Impacts of Hillslope Thaw Slumps on the Geochemistry of Permafrost Catchments (Stony Creek Watershed, NWT, Canada)

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Chemical Geology 356 (2013) 38–49

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Chemical Geology

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Impacts of hillslope thaw slumps on the geochemistry of permafrost catchments (Stony Creek watershed, NWT, Canada)

Laura Malone a, Denis Lacelle b,⁎, Steve Kokelj c, Ian D. Clark a a Department of Earth Sciences, University of Ottawa, Ottawa, Canada b Department of Geography, University of Ottawa, Ottawa, Canada c NWT Geoscience Ofﬁce, Government of the Northwest Territories, Yellowknife, Canada article info abstract

Article history: Retrogressive thaw slumps are one of the most dramatic thermokarst landforms in periglacial regions. This study Received 24 December 2012 investigates the impacts of one stable and two active thaw slumps on the geochemistry of streams in the Stony Received in revised form 12 July 2013 Creek watershed (Peel Plateau, NWT, Canada). The objective of this study is to elucidate the geochemical process- Accepted 13 July 2013 es associated with ground ice ablation in retrogressive thaw slumps and the geochemical evolution of slump run- Available online 22 July 2013 off to streams. This is accomplished by describing the geochemical composition of runoff across active mega- Editor: J. Fein slumps, impacted and pristine tundra streams, as well as that of the ice-rich permafrost exposed in the slump headwalls. In the Stony Creek watershed, runoff derived from active and stable thaw slumps is characterized – Keywords: by a Ca(Mg) SO4 geochemical facies with conductivity and solute concentrations approximately one order of Retrogressive thaw slumps magnitude higher than in pristine streams. The elevated solute concentrations in the slump runoff are directly Periglacial streams related to thawing of highly weatherable Late Pleistocene age ice-rich and solute-rich permafrost exposed in Geochemistry the headwalls of slumps, which has solute concentrations nearly 100 times higher than those measured in the Western Arctic uppermost 1–2 m (i.e., above the early Holocene thaw unconformity). An examination of ionic relations revealed 2+ 2− 2+ 2+ 2− a strong relation between Ca and SO4 and (Ca +Mg )–SO4 , suggestive that sulfate dissolution is the main process responsible for the geochemical composition of slump impacted streams. Thaw slumps signiﬁcantly impact the geochemistry of streams, by increasing their solute load well above that of pristine streams along any reach of impacted streams. Unlike shallow active layer disturbances, the thaw slumps can degrade permafrost to depths of 10 m or more and the impacts of abundant slump activity on stream geochemistry can be detected at the 10^2 km2 watershed-scale. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved.

1. Introduction One of the most dramatic thermokarst landforms in periglacial regions are retrogressive thaw slumps, however their geomorphic and Across the Arctic, geomorphic disturbances causing the thawing of geochemical impacts on fluvial systems have only been recently inves- near-surface ice rich permafrost result in some of the most discernable tigated (i.e., Kokelj et al., 2013). Unlike active layer disturbances, thaw impacts to freshwater ecosystems (e.g., Kokelj and Burn, 2005; slumps may thaw the uppermost tens of meter of ice-rich permafrost Mesquita et al., 2010; Thienpont et al., 2013). For example, active and enlarge for decades (Burn and Lewkowicz, 1990). Some of the larg- layer deepening and active layer detachment slides have been found est thaw slumps ever observed (up to 40 ha), termed “mega-slumps”, to modify the total suspended sediments and geochemistry of fluvial have recently developed along valley sides and open slopes of fluvial in- systems (Kokelj and Lewkowicz, 1999; Lamoureux and Lafrenière, cised ice-rich landscapes across northwestern Canada (Lacelle et al., 2009). During active layer deepening, solute sequestered in the shallow 2010; Kokelj et al., 2013). Slump growth is primarily driven by ablation permafrost may be released, resulting in increasing ionic concentrations of ground ice exposed in the headwall, which leads to progressive in streams, rivers and lakes (English et al., 1991; Kokelj and Burn, 2003; backwasting of the ice-rich headwalls (Lewkowicz, 1987). The thawed Keller et al., 2007, 2010). In watersheds containing active layer detach- materials accumulate as a mud-slurry on the slump floor and may be ment slides, the suspended sediment and solute concentrations in transported downslope by debris flows which can enter water bodies, streams situated immediately downstream of the slides may be signifi- such as streams, lakes or the ocean. Once the ice-rich headwall of cantly higher than in the adjacent landscape (Kokelj and Lewkowicz, a slump is buried by thawed sediments, the meltwater supply from 1999; Bowden et al., 2008; Lamoureux and Lafrenière, 2009; Stewart headwall ablation ceases and the slump and debris flow stabilizes. and Lamoureux, 2011; Dugan et al., 2012; Lewis et al., 2012). Recent studies support the idea that the growth of thaw slumps along lakeshores can modify water quality of lakes (Kokelj et al., 2009), with fi ⁎ Corresponding author. signi cant impacts to lake ecosystems (Mesquita et al., 2010; E-mail address: [email protected] (D. Lacelle). Thienpont et al., 2013). In streams impacted by hillslope thaw slumps,

0009-2541/$ – see front matter. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.chemgeo.2013.07.010 L. Malone et al. / Chemical Geology 356 (2013) 38–49 39

