Warming and wetting in the western Canadian Arctic are
accelerating thaw-driven mass wasting by permafrost thaw slumps, increasing
total organic carbon (TOC) delivery to headwater streams by orders of
magnitude primarily due to increases in particulate organic carbon (POC).
Upon thaw, permafrost carbon entering and transported within streams may be
mineralized to CO2 or re-sequestered into sediments. The balance
between these processes is an important uncertainty in the
permafrost–carbon–climate feedback. Using aerobic incubations of TOC from
streams affected by thaw slumps we find that slump-derived organic carbon
undergoes minimal (∼ 4 %) oxidation over a 1-month period,
indicating that this material may be predominantly destined for sediment
deposition. Simultaneous measurements of POC and dissolved organic carbon
(DOC) suggest that mineralization of DOC accounted for most of the TOC loss.
Our results indicate that mobilization of mineral-rich tills in this region
may protect carbon from mineralization via adsorption to minerals and
promote inorganic carbon sequestration via chemolithoautotrophic processes.
With intensification of hillslope mass wasting across the northern
permafrost zone, region-specific assessments of permafrost carbon fates and
inquiries beyond organic carbon decomposition are needed to constrain
drivers of carbon cycling and climate feedbacks within stream networks
affected by permafrost thaw.
Introduction
Permafrost soils comprise the single largest pool of terrestrial organic
carbon (OC)
(Schuur
et al., 2015; Hugelius et al., 2014), half of which may be vulnerable to
rapid mobilization into modern biogeochemical cycles via abrupt thaw
processes (Turetsky et al.,
2020; Olefeldt et al., 2016). Permafrost dissolved organic carbon (DOC),
typically defined as compounds < 0.7 µm, is often highly
susceptible to biotic mineralization into CO2 within aquatic systems
(Vonk
et al., 2015a; Littlefair and Tank, 2018; Abbott et al., 2014; Mann et al.,
2015). Abrupt thaw can mobilize orders of magnitude more particulate organic
carbon (POC; typically > 0.7 µm) than DOC, yet the
biodegradability of permafrost POC is not well understood
(Shakil
et al., 2020; Tank et al., 2020; Vonk et al., 2015b).
Suspended particles can be important sites for mineralization
(Attermeyer et al., 2018) or mineral
protection
(Hemingway et
al., 2019; Opfergelt, 2020; Groeneveld et al., 2020). In addition to
molecular composition and a host of environmental factors that typically
affect organic matter decomposition (e.g., microbial activity, nutrient
availability) (Kothawala et al., 2021),
mineralization of POC in stream networks depends on transport vs.
deposition. When settled out, mineralization of POC can be reduced by
∼ 50 % (Richardson et al., 2013) or more,
particularly if contained in anoxic sediments
(Peter et al., 2016), though carbon
release can shift to be in the form of methane
(Schädel et al., 2016). Fractions of POC
with different density and size therefore not only experience a different
settling and transport trajectory but also may have differing processes and
rates affecting OC dynamics
(Tesi et al., 2016). If
biodegradability varies across size and density fractions, this could alter
realized mineralization during transport relative to measurements on bulk OC
(Tesi et al., 2016).
Warming and intensifying precipitation across the ice-rich terrain of the
Peel Plateau in western Canada have triggered an acceleration of thaw-driven
landscape erosion in the form of retrogressive thaw slumps (hereafter,
slumps) (Kokelj et al.,
2021). Thaw slumping along stream sites in this region can increase total
organic carbon (TOC) yields by orders of magnitude almost entirely due to
increases in POC (Shakil et al., 2020). Slump-derived DOC in the region is
relatively more labile than background DOC, as shown by chemical composition
and incubations
(Littlefair et
al., 2017; Littlefair and Tank, 2018). Slump-derived POC chemical
composition suggests lower bioavailability as POC sources shift from the
active layer and some periphyton material to Pleistocene-aged organic carbon
and petrogenic organic carbon mobilized from permafrost
(Shakil et al., 2020;
Bröder et al., 2021). However, POC bioavailability has not been
experimentally assessed. Given that slump-derived carbon occurs almost
entirely as POC in this region (Shakil et
al., 2020), understanding the fate of this carbon remains a critical
knowledge gap.
Our objectives were to assess the potential for slump POC to be mineralized
to CO2 during transport in streams. To do this, we undertook
experimental incubations to (a) determine whether slump POC differs in
biodegradability from POC present in unimpacted waters and (b) quantify and
assess the biodegradability of slump-POC fractions relative to their
transport potential. This work provides insight into the fate of an
understudied component of permafrost-mobilized OC.
MethodsRegion and field sampling
Slumping occurs across the Peel Plateau (Fig. 1) and typically mobilizes
terrestrial material from three distinct sources: (1) Pleistocene-age tills
that have remained preserved within permafrost since deposition by the
Laurentide Ice Sheet and subsequent permafrost aggradation; (2) Holocene-age
permafrost developed from tills following active layer deepening and/or
slumping in previous warm periods, followed by permafrost aggradation during
a cooler climate; and (3) a contemporary active layer. Thus, the relative
contribution of biogeochemical substrate from these three terrestrial
sources to streams can depend on thaw depth
(Shakil et al., 2020;
Bröder et al., 2021). Source composition can also vary west–east as
vegetation (elevation) and geology change along this gradient
(Duk-Rodkin and Hughes, 1992; Norris, 1985).
Sampling occurred during July–August, within the Stony Creek and Vittrekwa
River watersheds of the Gwich'in Settlement Region on the Peel Plateau (Fig. 1). In 2015, substrate from streams near and within three slump sites (HA,
HB, HD) was used to test if mobilization of slump POC and nutrients affect
biodegradability of OC in streams. Stream water samples were obtained from
channelized runoff within each thaw slump (IN), a downstream location where
all runoff entered the valley-bottom stream (DN), and an unimpacted
reference stream upstream of slump inflow (UP) (Fig. 1b). Site HD-UP
experienced some encroachment of slump runoff and thus was not a fully
unimpacted site (Fig. S4). In 2016, substrate was collected upstream of,
within, immediately downstream of, and 2.79 km downstream of slump site SE to assess
variations in biodegradability with transport potential. In 2019, substrate
was collected within and downstream of slump FM3 to follow up results from
2015 and 2016 (details below). Slump sites had varied elevation and
morphology, with maximum headwall heights (Fig. 1a) ranging from 7.1 to 23 m
(see Shakil et al., 2020). All samples were
processed (i.e., filtered) within 24 h of collection, apart from
within-slump and downstream samples used for adding particles to
“unfiltered” treatments in 2016 that were stored in the dark at
4 ∘C until the start of the experiment. Experiments were initiated
within 24 h (2015, 2019) or 48 h (2016) after processing. The extra
hold time for the 2016 experiment was due to the extra time needed for size
fractionation of samples (see below and Supplement S2). For further
sample collection details, see Supplement S1.
Biodegradation experimentsEffects of POC source, dissolved constituents, and settling (2015)
To test the effect of POC source (Table 1), we incubated unfiltered upstream
water (upstream POC; treatment “UU”) and filtered upstream water with a 2 mL addition of slump runoff (slump POC; “SU”) in 120 mL glass serum
bottles for 7 d at ca. 20 ∘C in the dark, with continuous
end-over-end rotation (4 rpm; Richardson et al., 2013) (Supplement S2,
Fig. 2). Control bottles accounted for DOC contained in filtered upstream
water alone (no POC control; “UF”). Additionally, we tested for (a) the
effect of particle deposition by allowing a replicate set of SU bottles to
settle out (“SS”) and (b) the slump-derived release of additional solutes
(e.g., nutrients) by mixing slump POC with filtered downstream water
(“SD”). Bottles were filled to have no headspace.
Variability as a function of transport potential (2016)
SE within-slump runoff was split into three sieve size fractions (63–2000, 20–63, < 20 µm) by sieving a 0.5 mL
aliquot and adding the resultant size fractions to filtered downstream water
in 60 mL glass biological oxygen demand (BOD) bottles (Supplement S2, Fig. 2). An unfractionated
control (0.5 mL of slump runoff in 60 mL downstream water) was also created, and bottles
were incubated for 8 d in the dark at ca. 20 ∘C as above. Since
relative concentrations of each size fraction were maintained, the < 20 µm fraction had orders of magnitude greater total suspended solid
(TSS) concentrations than the two larger fractions (Table S2). We also
incubated filtered and unfiltered (but diluted; Table S2) stream water from
sites upstream, downstream, and 2.79 km downstream of SE to accompany size
fraction incubations. We characterized POC differences between size
fractions using (a) 14C age, (b) percent POC (%POC; POC : TSS), and
(c) absorbance and fluorescence spectra of base-extracted particulate
organic matter (BEPOM)
(Osburn et al.,
2012) (Supplement S3). Bottles were filled to have no headspace.
Flow chart for processing of (a) 2015 and (b) 2016 experiment.
Note that time point two for 2015 occurred shortly after time point one due to
rapid oxygen loss. Due to this, and the fact that some bottle replicates had
to be removed because of anoxia, only data for time point one are presented
in the main paper. Analyses show particulate (POC) and dissolved (DOC)
organic carbon and dissolved organic matter (DOM) optics (SUVA254).
