Spatial and Temporal Assessment of Mercury and Organic Matter inThermokarst Affected Lakes of the Mackenzie Delta Uplands, NT,CanadaRamin Deison,*,† John P. Smol,‡ Steve V. Kokelj,§ Michael F. J. Pisaric,∥ Linda E. Kimpe,†

Alexandre J. Poulain,† Hamed Sanei,⊥ Joshua R. Thienpont,‡ and Jules M. Blais†

†Program for Chemical and Environmental Toxicology, Department of Biology, University of Ottawa, ON, K1N 6N5, Canada‡Paleoecological Environmental Assessment and Research Lab (PEARL), Department of Biology, Queen’s University, Kingston, ON,K7L 3N6, Canada§Cumulative Impact Monitoring Program, Aboriginal Affairs and Northern Development Canada, NWT Geoscience Office, P.O. Box1500, Yellowknife, NT, X1A 2R3, Canada∥Department of Geography and Environmental Studies, B353 Loeb Building, Carleton University, 1125 Colonel By Drive Ottawa,ON, K1S 5B6, Canada⊥Geological Survey of Canada-Calgary, 3303 33rd Street NW, Calgary, AB, T2L 2A7, Canada

*S Supporting Information

ABSTRACT: We examined dated sediment cores from 14 thermokarstaffected lakes in the Mackenzie Delta uplands, NT, Arctic Canada, using acase-control analysis to determine how retrogressive thaw slumpdevelopment from degrading permafrost affected the delivery of mercury(Hg) and organic carbon (OC) to lakes. We show that sediments fromthe lakes with retrogressive thaw slump development on their shorelines(slump-affected lakes) had higher sedimentation rates and lower total Hg(THg), methyl mercury (MeHg), and lower organic carbon concen-trations compared to lakes where thaw slumps were absent (referencelakes). There was no difference in focus-corrected Hg flux to sedimentsbetween reference lakes and slump-affected lakes, indicating that thelower sediment Hg concentration in slump-affected lakes was due todilution by rapid inorganic sedimentation in the slump-affected lakes.Sedimentation rates were inversely correlated with THg concentrations in sediments among the 14 lakes considered, andexplained 68% of the variance in THg concentration in surface sediment, further supporting the dilution hypothesis. We observedhigher S2 (algal-derived carbon) and particulate organic carbon (POC) concentrations in sediment profiles from reference lakesthan in slump lakes, likely because of dilution by inorganic siliciclastic matter in cores from slump-affected lakes. We concludethat retrogressive thaw slump development increases inorganic sedimentation in lakes, and decreases concentrations of organiccarbon and associated Hg and MeHg in sediments.

■ INTRODUCTION

About one-fifth of the terrestrial surface of the Earth isunderlain by some form of permafrost, including 22% of theland area in the Northern Hemisphere.1 In response to climatewarming, there has been a steady increase in permafrosttemperatures across the circumpolar Arctic including Alaska,2

western Canada,3 and Siberia.4 The degradation of permafrostcan rapidly modify polar landscapes 5 and thermokarstprocesses will continue to accelerate with climate warming.Consequent to thaw, significant changes in hydrology andorganic carbon and nutrient pathways are expected tosignificantly impact northern freshwater resources.1

Retrogressive thaw slumps are a form of degraded permafrostor thermokarst, which can impact ice-rich slopes adjacent tolakes,6 streams,7 and coastlines8 across the circumpolar North.

Recent studies show that these slumps result in increasedweathering of minerogenic deposits, which introduce materialsand ions into freshwater systems that were previously trappedin the frozen ice and soil.3,9 This process leads to changes inchemistry of freshwaters, including increases in concentrationsof base ions such as Na+, K+, Mg2+, SO4

−2, Cl−, and HCO3−,

and decreases in dissolved organic carbon (DOC).9 The extentto which thawing permafrost will change the amount of metallicand organic substances entering freshwater lakes is a significantgap in our knowledge of how northern ecosystems will respond

Received: February 28, 2012Revised: June 25, 2012Accepted: July 27, 2012Published: July 27, 2012

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to climate warming, although geochemical patterns in severalnorthern rivers suggest that deeper flowpaths can increase theamount and age of DOC from terrestrial to aquaticenvironments.10