Kokelj et al. (2013) showed that the suspended sediment was orders of and others that incise ice-rich morainal deposits along the eastern foot- magnitude higher than pristine tundra streams and displayed diurnal hills of the Richardson Mountains and Mackenzie Mountains contribute oscillations that correlated with solar insolation and associated significant discharge and sediment loads to the delta of the Mackenzie ground ice ablation. Solute concentrations were also notably elevated River (Carson et al., 1999). The geology of the area is dominated by in slump impacted streams. These conditions appear to be unique to Lower Cretaceous marine shale and siltstone bedrock of the Arctic ice-rich permafrost landscapes impacted by thaw slumps. Red River Formation. The bedrock is overlain by up to 60 m of Late This study builds on the findings of Kokelj et al. (2013) conducted in Pleistocene glacial, glacio-fluvial and glacio-lacustrine sediments hosting the Stony Creek watershed, Peel Plateau NWT (Fig. 1). Here, we eluci- ice-rich permafrost (Norris, 1984; Duk-Rodkin and Hughes, 1992). The date the geochemical processes associated with ground ice ablation in geochemical analyses of the till veneer on the Peel Plateau indicate retrogressive thaw slumps and the geochemical evolution of slump that, SiO2 (60%), Al2O3 (13%) and Fe2O3tot (5%) are the dominant con- runoff to streams. This objective is accomplished by characterizing the stituents, whereas the abundance of CaO is low (b 1%) (Lacelle et al., geochemical composition of the ice-rich permafrost exposed in the in press). headwalls of thaw slumps, runoff across the floor of active and stable The regional climate consists of long cold winters and short cool thaw slumps, as well as that of impacted and pristine tundra streams. summers. The mean annual air temperature at Fort McPherson We also characterize the physical and geochemical characteristics of (1986–2010) is −6.8 °C, with mean annual summer air temperatures sediments exposed in the headwalls and those transported in the of 13.5 °C (Environment Canada, 2010). July is the warmest month with debris-flows and streams, which represent a source of leaching of sol- average temperatures of 15 °C, whereas the coldest temperatures occur utes following thaw. Based on the results, we discuss potential process- in January with an average of −27 °C. The number of thaw degree- es that lead to the elevated solute concentration in the slump runoff days, a climate index of temperature intensity, is showing a decreasing and assess the extent to which the contribution of slump runoff is ob- trend over the period of available data (−6 °C per year between 1986 servable in streams. Overall, our study illustrates the linkages between and 2010). Total precipitation at Fort McPherson (1986–2010) averaged thawing of the uppermost tens of meters of ice-rich permafrost exposed 295 mm, with rainfall representing 148 mm. Maximum annual rainfall in thaw slumps and the geochemical evolution of slump runoff to values can approach 250 mm (Environment Canada, 2010). Vegetation streams. consists of open forest-tundra woodlands in the valley bottoms transi- tioning to shrub and dwarf shrub tundra at higher elevations (Smith 2. Study area et al., 2004). These climatic conditions ensure the presence of continuous perma- 2.1. General setting frost across the Peel Plateau and Richardson Mountains. The thickness of permafrost reaches 100 m at Inuvik, and has been estimated at 400 m The Stony Creek watershed (67°25′N; 134°51′W; 1100 km2)issitu- for a nearby site 30 km west of Fort McPherson (Smith and Burgess, ated southwest of the hamlet of Fort McPherson. The watershed drains 2000). The active layer thickness on the Peel Plateau ranges from the Peel Plateau, which represents the eastern foothills of the Richardson about 50 cm on vegetated surfaces in fine-grain tills or in organic de- Mountains, NWT (Fig. 2). Elevations in the Stony Creek watershed range posits, to more than 100 cm in disturbed areas and gravelly floodplains from about 800 m in the mountains to less than 10 m above sea level at (Smith and Burgess, 2000). Regional active layer monitoring shows the confluence with the Peel River. The gravel bed streams of the Stony significant year to year variability, and longer-term trends vary with Creek watershed run parallel to the Dempster highway. This watershed ecological change, soil moisture and climate (Brown et al., 2003). In areas with stable vegetation conditions, the inter-annual variation in thickness of active layer is largely dependent on the thaw degree-days (Smith et al., 2009). Considering that the thaw degree-days is showing a declining trend since 1986 (p = 0.07), the active layer thickness is expected to show a decreasing trend in the study area. This is supported by the declining trend in historical active layer thickness measurements from Rengleng River situated 50 km north-east of the study area (Brown et al., 2003).

2.2. Description of studied thaw slumps

Based on historical air photographs (1944–2010), more than 75 thaw slumps were active in the Stony Creek watershed. This study fo- cuses on one stable and two active thaw slumps located along the hillslope of Dempster Creek (unofﬁcial name), a small tributary of Stony Creek. These slumps were selected due to their accessibility from the Dempster Highway and the difference in their surface area (Fig. 2). The Charas slump (FM2) is located ca. 2 km north of the Dempster Highway and in 2008 measured ca. 1 km wide. This slump is the largest, most dynamic slump in the Stony Creek watershed and has been visible as a much smaller feature on the earliest air photographs for the region (1944). The slump headwall is 25–30 m high and contains sections of silt-rich reticulate ice, vertical ice veins passing through sections of silt, and laminated ice-rich diamicton which are all overlain by a continuous layer of till varying in thickness from 30 cm to ca. 1 m. An gently undulating thaw unconformity is observed 1.5 to 2 m below the surface. In 2008, a stable portion of the slump was distinguishable by the vegetation re-colonizing the slump ﬂoor. Following a high intensity rainfall Fig. 1. Map of northern Yukon and adjacent Northwest Territories showing location of event (92 mm received on August 3rd, 2012), this stabilized portion Stony Creek watershed on the Peel Plateau (gray box). of the slump was re-activated. The slump runoff travels from headwall, 40 L. Malone et al. / Chemical Geology 356 (2013) 38–49

