Introduction
Permafrost contains an estimated 1140–1476 Pg of soil organic
carbon (OC; ,
). It is considered to be a vulnerable
carbon (C) pool in a warming climate , as both models and observations indicate
that permafrost is warming and thawing in many regions
. Large uncertainties remain about the
magnitude, timing, and form of C loss to the atmosphere from
thawing permafrost; however, some studies suggest thawing
permafrost will release 19–208 Pg C by 2050 . A substantial portion (∼450 Pg) of the
deep permafrost C pool is stored in permafrost soils of the
yedoma region, both in undisturbed yedoma and in the
organic-rich sediments of thermokarst lake basins in the yedoma
region . Yedoma refers to the icy
late-Pleistocene loess-dominated permafrost soil type that
occurs predominantly in previously unglaciated lowland regions
of Beringia (NE Siberia, Alaska, and NW Canada;
, ;
, ). Typical yedoma
deposits in Alaska are 10–30 m thick, but can reach local
thicknesses greater than 60 m . Late Pleistocene
yedoma ice wedges range from 2 to 6 m in width and can extend
tens of meters below the ground surface
. In addition to being ice-rich, the OC
content of yedoma is high (2–5 %) for mineral soils
, and yedoma soils typically contain OC contents
10–30 times higher than that of similar loess-dominated
non-permafrost soils .
In a warming climate, or when subjected to ground-surface
disturbance, ice-rich yedoma is prone to thermokarst lake
formation, a process by which the melting of massive ground ice
forms depressions that fill with water . In
yedoma-dominated regions, up to 90 % of all lakes are
thermokarst lakes . Thermokarst lakes
strongly alter the local thermal balance of the land surface and
transfer heat from the water body into the underlying ground
more effectively than other land cover types , especially when the depth of a lake exceeds the
thickness of the winter lake ice. The enhanced heat flux
triggers rapid permafrost thaw and talik (thaw bulb) formation
underneath the lake , which can lead to the
mobilization of freshly thawed OC from yedoma
.
As permafrost thaws, it releases previously frozen OC, which can
subsequently be processed by microorganisms that produce methane
(CH4) and carbon dioxide (CO2). It is
estimated that arctic systems annually emit 15–50 Tg of
CH4 , a potent greenhouse gas which
has 34 times more global warming potential than CO2
over a 100-year time period . Many lakes formed
in permafrost-dominated landscapes, particularly yedoma-type
thermokarst lakes, have high rates of CH4 emission
. The CH4
emitted from arctic lakes largely originates in terrestrial
sources such as the Holocene soils of the lakes' watersheds
, the thaw of Holocene- and Pleistocene-aged
permafrost soil beneath and surrounding the lakes
, and decomposition of contemporary organic matter
(OM) in the lakes .
The location of CH4 production in thermokarst lake
systems is not well understood. Using radiocarbon dating, stable
isotopes, and spatial mapping of CH4 emissions within
lakes, concluded that the highest rates of
CH4 emission occur along thermokarst margins,
originating from actively expanding taliks. Numerical modeling
of CH4 production in thermokarst lakes confirmed
field-based observations that CH4 production is
concentrated along permafrost thaw margins of lakes
. Surface lake sediments contain contemporary
OM as well as re-deposited, thawed permafrost OM. Both sources
may fuel CH4 production. At greater sediment depths,
permafrost thaw in taliks under thermokarst lakes also supplies
substrate for methanogenesis. However, the labile fraction of
thawed permafrost OM is in limited supply so, given enough time,
CH4 production in thawed permafrost sediments can
diminish .
The objective of this study was to constrain the location and
magnitude of CH4 production in a thermokarst lake
environment. We conducted long-term anaerobic incubations using
sediment samples collected from a deep thermokarst lake core in
the center of the lake that extended from the sediment surface,
through the talik, and into the yedoma permafrost underlying the
talik. We compared CH4 production rates from the lake
core to those of samples collected from an adjacent permafrost
tunnel, which extended through yedoma deposits into the
underlying gravel. We also measured the thickness of the talik
at various locations in the lake to help constrain the spatial
variability of CH4 production in the lake.
Methods
Study site
The Vault Creek permafrost tunnel and Vault Lake (informal name;
65.0293∘ N, 147.6987∘ W) are located
approximately 40 km north of Fairbanks, Alaska, USA, in
a region characterized by discontinuous permafrost
(Fig. ). The region experiences a continental climate
with a mean annual air temperature of -2.39 ∘C and
274.6 mm mean annual precipitation (Fairbanks International
Airport 1981–2010, National Climate Data Center). The 220 m
long Vault Creek permafrost tunnel, previously described by
, extends through a 25 m thick yedoma sequence
and a 15 m thick gravel horizon down to bedrock. The tunnel
entrance is secured by a steel tube, making the top 8 m of the
profile inaccessible to sampling. Vault Lake (3200 m2,
4.6 m maximum depth, 3.7 m average depth; Walter Anthony, unpublished data), located within
60 m of the subsurface tunnel, is a first-generation
thermokarst lake formed by the melting of permafrost ground ice,
including massive ice wedges. Steep, eroding bluffs, tilted
spruce trees along the margins, and numerous CH4
bubbling seeps across the lake surface indicate that the lake is
still actively deepening and laterally expanding.