Flow chart for 2019 is provided in Appendix D.
Measurements
We measured (a) concentrations of DOC, POC, TOC, and SUVA254 (an
optical proxy for dissolved organic matter (DOM) aromaticity)
(Weishaar et al., 2003) at the beginning and end of
incubations and (b) O2 concentrations approximately daily (PreSens,
Fibox 4, SP-PSt3-NAU-D5-YOP) to provide insight on rate of change (Richardson
et al., 2013). We initially assumed heterotrophic breakdown of OC would be
the dominant O2 consumption pathway as respiratory quotients across
several freshwater sites have been noted to vary around 1
(Berggren et al., 2012). Incubation O2 concentrations
presented never dropped below 2 mg L-1, a threshold well above O2
limiting concentrations for different bacterial species
(Stolper et al., 2010, and references therein). One
exception was one of four replicates for the SE unfractionated treatment,
which was removed and replaced with the mean of replicates for statistics.
Our experiments aimed to mimic conditions downstream of slump inflows; thus
slump-affected incubations had POC concentrations ranging from 1.4 to 18.6
times DOC concentrations. Samples for TSS concentration were collected
alongside POC. Further details are available in Supplement S3.
Follow-up experiments
To assess processes that could consume O2 and/or generate OC (due to
O2 losses coupled with OC gains observed in 2015 and 2016), we undertook
two follow-up experiments. First, we combined 0.15 mg of sterilized HD
debris tongue sediments (collected in
2016) (Zolkos and Tank, 2020) with
18.2 Ω Milli-Q (MQ) water to assess abiotic O2 loss. MQ water
was sourced from a machine with a carbon filter and was quality controlled
to have less than 10 ppb TOC. We incubated 60 mL glass BOD bottles on a shaker table in the dark at ca. 20 ∘C
for 7 d, monitoring O2 as above. Second, we measured the change in
dissolved and particulate inorganic carbon (DIC, PIC), in addition to the change
in DOC and POC, in an incubation combining FM3 slump runoff with downstream
water, including sterilized replicates. The treatments were designed to
replicate treatment “SD” in 2015. Sterilization was achieved by
autoclaving and adding ZnCl2 as a poison and was validated using plate
counts (Supplement S4). We hypothesized that chemosynthesis associated
with nitrification and sulfide oxidation (Eqs. 1–2) could generate OC, and
so we additionally measured dissolved inorganic nitrogen (NH4+,
NO3-, NO2-) via automated colorimetry and sulfate
(SO42-) via ion chromatography at the Canadian Association of
Laboratory Accreditation (CALA)-certified Biogeochemical Analytic Service
Laboratory (BASL; University of Alberta, further details in Supplement S3.3).
1aNH3+1.5O2↔NO2-+H++H2O(Stumm and Morgan, 2012)1bNO2-+0.5O2↔NO3-(Stumm and Morgan, 2012)CO2+H2S+O2+H2O↔CH2O+H2SO4(Klatt and Polerecky, 2015)
Note that Eq. (2) is a general equation of chemolithoautotrophic reduced
sulfur oxidation that can have a variable stoichiometry and assumes sulfur
oxidizing bacteria exclusively produce SO42- rather than
both SO42- and S0
(Klatt and Polerecky,
2015; Nelson et al., 1986). Equation (3) shows a net reaction for a model of
pyrite oxidation at circumneutral pH
(Percak-Dennett et al.,
2017). We note that pH in the streams in this study can be quite variable
but tend to be circumneutral, often varying around pH 7 and mostly ranging
from pH 6 to 8 (see Supplement data of Shakil et al., 2020). This sulfide
oxidation can generate sulfuric acid that can weather carbonates (e.g., Eq. 4) or silicates
(Zolkos
and Tank, 2020; Zolkos et al., 2020).
3FeS2+3.75O2+3.5H2O↔2H2SO4+Fe(OH)3(Percak-Dennett et al.,
2017)4H2SO4+2Ca,MgCO3↔2Ca2+,Mg2++SO42-+2HCO3-(Calmels et
al., 2007)(Zolkos and Tank, 2020)
Sediments in sterilized and unsterilized bottles were additionally
characterized using X-ray diffraction (XRD) (Supplement S3.4), while
absorbance and fluorescence spectra of BEPOM and DOM were assessed at the
beginning and end of the experiment (see Supplement for further details).
Data analyses
Two-way ANOVAs, with site and treatment as fixed effects, were used to
assess the effect of POC presence (UF vs. SU), source (UU vs. SU), dissolved
matrix (SU vs. SD), and settling (SU vs. SS) on percent changes in OC (DOC,
POC, and TOC), DOM aromaticity (SUVA254), and O2 loss rate. One-way
ANOVAs were also used to assess differences in the aforementioned changes
between size fractions (2016 experiment). Two-way ANOVAs were used to assess
the effect of distance and filtration (fixed effects) for the 2016 transect
experiments. Significant ANOVA tests were followed up with Tukey-adjusted
pairwise t tests (Zar, 2010). We also calculated 95 %
confidence intervals to evaluate whether OC changes significantly differed
from zero. Principal component analyses were used to visualize differences
in optical indices between size fractions (2016) and changes in DOM and
BEPOM (2019), following the calculation of SUVA254 (DOM only;
Weishaar et
al., 2003; Poulin et al., 2014) and slope ratios
(Helms et al., 2008) on
absorbance data, as well as humification indices (HIX; Ohno, 2002),
biological indices (BIX;
Huguet
et al., 2009), the proportion of peaks C and M (Coble, 2007),
and previously identified BEPOM peaks
(Shakil et al., 2020) on fluorescence data
(Supplement 3.6–3.9). To assess factors controlling in situ CO2
and O2 dynamics we calculated departures of O2 and CO2 from
atmospheric equilibrium (Vachon et al., 2019) using 2015 in situ measurements
of dissolved O2 at several slump sites
(Shakil et al., 2020) and coupled CO2
departures (Zolkos et al., 2019). For
further details, see Appendix A.
ResultsEffects of POC presence and source (2015)
Across experiments, declines in POC were not observed, and in some cases,
POC increased (Fig. 3). Slump runoff addition into
filtered upstream water (SU) did not significantly alter %ΔDOC
(the percent change in DOC from beginning to end of the experiment), ΔSUVA254 (absolute change in DOM aromaticity), or %ΔTOC
(p> 0.05; Table A1, Fig. 3) relative to
the upstream filtered control (UF). Similarly, POC source (slump, SU, vs.
unfiltered upstream, UU) did not significantly affect %ΔTOC or
ΔSUVA254 (p> 0.05; Table A1,
Fig. 3). However, %ΔPOC was
significantly lower when particles were sourced from slump runoff (SU vs.
UU; p< 0.001; Table A1) potentially because particle concentrations
were orders of magnitude lower in upstream bottles (Table S2). DOC increased
in the presence of upstream particles (UU) but decreased in the presence of
slump particles (SU), though this difference was marginally insignificant
(Table A1, p=0.053). Despite no effect on %ΔTOC (Table A1),
the addition of slump particles (SU) did significantly increase rates of
O2 consumption compared to upstream filtered and unfiltered treatments
(UF and UU; Fig. 3a–c), though this effect was
dependent on slump site (significant interactions; Table A1; note lack of
increase for site HD where slump runoff encroached the upstream site).
Summary of experiments and main results with reference to figures
and text sections for details.
YearField samplingExp. detailsNo. daysTestTreatmentsMain resultConsiderations2015UP, IN, and DNof slumps HA,HB, and HDSect. 2.2.1, Fig. 27POC presenceSU (slump in filtered upstream) vs. UF (filtered upstream)(1) Between treatments: no effecton %ΔTOC but increased O2 loss; (2) within treatments: no sig. TOC loss (Fig. 3, Table A1)POC sourceSU vs. UU (unfiltered upstream)Dissolved constituentsSU vs. SD (slump in filtered downstream)(1) Between treatments: no effecton %ΔTOC or O2 loss; (2) within treatments: no sig.TOC loss (Fig. 3, Table A1)SettlingSU vs. SS (SS off rotator to allow settling)(1) Between treatments: noeffect on %ΔTOC, reduced O2loss; (2) within treatments: no sig.TOC loss (Fig. 3, Table A1)2016UP, IN, DN,and 2.79 kmDN ofslump SESect. 2.2.2, Fig. 28Biodegradability vs. transport potentialSieve size fractions: (1) 2000–63 µm (2) 63-20 µm (3) < 20 µm). + unfractionated referenceNo sig. diff. in %ΔTOC or %ΔPOC changes between sizefractions but sig. TOC gain andlargest DOC loss for particles < 20 µm (Fig. 3, Table A2)↓ [TSS] in two larger size fractions relative to smallest, two largest also within MQ errorTransect validationUnfiltered and filtered DNvs. 2.79 km DN. + UPreferenceChange in downstream distance:no effect on %ΔTOC; withintreatments: no sig. TOC loss (Fig. 3, Table A3)UP and 2.79 km DN within MQ blank error2018HD debristongue materialSect. 2.2.47Abiotic O2 lossSterilized debris tonguesediments in Milli-Q (MQ) water vs. MQ controlRapid O2 loss in absence ofmicrobial activity (Fig. S3a)MQ may accelerate weathering, HD debris sediments an extreme weathering endmember2019IN and DNof slump SCSect. 2.2.4, Fig. D127Paired inorganic carbon changes and chemo- lithotrophyUnsterilized mimic of SD(2015 treatment) vs.sterilized SD. + MQ blanksPrior gains not replicated but only ∼ 4 % of TOC mineralized, greater sulfate gains and nitrification in unsterilized treatments(Fig. 5)SC slump in a differentlandscape type than SE andHA where prior gains wereobserved, sterilizationtreatments may not bea true abiotic control
(a–c) Modelled (line) O2 (mg L-1) across combinations of
source material and settling effects. Percent change in DOC, POC, and TOC in
comparison to (d–f) combinations of source material and settling effects,
(g) geometric mean particle size, and (h–i) distance from slump site.