Fig. 2. Top) Map of Stony Creek watershed on the Peel Plateau showing location of active and stabilized thaw slumps as well as stream monitoring sites within catchments impacted to varying degree by thaw slumps. (PMC: Poor Man's Creek; DC: Dempster Creek; SC: Stony Creek). PMC1 represents a pristine tundra stream; PMC3 is situated downstream of a mega-slump; PMC4 is situated at the confluence with Dempster Creek). Bottom) Quickbird satellite image (September 2008) showing Charas and Melanie slumps along Dempster Creek and location of sampling sites of slump runoff along debris flows and a stream flowing above a stable slump (SS1). down the slump floor into a massive debris flow that extends into slump, active since at least 1958, although with a much smaller width Dempster Creek. The debris flow is at least 2 m thick and in 2010 ex- (160 m) relative to the Charas mega-slump. The headwall is about tended almost 2 km downvalley. Historical air photographs show that 10 m high, and consists of ice-poor and ice-rich diamicton with reticu- the debris flow is dynamic and has advanced progressively downvalley late ice in the uppermost 2 m, corresponding to a thaw unconformity. (Kokelj et al., 2013). Sometime between 2000 and 2008, the debris flow Lacelle et al. (in press) obtained an age of 7890 ± 250 14CyrBPat also blocked a portion of Dempster Creek, which generated the mud- 1.5 m below the surface, suggesting that the thaw unconformity is asso- dammed lake upstream of the Charas debris-flow observed in Fig. 2. ciated with the early Holocene warm interval. According to Burn However, the 92 mm rain event received on August 3rd, 2012 caused (1997), the early Holocene thaw unconformity in the western Canadian a breach and the lake emptied catastrophically the same day. Arctic was ca. 2.5 times greater than present-day thicknesses of the ac- The Melanie slump (FM3) is located 1.8 km upstream of the Charas tive layer, similar to the position of the thaw unconformity observed at slump, 1.3 km north of the Dempster highway. This is also a very large our study sites. The runoff from the Melanie slump also extends through L. Malone et al. / Chemical Geology 356 (2013) 38–49 41 amassivedebrisflow into the mud-dammed lake upstream of the Cha- bottles and in 2010 required up to 1 week to settle before the water ras slump debris flow. For both active slumps, the runoff over the slump could be extracted, filtered and transferred into 30 mL polyethylene floor can be through channelized drainage via rills during dry periods or bottles for geochemical analyses. This challenge was rectified in 2011 as sheet flow during rain events. In the Charas slump, the water can also and an active filtering system was used which allowed to filter the move downslope as a highly viscous mudflow during periods of inten- slump runoff samples within 1–2days. sive ground ice thaw. The slump runoff, stream water samples and melted ice-rich perma-