There is concern that some of these thermokarst processesthat alter the flow of DOC, nutrients, soil temperature, andmicrobial processes may have a number of effects on themercury cycle.11 For example, recent increases in algalproductivity from pronounced climate warming in permafrostregions may increase contaminant delivery to lake sedi-ments,11−16 and may be one explanation for rising mercurylevels in Mackenzie River burbot (Lota lota).15 Several havesuggested that the delivery of mercury to aquatic sedimentsmay be strongly influenced by variations in the source, type,and quantity of autochthonous (produced within the lake)versus allochthonous (produced externally) organic matter.17

Likewise, some studies on northern lakes have reportedsignificant associations between temporal trends of mercuryand proxies for autochthonous organic matter.13−15 Thesestudies suggested that algal-derived carbon is primarilyresponsible for mercury scavenging in the water column andthis effect is likely on the rise in recent decades due to climate-related increases in algal productivity. However, a recentmultisite comparative analysis across the Canadian Arctic byKirk et al.18 suggested that algal scavenging does not alwaysexplain Hg deposition to sediments. The algal scavenginghypothesis has been particularly contentious18−20 because itholds direct industrial emission responsible for only a fraction(less than 40%) of increased mercury in the 20th century insediment archives.13,14 Despite the controversy, the algalscavenging hypothesis may offer one explanation for mercuryincreases in some Arctic lake sediment archives despitedecreasing atmospheric mercury concentrations.Here we assessed mercury sedimentation in lakes with

catchments affected by thawing permafrost in a case-controlanalysis of lakes where retrogressive thaw slumps are presentand absent (Figure 1). This comparative study design isintended to provide an indication of the influence of thawslump development on mercury and organic carbon delivery tolake sediments in remote thermokarst impacted landscapes. Inaddition, we investigated the correlation between Hg and S2carbon as predicted by the algal scavenging hypothesis bothspatially among lakes, as well as temporally within lakesediment profiles.

■ METHODSStudy Area. We examined sediment cores of fourteen lakes

along a transect situated east of the Mackenzie Delta, fromInuvik to Richards Island (Figure 1). These lakes were chosenfollowing an analysis of aerial photographs and field surveys.3

Seven study lakes have retrogressive thaw slumps on theirshorelines (i.e., degraded permafrost, classified here as ‘slump-affected lakes’) and the other seven lakes are in undisturbedcatchments, referred to as “reference lakes”. The location andcharacteristics of the study lakes are given in Table 1, anddetailed water chemistry is in Supporting Information Table S1.A detailed analysis of lake sediment profiles is performed on asubset of 8 of these lake sediment cores.Sample Collection and Preparation. Sediment cores

measuring 35−40 cm in length were recovered from the centerof the fourteen lakes in the summers of 2007−2008 using aGlew gravity corer and Lexan core tubes. All the cores weresubsampled at 0.25 cm intervals between 0 and 15 cm and at

0.5 cm intervals from 15 cm to the bottom with a vertical coreextruder.21 The sediment samples were placed into airtightcentrifuge tubes and plastic bags, placed on ice, and transportedin a dark cooler to the laboratory the same day. Sediments werethen stored in a freezer for future analysis. The sediments weresubsampled and freeze-dried for 2 to 3 days, and the driedsediments were ground for radiometric analysis, THg, MeHg,and Rock-Eval analysis.

210Pb Inventories and Sedimentation Rates. Sedimentcores were radiometrically dated using gamma (γ) spectrom-etry. The fourteen sediment cores were analyzed for the activityof 210Pb, 137Cs and 226Ra. Analysis of 210Pb was performed at12−15 selected depth intervals in the sediment cores todetermine the sediment age, and the sediment accumulationrate.22 Detailed methods are provided in SupportingInformation.

THg Analysis. Homogenized freeze-dried sediment sampleswere analyzed for THg using an automatic mercury analyzerbased on thermal decomposition, dual step gold amalgamationand detection via cold-vapor atomic absorption using a Sp-3Dmercury analyzer (Nippon Instrument Corp.) with detectionlimit of 0.01 ng per sample size. Sample mass ranged between25 and 30 mg. The accuracy of our analysis was estimated byrunning blanks and spikes as well as two reference materialsduring the analytical procedure. Spikes from a stock of MercuryReference Solution (certified 1000 μg g−1 ± 1%; FisherScientific CSM114−100) were brought to a concentration of

Figure 1. Study lakes in the Mackenzie Delta near Inuvik. Lakesdenoted “a” are reference lakes, and those denoted “b” are slump-affected lakes. Inset shows a photograph of a thaw slump at Lake 14b.