Vertical errors are 95 % confidence intervals with asterisks marking
significant differences from zero. Horizontal errors (e) are particle-size geometric standard deviation. Codes (a–f): filtered (UF) and unfiltered (UU)
upstream, slump material in upstream (SU) and downstream (SD) filtrate, and
SU settled out (SS). HA, HB, and HD are slump sites. For measured O2
data, see Figs. S1 (2015), S2 (2016), and S3 (2018–2019).
Effects of background dissolved constituents and settling (2015)
Changing filtrate to downstream water, which has higher ion and nutrient
concentrations (Shakil et al., 2020), had no significant effect on any
parameter measured in the experiment (SU vs. SD; Table A1). Allowing slump
particles to settle (SU vs. SS) did not affect %ΔTOC but did
significantly reduce O2 consumption rates. Mean DOC concentrations also
switched from decreasing (SU) to increasing (SS)
(Fig. 3d), though the difference was not
significant (p=0.10; Table A1).
Variability dependent on transport potential (2016)
Based on 14C age, %POC, and the relative contribution of fluorescent
peak C (Coble, 2007), larger particle size fractions appeared
to be associated with fresher terrestrial-origin organic matter than smaller
size fractions (Appendix B). POC associated with particles < 20 µm dated to > 27 000 cal BP, while POC associated with
particles ranging from 20–63 and 63–2000 µm dated to
∼ 19 600 and ∼ 8000 cal BP respectively
(Table B1). The majority of POC (73 %; Table B1) was associated with
particles less than 20 µm.
Although bottles containing particles < 20 µm displayed
significant gains in TOC (95 % CI; Fig. 3g),
ANOVA analyses did not uncover a significant difference in %ΔTOC
or %ΔPOC between size fractions (Table A2). DOC losses occurred
in all treatments downstream of slumps, to a greater degree when particles
were present (i.e., unfiltered treatments) (Fig. 3h–i; p< 0.05; Table A3), and were significantly greater when
particles were < 20 µm (p< 0.05;
Fig. 3g, Table A2). Increases in SUVA254 were
also significantly greater for < 20 µm particle treatments
(p< 0.05; Tables A2 and C1), as were TSS concentrations (Table S2). The percent DOC loss was also significantly greater 2.79 km downstream of slump SE
compared to immediately downstream (Table A3).
O2 vs. carbon
Change in O2 and TOC generally did not follow the 1:1 trend expected if
heterotrophic respiration dominated metabolic processing
(Fig. 4a–c). The greatest deviations from 1:1
were observed in treatments containing slump runoff, in which despite large
losses in O2, we saw non-significant changes to gains in TOC. Increases
in TOC from upstream, filtered, and 2.79 km downstream bottles were within
the range of experimental blanks (Fig. 4c).
Although the rate of O2 consumption within and across experiments was
generally greater in treatments with greater initial TSS
(Fig. 4d), there was no consistent relationship
between TOC changes and initial TSS (Fig. 4e). However, some of the greatest
TOC increases occurred during incubations of slump SE particles < 20 µm and slump HA particles in upstream (SU) and downstream (SD)
filtrate, treatments amongst those with the greatest initial TSS
(Fig. 4a, b, e).
In situ comparisons of O2 vs. CO2 showed within-slump samples to
have the greatest excess CO2, with several samples substantially
departing from the 1 CO2 : -1 O2 stoichiometry associated with
heterotrophic respiration (Fig. 4f). In contrast,
several downstream and upstream sites displayed measurements close to
atmospheric equilibrium for CO2 but were supersaturated for O2
potentially due to temperature changes and lower solubility of O2
(Vachon et al., 2020).
Concentration changes in TOC vs. changes in dissolved O2 for
(a) 2015 experiments, (b) 2016 site SE fractionation experiment, and (c) 2016 site SE transect experiment. Dashed lines in (a)–(c) represent predicted
loss of OC for respiratory quotient = 1, and error bars show 95 %
confidence intervals. (d–e) Exponential rate of O2 consumption (k) or
changes in TOC vs. initial TSS for treatments across experiments, excluding
filtered, settled, and sterilized treatments from (a)–(c). SE-Frac indicates the
fractionated treatments from panel (b). (f) Departures of O2 and
CO2 from atmospheric equilibrium in samples collected upstream (UP),
downstream (DN), and within (IN) a series of slump sites on the Peel Plateau.
Error bars in (d) and (e) show standard error of the mean. Codes in (a) are filtered (UF) and unfiltered (UU) upstream, slump material in upstream (SU)
and downstream (SD) filtrate, and SU settled out (SS). HA, HB, and HD are
slump sites.
Follow-up experimentsSterilized debris sediments in Milli-Q water (2018)
Oxygen was completely consumed (∼ 226 µM) in bottles
containing sterilized HD debris tongue material suspended in MQ water within
4–5 d, exceeding the O2 loss rates previously observed (Table S1,
Fig. S3a). Bottles containing sterilized sediments had lower pH (5.52–6.09) following incubation than MQ controls (6.53–6.91).
Inorganic carbon changes and potential chemolithoautotrophy (2019)
Oxygen consumption occurred in sterilized treatments (-15 ± 6 µM, mean ± 95 % CI, t=27 d) but was more pronounced in
unsterilized bottles (-124 ± 15 µM). The pronounced O2
decline in unsterilized bottles was accompanied by a significant loss of DOC
(-83 ± 26 µM) and a non-significant loss of POC (-85 ± 261 µM), balancing to a non-significant loss of TOC (-170 ± 262 µM, Fig. 4). Total inorganic carbon (TIC) increased significantly in
unsterilized treatments, driven by increases in DIC. In sterilized bottles,
modest O2 losses were accompanied by significant DOC losses (-93 ± 51 µM) and significant POC gains (141 ± 33 µM),
balancing to a modest non-significant gain in TOC (48 ± 66 µM).
TIC in sterilized bottles had a minor significant decrease (-25 ± 20 µM), driven by losses in DIC.
Ammonium (NH4+) decreased from 8.68 ± 0.47 µM to
below detection (0.2 µM) in unsterilized bottles, while
NO2- and NO3- increased by 2.58 ± 2.36 and 1.33 ± 1.67 µM respectively. In sterilized bottles,
NH4+ increased by 6.74 ± 1.80 µM alongside negligible
changes in NO3- and NO2-. Sulfate (SO42-)
generation was greater in unsterilized (90 ± 6 µM) than
sterilized (54 ± 23 µM) bottles, but for both treatments
SO42- increased more than would be expected via pyrite oxidation
(Fig. 5d, based on oxygen stoichiometry in Eq. 3).
However, the only sulfur-bearing mineral detected in sediments (XRD; 1 wt %–5 wt %
detection limit) was pyrite (Table S3).
A biplot of principal component analysis (PCA) components 1 and 2 did not reveal any shifts in BEPOM or DOM
optical characteristics during incubation of unsterilized treatments
(Fig. 5d–e). However, DOM from sterilized
treatments shifted towards lower molecular weight (SR, slope ratio) and lower
aromatic material (SUVA254) of greater biological origin (BIX). PC1
separated DOM in sterilized and unsterilized bottles, suggesting the
sterilization processes increased the proportion of simple compounds. Since
the sterilization process itself appears to increase the proportion of
simple compounds, the results caution against its use as an abiotic
baseline.
Changes in millimolar concentrations of (a) organic carbon; (b) inorganic carbon; (c) ammonium (NH4+), nitrite (NO2-),
and nitrate (NO3-); and (d) sulfate (SO42-) in 2019 test
of interferences. Note difference in scales between panels. Blue shading
highlights potential carbon gains or losses based on O2 loss and a
respiratory quotient of 1 (a–b) and potential SO42- generated from
pyrite oxidation (d; Eq. 3). Error bars and height of blue shading both show
a range representing 95 % confidence intervals. (e–f) PCA biplots of
components 1 and 2 showing variation in POM (e) and DOM (f) optical
properties. Grey circle outlines circle of equilibrium contribution, and plot is shown in scaling 1. Abbreviations of optical indices are provided in Table S5.