Location map (a), study site overview (b),
and Vault Lake morphology shown in cross section along the long
axis of the lake (c) for Vault Lake and Vault Creek
permafrost tunnel, Alaska, USA (65.0293∘ N,
147.6987∘ W). Height of the thaw bluffs was measured using
differential
GPS (DGPS). Depth of the Vault Lake talik was measured using the
delineated borehole transect (c). Thaw bluff heights, lake
depth, talik depths, and distance between boreholes (b, c)
are shown to scale. Vault Lake is a 3230 m2, 3.7 m deep
thermokarst lake with thermokarst bluffs (d) ranging from
0.2 to 4.5 m in height and a 5.9 m deep talik underneath. The
Vault Lake core was collected from the center of the lake
(BH14). The Vault Creek permafrost tunnel extends 220 to 40 m
depth below the ground surface. Ice wedges (e) are present
to 23 m depth. Tunnel sampling sites are marked as red dots;
numbers adjacent to the dots represent incubation sample
IDs. Photographs by K. M. Walter Anthony (d) and
J. K. Heslop (e).
We measured lake and talik depth in March 2013 by drilling
boreholes through thawed sediments to the permafrost table at
six points along a transect spanning the long axis of the lake
and at eight additional points distributed across the lake. Lake
area and thermokarst bank height were measured by differential
GPS (Leica Viva GS15, Leica Geosystems, Norcross, Georgia, USA)
in November 2013. Talik temperatures were measured in galvanized
steel tubes placed in two boreholes, one near the center of the
lake (borehole (BH) 13) and the other 6.1 m from an actively
expanding thermokarst margin (BH10). We installed temperature
sensors (Onset TMCx-HD, accuracy ±0.21 ∘C, Onset
Corporation, Bourne, Massachusetts, USA) at four depths below the
sediment–water interface within the talik (BH10: 0.5, 1.0, 6.2,
and 8.85 m; BH13: 0.5, 1.0, 5.7, and 6.2 m). With the
exception of a missing-data period between 25 July 2014 and 11 November 2014,
temperatures were recorded hourly from May 2013
through December 2014.
Sediment sample collection, preparation, and
characterization
During March 2013, a 590 cm long sediment core was collected
from the center of Vault Lake (BH14, 4.0 m water depth). Using
a Boart Longyear diamond core drilling system, continuous
sediment cores were retrieved by percussion coring with a split
spoon sampler. Sediment core sections were retrieved in 6 cm
diameter clear plastic liners inside of core barrels in
approximately 60 cm intervals from the same hole. Casing inside
the hole ensured that adjacent sediments did not slough. The sediment core captured nearly the entire sequence of
thawed lake sediments in the talik (550 cm) and the top 40 cm
of permafrost beneath the talik, though several short sections
of core were lost from tubes during retrieval
(Fig. ). Thawed core sections were sealed and stored
in the laboratory at 3 ∘C. The permafrost section was
sealed and stored in the laboratory at -10 ∘C.
We measured magnetic susceptibility on the intact Vault Lake
core using a loop sensor on an automated core logger (Geotek
MSCL-X, Geotek Limited, Daventry, Northamptonshire, United
Kingdom) at the Limnological Research Center Core (LacCore)
Facility in the University of Minnesota, Minneapolis, Minnesota,
USA. Then we split the core lengthwise and immediately sealed
and archived one half of the core at 3 ∘C for later use
in anaerobic incubations. This first half of the core was sealed
with four layers of oxygen- (O2) and moisture-barrier film
(Krehalon PC101, Filcon, Clare, Michigan, USA).
On the second half of the core, we conducted initial core
descriptions and made the following measurements at LacCore. We
imaged the core using a line scan camera (Geotek Geoscan-III,
Geotek Limited, Daventry, Northamptonshire, United
Kingdom). High-resolution magnetic susceptibility was measured
in 0.5 cm intervals using a point sensor on an automated core
logger (Geotek MSCL-XYZ, Geotek Limited, Daventry,
Northamptonshire, United Kingdom). We sampled sediments in
10 cm intervals along the core at a known volume (3 cm3). We
weighed samples at field moisture, then dried them at
105 ∘C for 48 h and reweighed them to determine the
weight loss compared to the total weight of the wet sample
(gravimetric water content) and dry sediment weight per unit
volume (dry bulk density). Smear slides were created from
samples taken every 10 cm along the core and analyzed under
a microscope to quantify the relative abundance of organic and
mineral matter, sponge spicules, and diatoms.
We quantified plant macrofossils in a subset of lake sediment
core samples (Table S1 in the Supplement). Macrofossil samples were sieved using
a 250 µm sieve, and the remaining plant material was
examined in a petri dish in a slurry with deionized water using
a binocular microscope. Relative percentages of each macrofossil
type were calculated for each sample. Any macrofossils worth
noting (seeds, leaves, needles, etc.) that did not comprise
a large enough fraction of the sample were counted separately
and simply noted as present.
In addition to the lake sediment core, triplicate samples of
permafrost soils exposed in the Vault Creek tunnel adjacent to
the lake were collected using a 75 cm3 hole saw mounted
on a hand-held hammer drill from 16 distributed depths along the
tunnel walls. The sediment samples represented the accessible
profile from 9 to 40 m vertical depth beneath the ground
surface. Permafrost samples were collected from both the yedoma
horizon and the silty matrix of the underlying gravel
horizon. Permafrost samples were stored in the laboratory at
-10 ∘C until further analyses.
The Vault Lake core with the five facies (Organic-rich mud,
Lacustrine silt, Taberite, Recently thawed taberite, and
Transitional permafrost) delineated. Core depth values are
indicated; core subsampling locations for incubations are marked
using red arrows with bold numbers indicating incubation sample
IDs; locations of picked and 14C dated macrofossil are shown
by blue arrows and calibrated 14C ages (calendar years
BP). Labeled breaks represent gaps in the sediment
core.
