Article pubs.acs.org/est
Spatial and Temporal Assessment of Mercury and Organic Matter in Thermokarst Affected Lakes of the Mackenzie Delta Uplands, NT, Canada Ramin 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. Box 1500, 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 thermokarst affected lakes in the Mackenzie Delta uplands, NT, Arctic Canada, using a case-control analysis to determine how retrogressive thaw slump development from degrading permafrost affected the delivery of mercury (Hg) and organic carbon (OC) to lakes. We show that sediments from the 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 concentrations compared to lakes where thaw slumps were absent (reference lakes). There was no difference in focus-corrected Hg flux to sediments between reference lakes and slump-affected lakes, indicating that the lower sediment Hg concentration in slump-affected lakes was due to dilution by rapid inorganic sedimentation in the slump-affected lakes. Sedimentation rates were inversely correlated with THg concentrations in sediments among the 14 lakes considered, and explained 68% of the variance in THg concentration in surface sediment, further supporting the dilution hypothesis. We observed higher S2 (algal-derived carbon) and particulate organic carbon (POC) concentrations in sediment profiles from reference lakes than in slump lakes, likely because of dilution by inorganic siliciclastic matter in cores from slump-affected lakes. We conclude that retrogressive thaw slump development increases inorganic sedimentation in lakes, and decreases concentrations of organic carbon and associated Hg and MeHg in sediments.
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INTRODUCTION About one-fifth of the terrestrial surface of the Earth is underlain by some form of permafrost, including 22% of the land area in the Northern Hemisphere.1 In response to climate warming, there has been a steady increase in permafrost temperatures across the circumpolar Arctic including Alaska,2 western Canada,3 and Siberia.4 The degradation of permafrost can rapidly modify polar landscapes 5 and thermokarst processes will continue to accelerate with climate warming. Consequent to thaw, significant changes in hydrology and organic carbon and nutrient pathways are expected to significantly impact northern freshwater resources.1 Retrogressive thaw slumps are a form of degraded permafrost or thermokarst, which can impact ice-rich slopes adjacent to lakes,6 streams,7 and coastlines8 across the circumpolar North. © 2012 American Chemical Society
Recent studies show that these slumps result in increased weathering of minerogenic deposits, which introduce materials and ions into freshwater systems that were previously trapped in the frozen ice and soil.3,9 This process leads to changes in chemistry of freshwaters, including increases in concentrations of base ions such as Na+, K+, Mg2+, SO4−2, Cl−, and HCO3−, and decreases in dissolved organic carbon (DOC).9 The extent to which thawing permafrost will change the amount of metallic and organic substances entering freshwater lakes is a significant gap in our knowledge of how northern ecosystems will respond Received: Revised: Accepted: Published: 8748
February 28, 2012 June 25, 2012 July 27, 2012 July 27, 2012 dx.doi.org/10.1021/es300798w | Environ. Sci. Technol. 2012, 46, 8748−8755
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to climate warming, although geochemical patterns in several northern rivers suggest that deeper flowpaths can increase the amount and age of DOC from terrestrial to aquatic environments.10 There is concern that some of these thermokarst processes that alter the flow of DOC, nutrients, soil temperature, and microbial processes may have a number of effects on the mercury cycle.11 For example, recent increases in algal productivity from pronounced climate warming in permafrost regions may increase contaminant delivery to lake sediments,11−16 and may be one explanation for rising mercury levels in Mackenzie River burbot (Lota lota).15 Several have suggested that the delivery of mercury to aquatic sediments may 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 reported significant associations between temporal trends of mercury and proxies for autochthonous organic matter.13−15 These studies suggested that algal-derived carbon is primarily responsible for mercury scavenging in the water column and this effect is likely on the rise in recent decades due to climaterelated increases in algal productivity. However, a recent multisite comparative analysis across the Canadian Arctic by Kirk et al.18 suggested that algal scavenging does not always explain Hg deposition to sediments. The algal scavenging hypothesis has been particularly contentious18−20 because it holds direct industrial emission responsible for only a fraction (less than 40%) of increased mercury in the 20th century in sediment archives.13,14 Despite the controversy, the algal scavenging hypothesis may offer one explanation for mercury increases in some Arctic lake sediment archives despite decreasing atmospheric mercury concentrations. Here we assessed mercury sedimentation in lakes with catchments affected by thawing permafrost in a case-control analysis of lakes where retrogressive thaw slumps are present and absent (Figure 1). This comparative study design is intended to provide an indication of the influence of thaw slump development on mercury and organic carbon delivery to lake sediments in remote thermokarst impacted landscapes. In addition, we investigated the correlation between Hg and S2 carbon as predicted by the algal scavenging hypothesis both spatially among lakes, as well as temporally within lake sediment profiles.