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50 ng g−1 and were tested every 5 samples. The averagerecovery of the spikes was 102% ± 5 standard deviation (SD),(n = 12). According to procedural blanks, no contaminationwas observed during THg analysis. Reference materials weretested every 4−5 samples and average percentage recovery forMESS-3 (Marine Sediment Certified Reference Materials fromNational Research Council, Ottawa) with concentration of 91± 9 ng g−1 was 96% ± 4 (SD) (n = 35).MeHg Analysis. Methylmercury concentration in the

homogenized freeze-dried sample sediments were determinedby capillary gas chromatography coupled with atomicfluorescence spectrometry (GC-AFS) as described by Cai etal.,23 with a detection limit of 0.02 ng per sample size. Samplemass ranged between 0.4 and 1 g. Further details are providedin Supporting Information.Organic Geochemistry (Rock-Eval Analyses). We

applied Rock-Eval 6 (Vinci Technologies, France) for thequantitative and qualitative study of organic matter (OM) inthe recent sediments. The Rock-Eval 6 method consists ofpyrolysis (under inert conditions) and then oxidation, bothperforming a temperature programmed heating of the sedi-ments (30−50 mg) at a rate of 25 °C per minute. Detailedmethods are provided in Supporting Information.Inferred Chlorophyll a in Sediments. Sedimentary

chlorophyll a content was inferred using visible reflectancespectroscopy (VRS), a method that provides an indication ofoverall lake production.24 Samples were freeze-dried, sieved(125 μm), and analyzed on a FOSS NIRSystems Model 6500rapid content analyzer. The portion of the electromagneticspectrum from 650 to 700 nm was analyzed in order to detectboth chlorophyll a and its derivatives (pheophytin a andpheophorbide a), allowing VRS-chlorophyll a to track changesin production over time and not strictly diagenesis.24,25

Statistical Analyses. All data were analyzed using OriginLab 7 software (Origin Lab Corporation MA, USA). Simplelinear regressions analysis was used to analyze the relationshipsbetween THg, S2, inferred chlorophyll a, and sedimentation

rates. Student t test was used to test the difference betweenslump-affected and reference lakes with respect to THg, S2,inferred chlorophyll a, and sedimentation rates.

■ RESULTS AND DISCUSSION210Pb Dating and Sedimentation. 210Pb activities in most

cores declined log−linearly with depth until reaching back-ground, which was determined by secular equilibrium with226Ra (Supporting Information Figure S1). Several coresshowed disturbance in their excess 210Pb profiles, particularlyin 9b and 14b, suggesting fluctuations in sedimentation at thoseintervals.Focus-corrected sedimentation rates in slump-affected lakes

(269 ± 66 SD g m−2 year−1) were more than double those inreference lakes (120 ± 37 g m−2 year−1) indicating higherincoming material to lakes disturbed by retrogressive thawslumps. Higher sedimentation rates in slump-affected lakes aredue to sediment inputs from growing thaw slumps, which haveincreased in activity during the late 20th Century in associationwith rapidly warming air and permafrost temperatures.6

Inorganic sedimentation rates largely accounted for differ-ences in sedimentation rates, with 253 ± 65 SD g m−2 year−1 inslump-affected lakes compared to 104 ± 34 SD g m−2 year−1

(t(2),12 = 5.32, p < 0.001). The focus-corrected TOC flux wassimilar between slump-affected lakes (16.3 ± 9.4 SD g m−2

year−1) and reference lakes (15.6 ± 6.8 SD g m−2 year−1, t(2),12= 1.78, p = 0.85), indicating fairly uniform organicsedimentation rates regardless of whether the lakes weredisturbed by retrogressive thaw slumps or not. Likewise, fluxesof S1, S2, and residual carbon (RC) showed no differencebetween slump-affected lakes and reference lakes (S1, slump-affected = 0.11 ± 0.12 SD g m−2 year−1, reference = 0.18 ± 0.12SD g m−2 year−1, t(2),12 = 1.15, p = 0.27; S2, slump-affected =3.09 ± 2.27 SD g m−2 year−1, reference = 3.07 ± 1.89 g m−2

year−1, t(2),12 = 0.012, p = 0.99; RC, slump-affected = 11.6 ±6.22 g m−2 year−1, reference = 10.9 ± 4.1 SD g m−2 year−1,t(2),12 = 0.23, p = 0.81). This apparent “dilution” of organic