Sediment facies classification
Using the imagery, smear slides, macrofossil data, and our
knowledge of thermokarst lake sediment facies classification
based on , , and
, we classified the Vault Lake core
into five facies for subsampling: Organic-rich mud, Lacustrine
silt, Taberite, Recently thawed taberite, and Transitional
permafrost (Fig. ).
Organic-rich mud consisted of the top 152 cm thick section of
the lake core containing alternating layers of dark,
organic-rich sediments, some peaty layers with variable sized
plant debris, and lighter, mineral silt dominated layers. Smear
slides and macrofossil analyses revealed relatively higher
abundances of aquatic and terrestrial macrofossils indicative of
the thermokarst-affected late-Holocene black spruce ecosystem
with peaty, organic rich soil that is still observed around the
lake today. Aquatic indicators included spicules, diatoms,
Daphnia ephippia, and benthic mosses. Terrestrial
macrofossils included mosses (Aulacomnium palustre,
Sphagnum spp., Tomenthypnum nitens,
Polytrichum spp.), and roots and leaves of ericaceous
shrubs, and spruce (Picea spp.) needles (Table S1).
The Lacustrine silt facies, 178 cm thick (153–330 cm) and
underlying the Organic-rich mud, consisted of massive mineral
sediment with occasional peat balls, representing material that
sloughed off exposed thermokarst margins of the lake. Since
sediments of Lacustrine silt were exposed to the lake water
column during erosion and re-deposition, they contained some,
albeit fewer, aquatic indicators (spicules, diatoms, and
Daphnia ephippia). Few other macrofossils were found in
the silt-dominated matrix. These were primarily undifferentiated
organic detritus with a few fragments of terrestrial mosses,
bark, and ericaceous rootlets.
Taberite sediments (331–550 cm), representing yedoma sediment
which thawed in situ and remained underneath the lake
, were identified as
massive, mostly mineral (silt)-dominated sediments. We did not
find diatoms or any other aquatic indicators in this facies. The
little OM that was present was dominated by fine,
indistinguishable detritus. The bottom 43 cm of the
taberite (508–550 cm), representing the most recently thawed sediments,
was designated as Recently thawed taberite.
Beneath the thawed portion of the lake core, we sampled 40 cm
of transitional permafrost (551–590 cm), which is close to the
thaw transition with a large amount of unfrozen water in the
inter-pore space but with numerous small lenses of bulk ice
still present . The Transitional
permafrost section of our core was ice-bearing, silt-dominated
soil with few organic remains, identified as graminoid detritus
indicative of the cold, dry Pleistocene steppe ecosystem.
Lake age
We estimated the age of Vault Lake by obtaining accelerator mass
spectrometry (AMS) radiocarbon dates on terrestrial plant
macrofossils picked from the lake center sediment core. Samples
were analyzed at the National Ocean Sciences AMS (NOSAMS)
facility (Table ). All radiocarbon ages were
calibrated to calendar 2σ years before present (BP) using
Calib 7.0 .
Radiocarbon ages of macrofossils picked from the Vault Lake sediment core,
calibrated to calendar 2σ years before present (BP) using Calib 7.0 .
Depth in
Lab ID
NOSAMS
14C age
Calibrated age
Average age
Material dated
core
ID
(yr BP)
(cal yr BP)
(cal yr BP)
(cm)
45
VAULT13-14A-1G-1-W
122576
150±25
172–223
195
Bryophyte
48–50 cm
(Aulacomnium palustre)
62
VAULT13-14A-3N-1-W
122577
240±20
285–303
295
Bryophyte
10.5–12.5 cm
(Aulacomnium palustre),
leaf fragments from ericaceous
shrubs or Betula nana,
Picea needles
72
VAULT13-14A-3N-1-W
122578
320±30
356–432
394
Bryophyte
20.5–22.5 cm
(Aulacomnium palustre)
144
VAULT13-14A-4N-1-W
122579
170±20
170–214
190
Bryophyte
8–19 cm
(Aulacomnium palustre),
leaf fragments from ericaceous
shrubs or Betula nana,
Picea needles
214
VAULT13-6N-1-W
122580
315±20
375–429
405
Leaf fragments from ericaceous
1–3 cm
shrubs or Betula nana
Geochemical analyses
Sediment samples (3 cm3) from both the core and tunnel were
oven-dried (105 ∘C for 48 h), homogenized using
a mortar and pestle, and analyzed for total C
(Ctot) and nitrogen (Ntot),
Ctot : Ntot ratios, and
isotope ratios δ15Ntot using an
elemental analyzer (Finnigan DeltaPlus XP, Thermo Scientific)
coupled to a Costech ECS4010 Elemental Analyzer (Costech
Scientific, Valencia, California, USA) at the University of
Alaska Stable Isotope Facility, Fairbanks, Alaska, USA. A
subsample of the homogenized oven-dried sediments was acidified
using muriatic acid (31.45 % HCl), rinsed five times with deionized (DI)
water, and used to measure total organic carbon
(Corg) and δ13Corg on the
same elemental analyzer. Measurement of an internal laboratory
standard (peptone, n=7) indicated measurement precision of
≤0.4 ‰ for both sets of C and N
isotopes. Sediment Ctot, Corg, and
Ntot contents are reported in weight percentage
(wt %). δ13Corg and
δ15Ntot contents are reported in parts per
mil (‰). All δ13Corg and
δ15Ntot values are expressed relative to
Vienna Pe Dee Belemnite (VPDB) and ambient air, respectively. We
report all results in mean ± standard deviation (SD).