Figure 1. Study lakes in the Mackenzie Delta near Inuvik. Lakes denoted “a” are reference lakes, and those denoted “b” are slumpaffected lakes. Inset shows a photograph of a thaw slump at Lake 14b.
0.5 cm intervals from 15 cm to the bottom with a vertical core extruder.21 The sediment samples were placed into airtight centrifuge tubes and plastic bags, placed on ice, and transported in a dark cooler to the laboratory the same day. Sediments were then stored in a freezer for future analysis. The sediments were subsampled and freeze-dried for 2 to 3 days, and the dried sediments were ground for radiometric analysis, THg, MeHg, and Rock-Eval analysis. 210 Pb Inventories and Sedimentation Rates. Sediment cores were radiometrically dated using gamma (γ) spectrometry. The fourteen sediment cores were analyzed for the activity of 210Pb, 137Cs and 226Ra. Analysis of 210Pb was performed at 12−15 selected depth intervals in the sediment cores to determine the sediment age, and the sediment accumulation rate. 22 Detailed methods are provided in Supporting Information. THg Analysis. Homogenized freeze-dried sediment samples were analyzed for THg using an automatic mercury analyzer based on thermal decomposition, dual step gold amalgamation and detection via cold-vapor atomic absorption using a Sp-3D mercury analyzer (Nippon Instrument Corp.) with detection limit of 0.01 ng per sample size. Sample mass ranged between 25 and 30 mg. The accuracy of our analysis was estimated by running blanks and spikes as well as two reference materials during the analytical procedure. Spikes from a stock of Mercury Reference Solution (certified 1000 μg g−1 ± 1%; Fisher Scientific CSM114−100) were brought to a concentration of
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METHODS Study Area. We examined sediment cores of fourteen lakes along a transect situated east of the Mackenzie Delta, from Inuvik to Richards Island (Figure 1). These lakes were chosen following an analysis of aerial photographs and field surveys.3 Seven study lakes have retrogressive thaw slumps on their shorelines (i.e., degraded permafrost, classified here as ‘slumpaffected lakes’) and the other seven lakes are in undisturbed catchments, referred to as “reference lakes”. The location and characteristics of the study lakes are given in Table 1, and detailed water chemistry is in Supporting Information Table S1. A detailed analysis of lake sediment profiles is performed on a subset of 8 of these lake sediment cores. Sample Collection and Preparation. Sediment cores measuring 35−40 cm in length were recovered from the center of the fourteen lakes in the summers of 2007−2008 using a Glew gravity corer and Lexan core tubes. All the cores were subsampled at 0.25 cm intervals between 0 and 15 cm and at 8749
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Table 1. Location and Physical Characteristics of the 14 Study Lakes Located in the Uplands Directly to the East of the Mackenzie River Delta NWT, Canadaa lake
latitude (°N)
longitude (°W)
Aob (ha)
CAc (ha)
2a 5a 6a 7a 9a 14a 36a
68°50′ 26.7″ 68°33′4.20″ 68°35′25.73″ 68°36′18.47″ 68° 58′ 05.8″ 68° 31′ 02.7″ 68° 30′ 10.4″
133° 66′ 07.1″ 133°38′23.39″ 133°38′33.40″ 133°35′27.14″ 133° 53′ 53.0″ 133° 44′ 55.4″ 133° 42′ 02.2″
2 2.9 3.6 1.4 3.1 3.4 0.8