A stable slump is situated 500 m upstream of the Charas slump and frost were analyzed for major ions. The concentration of cations (Altot, 2+ 2+ 2+ + + 2+ is approximately the same size as the Melanie slump. Based on historical Astot,Ba ,Ca ,Fetot,Mg ,Mntot,Na ,K and Zn )wasdeter- air photographs, the slump has been stable since at least 1944. The mined on acidified samples (pH 2) using a Vista-Pro Inductively slump floor is colonized by willows and spruce vegetation, similar to Coupled Plasma Optical Emission Spectrometer (ICP-OES) at the Uni- 2− − − the surrounding valley bottoms. A small stream dissects the stable versity of Ottawa. The concentration of anions (SO4 ,NO3 and Cl ) slump floor and flows into the mud-dammed lake. was determined by ion chromatography (Dionex ICS-2100). Select samples (10% duplicate in each run) were run in duplicate and the analytical 3. Methodology reproducibility was 5%. Alkalinity titrations were performed in the field using a Hach® Digital titrator kit to the pH endpoint (b4.3) and results − 3.1. Sample collection presented with respect to HCO3 . The mineral saturation index (SI) of all samples was calculated using PHREEQC, a hydrogeochemical program The characteristics of runoff along active and stable debris slumps (Parkhurst and Appelo, 1999). The solution is undersaturated if SI and the associated impacts to the Stony Creek watershed were assessed is b1 at equilibrium, if SI = 1, or supersaturated if SI N 1. The results by sampling and analyzing the inorganic geochemistry of: i) slump run- of the geochemical composition and saturation index of all samples off from two active thaw slumps of different sizes (Charas and Melanie: are presented in Appendix A. slump runoff is defined as the water flowing over the slump floor and The suspended sediments in the slump runoff and impacted streams debris flow); ii) runoff from streams flowing over the scar surface were analyzed for particle size distribution, mineralogy and geochemis- of the stabilized slump; iii) streams impacted to various extents by try. The particle size distribution of select sediment samples in the de- slump runoff within the watershed; and iv) pristine tundra streams. bris flows (n = 8) was determined using a Microtrac S3500 laser To elucidate the source of solutes in the slump runoff, the ice-rich per- particle size analyzer at the University of Ottawa and reported in three mafrost exposed in the headwall of both slumps was also sampled grain size class: N63 μmsand,b63 μm silt N 2 μm, and b2 μm clay. Pow- and analyzed to determine its geochemical composition. Kokelj et al. der X-ray diffraction (XRD) was performed to determine the specific (2013) monitored hourly variations in turbidity and conductivity in mineralogy of the sediment (n = 8). The samples were prepared by pristine and slump impacted streams along Poor Man's creek, (PMC1 grinding them into a fine powder with an agate mortar and pestle. and PMC3, respectively; Fig. 2). Here, we supplement this dataset The XRD uses a Philips double goniometer X'Pert system and the analy- with hourly measurement of conductivity from the stream flowing sis was set with a scan step of 1 s and step size 0.02. Measurements over the stable slump (SS1) using a Hobo conductivity data logger were carried out between 10° and 85° and at 40 V/45 mA on a continu- (model U24-001). ous goniometer scan with the sample mount spinning. Select sediment Sample collection took place during the months of June and August samples (n = 4) were also analyzed by a Philips PW2400 X-Ray Fluo- in 2010 and 2011 (Fig. 2). Slump runoff samples (n =16in2010; rescence with a Rh tube and 30 position sample changer at the Univer- n = 27 in 2011) were collected from near the base of the headwall of sity of Ottawa. Philips SuperQ/Quantitative and SemiQ/Qualitative Charas and Melanie slumps, as well as at 3–4 locations along the active software (ver. 2.1D) provided quantitative analysis of the geochemical debris flow of both slumps, depending on accessibility to the rills. Stable composition of the samples. The grain size distribution and geochemical slump runoff samples were collected in 2011 at the confluence with the composition of the sediments are presented in Appendix A. mud-dammed lake. Water samples from pristine and impacted streams To further investigate the mineralogy of the ice-rich permafrost and within the Stony Creek watershed were collected twice during the sum- suspended sediments, the sediments were observed under scanning mer using helicopter support. Pristine stream samples (n = 17 in 2010; electron microscopy and energy dispersive spectrometry (SEM-EDS). n = 8 in 2011) were collected at locations where there is no contribu- The suspended sediments from the slump runoff were oven dried and tion of slump runoff to the stream and represent the background observed under a Jeol 6610LV scanning electron microscope equipped geochemistry of streams in the watershed. Impacted stream samples with an Oxford Inca large area SSD detector for qualitative chemical (n = 8 in 2010; n = 8 in 2011) were collected in stream segments sit- analysis by energy dispersive X-ray spectrometry at the University of uated downstream of a slump and as such receive direct slump runoff Ottawa. Imaging was done in both backscattered electron mode and contribution. Ice-rich permafrost samples (ca. 100 cm2) exposed along secondary electron mode. For observations, samples were mounted on headwalls were collected along two vertical sections at the Melanie sample holders by placing the material directly onto carbon tape, and thaw slump (30–100 cm and 530–600 cm below the surface) and were then carbon coated. along accessible sections in the lowermost 2 m of the headwall in both thaw slumps using an ice pick or chainsaw. The samples were 3.3. Statistical analysis melted in sealed plastic bags and all yielded excess water. The excess water was filtered within a few hours and stored in HDPE bottles until To evaluate differences in conductivity and the concentrations of analysis. major ions between slump runoff, impacted and pristine streams, we performed a paired t-test using Aable statistical software. The results 3.2. Analytical measurements are presented in Appendix A.

At each stream and slump runoff sampling location, pH and conduc- 4. Results tivity were measured in situ using a VWR SP90M5 symphony multi- meter calibrated daily with pH 4, 7 and 10 buffer solutions. Waters 4.1. Relation between in situ conductivity and total dissolved solids from impacted and pristine streams were collected unacidified in 30 mL pre-cleaned amber polyethylene bottles and filtered through To assess the potential effect of in situ leaching of sediments caused 0.45 μm pore diameter cellulose nitrate membrane filters. Due to their by the 1 to 7 days delay in filtering the slump runoff water samples, high sediment load, the slump runoff samples were collected in 1 L which could have caused the measured dissolved solutes to be higher 42 L. Malone et al. / Chemical Geology 356 (2013) 38–49 than at the time of sampling, we compared the in situ electrical conduc- facies, displayed a slope value of 0.64, similar to low salinity waters tivity (EC) measurements done at the time of sampling with the sum of (EC = 0.55 TDS; Drever, 1997), whereas the impacted streams (charac- 2+ 2+ 2− major ions determined in the laboratory (Ca ,Mg ,Fetot,Na,SO4 , terized also by a Ca(Mg)–SO4 geochemical facies) display a slope of − − Cl ,NO3 ). Electrical conductivity is dependent on the concentration 0.79, as expected for high SO4 waters (EC = 0.75 TDS; Drever, 1997). of ionic species in solution and is therefore expected to be correlated The 2010 and 2011 slump runoff samples fall along a slope similar to with the total dissolved solids (TDS) (Wood, 1976). Fig. 3 shows that the impacted streams. However, 7 out of the 27 samples in 2011 plot the electrical conductivity and total dissolved solids of the water samples above the regression line due to higher TDS values, suggesting that are highly correlated (r2 N 0.76) and the different water types display re- leaching occurred in the sampling bottle following the collection of the gression slope values that are consistent with their dissolved solute con- water samples and prior to filtering. These 7 slump runoff samples centration and geochemical facies. The pristine tundra streams, which were removed from the results presented below. have the lowest TDS and characterized by a Ca(Mg)–SO4 geochemical 4.2. Geochemistry of ice-rich permafrost exposed in headwalls