Table 1. Location and Physical Characteristics of the 14 Study Lakes Located in the Uplands Directly to the East of theMackenzie River Delta NWT, Canadaa

lake latitude (°N) longitude (°W) Aob (ha) CAc (ha) ASd (ha) slump status Zm

e (m) FFf average sed. Rateg (g m−2 year−1)

I. reference lakes2a 68°50′ 26.7″ 133° 66′ 07.1″ 2 17.2 6.1 2.30 675a 68°33′4.20″ 133°38′23.39″ 2.9 20.9 10.9 0.96 1326a 68°35′25.73″ 133°38′33.40″ 3.6 19.7 2.3 0.94 1217a 68°36′18.47″ 133°35′27.14″ 1.4 18.1 2.7 2.45 349a 68° 58′ 05.8″ 133° 53′ 53.0″ 3.1 29.3 2.7 4.57 2014a 68° 31′ 02.7″ 133° 44′ 55.4″ 3.4 33.5 7.5 0.87 21036a 68° 30′ 10.4″ 133° 42′ 02.2″ 0.8 6.6 9.5 0.77 119

II. slump-affected lakes2b 68° 50′ 72.8″ 133° 67′ 03.6″ 4.9 15.9 0.95 stable 3.4 0.46 5565b 68°32′14.96″ 133°39′27.41″ 2.8 27.7 2.02 stable 9 5.00 616b 68°35′17.78″ 133°38′16.95″ 1.2 7.5 0.81 stable 2 0.55 5287b 68°36′32.23″ 133°35′12.77″ 3.1 34.7 1.13 active 5 0.89 2719b 68° 58′ 14.1″ 133° 53′ 59.3″ 3.6 7.2 2.5 active 3 1.18 24014b 68° 31′ 02.7″ 133° 44′ 55.4″ 9.2 45.1 2.4 active 10.5 0.05 659436b 68° 30′ 09.6″ 133° 42′ 05.2″ 3.9 24.4 4.9 stable 7.4 0.51 290

aMorphometric data and slump activity were determined from air photo analyses and ground surveys undertaken during 2001−2005. Seven studylakes have retrogressive thaw slumps (i.e. degraded permafrost), and the other seven lakes are in undisturbed (reference) catchments. Slump-affectedlakes are denoted by the letter b, whereas reference lakes are identified by the letter a.9 bAo = lake surface area. cCA = catchment area. dAS = area ofretrogressive thaw slump. eZm = maximum depth of lake. fFF = focusing factor. gSedimentation rate was calculated using the CFCS model (Appleby2001).

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carbon by inorganic sediments was further evident by theinverse correlation between TOC concentration and sedimen-tation rate (r = −0.66, F1,12 = 9.3, p = 0.01, SupportingInformation Figure S2A). Slump-affected lakes had significantlylower TOC concentration and higher sedimentation rates,suggesting that accelerated deposition of inorganic sedimentsfrom retrogressive thaw slumps diluted the sedimentary organiccarbon. This inorganic sediment dilution effect was evident forother organic fractions as well (e.g., S2, Supporting InformationFigure S2B).Mercury in Surface Sediments. Mercury concentration in

surface sediments was significantly correlated with total organiccarbon (TOC) (r = 0.73, p < 0.05, Supporting InformationFigure S2A), and S2 carbon (r = 0.60, p < 0.05, SupportingInformation Figure S2B) but not as strongly related to inferredchlorophyll a (r = 0.50, p = 0.06, Supporting InformationFigure S2C). The strong relationship between Hg, TOC and S2indicates their important role in adsorbing Hg and depositing itto sediments. Likewise, there was a significant correlationbetween Hg and RC in surface sediments (r = 0.77, p = 0.001).RC partially represents the quantity of thermally resistantorganic material, which only decomposes during high temper-ature oxidation such as in forest fires and domestic woodburning, which may partly explain the correlation between theHg and RC.26