17.2 20.9 19.7 18.1 29.3 33.5 6.6
2b 5b 6b 7b 9b 14b 36b
68° 50′ 72.8″ 68°32′14.96″ 68°35′17.78″ 68°36′32.23″ 68° 58′ 14.1″ 68° 31′ 02.7″ 68° 30′ 09.6″
133° 67′ 03.6″ 133°39′27.41″ 133°38′16.95″ 133°35′12.77″ 133° 53′ 59.3″ 133° 44′ 55.4″ 133° 42′ 05.2″
4.9 2.8 1.2 3.1 3.6 9.2 3.9
15.9 27.7 7.5 34.7 7.2 45.1 24.4
ASd (ha)
slump status
Zme (m)
FFf
average sed. Rateg (g m−2 year−1)
6.1 10.9 2.3 2.7 2.7 7.5 9.5
2.30 0.96 0.94 2.45 4.57 0.87 0.77
67 132 121 34 20 210 119
3.4 9 2 5 3 10.5 7.4
0.46 5.00 0.55 0.89 1.18 0.05 0.51
556 61 528 271 240 6594 290
I. reference lakes
II. slump-affected lakes 0.95 stable 2.02 stable 0.81 stable 1.13 active 2.5 active 2.4 active 4.9 stable
a
Morphometric data and slump activity were determined from air photo analyses and ground surveys undertaken during 2001−2005. Seven study lakes have retrogressive thaw slumps (i.e. degraded permafrost), and the other seven lakes are in undisturbed (reference) catchments. Slump-affected lakes 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 of retrogressive thaw slump. eZm = maximum depth of lake. fFF = focusing factor. gSedimentation rate was calculated using the CFCS model (Appleby 2001).
50 ng g−1 and were tested every 5 samples. The average recovery of the spikes was 102% ± 5 standard deviation (SD), (n = 12). According to procedural blanks, no contamination was observed during THg analysis. Reference materials were tested every 4−5 samples and average percentage recovery for MESS-3 (Marine Sediment Certified Reference Materials from National 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 determined by capillary gas chromatography coupled with atomic fluorescence spectrometry (GC-AFS) as described by Cai et al.,23 with a detection limit of 0.02 ng per sample size. Sample mass ranged between 0.4 and 1 g. Further details are provided in Supporting Information. Organic Geochemistry (Rock-Eval Analyses). We applied Rock-Eval 6 (Vinci Technologies, France) for the quantitative and qualitative study of organic matter (OM) in the recent sediments. The Rock-Eval 6 method consists of pyrolysis (under inert conditions) and then oxidation, both performing a temperature programmed heating of the sediments (30−50 mg) at a rate of 25 °C per minute. Detailed methods are provided in Supporting Information. Inferred Chlorophyll a in Sediments. Sedimentary chlorophyll a content was inferred using visible reflectance spectroscopy (VRS), a method that provides an indication of overall lake production.24 Samples were freeze-dried, sieved (125 μm), and analyzed on a FOSS NIRSystems Model 6500 rapid content analyzer. The portion of the electromagnetic spectrum from 650 to 700 nm was analyzed in order to detect both chlorophyll a and its derivatives (pheophytin a and pheophorbide a), allowing VRS-chlorophyll a to track changes in production over time and not strictly diagenesis.24,25 Statistical Analyses. All data were analyzed using Origin Lab 7 software (Origin Lab Corporation MA, USA). Simple linear regressions analysis was used to analyze the relationships between THg, S2, inferred chlorophyll a, and sedimentation
rates. Student t test was used to test the difference between slump-affected and reference lakes with respect to THg, S2, inferred chlorophyll a, and sedimentation rates.