Anaerobic laboratory incubations
Sediment slurries were prepared and incubated in triplicate for
21 depths along the Vault Lake core and 16 depths along the
Vault Creek permafrost tunnel (Figs.
and ; Table ). We homogenized sediment
samples under anaerobic conditions with O2-free, sterilized
DI water while flushing the slurry with ultra-high-purity (UHP)
N2 gas (Air Liquide, Houston, Texas, USA) in a solution
container. Subsamples of slurry were oven-dried (105 ∘C
for 48 h) and analyzed for dry sediment Ctot,
Corg, Ntot,
δ13Corg, and δ15Ntot
contents using the methods outlined above. Fifty milliliters of the
anaerobic slurry was transferred to 100 mL glass serum bottles
(Wheaton, Millville, New Jersey, USA) using a pipette. Serum
bottles were degassed using a constant stream of UHP N2 gas
and sealed with butyl rubber stoppers (Bellco Glass, Vineland,
New Jersey, USA). The slurry in each incubation bottle was
reduced by injecting L-cysteine (Sigma-Aldrich, St. Louis,
Missouri, USA) to a concentration of 0.025 % (wt / v;
, ). Anaerobic
conditions in the bottles were subsequently verified by
measuring O2 concentrations in the headspace using gas
chromatography (Shimadzu GC-2014, Shimadzu, Kyoto, Japan). We
incubated the bottles at 3 ∘C until linear
CH4 production rates were achieved in all Vault Lake
sediment core incubation bottles. The bottles remained sealed to
maintain anaerobic conditions throughout the incubation period,
which was 175 days for lake sediments and 220 days for
permafrost tunnel samples.
Vault Lake core and Vault Creek permafrost tunnel facies,
their depths and thicknesses, and the representative number of
samples used in anaerobic laboratory incubations.
Profile
Facies
Depth
Thickness
Incubation
(cm)
(cm)
samples (n)
Vault L. core
Organic-rich mud
0–152
152
6
Lacustrine silt
153–330
178
3
Taberite
331–507
177
6
Recently thawed taberite
508–550
43
4
Transitional permafrost
551–590
40
3
Vault Cr. tunnel
Permafrost (silt)
0–2400
2400
7
Permafrost
2400–4000
1600
9
(Silty matrix of gravel)
We measured headspace CH4 concentrations in each
incubation bottle every 30 days using gas chromatography
(Shimadzu GC-2014, Shimadzu, Kyoto, Japan). CH4
production potential rates were calculated by the slope of the
CH4 concentration in headspace over time. We
normalized CH4 production rates across incubation
bottles in two ways: dividing the CH4 production rates
by the mass of dry sediment and the mass of Corg in
each bottle. CH4 production rates are reported in
units of µg C-CH4
gdw-1d-1 and µg C-CH4 gCorg-1d-1. We
calculated whole sediment-column CH4 production
(µg C-CH4 cm-2d-1) for the
center lake core as the sum of facies' products of mean
CH4 production (µg C-CH4
gdw-1d-1), dry bulk density
(gcm-3), and facies thickness (cm). Using
a conversion factor of 0.01, we report whole sediment-column
CH4 production in units of g C-CH4
m-2d-1.
Statistics
Sediment characteristics (dry bulk density, gravimetric water
content, Ctot, Corg, Ntot,
Ctot:Ntot,
δ13Corg, and
δ15Ntot) and CH4 production
rates (µg C-CH4
gdw-1d-1, µg C-CH4
gCorg-1d-1) were tested for normal
distribution using the Jarque–Bera test (MATLAB R2013a Student
Version, MathWorks, Natick, Massachusetts, USA). All parameters
except CH4 production rate expressed as µg C-CH4 gCorg-1d-1 were
non-normally distributed (α=0.05); therefore,
differences among facies were tested for statistical
significance using Wilcoxon rank sum tests (MATLAB R2013a
Student Version). Differences were considered statistically significant when
p≤0.05. Spearman's rank correlation coefficients (MATLAB
R2013a Student Version)
were used to determine correlations between explanatory
variables (Ctot, Corg, Ntot,
and Ctot:Ntot ratios) and
anaerobic CH4 production (µg C-CH4 gdw-1d-1) in our incubations.
Results
Lake age, morphology, and talik temperatures
Calibrated ages of macrofossils picked from the Organic-rich mud
facies of the Vault Lake core ranged from 190±20 to 405±25 calendar years BP (Fig. ; Table ).
Lake water depths determined at the borehole locations ranged
from 0.7 to 4.6 m (mean ± SD 2.9±1.1 m, n=14
boreholes). Talik depths below the
sediment–water interface ranged from 0 to 8.8 m (mean ±
SD 5.9±2.1 m, n=14 boreholes). Figure c
shows borehole data for the long axis of the lake, including the
lake center borehole (BH 14) from which our sediment core was
taken. The talik was ∼50 % thicker adjacent to the
southern thermokarst margin than it was in the lake center.
Vertical profile temperatures measured along the borehole in the
center of Vault Lake, where water depth was 4.0 m and talik
thickness was 5.7 m, ranged from -0.40 to 4.22 ∘C
(Fig. a). In the borehole adjacent to the southwest
thermokarst margin (6.1 m offshore), talik thickness was
greater (8.6 m), lake water shallower (1.4 m), and sediment
temperatures were warmer than in the lake center (-0.40 to
14.00 ∘C; Fig. b). In both profiles,
temperatures in the shallower sediment depths (-0.5 to
-1.0 m (0 m = sediment/water interface), temperature range
0.14 to 14.00 ∘C, mean annual temperature
3.57 ∘C) were warmer and showed clear seasonal
variations (Fig. a and b). Temperatures along the
thaw boundary (-5.70 to -8.85 m, temperature range -0.40
to 2.07 ∘C, mean annual temperature 0.13 ∘C)
were colder and relatively more consistent throughout the year.
