depth; borehole F3, 55 -154 cm depth).

Slope morphology, solifluction lobe development and syngenetic permafrost aggradation
The slope, the central depression and the peripheral ridges of the lobe had contrasting ice-rich and ice-poor cryofacies, each possessing predominant cryostructures and different grain size and ice contents.As mass movements occurred in a host material dominantly made of coarse sediments, slow accumulation of new material on the slope surface led to the syngenetic development of permafrost and to the aggradation of ground ice over the pre-existing slope surface.The movement of material, saturated by the inflow of meltwater from upstream snowbanks, resulted from numerous near-surface freeze-thaw cycles causing vertical and lateral sorting, re-shaping the D r a f t morphology of the slope.In the frost-sorting process, coarser material was brought on the peripheral sections to form the frontal and longitudinal ridges of coarse gravel partially surrounding the central depression of the lobe; similar processes were described by Harris (1987) and Harrison and Macklin (1991).The morphology of the lobe gave rise to two contrasting cryofacies: 1) A syngenetic ice-rich cryofacies developed in the central depression of the lobe, consisting of sediments with layered and lenticular cryostructures typical of syngenetic permafrost (Shur et al. 2004).The distinct ice layers ("ice belts") observed in borehole F1 reflects former positions of the permafrost table at different stages of permafrost aggradation (Fortier et al. 2008;Kanevskiy et al. 2008Kanevskiy et al. , 2011)).The occurrence of ice crusts preferentially located on the upper surface of gravels (Fig. 10e) and of vertically elongated air bubbles stretched upward are indicators of a freezing front progressing from the bottom upwards, which suits the common process of syngenetic permafrost aggradation.The higher thermal conductivity of gravels compared to the ground matrix and the bottom-up freezing direction establish cryosuction conditions on the top of the gravel, which are responsible for the development of ice crusts on their upper surface.The freezing direction is also confirmed from the air bubbles oriented upward.Finer frost-susceptible material (silt and sand) transported by snowmelt water from upslope was trapped in the saturated central depression of the lobe (Woo and Steer 1986;Woo and Xia 1995) and promoted ice segregation as the permafrost aggraded upward, which explains the distinct ice-rich cryostructures forming the cryofacies of the central depression.Moreover, the growth and accumulation of mosses and plants at the surface contributed to develop an intermediate layer, which is established by a decrease of active layer depth and the aggradation of near surface ground ice (Shur 1988;Shur et al. 2011;French and Shur 2010).Interestingly, this means that over the years, a portion of snowmelt water derived from upstream snowbank was re-frozen and incorporated into newly formed permafrost, thereby increasing the ice content of slopes on the long term.We estimated the volume of ground ice and void ratio of frozen sediment (volume of ice to volume of dry sediment) under the lobe.It was found that the void ratio of frozen sediment varied from 1.08 for the ice-rich cryofacies of the central depression to 0.30 for the ice-poor cryofacies of the ridges.Per cubic meter, the permafrost recently formed under the central depression therefore contained more than three times the amount of ice than the permafrost under the ridges.