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RESULTS AND DISCUSSION Pb Dating and Sedimentation. 210Pb activities in most cores declined log−linearly with depth until reaching background, which was determined by secular equilibrium with 226 Ra (Supporting Information Figure S1). Several cores showed disturbance in their excess 210Pb profiles, particularly in 9b and 14b, suggesting fluctuations in sedimentation at those intervals. Focus-corrected sedimentation rates in slump-affected lakes (269 ± 66 SD g m−2 year−1) were more than double those in reference lakes (120 ± 37 g m−2 year−1) indicating higher incoming material to lakes disturbed by retrogressive thaw slumps. Higher sedimentation rates in slump-affected lakes are due to sediment inputs from growing thaw slumps, which have increased in activity during the late 20th Century in association with rapidly warming air and permafrost temperatures.6 Inorganic sedimentation rates largely accounted for differences in sedimentation rates, with 253 ± 65 SD g m−2 year−1 in slump-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 was similar 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 organic sedimentation rates regardless of whether the lakes were disturbed by retrogressive thaw slumps or not. Likewise, fluxes of S1, S2, and residual carbon (RC) showed no difference between slump-affected lakes and reference lakes (S1, slumpaffected = 0.11 ± 0.12 SD g m−2 year−1, reference = 0.18 ± 0.12 SD 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 8750
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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 total organic carbon in surface sediments, (B) the S2 (algal derived) carbon in surface sediments and (C) Hg concentration. Note that sedimentation rate in slump-affected lakes tends to be higher than in reference lakes, and organic carbon and Hg is lower and ‘diluted’ by the higher inorganic sediment flux seen in slump-affected lakes.
carbon by inorganic sediments was further evident by the inverse correlation between TOC concentration and sedimentation rate (r = −0.66, F1,12 = 9.3, p = 0.01, Supporting Information Figure S2A). Slump-affected lakes had significantly lower TOC concentration and higher sedimentation rates, suggesting that accelerated deposition of inorganic sediments from retrogressive thaw slumps diluted the sedimentary organic carbon. This inorganic sediment dilution effect was evident for other organic fractions as well (e.g., S2, Supporting Information Figure S2B). Mercury in Surface Sediments. Mercury concentration in surface sediments was significantly correlated with total organic carbon (TOC) (r = 0.73, p < 0.05, Supporting Information Figure S2A), and S2 carbon (r = 0.60, p < 0.05, Supporting Information Figure S2B) but not as strongly related to inferred chlorophyll a (r = 0.50, p = 0.06, Supporting Information Figure S2C). The strong relationship between Hg, TOC and S2 indicates their important role in adsorbing Hg and depositing it to sediments. Likewise, there was a significant correlation between Hg and RC in surface sediments (r = 0.77, p = 0.001). RC partially represents the quantity of thermally resistant organic material, which only decomposes during high temperature oxidation such as in forest fires and domestic wood burning, which may partly explain the correlation between the Hg and RC.26
Likewise, MeHg in surface sediments was significantly correlated to TOC (r = 0.78, p < 0.01, Supporting Information Figure S3), S1 carbon (r = 0.71, p < 0.01), and S2 carbon (r = 0.79, p < 0.05), indicating an important association between MeHg formation and organic carbon content, which is in turn influenced by the permafrost disturbance status of these lakes. Taken together, the results suggest that dilution of organic matter by rapid influx of inorganic sedimentation in thaw slump lakes coincides with reduced mercury and MeHg concentrations in these sediments. However the proportion of MeHg to THg was similar between reference lakes (0.0046 ± 0.037 SD) and slump-affected lakes (0.0036 ± 0.0015 SD) indicating no difference in the ability to generate MeHg among sediment types (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-affected lakes (0.07−0.12 μg/g dw), (t = 4.99, p < 0.01). A strong inverse correlation was found between mercury concentration and sedimentation rate in our study lakes (R2 = 0.68, P < 0.01, Figure 2C), showing the influence of inorganic sediment dilution on mercury. Mercury concentrations in clays collected within the thaw slump scars were very low (75 ng g−1 ± 67 SD, n = 22) when compared to mercury concentrations in lake sediments (Figure 2C), so the influx of these inorganic materials following thaw slump development would likely 8751
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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 cores depth 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 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 cores depth in 4 slump-affected lakes in the Mackenzie Delta.