Vertical profile temperatures measured in the lake center
(<1 m from BH14; a) and 6.1 m offshore of the
southwestern thermokarst margin (b). Negative depth
values indicate depth from the sediment–water interface (0 m) at
each of the examined boreholes. Loggers did not record
temperature between 25 July and 11 November 2014.
The temperature data also indicated a strong thermal lag and
lateral offset in the propagation of summer heat into deeper
sediments and the lake center. The maximum temperature in the
near-shore surface sediments (14.00 ∘C at -0.5 m)
occurred on 10 August 2013, while maximum temperature at
-6.2 m (2.07 ∘C) and -8.9 m (-0.03 ∘C)
depths in the same vertical profile occurred on 22 October 2013.
In the lake center, where water depth was deeper, the
maximum temperature of 4.22 ∘C occurred on 24 September 2013 at -0.5 m sediment depth, and also showed a thermal lag
of heat propagation to greater depths in the same profile.
Sediment attributes
Sediment properties in the Vault Lake core and Vault Creek
permafrost tunnel varied by facies and are summarized in
Table . Magnetic susceptibility (MS) results indicate
two points with low MS values at approximately 100 and
400 cm depth in the lake core (Fig. ). These points
are associated with diamagnetic materials in the core,
potentially indicative of tephra. Tephra layers were observed in
the Vault Creek permafrost tunnel, albeit at greater depths
below the surface . The Organic-rich mud facies
had the lowest bulk density values in the lake core (Table ). Differences in dry bulk density values between
the remaining facies and between the silt and silty matrix of
the gravel horizons of the permafrost tunnel were not
statistically significant. Organic-rich mud had higher
gravimetric water content values and Taberite had lower values
than the remainder of the core (Table ). High dry
bulk density values had a strong linear correlation with low
gravimetric water contents in samples (R2=0.74).
Summary of dry bulk density (ρ) and gravimetric water content (W) data
from the Vault Lake core and the Vault Creek permafrost tunnel. Data are presented as mean ± SD.
Sample
Sediment properties
Profile
Facies
ρ (gcm-3)
W (gwatergsediment-1)
Vault L. core
Organic-rich mud
0.79±0.29a
0.96±0.39a
Lacustrine silt
1.32±0.21
0.35±0.19
Taberite
1.22±0.33
0.29±0.04b
Recently thawed taberite
1.36±0.15
0.38±0.05
Transitional permafrost
1.29±0.06
0.52±0.10
Vault Cr. tunnel
Permafrost (silt)
0.89±0.28
0.91±0.40
Permafrost (Silty matrix of gravel)
1.28±0.21
0.44±0.20
a, b Different letters indicate significant
differences from other facies in the same profile at the α=0.05 level. Vault L. core and Vault Cr. tunnel profiles were
analyzed separately.
Depth profiles for magnetic susceptibility (MS), wet bulk
density (ρwet), dry bulk density
(ρdry), and gravimetric water content (W) in the
Vault Lake sediment core. Two MS values at 95.4 and 405.4 cm were
-493.1 and -488.0 m3kg-1, respectively (not
shown).
Sediment geochemical parameters varied among the lake core and
permafrost tunnel facies (Fig. ) and are summarized
in Table . The Organic-rich mud facies had the
highest soil C concentrations (mean ± SD Ctot6.01±1.90; Corg3.83±1.66) in the core
(Table ), while the Taberite facies had the lowest
C concentrations (mean ± SD Ctot1.21±0.44;
Corg0.84± 0.45). The remaining lake core facies
grouped together had lower C concentrations (Ctot1.24–2.52 %; Corg0.84–1.52 %). Table shows the C
concentrations for individual facies, but C concentrations among
the Lacustrine silt, Recently thawed taberite, and Transitional
permafrost were not significantly different from each other.
Summary of geochemical properties and stable isotopes measured on sediment
samples from the Vault Lake core and Vault Creek permafrost tunnel. Data are presented as mean ± SD.
Sample
Geochemistry
Profile
Facies
Ctot (% wt)
Corg (% wt)
Ntot (% wt)
Ctot:Ntot
δ13Corg (‰)
δ15Ntot (‰)
Vault L. core
Organic-rich mud
6.01±1.90a
3.83±1.66a
0.40±0.13a
15.0±2.5a
-28.11±1.12
3.05±0.34a
Lacustrine silt
1.60±1.24
1.04±0.58
0.13±0.07
11.3±2.7
-26.85±0.92
2.49±0.90
Taberite
1.24±0.44b
0.84±0.45b
0.12±0.04b
10.8±1.9b
-26.58±0.44a
3.70±1.46a
Recently thawed taberite
2.04±0.79
1.36±0.80
0.18±0.10
12.2±2.4
-26.94±0.51
2.72±1.13
Transitional permafrost
2.52±1.10
1.52±1.18
0.18±0.05
14.0±1.9
-27.28±0.39
2.88±0.98
Vault Cr. tunnel
Permafrost (silt)
11.88±1.75b
2.21±0.80a
1.91±1.60
8.8±4.1
-27.02±0.40
2.21±0.80a
Permafrost
18.77±6.80a
1.31±1.66b
3.19±3.32
11.2±9.4
-27.63±0.60
1.40±1.55b
(Silty matrix of gravel)
a, b Different letters indicate significant
differences from other facies in the same profile at the α=0.05 level. Vault L. core and Vault Cr. tunnel profiles were
analyzed separately.