The ice-rich permafrost exposed in the headwalls of both thaw

slumps has a Ca(Mg)–SO4 geochemical facies with very low amount of − Fetot and HCO3 . The various ice-rich units exposed in the headwall of both slumps shows the same relative proportions of soluble ions (Fig. 4). However, a clear distinction in solute concentration is observed between near-surface and deep permafrost (i.e., permafrost above and below the early Holocene thaw unconformity, respectively) (Fig. 5). For example, at the Melanie slump, the concentration of Ca2+ ranges between 2 and 45 mg L−1 in the uppermost 1 m and the highest concentration is observed immediately below the ice-rich transient layer, consistent with other studies (i.e., Kokelj and Burn, 2003; Keller et al., 2007). By contrast, the Ca2+ concentration in permafrost below the early Holocene thaw unconformity, with values ranging between 300 to 730 mg L−1, is one order of magnitude higher. Although the near- surface permafrost was not sampled at the Charas slump (due to inac- cessibility), a similar range of ionic concentration is observed in the permafrost below the thaw unconformity as all samples were recovered along the base of the 25–30 m high headwall. A distinction in solute concentration is also observed between the clean and dirty ice units, although the difference is not as great as the one observed between the shallow and deep permafrost. The dirty ice units (i.e., reticulate ice, ice-rich diamicton) have the highest concentration of solutes (i.e. TDS = 2670 ± 750 mg L−1), whereas the massive ice units (i.e., ice veins) have the lowest solute concentrations (i.e. aver- −1 2− 2+ age TDS = 400 ± 290 mg L ). The concentrations of SO4 ,Ca and 2+ 2− Mg in the dirty ice units is up to 5 times higher (e.g. average SO4 : 1810 ± 530 mg L−1; Fig. 6) than the massive ice units (e.g. average 2− −1 − SO4 : 465 ± 410 mg L ). However, variation in Cl concentration 2− 2+ 2+ between ice types is less than that observed for SO4 ,Ca and Mg .

4.3. Geochemistry of slump runoff and streams

In situ measurements of pH and conductivity allow for an initial characterization of slump runoff, impacted and pristine streams. The pH values for the three groups range between 5.5 and 8.5 and no statistical differences are observed in the average values (Fig. 6). There exist a few outliers with pH values near 4.5 and these samples were measured from pristine streams in June receiving contribution from a melting late- lying snowbank. In situ conductivity measurements are one of the most clear indi- cators of slump runoff contribution to surface waters (Fig. 6). The average conductivity for active and stable slump runoff (Charas: 1220 ± 785 μScm−1; Melanie: 1175 ± 910 μScm−1;stable slump: 990 μScm−1) is nearly an order of magnitude greater (statistically significant, p b 0.001) than the average conductivity value for pristine tundra streams (135 ± 97 μScm−1). The conductivity of slump impacted streams (average of 265 ± 170 μScm−1)reflects the variable contribution of slump runoff and unimpacted waters and is statistically different to pristine streams (p = 0.017). The temporal variation in con- Fig. 3. Relation between total dissolve solids (TDS) versus electrical conductivity (EC) for ductivity in impacted streams (PMC3) and stream flowing over stable pristine streams, impacted streams and slump runoff. Seven 2011 slump runoff samples slump (SS1) both display subtle diurnal oscillations and the values pro- deviate from the TDS–EC relation (circled samples) and these were not included in the results as the sediment-rich water samples most likely leached in the 1 L sampling bottles gressively increase during the summer, a trend not observed in pristine prior to filtering. streams (PMC1) (Fig. 7). However, the higher conductivity values in the L. Malone et al. / Chemical Geology 356 (2013) 38–49 43

Fig. 5. A) Distribution of major cations and gravimetric water content in the uppermost 90 cm of permafrost in Melanie thaw slump. B) Distribution of major cations and gravimetric water content between 530 and 560 cm below the surface. Note the change in range in solute concentration.

