PUBLICATIONS
Geophysical Research Letters
RESEARCH LETTER
10.1002/2016GL068977
Key Points:
• Methane sea-air flux in the East
Siberian Arctic shelf region appears
larger than other shelf seas
• An under-ice accumulation of
methane during ice-covered seasons
is rapidly released at ice melt
• Sea-air methane flux is regionally
dominated by turbulence-driven
diffusive fluxes, not bubble fluxes
Supporting Information:
• Supporting Information S1
Correspondence to:
B. F. Thornton and M. C. Geibel,
[email protected];
[email protected]
Citation:
Thornton, B. F., M. C. Geibel, P. M. Crill,
C. Humborg, and C.-M. Mörth (2016),
Methane fluxes from the sea to the
atmosphere across the Siberian shelf
seas, Geophys. Res. Lett., 43, 5869–5877,
doi:10.1002/2016GL068977.
Received 4 APR 2016
Accepted 9 MAY 2016
Accepted article online 11 MAY 2016
Published online 6 JUN 2016
Methane fluxes from the sea to the atmosphere
across the Siberian shelf seas
Brett F. Thornton1,2, Marc C. Geibel2,3, Patrick M. Crill1,2, Christoph Humborg2,3,
and Carl-Magnus Mörth1,2
1
Department of Geological Sciences, Stockholm University, Stockholm, Sweden, 2Bolin Center for Climate Research, Stockholm,
Sweden, 3Department of Environmental Science and Analytical Chemistry, Stockholm University, Stockholm, Sweden
Abstract The Laptev and East Siberian Seas have been proposed as a substantial source of methane (CH4)
to the atmosphere. During summer 2014, we made unique high-resolution simultaneous measurements of
CH4 in the atmosphere above, and surface waters of, the Laptev and East Siberian Seas. Turbulence-driven
sea-air fluxes along the ship’s track were derived from these observations; an average diffusive flux of
2.99 mg m 2 d 1 was calculated for the Laptev Sea and for the ice-free portions of the western East Siberian
Sea, 3.80 mg m 2 d 1. Although seafloor bubble plumes were observed at two locations in the study area,
our calculations suggest that regionally, turbulence-driven diffusive flux alone accounts for the observed
atmospheric CH4 enhancements, with only a local, limited role for bubble fluxes, in contrast to earlier reports.
CH4 in subice seawater in certain areas suggests that a short-lived flux also occurs annually at ice-out.
1. Introduction
Seafloor CH4 seeps have attracted increased attention following the suggestion that massive releases of
subsea CH4 may have led to rapid climate shifts in the distant past [Kennett et al., 2000; Nisbet, 1990].
Seeps of CH4 are found in shallow shelf seas worldwide [Hu et al., 2012; Kodovska et al., 2016; Skarke et al.,
2014] and are estimated to contribute 18–48 Tg yr 1 [Hornafius et al., 1999] globally to the water column.
The amount of CH4 which reaches the atmosphere from subsea seeps is limited by dissolution into the water
column and methanotrophic microorganisms in the water column [Leifer and Patro, 2002; McGinnis et al.,
2006; Osudar et al., 2015], at the seafloor [Marlow et al., 2014], or below the seafloor [Overduin et al., 2015].
Nonetheless, reports of high levels of CH4 in the atmosphere above the Laptev Sea (LS) and East Siberian Sea
(ESS) with average concentrations of 2.97 and 2.66 ppm respectively, far above then-mean latitudinal values
of approximately 1.85 ppm [Shakhova et al., 2010a, 2010b], suggested a surprisingly large CH4 source from
these seas. The Laptev and East Siberian Seas are wide and shallow, with continental shelf areas averaging
48 and 58 m average depths, respectively (collectively the East Siberian Arctic Shelf or ESAS) [Jakobsson,
2002], and partly underlain by thawing remnant permafrost from the Last Glacial Maximum [Nicolsky et al.,
2012; Romanovskii and Hubberten, 2001]. The changing state of this system [Overduin et al., 2007] kindled
interest in the possibility of present-day or near-future large-scale release of CH4 from the ESAS. Such CH4
could originate from long-locked reservoirs, either deep thermogenic sources [Cramer and Franke, 2005],
degradation of Pleistocene and Holocene organic material released and transported from thawing permafrost on land [Charkin et al., 2011] or coastal erosion [Günther et al., 2013], increased primary production
due to less ice coverage in recent years [Arrigo et al., 2008], or CH4 produced by modern subsea permafrost
degradation [Dmitrenko et al., 2011]. Such enhanced degradation could be driven by seawater warming via
Atlantic intrusion [Westbrook et al., 2009] or warming due to seasonal ice loss [Hölemann et al., 2011].
©2016. The Authors.
This is an open access article under the
terms of the Creative Commons
Attribution-NonCommercial-NoDerivs
License, which permits use and distribution in any medium, provided the
original work is properly cited, the use is
non-commercial and no modifications
or adaptations are made.
THORNTON ET AL.