Depth profiles for sediment geochemical characteristics:
total carbon (Ctot), organic carbon (Corg),
total N (Ntot), Ctot:Ntot ratios, δ13Corg, and
δ15Ntot. Values for the Vault Lake core are
in the top panel; values for the Vault Creek permafrost tunnel are in
the bottom panel.
High levels of Ctot were observed in the permafrost
tunnel (Table ), and differences between
relatively high levels of Ctot (mean 14.5 % wt)
and lower levels of Corg (mean 1.9 % wt)
indicate significant inorganic C content in the permafrost
tunnel soils (mean ± SD 86.5±8.4 % of total C). In
contrast, inorganic C was 33.4±17.8 % of total C in the
lake sediment core samples.
Among the lake core facies, the highest Ntot
concentrations occurred in the Organic-rich mud (0.40±0.13 % wt). The permafrost tunnel horizons also had high
Ntot concentrations (2.39±2.37 % wt). The
Taberite facies had the lowest
Ctot:Ntot ratios among all
facies and the Organic-rich mud had the highest
Ctot:Ntot ratios, but there
were no statistically significant differences among the other
core facies. The Taberite facies had higher
δ13Corg values (-26.58±0.44) than the
remaining lake sediment core facies (Table ).
CH4 production potentials and
depth-integrated CH4 production
Mean CH4 production potentials varied across the lake
core facies (Table ) and no CH4
production was observed in the thawed permafrost tunnel
samples. CH4 production potentials in the lake core
over the 175-day incubation period ranged from 0.002 to
8.08 µg C-CH4 gdw-1d-1
and 0.51 to 178.9 µg C-CH4
gCorg-1d-1 (Fig. ). The
highest production potentials were observed in the Organic-rich
mud and the lowest rates occurred in Taberites and Transitional
permafrost (Table ). We found that
Ctot (r=0.47, p=0.043) and Corg
(r=0.47, p=0.043) were positively correlated with
C-CH4 production. Total N and
Ctot:Ntot ratios were
unrelated to CH4 production potentials in our
incubations.
Summary of facies' CH4 production potentials and
depth-integrated CH4 production for the total sediment
column. Data are presented as mean ± SD. It should be noted
that, based on optical properties (Fig. ), dry bulk
density (mean ± SD, 1.31±0.07 gcm-3),
gravimetric moisture content (29±0.00 %), and
Corg (1.64 %) values measured on two samples in the
depth interval 77–97 cm of the core, which were similar to those
of Lacustrine Silt and very different from the remainder of the
Organic-rich mud facies segments, we applied CH4
production rates measured on Lacustrine silt samples to this 21 cm interval of the Organic-rich mud section. This was done because no
samples from this 21 cm thick, mineral-dominated segment of
organic-rich mud were represented in the
incubation.
Sample
CH4 production potentials
Sediment column CH4 production
(µgC-CH4d-1)
Profile
Facies
g dw-1
g Corg-1
g C-CH4
% Total
% Total
R
m-2d-1
C-CH4
column
(% production/
production
thickness
% thickness)
Vault L. core
Organic-rich mud
5.95±1.67a
125.9±36.2a
5.2
67
26
2.6
Lacustrine silt
0.45±0.39
24.6±12.8
1.1
14
30
0.5
Taberite
0.25±0.26b
17.9±13.6
0.5
7
30
0.2
Recently thawed taberite
1.18±0.61
59.6±51.5
0.7
9
7
1.2
Transitional permafrost
0.48±0.31
15.3±9.1b
0.2
3
7
0.5
a, b Different letters indicate significant differences
from other facies in the same profile at the α=0.05 level.
Depth profile for CH4 production potentials in
the Vault Lake core. Samples were anaerobically incubated at
3 ∘C. Methane production potentials are represented as
mean value ± SD among replicates and normalized per gram dry
weight sediment (a) and per gram organic carbon
(Corg; b).
The ratios (R) of facies' CH4 production potentials to
their thickness in the lake center sediment column revealed the
highest CH4 production per unit of sediment were in
the Organic-rich mud (R=2.6) and Recently thawed taberite
(R=1.2) facies, while R of the remaining facies was lower
(R=0.2 to 0.5; Table ). Specifically, the
Organic-rich mud facies, which represented 26 % of the
sediment column thickness, dominated (67 %) whole-column
CH4 production in the lake center sediment core
(5.2 g C-CH4 m-2d-1;
Table ). The Lacustrine silt facies, which
represented 30 % of the sediment column thickness, had the
second largest contribution (14 %; 1.1 g C-CH4
m-2d-1) to whole-column CH4
production. Results for other facies are shown in
Table .
Discussion
CH4 production potentials
Our study indicates that, in the center of the lake, the
Organic-rich mud facies contributed the most (67 %) to
whole-column CH4 production despite occupying a lesser
fraction (26 %) of sediment column thickness. This is
consistent with findings from a study examining an 8 m deep
Holocene permafrost core from the Lena Delta, in which the top
(125 cm) section of permafrost sediments were also found to
have the highest observed CH4 production in the
sediment column .
A main reason for the Organic-rich mud facies in our study
having high CH4 production potentials is its
relatively high OM content. Past studies have suggested that
CH4 production rates in natural ecosystems are
controlled by environmental conditions, including substrate
availability . Correlation analyses showed
that CH4 production in our study was positively
correlated to sediment Ctot and Corg
contents (p=0.043 for both). This indicates that facies with
higher C contents, such as the Organic-rich mud, would have
higher CH4 production potentials compared to other
facies.