begin to decrease with distance. At increasing distance from the head- Fig. 4. Schoeller diagram of A) average geochemical composition of various ground ice wall, the debris flow begins to receive contribution from pristine tundra types exposed in the headwall of Charas and Melanie thaw slumps; B) average geochemical composition of pristine tundra and impacted streams as well as slump runoff from streams, which dilutes the slump runoff ionic concentrations. By con- Charas and Melanie thaw slumps. trast, an increase in conductivity and dissolved ions is not observed at the Melanie mudflow because the mudflow is much smaller in length (800 m vs. 2000 m) and runoff was channelized in a rill, reducing inter- stream flowing over stable slump runoff were most likely due to its action between the surface water and debris flow materials (Fig. 2). proximity of a source of leaching sediments. The geochemical composition of impacted streams between the Cha- A Schoeller diagram of the average concentration of major ions for ras mudflow and the Peel River situated 22 km downstream allows to the slump runoff (active and stable), impacted streams and pristine assess the spatial extent of slump runoff contributions at the 1100 km2 streams shows a similar geochemical facies, that is a Ca(Mg)–SO4 facies watershed-scale (Fig. 8). The conductivity along Dempster Creek, (Fig. 4). With the exception of Fetot, where its concentration is highest in which receives contribution from the Charas and Melanie mudflows de- pristine streams, the slump runoff has the highest concentration of creases exponentially with dilution from 1800 to 200 μScm−1 at the specific dissolved solutes, whereas the pristine streams have the lowest. confluence with Poor Man's Creek, ca. 10 km downstream. At this con- 2− 2+ 2+ The concentrations of the dominant major ions (SO4 ,Ca and Mg ) fluence, the conductivity in Poor Man's and Stony Creek increases to are one order of magnitude higher in slump runoff than in pristine about 300 μScm−1, which suggests the contribution of other slump im- streams (Fig. 6). By contrast, variations in Cl− concentration between pacted sub-watersheds (i.e., Fig. 2). At all monitoring stations, the con- the three groups are not statistically different (Fig. 6). Potentially toxic ductivity of impacted streams was always higher than in pristine 2− solutes and dissolved metals such as iron and aluminum have concen- tundra streams. A similar trend is observed for SO4 concentration. tration b1mgL−1 whereas the concentration of arsenic and zinc is below detection for the majority of samples. 4.4. Physcial and geochemical characteristics of sediments in debris-flows Dissolved ions in runoff along the two mudflows vary with increas- and streams ing distance from the headwall (Fig. 8). Within the Charas mudflow, the conductivity and concentration of dissolved ions in runoff increase from The suspended sediment of the slump runoff reached values up to the headwall to approximately halfway down the mudflow and then 911 g L−1 whereas it is generally b0.1 g L−1 in pristine streams. In 44 L. Malone et al. / Chemical Geology 356 (2013) 38–49

Fig. 6. A) Box and whisker plots of pH and conductivity values of pristine tundra streams, impacted streams and slump runoff from Charas and Melanie thaw slumps. B) Box and whisker 2− 2+ − −1 plots of concentration of SO4 ,Ca and Cl (in mmol L ) in pristine tundra streams, impacted streams, slump runoff from Charas, Melanie thaw slump, and ground ice exposed in headwalls of both slumps. impacted streams, the suspended sediment load is one to three orders The suspended sediments in the Charas slump runoff were also ob- of magnitude higher than in pristine waters and varies diurnally by up served under SEM-EDS (Fig. 9). The SEM imaging and EDS analysis to an order of magnitude following the patterns of net radiation and revealed the presence of framboidal pyrite, barite, Fe-hydroxides, mica ground ice ablation in thaw slumps (Kokelj et al., 2013). Particle size and quartz. The framboidal pyrite is covered in a thin layer of organic analysis showed that sediments transported in the Charas and Melanie matter, which suggests that it is post-depositional. The Fe-hydroxides slump runoff consist of a well-sorted silt, with an average grain size showed a texture similar to the framboidal pyrite. Euhedral–subhedral value of 20.27 μm and 12.28 μm, respectively. barite crystals were found in all samples, which would explain the high XRD analysis indicates that the major minerals in the slump concentrations of Ba2+ detected in the XRF analysis. However, due to runoff consist of quartz, muscovite, ferroan clinochlore and albite. The the insoluble nature of barite (Ksp = 10−10)itisnotlikelytobethe 2− geochemistry of these sediments determined from XRF analysis is source of SO4 found in the slump runoff waters as the concentration −1 dominated by SiO, Fe2O3tot,K2O, MgO and CaO, which is similar to that of Ba in our samples was low (maximum value: 0.06 mg L ). None of measured from the till veneer (i.e., Lacelle et al., in press). the samples analyzed by SEM revealed the presence of anhydrite or L. Malone et al. / Chemical Geology 356 (2013) 38–49 45

Fig. 7. A) Speciﬁc conductivity in pristine (PMC1), slump impacted stream (PMC3) along Poor Man's Creek (data from Kokelj et al., 2013)andfromastreamﬂowing over a stable slump (SS1). Measurements were performed between June and August 2010. B) Daily rainfall measured at an automated meteorological station installed between Charas and Melanie slumps. gypsum and compositional mapping revealed no zones where high Ca2+ correlated with high SO42−. This might be attributed to the high solubil- ity of gypsum (Ksp = 10−4.60) and anhydrite (Ksp = 10−4.50)andthat the water samples were all undersaturated with respect to these minerals.