Discussion
Our incubations, coupled with multiple studies examining slump-POC
composition
(Shakil
et al., 2020; Bröder et al., 2021; Keskitalo et al., 2021) indicate that
slump-derived POC in glacial landscapes of western Canada has low
biodegradability on the Peel Plateau. We found no significant losses of POC
or TOC or evidence that %ΔTOC increased due to the presence of
slump-derived POC. This finding was consistent across slump sites and for
varying size fractions and distances downstream of slump inputs. While
%ΔTOC did not significantly differ across size fractions of slump
SE particles, particles most likely to remain in transport (< 20 µm) enabled the greatest loss in DOC (linked to greater TSS) and
significant gains in TOC. A coupled transect experiment showed that
downstream of SE, TOC gains were not significant or were within error of
blanks (2.79 km downstream), though both downstream treatments had orders of
magnitude lower TSS than the < 20 µm size fraction treatment
sourced directly from slump runoff.
The lack of loss in TOC and POC contrasted with elevated O2 consumption
rates in incubations using water collected within and downstream of slumps,
except for slump HD, where slump runoff encroached into the upstream site
(Fig. S4). Despite a lack of TOC or POC loss, oxygen consumption rates in
treatments containing particles were always elevated relative to their DOC
controls, highlighting that oxygen consumption could not be solely accounted
for by DOC mineralization. Instead, TOC gains suggest the potential for
chemoautotrophic carbon sequestration. Further, abiotic processes (e.g.,
mineral oxidation) appear to have the potential to consume oxygen rapidly
enough to decouple oxygen and carbon dynamics from the 1:1 relationship
associated with heterotrophic respiration, as suggested by sterile
incubations of HD debris material. The excess in situ CO2
concentrations we observe are likely from mineral weathering that can
generate CO2 in this system (Zolkos et al.,
2018).
Our finding of low POC biodegradability is likely conservative since
incubations focus on the most labile period (initial 7–27 d)
(Richardson et al., 2013). Our longest incubation (27 d;
Fig. 5 non-sterile) did not show significant TOC
losses, though 95 % error spanning losses expected from a 1:1 relationship
with O2 suggests that detection of change may be masked by error in POC
measurements (Fig. 5a). Using ΔTIC from
our 2019 experiment as an alternate metric of carbon mineralization, we
estimate that a maximum of 4 % of the initial TOC pool may have been
mineralized within 27 d. The difference in the landscape position and slump
morphology of the slump site (SC is further east than slumps HA, HB, and SE)
and the longer time period of the incubation (27 d vs. 7–8 d) could
have both played a role in the average loss in TOC vs. the gain seen in
previous comparable experiments in which slump material was added to downstream
water. Time series experiments on TOC degradation from similarly
glacially conditioned Qikiqtaruk (Herschel Island) indicate that CO2
production tied to organic matter mineralization ceased by the end of a
120 d incubation, while more than half (∼ 58 %) of
total CO2 was produced within the first 27 d (Tanski et al., 2019).
Assuming a similar rate for OC mineralization, we can scale up our findings
beyond the time of our incubation to estimate that ∼ 7 % of
slump-derived TOC on the Peel Plateau may be mineralized during the entirety
of time it is transported in streams. Our findings of minimal TOC
mineralization are consistent with measures of little change in CO2
concentrations downstream relative to upstream of slumps
(Zolkos et al., 2019) despite orders of
magnitude increases in POC and thus TOC
(Shakil et al., 2020).
While the upper bound of 7 % TOC mineralization calculated from
observations in this study is elevated relative to slow rates of
mineralization within permafrost (Leewis et al., 2020), it is considerably lower than estimates that greater than 60 % of TOC mobilized
by hillslope abrupt thaw will be mineralized on a decadal timescale (e.g.,
Table S1 in Turetsky et al., 2020), which have been based on mineralization
rates observed for DOC in Pleistocene Yedoma landscapes (Vonk et al., 2013).
Although similarly elevated rates of DOC mineralization have been noted in
other studies (e.g., Spencer et al., 2015; Abbott et al., 2014), these
DOC-specific findings have not been consistent in landscapes across the
Arctic (e.g.,
Burd
et al., 2020; Wickland et al., 2018) likely due to differences in landscape
factors and permafrost composition (Tank et al.,
2020). Past studies have also generally not included POC within their
assessments of permafrost carbon mineralization, even though POC
concentrations within thaw streams can be orders of magnitude greater than
for DOC (Vonk et al., 2013;
Shakil et al., 2020). Notably, percent TOC loss from this study is
substantially lower than percent DOC loss previously observed for this study
area (Littlefair and Tank, 2018) likely due to
substantial differences in biogeochemical processes occurring in the DOC vs.
POC pool upon thaw, interaction with mineral surfaces exposed (see further
discussion below), and contrasting headwall sources
(Shakil et al., 2020). Our loss estimate is
comparable to Tanski et al. (2019), who
observed 2 % to 9 % loss rates for incubations of permafrost TOC mixed
with seawater and incubated at 16∘C for 120 d. Clearly, our
results highlight the need to better understand the relative lability of
different organic matter fractions (i.e., DOC vs. POC) and how
mineralization rates of these fractions may vary with source (i.e., across
landscapes) and receiving environments (lacustrine, fluvial, marine). This
increased understanding appears critical for better constraining the
magnitude and effective time span of permafrost carbon degradation in Earth
system models.
This study and work by Tanski et al. (2019) both suggest DOC contributes substantially more to heterotrophic
CO2 production than POC in glacial margin landscapes even where
hillslope thermokarst increases fluvial POC by orders of magnitude (Shakil
et al., 2020). This seemingly contrasts with the protection of DOM by adsorption to
mineral surfaces (Littlefair et al.,
2017); however adsorption onto minerals tends to favour humic-like,
oxygen-rich compounds, typically considered recalcitrant, over protein-like
compounds (Groeneveld et al., 2020). Thus,
sorption could “sort” labile carbon into the dissolved phase and relegate
intrinsically recalcitrant (“humic-like”, aromatic) carbon to mineral
protection as POC. Evidence of this effect includes elevated lability of
slump-derived DOM relative to upstream DOM
(Littlefair et
al., 2017; Littlefair and Tank, 2018), low lability of slump POM
(Shakil et al., 2020; this study) and
striking compositional similarity of DOC from slump-impacted streams on the
Peel Plateau to that from other circumpolar regions with mineral soils
(Wologo et al.,
2021). However, when particles settle out, anoxia could result in release of
adsorbed DOC to the overlying water column
(Peter et al., 2016), which may explain
the switch from mean DOC decreases to DOC increase in comparisons between
rotated and settled slump treatments. Since both selective sorption in the
water column and desorption in sediment deposits are likely to happen along
the aquatic continuum, it may be difficult to detect or quantify either of
these processes in situ. Sorption appeared to occur in 2019 sterilized bottles as
DOC concentrations declined but POC increased, and DOM aromaticity and
molecular weight decreased. No change in DOM in unsterilized bottles may
reflect sorption (loss of aromaticity) and degradation (gain of aromaticity)
acting simultaneously, which further underlines challenges in quantifying
the process by measuring OC changes in bulk incubations and why DOC declines
were not consistently followed by increases or decreases in SUVA254
across experiments, even when losses were consistent as in 2016.
We note that rapid within-slump processing of labile TOC fractions prior to
entrainment within streams may still occur, as supported by high
within-slump NH4+ concentrations
(Shakil et al., 2020) indicative of
decomposition (Tanski et
al., 2017), low representation of labile compounds in the slump scar zone
and stream sediments relative to headwall sources (active
layer) (Keskitalo et al., 2021),
and excess CO2 in within-slump rill water resulting from both
heterotrophic respiration and geogenic production
(Zolkos et al., 2019) (Fig. 3f). Past work
indicates that OC rapidly lost within-slump may predominantly originate from
the active layer (Bröder et al., 2021) and
Holocene-age permafrost in areas where organic material buried in colluvial
deposits from past slumping has preserved organics
(Lacelle et al., 2019). In addition to serving as a
possible marker for decomposition, high concentrations of NH4+ may
stimulate nitrification and associated chemosynthetic carbon sequestration.
Though we did not see significant TOC gains in our 2019 experiment, ammonia
loss coupled with nitrite production suggests active nitrification.
Nitrifying bacteria have slow growth rates
(Sinha and Annachhatre, 2007; Bock
and Wagner, 2013), with the molar ratio of NH4+ consumed to carbon
fixed ranging from 25–100 (Ward, 2013). Using this stoichiometry and
initial NH4+ concentrations estimated across 2015–2016 incubations
(Table S4) indicates that nitrification would be unlikely to fix more than 1 µM of carbon, in comparison to OC gains of 601 ± 459 µM
(mean ± 95 % CI of SE < 20 µm incubation; Fig. 3b,
Table S4). Chemolithoautotrophy by sulfur oxidizing bacteria can also
sequester carbon (Klatt and Polerecky, 2015)
with the ratio of CO2 sequestered to O2 consumed ranging from 0.09 to 0.41 for aerobic thiosulfate oxidizers (Klatt and Polerecky, 2015). The
process has been noted to be an important carbon sequestration mechanism in
mine tailings (Li et al., 2019). Although the role of aerobic microorganisms
in sulfide oxidation is commonly associated with acidic-pH conditions as in
Li et al. (2019), this process can also occur at circumneutral pH
(Percak-Dennett et al., 2015). Given the high sediment concentration in
streams affected by slumping (can exceed 800 g L-1; Shakil et al.,
2020) and the prevalence of sulfide minerals and oxidation across the Peel
Plateau (Zolkos et al., 2018), chemolithoautotrophy
associated with sulfide oxidation is a mechanism worth exploring as a
counterbalance to OC mineralization. Precise techniques such as isotope
labelling (Spona-Friedl et al., 2020) and the
tracking of genes associated with carbon fixation processes
(Percak-Dennett et al.,
2017) may circumvent challenges associated with POC measurement errors and
tracking multiple processes acting on OC end-point measurements.