Following the Organic-rich mud facies, relatively high
CH4 production potentials were also observed in the
Recently thawed taberite facies. However, the narrowness
of the Recently thawed taberite in the center of the lake
limited its overall contribution to total sediment column
CH4 production potentials. The thickest sequence,
which consisted of combined Lacustrine silt and Taberite facies
(60 % of total core thickness), had low CH4
production potentials, contributing only 21 % of whole
sediment column CH4 production potential. Our results
of higher CH4 production in the Recently thawed
taberite facies compared to the Lacustrine silt and Taberite
facies are consistent with model simulations of CH4
production in a thermokarst lake that showed CH4
production among the thawed yedoma horizons to be highest along
the talik's downward progressing thaw boundary
. Assuming homogenous C
contents along the full yedoma profile in numerical modeling, the higher CH4
production at the thaw boundary was explained by fresh OM made
available to microbial decomposition by permafrost
thaw. Conversely, lower CH4 production in the
overlying mineral-dominated sediments, which represent
permafrost that thawed earlier, is explained by earlier
microbial decomposition that previously exhausted a large
fraction of the labile C pool of permafrost sediment OM.
Total soil OC consists of various OC pools with turnover times
ranging from less than a year to up to thousands of years
. OC pool sizes and turnover times
significantly impact how OC behaves in the global C cycle and
remain a significant uncertainty in estimating how permafrost OC
will be processed as it thaws . In our study, CH4 production potential
rates in the Recently thawed taberite facies, which we estimate
thawed during approximately the previous decade based on
downward talik growth rates determined through numerical
modeling of a similar yedoma thermokarst lake system
, were approximately 4.7 times higher than
those in the overlying taberite, which we estimate thawed over
longer periods of time (up to 400 years). This suggests that,
upon thaw, labile fractions of OC in the talik are depleted over
decadal to century timescales and the remaining OC pool is less
susceptible to processing upon thaw. Prior studies of Siberian
yedoma suggest that approximately 30 % of the yedoma C pool is
bioavailable upon thaw under anaerobic conditions in yedoma-lake
taliks . Under aerobic conditions,
5–30 % of the total C pool in both organic and mineral
circumpolar permafrost samples are estimated to have a rapid
turnover time (mean 0.35±0.6 years) upon thaw
. Remaining permafrost C contains
10–90 % “slow” C (mean turnover time 7.21±4.32
years) and 5–85 % “passive” C (mean turnover time
>2500 years; ,
) under aerobic decomposition
regimes. However, further research is necessary to determine the
relative sizes of permafrost C pools and better assess what
proportion of permafrost OC can be processed into CH4
upon thaw in an anaerobic thermokarst lake environment.
Within the Organic-rich mud facies, we observed higher
CH4 production potentials (g Corg-1)
near the surface of the Vault Lake core and slightly decreasing
CH4 production potentials with depth through the
facies. Surface lake sediments originate from both allochthonous
(terrestrial soils and vegetation) and autochthonous (i.e., lake
biota) sources . Surface sediments typically contain the most
recently deposited materials while deeper sediments represent
older deposited materials . The
higher CH4 production near the surface of the
Organic-rich mud facies may be explained by these more
recently deposited surface sediments containing fresher, more
labile substrates than the older, underlying sediments. Another
possibility is that autochthonous organic matter in the surface
sediments provides a labile C substrate that may prime decomposition
of more recalcitrant allochthonous C in the sediments, leading to
higher total CH4 production than in underlying sediments
that do not receive the autochthonous C.
CH4 production potentials versus observed emissions
Laboratory incubations measure maximum CH4 production
potentials, while CH4 emissions observed in the field
represent in situ production minus CH4 consumption,
dissolved CH4, and trapped CH4 accumulating
in the system . Because of this, the CH4 production
potentials measured in our study may be higher than in situ
CH4 production in the Vault thermokarst lake system,
possibly explaining the discrepancies among potential
CH4 production rates for the total lake center
sediment core in our study (2819 g CH4
m-2yr-1) and observed, lower CH4
emission rates from across Vault Lake (41 g CH4
m-2yr-1; ,
) and observations in the
literature for northern (>54∘ N) lakes (∼7 g
CH4 m-2yr-1;
, ). It is possible
that CH4 emissions at Vault Lake are underestimated due to
lake sediments storing large quantities of CH4 which are
released during rare extreme-low pressure events and are unlikely to
be captured by the ebullition ice-bubble surveys combined with
bubble trap measurements utilized by . Other
possible explanations for higher potential CH4
production rates observed by laboratory incubations in this
study compared to emissions observed through field measurements
are poor representation of spatial heterogeneity in the lake by
a single lake center core and CH4 oxidation. Aerobic
and anaerobic oxidation in sediments and the water column
consumes a significant fraction of CH4 produced in
lakes . A simulated CH4
production study found CH4 production in a modeled
thermokarst lake talik was up to 10 times higher than observed
emissions in the field . Produced
CH4 may also be oxidized in anaerobic environments
, but the
magnitude of anaerobic CH4 oxidation in lake
environments is not well understood. The combined effects of
aerobic and anaerobic CH4 oxidation may also account for,
in part, the higher talik CH4 production
potentials compared to lake emissions observed by
.
Differences between our incubation temperature (3 ∘C)
and actual temperatures in the talik environment may also lead
to some differences between the CH4 production
potentials observed in our incubations and in situ CH4
production at Vault Lake. Observed annual temperatures under
Vault Lake ranged from -0.4 to 14.0 ∘C
(mean ± SD 1.61±2.80 ∘C). Microorganisms show
increased methanogenesis with temperature increases
. Therefore,
depending on the actual temperatures throughout the talik
profile, CH4 production rates at a given time at Vault
Lake may be higher or lower than those measured at our reference
incubation temperature of 3 ∘C.