Likewise, MeHg in surface sediments was significantlycorrelated to TOC (r = 0.78, p < 0.01, Supporting InformationFigure S3), S1 carbon (r = 0.71, p < 0.01), and S2 carbon (r =0.79, p < 0.05), indicating an important association betweenMeHg formation and organic carbon content, which is in turninfluenced by the permafrost disturbance status of these lakes.Taken together, the results suggest that dilution of organicmatter by rapid influx of inorganic sedimentation in thaw slumplakes coincides with reduced mercury and MeHg concen-trations in these sediments. However the proportion of MeHgto THg was similar between reference lakes (0.0046 ± 0.037SD) and slump-affected lakes (0.0036 ± 0.0015 SD) indicatingno difference in the ability to generate MeHg among sedimenttypes (t10 = 0.6, p = 0.55).Mercury concentrations in surface sediments in reference

lakes were higher (0.12−0.30 μg/g dw) than slump-affectedlakes (0.07−0.12 μg/g dw), (t = 4.99, p < 0.01). A stronginverse correlation was found between mercury concentrationand sedimentation rate in our study lakes (R2 = 0.68, P < 0.01,Figure 2C), showing the influence of inorganic sedimentdilution on mercury. Mercury concentrations in clays collectedwithin the thaw slump scars were very low (75 ng g−1 ± 67 SD,n = 22) when compared to mercury concentrations in lakesediments (Figure 2C), so the influx of these inorganicmaterials following thaw slump development would likely

Figure 2. Sedimentation rate in sediment cores from 14 study lakes (focus corrected to 50 Bq m−2 year−1 excess 210Pb) plotted against (A) the totalorganic carbon in surface sediments, (B) the S2 (algal derived) carbon in surface sediments and (C) Hg concentration. Note that sedimentation ratein slump-affected lakes tends to be higher than in reference lakes, and organic carbon and Hg is lower and ‘diluted’ by the higher inorganic sedimentflux seen in slump-affected lakes.

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Figure 3. Temporal distribution of sediment mercury (ng g−1 DW), TOC (%), algal derived (S2) carbon (mg HC g−1 DW), inferred chlorophyll a(μg g−1 DW), sedimentation rate (focus corrected to 50 Bq m−2 year−1 excess 210Pb), percent water (%), and RC plotted against sediment coresdepth in 4 reference lakes in the Mackenzie Delta.

Figure 4. The temporal distribution of sediment mercury (ng g−1 DW), TOC (%), algal derived (S2) carbon (mg HC g−1 DW), inferred chlorophylla (μg g−1 DW), sedimentation rate (focus corrected to 50 Bq m−2 year−1 excess 210Pb), percent water (%), and RC plotted against sediment coresdepth in 4 slump-affected lakes in the Mackenzie Delta.

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reduce the overall mercury concentration in sediment. Withinthe two lake categories, mercury concentration was correlatedwith sedimentation rate in reference lakes (R2 = 0.89, p < 0.05,Figure 2C) but no correlation was found for slump-affectedlakes, which might have resulted due to variable thaw slumpactivity among slump-affected lakes. Mercury concentrations inslump-affected lakes were comparable to other studies in theCanadian Arctic,13,15,27 but were generally higher in referencelakes. There was no difference in focus corrected mercury fluxbetween reference lakes (25.35 ± 3.01 μg m−2 year−1) andslump-affected lakes (26.61 ± 6.92 μg m−2 year−1), indicatingthat changes to limnetic conditions from thaw slump activity(e.g., higher ionic strength, lower DOC, Supporting Informa-tion Table S1) did not significantly alter Hg deposition rates tolake sediments, and further supported our contention that thedifference in Hg concentration in sediments between lake typeswas due to dilution by rapid inorganic sedimentation in theslump-affected lakes.MeHg in surface sediments (0−5 cm) from all six reference

lakes (0.94 ± 0.54 ng g−1d.w.) was substantially higher than inslump-affected lakes (0.35 ± 0.20 ng g−1dw, t(2), 13 = 3.28, p =0.01). We also found a strong correlation between THg andMeHg in surface sediments (r = 0.88, p = 0.01, SupportingInformation Figure S2), suggesting that Hg methylation may beHg-limited.Mercury in sediment profiles. In profile, sedimentary Hg

and TOC in reference lakes tended to show modest increasesto the surface (Figure 3), though not to the same extent asthose described in other regions closer to active Hg emissions.Enrichment factors (EF) ranged from 1.27 to 2.50 in lakes 14aand 36a, respectively, which was close to those foundthroughout the Arctic by Kirk et al 18 and others13,27 (∼1.1−2.6).Mercury concentrations in slump-affected lakes did not show