Syngenetic ice aggradation in solifluction lobe depression and the thermal resistance of permafrost
The period spanning 2008 to 2012 showed a succession of warm years on WHI (Paquette et al. 2015).The summer 2011 saw an active layer deepening of + 23 cm (106 cm depth) compared to 2010 (83 cm depth) (Table 2), which was caused by a high TI and a particularly high amount of incoming solar radiation to the ground surface (1862 MWm -2 ) (Paquette et al. 2015).The subsequent summer ( 2012) was marked by a particularly low amount of incoming solar radiation (1542 MWm -2 ) (Paquette et al. 2015), resulting in the thinning of the active layer by -29 cm (77 cm depth).
The active layer deepening of 2011 was detected on the slopes adjacent to the solifluction lobe by the presence of thaw unconformities at depths of 35 and 55 cm (Fig. 10a, c).In the central depression of the lobe, the 2011 thaw unconformity was estimated at 62 cm depth, which corresponds to the boundary between ice-poor sediments (mainly with porous invisible cryostructures) and ice-rich sediments (with layered, lenticular and crustal cryostructures).The shallower depths of thaw-unconformities on slopes and under the central depression, in comparison to the maximum depth of thaw at the SILA meteorological station in 2011, is likely explained by the ice-rich nature of the upper permafrost (e.g.ice wedge in F2), which required larger heat input to overcome the latent heat of ice needed to thaw the permafrost.The conditions during the summer 2011 led to active layer deepening, partial drainage of the excess water through the ridges and thus to the development of an ice-poor facies after freeze-back of the active layer.
In the peripheral ridges, the lower ice content of the material was caused by the effective drainage of ground water (short residence time of water) through porous coarse gravels with high hydraulic conductivity, prior to upward freezing and progressive permafrost aggradation.The thinner snow cover over the wind-swept ridges and its rapid melting in spring, along with the D r a f t absence of vegetation increased the amount of solar radiation received.This, along with the low ice content of the ground material caused a deeper thaw depth in the ridges than in the other subunits.This is supported by the thaw depths measured on June 18, 2013 (Fig. 11), which were deeper under the ridges (31 cm) than at the SILA meteorological station (20.5 cm) and elsewhere on the transect (slope -23 cm; central depression -14 cm).Based on these measurements and on the comparison with the maximum thaw depth at the SILA meteorological station, the 2012 maximum thaw depth on the peripheral ridge (borehole F7) was estimated to be around 87 cm.It was also assumed that the 2011 thaw depth under the ridges had been deeper due to the low ice content (smaller latent heat effect) of the permafrost, although no cryostratigraphic markers for the year 2011 were observed to confirm this assumption.
The higher volumetric ice content in the central depression of the lobe increases the latent heat damping effect and act as a thermal buffer reducing active layer deepening.The low ice content of the ridges permitted a greater deepening of the active layer.The energy (E) required to melt the ice in a cubic meter of permafrost can be calculated using: where H f is the heat of fusion of ice (334 kJ kg -1 ) and D i is the ice density (917.5 kg m -3 ).Assuming an ice temperature of 0 °C, calculations using the mean VIC of the different sections of the lobe (central depression: 52 % -ridge: 23 %), results in a 1.7 times greater E value under the central depression (159 351.4 kJ) than under the ridges (91 933.5 kJ).The cryostratigraphy therefore indicates that warm summers have a greater effect on the increase of active layer depths in the icepoor ridges than in the central longitudinal depression and in the adjacent slopes.Under future scenarios of climate change and generalized active layer deepening in the high Arctic, the thawing of ice-rich zones under the central depressions should translate into a larger thaw subsidence in comparison to the ridges and in an increased drainage of meltwater through the ridges downslope.