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Sedimentation rates were variable in many of these cores and were occasionally observed to spike at intervals, often with a corresponding drop in sedimentary organic content and water content (Figures 3 and 4). This might be expected due to a sudden dilution of organic matter from an influx of inorganic sediment as also recorded in the spatial analysis of surface sediments (Figure 2). Examples are evident in cores from reference lakes and slump-affected lakes: Lakes 9a (at 7−12 cm), 36a (5−10 cm), 2b (3−7 cm), and 14b (0−1.5 cm). S2 carbon was inversely correlated with 210Pb-derived sedimentation rates (based on the constant rate of supply model) in some of these cores, particularly those where sedimentation rates exceeded 1,000 g m−2 year−1 (Supporting Information Figure S4). These correlations may have resulted from organic matter dilution during periods of rapid inorganic sedimentation such as those that occur from retrogressive thaw slumping (Figure 2). Relationship between Hg and S2. Significant correlations between Hg and S2 in previous studies13,14,16,17,19 emphasized the role of algal scavenging of mercury. Other studies observed that Hg was only correlated with S2 in a subset of sediment cores, suggesting that algal scavenging may not explain Hg deposition to sediments in some cases.18 We showed a significant correlation between Hg and S2 across all surface sediments (Supporting Information Figure S2B); however, we only observed positive correlations between Hg and S2 in 5 of our 8 sediment profiles (Supporting Information Figure S5). These results suggest that other factors besides algal scavenging affect Hg delivery to sediments, which may include dilution by inorganic sedimentation and catchment erosion (Figure 2). Further in-depth studies are warranted to understand the importance and mechanism of autochthonous organic production, or algal scavenging, in influencing Hg delivery to sediments. S2 Carbon and Inferred Chlorophyll a in Surface Sediments. Average inferred chlorophyll a concentration in the surface sediments of our reference lakes (0.034 ± 0.017 mg g−1 DW) was slightly higher than the slump-affected lakes (0.019 ± 0.019 017 mg g−1 DW) but was not statistically different (t = 1.70, p > 0.05). S2 carbon in surface sediments was significantly correlated with inferred chlorophyll a (r = 0.83, p < 0.0002, Supporting Information Figure S6). This would be expected because S2 fraction of organic carbon is primarily of algal origin and sedimentary inferred chlorophyll a is also typically of autochthonous origin (ref 24 and papers cited therein), though both of these constituents should also be susceptible to some degree of degradation/modification in sediments.24,28 S2 and inferred chlorophyll a were significantly correlated in 7 of 8 lake sediment profiles (r = 0.44−0.94, p < 0.05) but no correlation was observed in lake 2a (r = 0.47, p > 0.05) (Supporting Information Figure S7). Hg and inferred chlorophyll 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 and composition of organic matter is presented by plotting the quantity of S2 as a function of TOC (Supporting Information Figure S8), representing the proportion of hydrogen-rich organic matter dominantly composed of autochthonous (mainly algal-derived) matter relative to the TOC in the sediments.29 Kerogens showing highest S2/TOC are classified
reduce the overall mercury concentration in sediment. Within the two lake categories, mercury concentration was correlated with sedimentation rate in reference lakes (R2 = 0.89, p < 0.05, Figure 2C) but no correlation was found for slump-affected lakes, which might have resulted due to variable thaw slump activity among slump-affected lakes. Mercury concentrations in slump-affected lakes were comparable to other studies in the Canadian Arctic,13,15,27 but were generally higher in reference lakes. There was no difference in focus corrected mercury flux between reference lakes (25.35 ± 3.01 μg m−2 year−1) and slump-affected lakes (26.61 ± 6.92 μg m−2 year−1), indicating that changes to limnetic conditions from thaw slump activity (e.g., higher ionic strength, lower DOC, Supporting Information Table S1) did not significantly alter Hg deposition rates to lake sediments, and further supported our contention that the difference in Hg concentration in sediments between lake types was due to dilution by rapid inorganic sedimentation in the slump-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 in slump-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 and MeHg in surface sediments (r = 0.88, p = 0.01, Supporting Information Figure S2), suggesting that Hg methylation may be Hg-limited. Mercury in sediment profiles. In profile, sedimentary Hg and TOC in reference lakes tended to show modest increases to the surface (Figure 3), though not to the same extent as those described in other regions closer to active Hg emissions. Enrichment factors (EF) ranged from 1.27 to 2.50 in lakes 14a and 36a, respectively, which was close to those found throughout 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 comparable to other studies in the Canadian Arctic of lakes similar to