pronounced surface enrichment (Figure 4) but were com-parable to other studies in the Canadian Arctic of lakes similarto our reference lakes,13,15,27 whereas reference lakes had higherHg than their slump-affected counterparts in all our study lakesexcept 14a/b.Similarities between vertical profiles of TOC, S2 carbon,

inferred chlorophyll a and THg were generally apparent withinthe sediment cores of reference lakes (Figure 3). For example,Lake 2a showed little fluctuation in these four constituents,whereas Lake 9a and 36a showed surface enrichment in all ofthem. Lake 14a had moderate surface enrichment of TOC, S2,and inferred chlorophyll a and Hg surface enrichment.Similarly, slump-affected lakes showed less surface enrichmentin Hg, TOC, S2, and inferred chlorophyll a in Lakes 2b, 9b, and36b compared to reference lakes, whereas 14b showed thelowest surface Hg enrichment (Figure 4). Enrichment factorsfor Hg were 1.16−1.48 and were similar to values reported inother Arctic regions (1.1−2.6).13,18,27These results corroborated the analysis of surface sediments

that showed a dilution of sedimentary organic matter byinorganic siliciclastic materials derived from thaw slumps inthese thermokarst-affected lakes. S2 concentrations in theselakes were generally similar to those reported in other Arcticsediment cores (0.3−15 mg g−1),13−15 however Lakes 9a and14a had 2 to 3 times higher S2 concentrations (41 and 31 mgg−1 respectively). It is unlikely that autochthonous organicproduction is higher in these lakes, given their oligotrophicstatus.

Sedimentation rates were variable in many of these cores andwere occasionally observed to spike at intervals, often with acorresponding drop in sedimentary organic content and watercontent (Figures 3 and 4). This might be expected due to asudden dilution of organic matter from an influx of inorganicsediment as also recorded in the spatial analysis of surfacesediments (Figure 2). Examples are evident in cores fromreference lakes and slump-affected lakes: Lakes 9a (at 7−12cm), 36a (5−10 cm), 2b (3−7 cm), and 14b (0−1.5 cm). S2carbon was inversely correlated with 210Pb-derived sedimenta-tion rates (based on the constant rate of supply model) in someof these cores, particularly those where sedimentation ratesexceeded 1,000 g m−2 year−1 (Supporting Information FigureS4). These correlations may have resulted from organic matterdilution during periods of rapid inorganic sedimentation suchas those that occur from retrogressive thaw slumping (Figure2).

Relationship between Hg and S2. Significant correla-tions between Hg and S2 in previous studies13,14,16,17,19

emphasized the role of algal scavenging of mercury. Otherstudies observed that Hg was only correlated with S2 in asubset of sediment cores, suggesting that algal scavenging maynot explain Hg deposition to sediments in some cases.18 Weshowed a significant correlation between Hg and S2 across allsurface sediments (Supporting Information Figure S2B);however, we only observed positive correlations between Hgand S2 in 5 of our 8 sediment profiles (Supporting InformationFigure S5). These results suggest that other factors besides algalscavenging affect Hg delivery to sediments, which may includedilution by inorganic sedimentation and catchment erosion(Figure 2). Further in-depth studies are warranted tounderstand the importance and mechanism of autochthonousorganic production, or algal scavenging, in influencing Hgdelivery to sediments.

S2 Carbon and Inferred Chlorophyll a in SurfaceSediments. Average inferred chlorophyll a concentration inthe surface sediments of our reference lakes (0.034 ± 0.017 mgg−1 DW) was slightly higher than the slump-affected lakes(0.019 ± 0.019 017 mg g−1 DW) but was not statisticallydifferent (t = 1.70, p > 0.05). S2 carbon in surface sedimentswas significantly correlated with inferred chlorophyll a (r =0.83, p < 0.0002, Supporting Information Figure S6). Thiswould be expected because S2 fraction of organic carbon isprimarily of algal origin and sedimentary inferred chlorophyll ais also typically of autochthonous origin (ref 24 and paperscited therein), though both of these constituents should also besusceptible to some degree of degradation/modification insediments.24,28

S2 and inferred chlorophyll a were significantly correlated in7 of 8 lake sediment profiles (r = 0.44−0.94, p < 0.05) but nocorrelation was observed in lake 2a (r = 0.47, p > 0.05)(Supporting Information Figure S7). Hg and inferredchlorophyll a were significantly correlated in 5 of 8 lakes (r =0.72−0.91, p < 0.005) and not correlated in the other 3 lakes(14a r = 0.09, p > 0.05; 2a r = 0.18, p > 0.005; and 14b r = 0.45,p > 0.005).