Conclusions
Permafrost thaw slumping is increasing TOC concentrations in streams across
the Peel Plateau (Canada) by orders of magnitude, almost entirely in the
form of POC
(Kokelj
et al., 2021; Shakil et al., 2020; Keskitalo et al., 2021). Across
incubations conducted including slump POC, we found a maximum of 4 % of
the initial TOC was lost within 27 d and estimate that this would scale
to approximately 7 % of slump-derived TOC being lost during transport in
streams. Changes in DOC and POC fractions suggest that this loss is
primarily driven by losses in DOC, with slump POC displaying low
biodegradability across our incubations. This finding, in addition to
previous findings that show that the majority of sediment lost from slumps
is quickly deposited in debris tongues rather than immediately transported
downstream
(Kokelj
et al., 2021; Shakil et al., 2020), suggests that the majority of the TOC
exiting the slump scar zone is likely to be sequestered in sediments after
mobilization to streams. While our experiments examine material exiting
rather than within the slump scar zone, thus missing potential within-slump
degradation, our estimates highlight that POC degradation rates may be much
lower than those for DOC – an important consideration for models of
permafrost organic matter degradation, particularly given the dominance of
organic matter export in the particulate form from thermokarst features. Our
results indicate that even when in suspension, thaw slump-derived POC on the
Peel Plateau may be subject to far slower rates of degradation than
estimates used in models of carbon release from abrupt permafrost
thaw (Turetsky et al., 2020), underlying the need to constrain regional
variation in a component of the carbon cycle that has undergone substantial
perturbation. Further, increased input of minerals alongside increases in
organic carbon into streams creates significant potential for carbon
sequestration via abiotic (sorption, mineral protection) and biotic
(chemolithotrophy) processes. Targeted investigations of these multiple
processes acting simultaneously on carbon dynamics require specific
quantification in landscapes experiencing rapid change.
Data analysis details and ANOVA results
The percent change in OC was used to measure differences in
biodegradability:
%ΔOC=OCTn-OCTOaverageOCTOaverage.
In Eq. (A1) above, OCTn is the DOC, POC, or TOC measured at an end
time point, and OCTOaverage is the mean OC measured at the beginning
of the experiment. Since multiple outcomes were tested for ANOVAs, p values
of main tests were corrected for false discovery rate using p.adjust from R package “emmeans” (Lenth, 2021). For 2015, main tests
were corrected for 19 tests since the SU treatment was tested in four
comparisons, and three to four outcomes were tested per comparison. For 2016, main
tests were corrected for four to five tests since four to five outcomes were tested per ANOVA.
Follow-up Tukey- or Games–Howell-adjusted pairwise t tests were conducted
only when an interaction or main test of interest was significant. Prior to
PCA we (a) used Pearson correlations to remove variables such that no
variables within the PCA had a Pearson correlation greater than 0.9, (b) log-transformed all variables to prevent skew, and (c) conducted a detrended
correspondence analysis to ensure linearity of the dataset.
Two-way ANOVAs of tests of sources, filtrate, and settling on biodegradability of POC in 2015 experiments. Follow-up Tukey-adjusted pairwise t tests are shown where significant interactions were present. Since SU treatments were tested three times and multiple outcomes were tested, p values reported in two-way ANOVAs were corrected for false discovery rate (19 tests). Degrees of freedom associated with treatment, site, interaction, and residuals are 1, 2, 2, and 12 respectively for all tests. P values below 0.05 are in bold font. Italic font is used to differentiate follow-up Tukey-adjusted pairwise t tests from two-way ANOVA tests.
EffectTreatment Site Treatment × Site testVariableEstimateErrorF/t*pFpFpControlΔSUVA254––1.450.481.640.371.420.66(SU-UF)%ΔDOC––0.540.750.020.980.540.81%ΔTOC––0.490.750.070.980.980.80ln(k)––305.570.006.430.048.920.04HA2.010.1711.80≪0.001––––HB2.020.1711.83≪0.001––––HD1.130.176.64≪0.001––––SourceΔSUVA2542.220.340.140.986.510.08(SU-UU)HA0.400.113.800.00––––HB-0.060.11-0.520.61––––HD-0.070.11-0.700.50––––%ΔDOC-7.062.468.220.051.130.471.530.66log(%POC + 50)-0.190.0427.400.005.080.072.390.42%ΔTOC––2.220.341.730.370.780.80ln(k)172.430.0043.210.0067.840.00HA1.690.1511.60≪0.001––HB1.900.1513.04≪0.001––HD-0.280.15-1.890.08––FiltrateΔSUVA254––2.520.3411.810.010.410.81(SD-SU)%ΔDOC––0.050.872.710.212.900.42%ΔPOC––0.110.852.660.210.200.89%ΔTOC––0.370.752.660.210.720.80k––0.240.8120.120.000.190.88SettlingΔSUVA254––0.100.8510.460.010.770.80(SS-SU)%ΔDOC––5.840.100.110.980.390.81%ΔPOC––0.380.751.080.470.210.88%ΔTOC––0.000.961.360.430.430.81ln(k)-0.670.0781.680.0011.470.012.460.42
*F values are reported for two-way ANOVAs, and t values are reported for
follow-up pairwise t tests.
Welch’s ANOVA and follow-up Games–Howell pairwise t tests of differences between size fractions (2016 experiment). P values for main ANOVAs are adjusted for false discovery rate to account for multiple outcomes tested (five tests). Follow-up tests were only conducted when main ANOVA tests showed a significant difference. Size fractions: SN = 63–2000 µm, SL = 20–63 µm, SMSC =< 20 µm. P values below 0.05 are in bold font.
Follow-up tests VariableFdfnumdfdenompEstimate95 % errortdfpΔSUVA25441.6325.80.001SL vs. SN2.00E-021.75E-010.465.20.894SMSC vs. SN3.80E-011.75E-017.145.00.002SMSC vs. SL3.60E-012.60E-018.626.0<0.001%ΔDOC22.2124.90.007SL vs. SN-6.09E+001.97E+012.004.80.200SMSC vs. SN-1.45E+012.00E+015.403.40.018SMSC vs. SL-8.39E+001.12E+015.204.30.012%ΔPOC2.8124.50.161%ΔTOC5.1324.40.089ln(k)66.6625.9<0.001SL vs. SN-4.34E-054.01E-03-0.019.00.992SMSC vs. SN3.87E-024.01E-039.669.0<<0.001SMSC vs. SL3.87E-024.01E-039.679.0<<0.001
Two-way ANOVAs for transect experiment examining effects of filtrations (unfiltered vs. filtered) and distance (immediately downstream vs. 2.79 km downstream) from slump SE and interactions between the two. Since multiple (four to five) parameters from the same experiment are tested, p values (in bold if less than 0.050) were corrected for false discovery rate. Degrees of freedom associated with distance, filtration, interaction, and residuals are 1, 1, 1, and 12 respectively for all tests, except %ΔPOC for which degrees of freedom for distance and residuals are 1 and 14 respectively.
VariableDistance Filtration Distance × filtration Follow-up tests FpFpFpEst.SEtdfpΔSUVA2540.210.6595.590.04812.440.013DN vs. 2.79 km DN (filtered)-0.090.04-2.174120.050DN vs. 2.79 km DN (unfiltered)0.120.042.815120.016%ΔDOC7.390.03117.000.00310.870.013DN vs. 2.79 km DN (filtered)0.671.640.409120.689DN vs. 2.79 km DN (unfiltered)-6.961.64-4.253120.001%ΔPOC*12.010.031––––%ΔTOC1.710.2691.990.1840.000.949ln(k)12.650.02055.960.0000.950.466
* Percent change in POC not tested for filtration or interaction effect
because negligible POC concentrations in filtered treatments.
Characterization of each size fraction
Size fractions with smaller particle sizes had lower organic matter content
(lower percent organic carbon), and associated organic matter had a greater
14C age (Table B1). Extraction of size fractions was conducted in
triplicate, but one replicate of the size fraction > 63 µm
had to be removed due to optical density concerns (Supplement S3.8). A
biplot of PCA components 1 and 2 that explained 92 % of the variation in
optical indices of BEPOM revealed that extractions of organic matter in the
clay and silt size fractions had a greater relative contribution of UVA
humic-like peak C (Fig. B1), which has been characterized as terrigenous
organic matter that has undergone less chemical reworking than peak A
(Stubbins et al., 2014). While peak C was not negatively correlated with peak
A, it was strongly correlated with total absorbance per unit of sediment
extracted (Table S6).