5. Discussion

5.1. Source of solutes in slump runoff Fig. 8. A) Variation in specific conductivity along Charas and Melanie debris flows. B) Var- 2− iations in specific conductivity and SO4 along a 22 km long stream gradient, from slump In the Stony Creek watershed, runoff derived from active and stable runoff near the headwall of Charas slump to impacted streams in Dempster Creek, Poor fl thaw slumps is characterized by a Ca(Mg)–SO4 geochemical facies with Man's Creek and in Stony Creek at the con uence with the Peel River. Dashed horizontal conductivity and solute concentrations approximately one order of lines represent the average value in pristine streams. C) Relative contribution of slump runoff from the Charas slump along Dempster Creek to the mouth of Stoney Creek. See magnitude higher than in pristine streams (Kokelj et al., 2013). Our Fig. 2 for location of streams sampling points. data shows that the elevated solute concentrations in the slump runoff are directly related to thawing of the ice-rich permafrost exposed in the headwalls of slumps. The processes responsible for the geochemical and other minor ions in our samples is very low (b1mgL−1) compared composition of slump runoff and streams can be assessed by examining to other permafrost regions where pyrite oxidation occurs (10 s to the ionic relations. Fig. 10 examines the potential of halite, gypsum, an- 100 s mg L−1;e.g.,Lacelle et al., 2007; Lacelle and Leveille, 2010). 2+ 2− hydrite, MgSO4, dolomite dissolution and pyrite oxidation as contribut- Conversely, a strong relation exists between Ca –SO4 and 2+ 2+ 2− ing processes. For example, the dissolution of gypsum can be expressed (Ca +Mg )–SO4 , suggestive of sulfate dissolution. The local geolo- by the following equation: gy consists of marine shales and siltstone and does not contain sulfate. The closest source of sulfate is located in the Mackenzie Valley east of → 2þ þ 2− þ : ð Þ CaSO42H2O Ca SO4 2H2O 1 the study area where two geological units contain sulfate, the Bear Rock Formation (with anhydrite) and the Saline River Formation 2+ 2− Assuming that Ca and SO4 in the waters originate from the dis- (with gypsum) (Michel, 1986). This sulfate source could have been solution of gypsum, their concentration (expressed in equivalent units) transported during the last glaciation by proglacial streams or the should plot close to the 1:1 line. The lack of relation between Na+–Cl− Laurentide Ice Sheet and incorporated in the local sediments. 2+ 2+ − and (Ca +Mg )–HCO3 suggest that halite and carbonate dissolu- However, sulfate minerals have not been detected by XRD analysis tion did not occur, respectively. Pyrite oxidation, although it remains a or observed under SEM-EDS. To elucidate this situation, we tested the possibility, most likely did not occur as revealed by the deviation of detection limit of XRD to identify gypsum in a sample. Five gypsum 2− Fetot and SO4 from the oxidation line. Further, oxidation of pyriferous standards were prepared to assess the presence of pure gypsum at con- shales releases a high amount of iron (2 mol of Fe2+) and acidity centrations of 0.5%, 1%, 2% and 5% mixed with pure quartz. The analysis (4 mol of H+) that can cause leaching of other minor ions from clay revealed that, even with 5% gypsum in the sample, the XRD analysis did + 2+ minerals, such as Al and Zn . However, the concentration of Fetot not detect gypsum. This suggests that if any of the analyzed sediment 46 L. Malone et al. / Chemical Geology 356 (2013) 38–49 samples contained b5% gypsum salts, they would not have been detected by the XRD analysis. Mixing of 10 g of sediments from two samples from the Charas mudflow with 40 mL of distilled water leached a −1 2− maximum of 1720 mg L of SO4 after 35 days, which represents a maximum gypsum content of 1.2% in the sediment sample. This concentration is well below the detection limit of XRD and could explain why gypsum was not detected by XRD analysis.

5.2. Thawing shallow versus deep permafrost and its impact on stream geochemistry

The distribution of ionic concentration with depth at the Melanie thaw slump showed that permafrost below the early Holocene thaw unconformity had solute concentrations nearly 100 times higher than those measured in the uppermost 1 m (Fig. 5). This strong contrast in ionic concentrations above and below the Holocene thermal unconformity is consistent with those measured in Late Pleistocene age frozen tills in the Mackenzie Delta region (i.e., Kokelj et al., 2002; Kokelj and Burn, 2003). Although it has been suggested that active layer disturbances and associated thawing of the solute-rich transient layer may modify the geochemistry of surface runoff and streams (i.e., Kokelj and Lewkowicz, 1999; Keller et al., 2007), the increase in ionic concentration in the shallow permafrost is subtle relative to thawing permafrost below the early Holocene thaw unconformity. This is because near-surface permafrost has experienced more frequent thawing which would lead to increased dissolution from infiltrating precipitation and removal by surface and subsurface runoff, in contrast to deeper permafrost. There- fore, the thawing of permafrost below the early Holocene thaw unconformity by intensive geomorphic disturbance will likely have a much greater geochemical impact on surface soils and water than a slight increase in active layer thickness. The mass-wasting associated with the growth of thaw slumps in the Stony Creek watershed is making im- mense quantities of previously frozen, highly weatherable sediments available for leaching and transport to hundreds of pristine streams and rivers (Kokelj et al., 2013). The dramatic increase in solute concentration below the early Holo- cene thaw unconformity suggests a headwall height threshold to observe the highest geochemical impacts in streams. Shallow slumps, such as the recently developed ones observed in the highlands of the Peel Plateau are unlikely to result in the same geochemical impacts as larger slumps with high headwalls that is exposing permafrost well below the thaw unconformity. The sedimentological contributions to streams with shallow or deep thaw slumps may be similar, but the volume of sediments and the magnitude of geomorphic impacts to fluvial systems are likely to also be much greater with large, mature thaw slumps. Large, deep features such as those we describe in this study, and that have developed throughout the Peel Plateau, likely sustained their growth due to sediment removal from the scar zone to the floodplain below during high intensity rain events (e.g., Lacelle et al., 2010; Kokelj et al., 2013).