our reference lakes,13,15,27 whereas reference lakes had higher Hg than their slump-affected counterparts in all our study lakes except 14a/b. Similarities between vertical profiles of TOC, S2 carbon, inferred chlorophyll a and THg were generally apparent within the 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 of them. Lake 14a had moderate surface enrichment of TOC, S2, and inferred chlorophyll a and Hg surface enrichment. Similarly, slump-affected lakes showed less surface enrichment in Hg, TOC, S2, and inferred chlorophyll a in Lakes 2b, 9b, and 36b compared to reference lakes, whereas 14b showed the lowest surface Hg enrichment (Figure 4). Enrichment factors for Hg were 1.16−1.48 and were similar to values reported in other Arctic regions (1.1−2.6).13,18,27 These results corroborated the analysis of surface sediments that showed a dilution of sedimentary organic matter by inorganic siliciclastic materials derived from thaw slumps in these thermokarst-affected lakes. S2 concentrations in these lakes were generally similar to those reported in other Arctic sediment cores (0.3−15 mg g−1),13−15 however Lakes 9a and 14a had 2 to 3 times higher S2 concentrations (41 and 31 mg g−1 respectively). It is unlikely that autochthonous organic production is higher in these lakes, given their oligotrophic status. 8753
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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 surface sediments 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 predominance of allochthonous organic matter to sediments of all lakes regardless 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 by normalizing the quantity of S2 to TOC (S2/TOC × 100), and is proportional to the kerogen elemental H/C ratio.31 OI, calculated as S3/TOC × 100, is proportional to the elemental O/C ratio of the kerogen.31 Well-preserved kerogens of dominantly autochthonous (algal) origin are known to have elevated HI values (hydrogen-rich; Type I and II kerogen) relative to terrestrially derived, and/or degraded and reworked organic matter.16,28,32 All surface sediments plotted in Type III kerogen space on the Van Krevelen diagram (Supporting Information Figure S9), further corroborating evidence that the majority of organic matter in these sediments is of allochthonous origin or has been exposed to a large degree of degradation within the water column. Our results show that sediments from lakes affected by thaw slumps contained lower concentrations of TOC and algal derived organic carbon (S2), lower mercury and MeHg concentrations, as well as higher total and inorganic sedimentation rates, which likely explain the dilution of organic materials and mercury in lakes where thaw slumps are present. Likewise our results showed significant correlations between mercury and S2 concentrations in 5 of the 8 lake sediment cores, suggesting that algal-derived materials may be sources of Hg to sediments, but other factors such as inorganic sedimentation rate and catchment erosion are likely mediating this effect. The effect of retrogressive thaw slump development on the mercury cycle should be compared with other manifestations of thermokarst, such as peat subsidence,33 which is much more likely to release significant quantities of Hg and MeHg to the aquatic environment.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This study was funded by a Natural Sciences and Engineering Research Council (Canada) Strategic Projects grant and a Northern Contaminants Program Grant to J.M.B., J.P.S., and M.F.J.P. We thank the Geological Survey of Canada for scientific and analytical support. Logistical support was provided by the Polar Continental Shelf Program (PCSP). We thank the Cumulative Impact Monitoring Program of Aboriginal Affairs and Northern Development Canada for additional research support, and Peter deMontigny for field assistance.
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ASSOCIATED CONTENT
S Supporting Information *
Analytical methods for Rock Eval analysis, radiometric measurements of lake sediments, and MeHg analysis of lake sediments, a table showing limnological characteristics of the 14 lakes considered in this study, and figures showing 210Pb and 226 Ra profiles for the 8 sediment cores, correlations between Hg in surface sediments and TOC, S2 carbon, and inferred chlorophyll a, the correlation between MeHg concentration in surface sediment and TOC, the correlation between S2 carbon and sedimentation rate in the 8 sediment core profiles, the correlation between Hg concentration between Hg and S2 in the 8 sediment core profiles, the correlation between S2 in surface sediments and inferred chlorophyll a in surface sediments of 14 lakes, correlations between S2 carbon and inferred chlorophyll a in the 8 sediment core profiles, kerogen type in surface sediments of the 14 lakes using S2:TOC graph,29 and kerogen type in 8 sediment core profiles using HI:OI pseudo Van Krevelen diagram. This information is available free of charge via the Internet at http://pubs.acs.org. 8754
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Environmental Science & Technology
Article
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