Characteristics of size fractions used in 2016 experiment.
%POC indicates POC : TSS. SD = standard deviation. SEM = standard error of the mean.
Size fraction categoryUnfractionated2000–63 µm63–20 µm< 20 µmMean particle size11.948.921.675.78SD3.784.62.92.79F14CNA0.41130.13320.056SDNA0.00190.00120.0006Cal BP (percentn/a8020–7926 (88.3 %)19 791–19 292 (95.4 %)27 640–27 250 (95.4 %)representation)7896–7871 (7.1 %)%POC1.63.121.351.67SEM0.030.090.020.02Percent of sum initialNA12 %15 %73 %POC across fractions
NA: not available. n/a: not applicable.
Principal component analysis of optical indices for BEPOM of
different size fractions. PCA is shown in scaling 1. The grey circle marks
the circle of equilibrium contribution. Abbreviations of optical indices are
as in Table S5; SN = 63–2000 µm, SL = 20–63 µm, SMSC =< 20 µm.
Absolute changes in SUVA254
Absolute changes in SUVA254 in
2015–2016 experiments.
YearTreatmentSiteΔSUVA25495 % errorn2015Filtered upstream (UF)HA0.080.363HB0.110.703HD0.020.083Unfiltered upstream (UU)HA-0.250.203HB-0.060.353HD-0.020.443Slump in filtered upstream (SU)HA0.160.473HB-0.110.163HD-0.090.153Slump in filtered downstream (SD)HA0.200.283HB-0.070.063HD0.040.123SU settle (SS)HA0.190.413HB-0.050.203HD-0.200.6022016UnfractionatedSE0.460.4142000–63 µmSE0.060.15463–20 µmSE0.090.104< 20 µmSE0.450.094UpstreamSE-0.070.114Upstream filtered controlSE-0.070.074DownstreamSE-0.100.174Downstream filtered controlSE0.070.0542.79 km downstreamS2-0.010.0342.79 km downstream filtered controlS2-0.050.074Flow chart of 2019 experiment
Flow chart for processing of 2019 experiment. Note that
only two bottle replicates were used to assess changes in base-extracted particulate organic
matter (BEPOM) for sterilized
treatments. Analyses show particulate (POC) and dissolved (DOC) organic
carbon, dissolved organic matter (DOM) optics (absorbance and fluorescence),
dissolved (DIC) and particulate (PIC) inorganic carbon, percent particulate
nitrogen (PN) and sulfur (PS), dissolved inorganic nitrogen (DIN),
SO42-, weathering ions, and BEPOM.
Data availability
Data are openly available through the Polar Data Catalogue (CCIN reference no.: 13237) (Shakil et al., 2021).
The supplement related to this article is available online at: https://doi.org/10.5194/bg-19-1871-2022-supplement.
Author contributions
SS and SET led the design of the study. SS led data collection, data
analysis and interpretation, and manuscript writing. JEV contributed to the
initial development of the idea, and JEV and SZ contributed to study design
and data interpretation. SZ contributed to laboratory methods for data
collection. All authors contributed to manuscript writing.
Competing interests
The contact author has declared that neither they nor their co-authors have any competing interests.
Disclaimer
Publisher’s note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Acknowledgements
This work took place within the Gwich'in Settlement Region, and we are
thankful for support from the Tetlit Gwich'in Renewable Resources Council
and Western Arctic Research Centre. We are further thankful for the field
assistance of Christine Firth, Elizabeth Jerome, Andrew Koe, Joyce Kendon,
Maya Guttman, Luke Gjini, and Lindsay Stephen. Maya Guttman and Joyce Kendon
also helped experiment set-up and sample processing. Hailey Verbonac
assisted with O2 measurements during our 2015 experiments conducted in
Inuvik. This paper also benefitted from helpful discussions with Steve Kokelj with regards to field sampling and perspectives on landscape changes
in the region, Matthias Koschorreck and Rafael Marcé with regards to
chemoautotrophic processes, and Alex Wolfe who first provided advice to
broaden consideration of what affects oxygen and carbon dynamics. The
rotator used for incubations was designed and manufactured by technical
services staff in the Department of Mechanical Engineering at the University
of Alberta, supervised by Roger Marchand. Funding for this study was
provided by the Natural Sciences and Engineering Research Council (NSERC),
Polar Continental Shelf Program (Natural Resources Canada), Campus Alberta
Innovates Program, ArcticNet, CICan Cleantech Internship Program,
Environment Canada Science Youth Horizons Internship, Northern Scientific
Training Program, University of Alberta and UAlberta North, and the Aurora
Research Institute. Personal support to Sarah Shakil was provided by NSERC and the
Garfield Weston Foundation. Research for this paper was conducted under NWT
research licences 15685 (2015), 15685 (2016), 15887 (2017), and 16575 (2019).
Financial support
This research has been supported by the Natural Sciences and Engineering Research Council of Canada (grant nos. 430696, 444873, CGS-M, CGS-D, and USRA), the Aurora Research Institute (Aurora Research Fellowship grant), the W. Garfield Weston Foundation (MSc Northern Research Scholarship grant), the University of Alberta (U of A Northern Research Award grant), the Environment Canada (Science Youth Horizons Internship GCXE16S064 and Cleantech Internship grant), the Arctic Institute of North America (Grant-in-Aid grant), and the ArcticNet (project #13 grant), the Polar Continental Shelf Program (Natural Resources Canada) and the Campus Alberta Innovates Program (University of Alberta).
Review statement
This paper was edited by Nicolas Brüggemann and reviewed by two anonymous referees.
ReferencesAbbott, B. W., Larouche, J. R., Jones, J. B., Bowden, W. B., and Balser, A.
W.: Elevated dissolved organic carbon biodegradability from thawing and
collapsing permafrost, J. Geophys. Res.-Biogeo., 119, 2049–2063,
10.1002/2014JG002678, 2014.Attermeyer, K., Catalán, N., Einarsdottir, K., Freixa, A., Groeneveld,
M., Hawkes, J. A., Bergquist, J., and Tranvik, L. J.: Organic Carbon
Processing During Transport Through Boreal Inland Waters: Particles as
Important Sites, J. Geophys. Res.-Biogeo., 123, 2412–2428,
10.1029/2018JG004500, 2018.Berggren, M., Lapierre, J.-F., and del Giorgio, P. A.: Magnitude and
regulation of bacterioplankton respiratory quotient across freshwater
environmental gradients, ISME J., 6, 984–993,
10.1038/ismej.2011.157, 2012.Bock, E. and Wagner, M.: Oxidation of Inorganic Nitrogen Compounds as an
Energy Source, in: The Prokaryotes, edited by: Rosenberg, E., DeLong, E. F.,
Lory, S., Stackebrandt, E., and Thompson, F., Springer Berlin Heidelberg,
Berlin, Heidelberg, 83–118,
10.1007/978-3-642-30141-4_64, 2013.Bröder, L., Keskitalo, K., Zolkos, S., Shakil, S., Tank, S. E., Kokelj,
S. V., Tesi, T., van Dongen, B. E., Haghipour, N., Eglinton, T. I., and
Vonk, J. E.: Preferential export of permafrost-derived organic matter as
retrogressive thaw slumping intensifies, Environ. Res. Lett., 16, 054059,
10.1088/1748-9326/abee4b, 2021.Burd, K., Estop-Aragonés, C., Tank, S. E., and Olefeldt, D.: Lability of
dissolved organic carbon from boreal peatlands: interactions between
permafrost thaw, wildfire, and season, Can. J. Soil Sci., 100, 1–13,
10.1139/cjss-2019-0154, 2020.Calmels, D., Gaillardet, J., Brenot, A., and France-Lanord, C.: Sustained
sulfide oxidation by physical erosion processes in the Mackenzie River
basin: Climatic perspectives, Geology, 35, 1003,
10.1130/G24132A.1, 2007.