CONCLUSION
Polar desert slopes are generally affected by solifluction processes.Our study of the cryostratigraphy of a stone-banked solifluction lobe on WHI revealed that the evolution of permafrost was greatly affected by slowly flowing and aggrading material on an ice-rich slope.To our knowledge, this study is the first to assess quantitatively the cryostratigraphy of polar desert solifluction lobes.This process, driven by the incoming water from snowbanks upstream, led to saturation and sorting of the sediment and formed a solifluction lobe.The movement of the lobe caused the syngenetic aggradation of permafrost, which cryostructures, cryofacies and ice content varied according to the morphology of the lobe.Our results showed the strong influence of solifluction lobe morphology on ice content of the permafrost with ice-poor zones under the peripheral ridges and ice-rich zones under the central depression of the lobe.On the long term, the movement of solifluction lobe promoted ice aggradation and increase of the total ice content in sections of the slope.
The ridges with their surficial coarse sediments of high hydraulic conductivity effectively drained subsurface water out of the lobe to create an ice-poor cryofacies with a void ratio of frozen sediment of 0.30.The saturated gravelly silty sand covered by mosses and plants in the central depression of the lobe resulted in a shallower active layer and in the formation of an ice-rich cryofacies with a void ratio of frozen sediment of 1.08.Our study showed that the energy required to melt the ice formed in the ice-rich zone of the central depression would be 1.7 times higher than in the ice-poor zones of the peripheral ridges.
In the context of climate change, active layer deepening in polar desert solifluction lobes will be temporarily buffered by the high ice content of the ice-rich zone formed in the central depressions, while the thawing of the ice-poor gravel of the ridges is expected to be faster.Overall, the ridges of             2. The active layer thickness in sandy gravel for the period 2006 -2014, interpolated from the thermal regime of a thermistor string connected to the SILA network weather station (CEN 2016).In 2013 active layer thicknesses at the slope outside of the lobe (S), at the peripheral ridges (R) and at the central depression of the lobe (C d ) were estimated based on analysis of cryostructures.
of a sandy-gravel beach deposit, provided by a thermistor string connected to a weather station(CEN 2016), indicated an interpolated thaw depth of 20.5 cm.Table2 presentsthe maximum thaw depth at the weather station between 2006 and 2014, along with the maximum active layer thicknesses estimated in 2013 by identifying thaw unconformities from the cryostratigraphy of the different sections of the lobe.The active layer thickness at the weather station site varied from 65 cm in 2006 to 106 cm in 2011, whereas in 2013 at the study site they ranged from 35 cm on the slope to 87 cm on the ridges.
syngenetic ice-poor cryofacies developed in the ridges of the lobe, as shown by ice crusts preferentially located on the upper surface of gravels.The high hydraulic conductivity of the gravel on the upper part of the ridges and the preferential subsurface flow path feature observed in the frontal ridge (Fig.6) promoted quick drainage of the sediment.This kept a low moisture content in the ridges prior to freeze-back, as shown by the dominance of porous cryostructure in ice-poor permafrost.Water drainage from the central depression through the layer of gravel, promotes a rapid subsurface flow towards water tracks networks located downslope, thus increasing the subsurface hydrological permeability of the slope.The cryostratigraphic profile of the solifluction lobe is shown in Fig.11.The succession of cryofacies with increasing VIC and grading into massive ice at depth observed in boreholes F2, F3 and F7, represents the predominant cryofacies of the slope.The ice-rich cryofacies observed in borehole F1 at depths of 62 -129 cm is characteristic of the central depression of the lobe and was surrounded by the ice-poor cryofacies found under the peripheral ridges.At depths of 129 -140 cm (borehole F1), the ice-poor cryofacies similar to the cryofacies found under the ridges (borehole F7), indicates that the slope was overridden by the gravelly front of the lobe.As the frontal portion of the lobe moved over the slope, the central depression stretched downslope and finer saturated material buried coarser material left behind by the advancing frontal ridge.

Figure 2 .
Figure 2. (a-b) Localization of Ward Hunt Island at the northern tip of Ellesmere Island (83°05'N,

Figure 4 .Figure 5 .
Figure 4. Central depression of solifluction lobes.(a) Sediments in the central depression are

Figure 6 .
Figure 6.Gravel conduit (15 cm diameter, fines were washed out) in the frontal ridge of the lobe

Figure 7 .
Figure 7. (a) Textural ternary diagram of the central depression, of the peripheral ridges and of the

Figure 8 .
Figure 8. Hydraulic conductivity (K s ) of sediments of the slope (boreholes F2, F3), of the central

Figure 6 .Figure 8 .
Figure 4. Central depression of solifluction lobes.(a) Sediments in the central depression are bordered by coarse gravel ridges, mostly devoid of vegetation; (b) Saturated fine-grained sediments in the depression create conditions favorable to vegetation growth; (c) Thick accumulation of bryophytes in the depression.Note the presence of water and fine-grained sediments at the surface.Cd -Central depression; F -Frontal ridge; L -Lateral ridge.The white arrows indicate flow direction.Date: 7 August 2015.(Photo credit: Daniel Fortier).Fig. 4 46x12mm (600 x 600 DPI)

Figure 10 .
Figure 8. Hydraulic conductivity (Ks) of sediments of the slope (boreholes F2, F3), of the central depression (borehole F1) and of the peripheral ridge (borehole F7) of the lobe according to the depth.Note the difference between the ridge material values at the surface (0 -20 cm) and at depth (60 -84 cm).Fig. 8 89x57mm (600 x 600 DPI)