Organic Carbon in Surface Sediments. Source andcomposition of organic matter is presented by plotting thequantity of S2 as a function of TOC (Supporting InformationFigure S8), representing the proportion of hydrogen-richorganic matter dominantly composed of autochthonous(mainly algal-derived) matter relative to the TOC in thesediments.29 Kerogens showing highest S2/TOC are classified

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as Type I (higher algal content), with Type II and Type III(increasing terrestrial organic content) having descending S2/TOC values.30 Sedimentary OM in the majority of surfacesediments in all lakes were classified as Type II and III kerogen,whether or not they were disturbed by thaw slumps(Supporting Information Figure S8), suggesting a predom-inance of allochthonous organic matter to sediments of all lakesregardless of permafrost status.Similarly, the organic matter composition may be further

characterized by hydrogen content as shown by hydrogen index(HI) and oxygen index (OI) parameters. HI is calculated bynormalizing the quantity of S2 to TOC (S2/TOC × 100), andis proportional to the kerogen elemental H/C ratio.31 OI,calculated as S3/TOC × 100, is proportional to the elementalO/C ratio of the kerogen.31 Well-preserved kerogens ofdominantly autochthonous (algal) origin are known to haveelevated HI values (hydrogen-rich; Type I and II kerogen)relative to terrestrially derived, and/or degraded and reworkedorganic matter.16,28,32 All surface sediments plotted in Type IIIkerogen space on the Van Krevelen diagram (SupportingInformation Figure S9), further corroborating evidence that themajority of organic matter in these sediments is ofallochthonous origin or has been exposed to a large degree ofdegradation within the water column.Our results show that sediments from lakes affected by thaw

slumps contained lower concentrations of TOC and algalderived organic carbon (S2), lower mercury and MeHgconcentrations, as well as higher total and inorganicsedimentation rates, which likely explain the dilution of organicmaterials and mercury in lakes where thaw slumps are present.Likewise our results showed significant correlations betweenmercury and S2 concentrations in 5 of the 8 lake sedimentcores, suggesting that algal-derived materials may be sources ofHg to sediments, but other factors such as inorganicsedimentation rate and catchment erosion are likely mediatingthis effect. The effect of retrogressive thaw slump developmenton the mercury cycle should be compared with othermanifestations of thermokarst, such as peat subsidence,33

which is much more likely to release significant quantities ofHg and MeHg to the aquatic environment.

■ ASSOCIATED CONTENT

*S Supporting InformationAnalytical methods for Rock Eval analysis, radiometricmeasurements of lake sediments, and MeHg analysis of lakesediments, a table showing limnological characteristics of the 14lakes considered in this study, and figures showing 210Pb and226Ra profiles for the 8 sediment cores, correlations between Hgin surface sediments and TOC, S2 carbon, and inferredchlorophyll a, the correlation between MeHg concentration insurface sediment and TOC, the correlation between S2 carbonand sedimentation rate in the 8 sediment core profiles, thecorrelation between Hg concentration between Hg and S2 inthe 8 sediment core profiles, the correlation between S2 insurface sediments and inferred chlorophyll a in surfacesediments of 14 lakes, correlations between S2 carbon andinferred chlorophyll a in the 8 sediment core profiles, kerogentype in surface sediments of the 14 lakes using S2:TOCgraph,29 and kerogen type in 8 sediment core profiles usingHI:OI pseudo Van Krevelen diagram. This information isavailable free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: Jules.Blais@uottawa.ca.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis study was funded by a Natural Sciences and EngineeringResearch Council (Canada) Strategic Projects grant and aNorthern Contaminants Program Grant to J.M.B., J.P.S., andM.F.J.P. We thank the Geological Survey of Canada forscientific and analytical support. Logistical support wasprovided by the Polar Continental Shelf Program (PCSP).We thank the Cumulative Impact Monitoring Program ofAboriginal Affairs and Northern Development Canada foradditional research support, and Peter deMontigny for fieldassistance.

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