Coble, P. G.: Marine Optical Biogeochemistry: The Chemistry of Ocean Color,
Chem. Rev., 107, 402–418, 2007.Duk-Rodkin, A. and Hughes, O. L.: Surficial geology, Fort McPherson-Bell
River, Yukon-Northwest Territories, Geol. Surv. Can., Series Map 1745A, 10.4095/184002, 1992.Groeneveld, M., Catalán, N., Attermeyer, K., Hawkes, J.,
Einarsdóttir, K., Kothawala, D., Bergquist, J., and Tranvik, L.:
Selective Adsorption of Terrestrial Dissolved Organic Matter to Inorganic
Surfaces Along a Boreal Inland Water Continuum, J. Geophys. Res.-Biogeo., 125, e2019JG005236, 10.1029/2019JG005236,
2020.Helms, J. R., Stubbins, A., Ritchie, J. D., Minor, E. C., Kieber, D. J., and
Mopper, K.: Absorption spectral slopes and slope ratios as indicators of
molecular weight, source, and photobleaching of chromophoric dissolved
organic matter, Limnol. Oceanogr., 53, 955–969,
10.4319/lo.2008.53.3.0955, 2008.Hemingway, J. D., Rothman, D. H., Grant, K. E., Rosengard, S. Z., Eglinton,
T. I., Derry, L. A., and Galy, V. V.: Mineral protection regulates long-term
global preservation of natural organic carbon, Nature, 570, 228–231,
10.1038/s41586-019-1280-6, 2019.Hugelius, G., Strauss, J., Zubrzycki, S., Harden, J. W., Schuur, E. A. G.,
Ping, C.-L., Schirrmeister, L., Grosse, G., Michaelson, G. J., Koven, C. D.,
O'Donnell, J. A., Elberling, B., Mishra, U., Camill, P., Yu, Z.,
Palmtag, J., and Kuhry, P.: Estimated stocks of circumpolar permafrost
carbon with quantified uncertainty ranges and identified data gaps,
Biogeosciences, 11, 6573–6593, 10.5194/bg-11-6573-2014,
2014.Huguet, A., Vacher, L., Relexans, S., Saubusse, S., Froidefond, J. M., and
Parlanti, E.: Properties of fluorescent dissolved organic matter in the
Gironde Estuary, Org. Geochem., 40, 706–719,
10.1016/j.orggeochem.2009.03.002, 2009.Keskitalo, K. H., Bröder, L., Shakil, S., Zolkos, S., Tank, S. E., van
Dongen, B. E., Tesi, T., Haghipour, N., Eglinton, T. I., Kokelj, S. V., and
Vonk, J. E.: Downstream Evolution of Particulate Organic Matter Composition
From Permafrost Thaw Slumps, Front. Earth Sci., 9, 181,
10.3389/feart.2021.642675, 2021.Klatt, J. M. and Polerecky, L.: Assessment of the stoichiometry and
efficiency of CO2 fixation coupled to reduced sulfur oxidation, Front.
Microbiol., 6, 484, 10.3389/fmicb.2015.00484, 2015.Kokelj, S. V., Kokoszka, J., van der Sluijs, J., Rudy, A. C. A.,
Tunnicliffe, J., Shakil, S., Tank, S. E., and Zolkos, S.: Thaw-driven mass
wasting couples slopes with downstream systems, and effects propagate
through Arctic drainage networks, The Cryosphere, 15, 3059–3081,
10.5194/tc-15-3059-2021, 2021.Kothawala, D. N., Kellerman, A. M., Catalán, N., and Tranvik, L. J.:
Organic Matter Degradation across Ecosystem Boundaries: The Need for a
Unified Conceptualization, Trends Ecol. Evol., 36, 113–122,
10.1016/j.tree.2020.10.006, 2021.Lacelle, D., Fontaine, M., Pellerin, A., Kokelj, S. V., and Clark, I. D.:
Legacy of Holocene Landscape Changes on Soil Biogeochemistry: A Perspective
From Paleo-Active Layers in Northwestern Canada, J. Geophys. Res.-Biogeo., 124, 2662–2679, 10.1029/2018JG004916, 2019.Lenth, R. V.: emmeans: Estimated Marginal Means, aka Least-Squares Means. R
package version 1.6.0, https://CRAN.R-project.org/package=emmeans, last access: 1 June 2021.Leewis, M.-C., Berlemont, R., Podgorski, D. C., Srinivas, A., Zito, P., Spencer, R. G. M., McFarland, J., Douglas, T. A., Conaway, C. H., Waldrop, M., and Mackelprang, R.: Life at the Frozen Limit: Microbial Carbon Metabolism Across a Late Pleistocene Permafrost Chronosequence, Front. Microbiol., 11, 1753, 10.3389/fmicb.2020.01753, 2020.Li, Y., Wu, Z., Dong, X., Xu, Z., Zhang, Q., Su, H., Jia, Z., and Sun, Q.: Pyrite oxidization accelerates bacterial carbon sequestration in copper mine tailings, Biogeosciences, 16, 573–583, 10.5194/bg-16-573-2019, 2019.Littlefair, C. A. and Tank, S. E.: Biodegradability of Thermokarst Carbon in
a Till-Associated, Glacial Margin Landscape: The Case of the Peel Plateau,
NWT, Canada, J. Geophys. Res.-Biogeo., 123, 3293–3307,
10.1029/2018JG004461, 2018.Littlefair, C. A., Tank, S. E., and Kokelj, S. V.: Retrogressive thaw slumps
temper dissolved organic carbon delivery to streams of the Peel Plateau,
NWT, Canada, Biogeosciences, 14, 5487–5505,
10.5194/bg-14-5487-2017, 2017.Mann, P. J., Eglinton, T. I., McIntyre, C. P., Zimov, N., Davydova, A.,
Vonk, J. E., Holmes, R. M., and Spencer, R. G. M.: Utilization of ancient
permafrost carbon in headwaters of Arctic fluvial networks, Nat. Commun., 6,
7856, 10.1038/ncomms8856, 2015.
Nelson, D. C., Jørgensen, B. B., and Revsbech, N. P.: Growth Pattern and
Yield of a Chemoautotrophic Beggiatoa sp. in Oxygen-Sulfide Microgradients,
Appl. Environ. Microbiol., 52, 225–233, 1986.Norris, D. K.: Geology of the Northern Yukon and Northwestern District of
Mackenzie Ottawa, Canada, Geol. Surv. Can., Series Map 1581A, 10.4095/120537, 1985.Ohno, T.: Fluorescence Inner-Filtering Correction for Determining the
Humification Index of Dissolved Organic Matter, Environ. Sci. Technol., 36,
742–746, 10.1021/es0155276, 2002.Olefeldt, D., Goswami, S., Grosse, G., Hayes, D., Hugelius, G., Kuhry, P.,
McGuire, A. D., Romanovsky, V. E., Sannel, A. B. K., Schuur, E. A. G., and
Turetsky, M. R.: Circumpolar distribution and carbon storage of thermokarst
landscapes, Nat. Commun., 7, 13043, 10.1038/ncomms13043,
2016.Opfergelt, S.: The next generation of climate model should account for the
evolution of mineral-organic interactions with permafrost thaw, Environ.
Res. Lett., 15, 091003, 10.1088/1748-9326/ab9a6d, 2020.Osburn, C. L., Handsel, L. T., Mikan, M. P., Paerl, H. W., and Montgomery,
M. T.: Fluorescence Tracking of Dissolved and Particulate Organic Matter
Quality in a River-Dominated Estuary, Environ. Sci. Technol., 46,
8628–8636, 10.1021/es3007723, 2012.Percak-Dennett, E., He, S., Converse, B., Konishi, H., Xu, H., Corcoran, A.,
Noguera, D., Chan, C., Bhattacharyya, A., Borch, T., Boyd, E., and Roden, E.
E.: Microbial acceleration of aerobic pyrite oxidation at circumneutral pH,
Geobiology, 15, 690–703, 10.1111/gbi.12241, 2017.Peter, S., Isidorova, A., and Sobek, S.: Enhanced carbon loss from anoxic
lake sediment through diffusion of dissolved organic carbon, J. Geophys.
Res.-Biogeo., 121, 1959–1977, 10.1002/2016JG003425,
2016.Poulin, B. A., Ryan, J. N., and Aiken, G. R.: Effects of Iron on Optical
Properties of Dissolved Organic Matter, Environ. Sci. Technol., 48,
10098–10106, 10.1021/es502670r, 2014.Richardson, D. C., Newbold, J. D., Aufdenkampe, A. K., Taylor, P. G., and
Kaplan, L. A.: Measuring heterotrophic respiration rates of suspended
particulate organic carbon from stream ecosystems: Measuring respiration
rates of POC, Limnol. Oceanogr. Methods, 11, 247–261,
10.4319/lom.2013.11.247, 2013.Schädel, C., Bader, M. K.-F., Schuur, E. A. G., Biasi, C., Bracho, R.,
Čapek, P., De Baets, S., Diáková, K., Ernakovich, J.,
Estop-Aragones, C., Graham, D. E., Hartley, I. P., Iversen, C. M., Kane, E.,
Knoblauch, C., Lupascu, M., Martikainen, P. J., Natali, S. M., Norby, R. J.,
O'Donnell, J. A., Chowdhury, T. R., Šantrůčková, H., Shaver,
G., Sloan, V. L., Treat, C. C., Turetsky, M. R., Waldrop, M. P., and
Wickland, K. P.: Potential carbon emissions dominated by carbon dioxide from
thawed permafrost soils, Nat. Clim. Change, 6, 950–953,
10.1038/nclimate3054, 2016.Schuur, E. A. G., McGuire, A. D., Schädel, C., Grosse, G., Harden, J.
W., Hayes, D. J., Hugelius, G., Koven, C. D., Kuhry, P., Lawrence, D. M.,
Natali, S. M., Olefeldt, D., Romanovsky, V. E., Schaefer, K., Turetsky, M.
R., Treat, C. C., and Vonk, J. E.: Climate change and the permafrost carbon
feedback, Nature, 520, 171–179, 10.1038/nature14338, 2015.Segal, R. A., Lantz, T. C., and Kokelj, S. V.: Acceleration of thaw slump
activity in glaciated landscapes of the Western Canadian Arctic, Environ.