5.3. Assessing the contribution of slump runoff to impacted streams Fig. 9. A) Scanning electron microscope image of framboidal pyrite grain found in slump runoff, Charas slump; B) Back-scatter electron image of framboidal pyrite grain found in The spatial variation in solute concentrations from Dempster Creek slump runoff, Charas slump; C) Scanning electron microscope image of euhedral pyrite to the mouth of Stony Creek shows a decreasing trend as the distance in slump runoff, Charas slump. from slumps increases. Geochemical analysis of water from different sources can be used to estimate the relative amount of those source water to a receiving body of water. Based on the geochemical analysis of slump runoff, impacted and pristine streams, there appears to be a The precision of this equation depends largely on the variability two component mixing between high-salinity slump runoff and low- in concentration of the two end-members and the difference in concen- salinity pristine streams. For a two-component mixing scenario, the tration between them. Therefore, the sensitivity of a two-component fraction of slump runoff (FSR) to impacted streams can be deﬁned as: mixing model can be determined from the following equation:

¼ ðÞ½−½=ðÞ½−½ ðÞ Sensitivity ¼ standard deviationSR=ðÞ½SR −½PS : ð3Þ fSR AF PS SR PS 2 where: [AF], [PS] and [SR] are the concentration of the ionic species in The two-component mixing model assumes that the value of slump the impacted streams (AF), pristine streams (PS) and slump runoff (SR). runoff remains temporally and spatially constant, which in slump L. Malone et al. / Chemical Geology 356 (2013) 38–49 47 impacted watershed would be unlikely. Therefore, the two-component of the Charas headwall (sample 11-LM-61), whereas the average value mixing equation is used to provide a first order estimate of the spatial of the pristine streams is used as the second end-member. Based on extent to which the slump runoff is impacting streams; the [SR] used Eq. (2), the relative contribution of slump runoff to the impacted for the slump runoff end-member is that from the Charas site and it water of Dempster Creek originating from the Charas slump ranges might be different for other slumps. Here, we calculate the relative con- from 100% at the base of the headwall and decreases to 7–12% at 6 km tribution of slump runoff from the Charas slump along Dempster Creek distance from the headwall (DC2), and reaches 1–3% at the confluence to the mouth of Stony Creek, a 22 km long segment (Fig. 8). Since the with Poor Man's Creek (PMC4). Considering that Dempster Creek mixing model also assumes that the end-members have a significant receives slump runoff contribution from only three active slumps, difference in concentration, we estimate the contribution of slump run- Charas, Melanie and an unnamed one, it is suggested that at 10–15 km 2− 2+ 2+ off to impacted streams using conductivity, SO4 ,Ca and Mg .The distance from a thaw slump, the geochemical impacts of slump runoff value of the slump runoff component is defined as measured at the base are still observable in the streams. At the mouth of Stony Creek, the

Fig. 10. Relation between major ion species in slump runoff, impacted and pristine tundra streams (undifferentiated) to assess the potential of halite, gypsum, anhydrite, MgSO4,dolomite dissolution and pyrite oxidation as contributing processes to the ionic concentration in the surface waters. 48 L. Malone et al. / Chemical Geology 356 (2013) 38–49 fraction of slump runoff increased to 3–5%, most likely due to the input References of nearby slump runoff (Fig. 2). Overall, the thaw slumps significantly impacts the geochemistry of streams, by increasing their solute load Bowden, W.B., Gooseff, M.N., Balser, A., Green, A., Peterson, B.J., Bradford, J., 2008. Sediment and nutrient delivery from thermokarst features in the foothills of the well above that of pristine streams along any reach of impacted streams. North Slope Alaska: potential impacts on headwater stream ecosystems. 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National Hydrology Research Institute, Environ- bances are typical of hundreds of other slumps of comparable magni- ment Canada, Saskatoon, Saskatchewan, pp. 75–88. tude that impact numerous streams throughout the lower Peel River Environment Canada, 2010. National climate data and information archive, Fort McPherson, NWT. http://climate.weatheroffice.gc.ca. watershed. Keller, K., Blum, J.D., Kling, G.W., 2007. Geochemistry of soils and streams on surfaces of Based on the results, the following conclusions can be reached: varying ages in Arctic Alaska. Arctic, Antarctic, and Alpine Research 39, 84–98. 1) The vertical distribution of ionic concentration of the ice-rich perma- Keller, K., Blum, J.D., Kling, G.W., 2010. Stream geochemistry as an indicator of increasing permafrost thaw depth in an arctic watershed. Chemical Geology 273, 76–81. frost exposed in the headwalls showed that permafrost stratigraphically Kokelj, S.V., Burn, C.R., 2003. Ground ice and soluble cations in near-surface permafrost, positioned below the early Holocene thaw unconformity has solute con- Inuvik, Northwest Territories, Canada. Permafrost and Periglacial Processes 14, 275–289. centrations nearly 100 times higher than those measured in the upper- Kokelj, S.V., Burn, C.R., 2005. Geochemistry of the active layer and near-surface perma- most 1–2 m; 2) Runoff derived from active and stable thaw slumps is frost, Mackenzie delta region, Northwest Territories, Canada. Canadian Journal of Earth Sciences 42, 37–48. characterized by Ca(Mg)–SO4 geochemical facies with conductivity Kokelj, S.V., Lewkowicz, A.G., 1999. Salinization of permafrost terrain due to natural and solute concentrations approximately one order of magnitude geomorphic disturbance, Fosheim Peninsula, Ellesmere Island. Arctic 52, 372–385. higher than in pristine streams. The elevated solute concentrations Kokelj, S.V., Smith, C.A.S., Burn, C.R., 2002. 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