Spatial patterns of CH4 production and
emission within lakes
Previous field and modeling studies found CH4
emissions to be up to an order of magnitude higher along
thermokarst margins of yedoma lakes than in lake centers
. While our sediment incubation
study was limited to a single lake center core, other physical
data measured along the thermokarst margin at Vault Lake lend
support to the findings of previous studies. The talik was
50 % deeper along the expanding thermokarst lake margin
(measured 6.1 m offshore; Sect. 3.1) compared to the lake
center core site. This suggests that the Recently thawed
taberite facies would be substantially thicker along the lake
margin than in the lake center, though individual facies
thicknesses were not measured. This would also be consistent
with field measurements of 14C-CH4 ages being
older (35 000 to 43 000 years old) along yedoma
thermokarst lake margins compared to lake centers
. Based on field
observations of cross-basin sediment stratigraphy in other
yedoma lakes , it
is possible that the overlying lake sediment (Organic-rich mud
facies) is thinner along the lake margin than in our lake center
core; however, observed >2 m thick
lake sediments within 15 m of the shore in much larger Siberian
yedoma thermokarst lakes. Regardless of relative facies
thicknesses, the thermokarst margin zone of the lake was more
recently converted from permafrost-dominated terrestrial
landscape into an open-water lake environment compared to the
lake center core location. This suggests that sediments along
the margin have had less time to decompose and, therefore,
should have an overall higher fraction of labile OM remaining,
consistent with higher total-column CH4 production
rates described in the literature.
CH4 production in thermokarst, permafrost,
and non-permafrost systems
Among long-term anaerobic incubations (>115 days), observed
CH4 production rates in the Vault Lake sediment core
incubations at 3 ∘C were comparable to rates observed
in incubations of shallow (<1 m) permafrost from non-lake
environments in Alaska (0.01 to 1.14 µg C-CH4 gdw-1d-1; ,
). However, these soil samples were incubated
at a significantly higher temperature (15 ∘C), which
would yield higher CH4 production rates than
incubations performed at 3 ∘C. Terrestrial soils from
other shallow (< 1 m) permafrost and active layer sites in
Alaska incubated at 5 ∘C produced
CH4 at rates approximately an order of magnitude lower
than the Vault Lake sediments in our study. Some of the sampling
locations of and are
underlain by yedoma-type permafrost; however, samples collected
from shallow surface depths (≤1 m) were likely disturbed and thawed at some
point during the Holocene, as indicated by the depth of
Pleistocene ice-wedge surfaces . Deeper Pleistocene-aged yedoma
soils (up to 5 m depth) from the Lena Delta region of Siberia
incubated at 4 ∘C had similar CH4 production
rates (approximately 0.1 to 1.3 µg C-CH4
gdw-1d-1; ,
) to the lake sediment facies dominated
by thawed yedoma in our study; however, these high rates in the
non-lake yedoma sediments in Siberia were only observed after
a significant lag time (average 963 days). Comparisons across
these studies suggest that undisturbed Pleistocene-aged yedoma
permafrost may have more biolabile OM than younger or previously
thawed yedoma permafrost soils. This conclusion is consistent
with findings of , in which yedoma samples had
the highest anaerobic C release per gram soil C among a variety
of mineral soil samples.
CH4 production rates in high latitude, non-permafrost
lake sediments (eight lakes in central Sweden) incubated at
4 ∘C were generally much lower (0.002 to
0.06 µg C-CH4 gdw-1d-1;
, ) than the rates we
observed in the Organic-rich mud facies of the Vault Lake sediment core. These
differences could be due to a combination of Vault Lake's yedoma
environment containing more biolabile OM derived from yedoma
permafrost thawing along lake margins as well as potentially
higher rates of Holocene-aged organic matter loading to Vault
Lake resulting from thermokarst expansion and high primary
production in and around the lake enhanced by nutrients released
from thawing yedoma .
It is interesting to note that studies of deep permafrost
(non-lake) sediments found that (a) no CH4 was
produced (, ; this
study), (b) CH4 production rates were an order of
magnitude lower , or (c) CH4 production
was only observed after a significant lag time
. In contrast, in studies of shallow-permafrost
sediments, CH4 production potentials were
observed in anaerobic incubations . We suggest a potential explanation for these
observations in the following section.
Role of modern methanogens in CH4 production from old C
The quantity of methanogens preexisting in soil samples can
influence the rate of methanogenesis in laboratory experiments
. In our study, all samples
collected from the Vault Lake core produced CH4 within
60 days of incubation, including the Transitional permafrost
samples at the bottom of the lake core. In contrast, no samples
collected from the Vault Creek permafrost tunnel had detectable
CH4 production during the observed 220 days of
incubation. A possible explanation for the lack of detectable
CH4 production in the permafrost tunnel could be
a paucity of viable methanogens naturally present in deep
permafrost soils . In previous anaerobic incubations of deep
permafrost, little or no CH4 production has been
observed and there was either no observed CH4
production (non-yedoma permafrost; ,
), a significant delay before detectable
CH4 production occurred (yedoma permafrost;
, ), or no
CH4 production until samples were inoculated with
modern lake sediments (yedoma permafrost;
, ; S. Zimov,
personal communication, 2002). Since we observed CH4 production in
the Transitional permafrost (thawing yedoma) beneath Vault Lake
but no CH4 production in the permafrost tunnel samples
(yedoma and underlying gravel horizons) it is possible that, in
thermokarst lake environments, CH4 produced from
yedoma OM requires the reproduction of modern and/or ancient
microbes along a thermally expanding substrate source as
permafrost thaws radially beneath lakes.