Res. Lett., 11, 034025, 10.1088/1748-9326/11/3/034025, 2016.Shakil, S., Tank, S. E., Kokelj, S. V., Vonk, J. E., and Zolkos, S.:
Particulate dominance of organic carbon mobilization from thaw slumps on the
Peel Plateau, NT: Quantification and implications for stream systems and
permafrost carbon release, Environ. Res. Lett., 15, 114019,
10.1088/1748-9326/abac36, 2020.Shakil, S., Tank, S., Vonk, J., and Zolkos, S.: Incubation Data Assessing Biodegradability of Organic Carbon Mobilized from Permafrost Thaw Slumps (Peel Plateau, NT, Canada), Waterloo, Canada: Canadian Cryospheric Information Network (CCIN), 10.21963/13237, 2021.Sinha, B. and Annachhatre, A. P.: Partial nitrification – operational
parameters and microorganisms involved, Rev. Environ. Sci. Biotechnol., 6,
285–313, 10.1007/s11157-006-9116-x, 2007.Spencer, R. G. M., Mann, P. J., Dittmar, T., Eglinton, T. I., McIntyre, C., Holmes, R. M., Zimov, N., and Stubbins, A.: Detecting the signature of permafrost thaw in Arctic rivers, Geophys. Res. Lett., 42, 2830–2835, 10.1002/2015GL063498, 2015.Spona-Friedl, M., Braun, A., Huber, C., Eisenreich, W., Griebler, C.,
Kappler, A., and Elsner, M.: Substrate-dependent CO2 fixation in
heterotrophic bacteria revealed by stable isotope labelling, FEMS Microbiol.
Ecol., 96, fiaa080, 10.1093/femsec/fiaa080, 2020.Stolper, D. A., Revsbech, N. P., and Canfield, D. E.: Aerobic growth at
nanomolar oxygen concentrations, P. Natl. Acad. Sci. USA, 107, 18755–18760,
10.1073/pnas.1013435107, 2010.
Stumm, W. and Morgan, J. J.: Aquatic Chemistry: Chemical Equilibria and
Rates in Natural Waters, John Wiley & Sons, 884 pp., ISBN 978-0-471-51185-4, 2012.Tank, S. E., Vonk, J. E., Walvoord, M. A., McClelland, J. W., Laurion, I.,
and Abbott, B. W.: Landscape matters: Predicting the biogeochemical effects
of permafrost thaw on aquatic networks with a state factor approach,
Permafr. Periglac. Process., 31, 358–370, 10.1002/ppp.2057,
2020.Tanski, G., Lantuit, H., Ruttor, S., Knoblauch, C., Radosavljevic, B.,
Strauss, J., Wolter, J., Irrgang, A. M., Ramage, J., and Fritz, M.:
Transformation of terrestrial organic matter along thermokarst-affected
permafrost coasts in the Arctic, Sci. Total Environ., 581/582, 434–447,
10.1016/j.scitotenv.2016.12.152, 2017.Tanski, G., Wagner, D., Knoblauch, C., Fritz, M., Sachs, T., and Lantuit,
H.: Rapid CO2 Release From Eroding Permafrost in Seawater, Geophys. Res.
Lett., 46, 11244–11252, 10.1029/2019GL084303, 2019.Tesi, T., Semiletov, I., Dudarev, O., Andersson, A., and Gustafsson, Ö.:
Matrix association effects on hydrodynamic sorting and degradation of
terrestrial organic matter during cross-shelf transport in the Laptev and
East Siberian shelf seas, J. Geophys. Res.-Biogeo., 121, 731–752,
10.1002/2015JG003067, 2016.Turetsky, M. R., Abbott, B. W., Jones, M. C., Anthony, K. W., Olefeldt, D.,
Schuur, E. A. G., Grosse, G., Kuhry, P., Hugelius, G., Koven, C., Lawrence,
D. M., Gibson, C., Sannel, A. B. K., and McGuire, A. D.: Carbon release
through abrupt permafrost thaw, Nat. Geosci., 13, 138–143,
10.1038/s41561-019-0526-0, 2020.Vachon, D., Sadro, S., Bogard, M. J., Lapierre, J.-F., Baulch, H. M., Rusak,
J. A., Denfeld, B. A., Laas, A., Klaus, M., Karlsson, J., Weyhenmeyer, G.
A., and Giorgio, P. A. del: Paired O2–CO2 measurements provide emergent
insights into aquatic ecosystem function, Limnol. Oceanogr. Lett., 5,
287–294, 10.1002/lol2.10135, 2020.Vonk, J. E., Mann, P. J., Davydov, S., Davydova, A., Spencer, R. G. M.,
Schade, J., Sobczak, W. V., Zimov, N., Zimov, S., Bulygina, E., Eglinton, T.
I., and Holmes, R. M.: High biolability of ancient permafrost carbon upon
thaw, Geophys. Res. Lett., 40, 2689–2693,
10.1002/grl.50348, 2013.Vonk, J. E., Tank, S. E., Mann, P. J., Spencer, R. G. M., Treat, C. C.,
Striegl, R. G., Abbott, B. W., and Wickland, K. P.: Biodegradability of
dissolved organic carbon in permafrost soils and aquatic systems: a
meta-analysis, Biogeosciences, 12, 6915–6930,
10.5194/bg-12-6915-2015, 2015a.Vonk, J. E., Tank, S. E., Bowden, W. B., Laurion, I., Vincent, W. F.,
Alekseychik, P., Amyot, M., Billet, M. F., Canário, J., Cory, R. M.,
Deshpande, B. N., Helbig, M., Jammet, M., Karlsson, J., Larouche, J.,
MacMillan, G., Rautio, M., Walter Anthony, K. M., and Wickland, K. P.:
Reviews and syntheses: Effects of permafrost thaw on Arctic aquatic
ecosystems, Biogeosciences, 12, 7129–7167,
10.5194/bg-12-7129-2015, 2015b.Ward, B. B.: Nitrification, in: Encyclopedia of Ecology, Elsevier, 351–358, 10.1016/B978-0-12-409548-9.00697-7, 2013.Weishaar, J. L., Aiken, G. R., Bergamaschi, B. A., Fram, M. S., Fujii, R.,
and Mopper, K.: Evaluation of Specific Ultraviolet Absorbance as an
Indicator of the Chemical Composition and Reactivity of Dissolved Organic
Carbon, Environ. Sci. Technol., 37, 4702–4708,
10.1021/es030360x, 2003.Wickland, K. P., Waldrop, M. P., Aiken, G. R., Koch, J. C., Jorgenson, M.
T., and Striegl, R. G.: Dissolved organic carbon and nitrogen release from
boreal Holocene permafrost and seasonally frozen soils of Alaska, Environ.
Res. Lett., 13, 065011, 10.1088/1748-9326/aac4ad, 2018.Wologo, E., Shakil, S., Zolkos, S., Textor, S., Ewing, S., Klassen, J.,
Spencer, R. G. M., Podgorski, D. C., Tank, S. E., Baker, M. A., O'Donnell,
J. A., Wickland, K. P., Foks, S. S. W., Zarnetske, J. P., Lee-Cullin, J.,
Liu, F., Yang, Y., Kortelainen, P., Kolehmainen, J., Dean, J. F., Vonk, J.
E., Holmes, R. M., Pinay, G., Powell, M. M., Howe, J., Frei, R. J.,
Bratsman, S. P., and Abbott, B. W.: Stream Dissolved Organic Matter in
Permafrost Regions Shows Surprising Compositional Similarities but Negative
Priming and Nutrient Effects, Global Biogeochem. Cy., 35, e2020GB006719,
10.1029/2020GB006719, 2021.
Zar, J. H.: Biostatistical Analysis, 5th Edn., Prentice Hall, Upper Saddle
River, N.J., 944 pp., ISBN 9780131008465, 2010.Zolkos, S. and Tank, S. E.: Experimental Evidence That Permafrost Thaw
History and Mineral Composition Shape Abiotic Carbon Cycling in
Thermokarst-Affected Stream Networks, Front. Earth Sci., 8, 152,
10.3389/feart.2020.00152, 2020.Zolkos, S., Tank, S. E., and Kokelj, S. V.: Mineral Weathering and the
Permafrost Carbon-Climate Feedback, Geophys. Res. Lett., 45, 9623–9632,
10.1029/2018GL078748, 2018.Zolkos, S., Tank, S. E., Striegl, R. G., and Kokelj, S. V.: Thermokarst
Effects on Carbon Dioxide and Methane Fluxes in Streams on the Peel Plateau
(NWT, Canada), J. Geophys. Res.-Biogeo., 124, 1781–1798,
10.1029/2019JG005038, 2019.Zolkos, S., Tank, S. E., Striegl, R. G., Kokelj, S. V., Kokoszka, J.,
Estop-Aragonés, C., and Olefeldt, D.: Thermokarst amplifies fluvial
inorganic carbon cycling and export across watershed scales on the Peel
Plateau, Canada, Biogeosciences, 17, 5163–5182,
10.5194/bg-17-5163-2020, 2020.