CH4 in surface waters enters the atmosphere via diffusion during ice-free conditions, with the flux rate controlled by temperature, relative sea-air concentrations, and wind [Wanninkhof, 1992, 2014]. It has also been suggested that shallow seawaters such as the ESAS are easily transited by sediment-released CH4-containing
bubbles [Leifer and Patro, 2002]; these bubbles released from the seafloor represent a short circuit, a rapid transport process for CH4 from seabed sources to the atmosphere which may avoid the water column loss processes
mentioned above. Such bubbling resulting in CH4 release to the atmosphere has been suggested to occur over
10% of the ESAS [Shakhova et al., 2010a]. Determining annual sea-air CH4 flux is complicated by ice cover, which
impedes and collects CH4 emissions for much of the year and may contribute to enhanced flux at ice-out. These
processes have been observed in lakes [Greene et al., 2014; Jammet et al., 2015; Lindgren et al., 2016].
CH4 FLUXES FROM SIBERIAN SHELF SEAS
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Here we combine unique data sets of spatial distributions of CH4 from continuous surface water and atmospheric measurements from the Swedish icebreaker Oden in the LS and ESS in July and August 2014 during
the SWERUS-C3 project. We report on the magnitude of CH4 transport to the atmosphere from any ESAS
sources. We do not speculate about any putative CH4 source because CH4 released to the atmosphere must
transit the water column and the sea-air interface. Our near-surface in situ approach integrates all local
CH4 sources.
2. Methods
We report here measurements from several overlapping segments of the cruise track divided into regions for
comparisons: (1) mostly ice-covered Arctic Ocean (AO) north of the Kara and LS, (2) the shelf break and upper
continental slopes to 500 m depth of the LS and ESS, (3) the ice-free segments of the LS and ESS, (4) the entire
LS (ice-free during this study), (5) the LS segments well removed from subsea gas seeps, (6) the ice-covered
segments of the ESS, and (7) the ice-free segments of the ESS.
Air was continuously pumped from four heights (averaging 9, 15, 20, and 35 m above sea level) during
SWERUS-C3. The exact heights varied with ship bunker load; when full at the beginning of the cruise, heights
were 70 cm lower, by late August, 60 cm higher than the listed heights. A 10 m meteorological tower provided mounts for the 15 and 20 m inlets. The 9 m inlet was mounted on a triangular bowsprit, 2.5 m forward
of the front deck rail. The 35 m inlet was located forward of the exhaust stacks on a cargo container atop
Oden’s bridge. At stations where the ship was anchored, the 15 m inlet was lowered to 4 m above the sea
surface. Air was pulled to the analysis system inside a lab; measurements were determined with a cavity
ring-down laser spectrometer (Model 0010, FGGA 24EP, Los Gatos Research (LGR), Mountain View,
California, USA). Raw measurements were collected at 1 Hz. Each inlet was monitored for 2 min at a time
and then switched to the next inlet in the sequence. The sequence for inlet sampling was 9, 20, 15 (or 4), then
35 m. The last 70 s of the 2 min dwell times after valve rotation are averaged for the values reported here. CH4
precision is 0.5 ppb; accuracy is 1.5 ppb. CO2 precision is 0.25 ppm; accuracy is 0.25 ppm. CH4 precision is
determined by target gas measurement variability and is similar to SD of 70 s averages. Target gases were
introduced every 2 h. Details are provided in the supporting information.
Wind speed and direction were used to filter the data to avoid air contaminated by the ship. We include data
collected when the relative wind speed was >2 m s 1, and the wind direction was >280° and <80° relative to
the ship’s bow. Any remaining observations with CO2 concentrations above 450 ppm were also removed.
After filtering, atmospheric CO2 and CH4 averaged 391.51 ± 0.58 ppm and 1.8834 ± 0.0026 ppm (both 2σ).
CH4 and CO2 in the surface water were continuously measured using the Stockholm University Water
Equilibration Gas Analyzer System (WEGAS; see supporting information). Seawater was collected from an
inlet at 8 m on the hull of Oden. After extraction with a showerhead equilibrator [Johnson, 1999] and drying,
CH4 and CO2 were analyzed by two cavity ring-down gas spectrometers (G2301 and G2131-i, Picarro, Inc,
Sunnyvale, California, USA). No special provision was made to exclude bubbles from the seawater intake.
The CH4 and CO2 concentrations measured by the WEGAS system thus include any bubbles in the sample
and will bias our flux calculation results toward higher values.
Ice coverage during the cruise was determined by visual observation and by satellite measurements from
the Special Sensor Microwave Imager/Sounder (SSMIS) instruments onboard the United States’ Defense
Meteorological Satellite Program satellites using a 91 GHz microwave radiometer channel for ice coverage
measurements. The analysis of these measurements was provided by the University of Bremen [Spreen
et al., 2008]. The resolution of these satellite data is approximately 13 × 15 km; ice coverage is reported as a
percentage within each grid cell.
CH4 mixing ratios in air and water CH4 concentration data, as well as ice coverage data for Oden’s position,
and other ancillary measurements, were integrated and extrapolated using R (version 3.1.2). The resulting
combined data set has a temporal resolution of approximately 10 s, which corresponds to an average spatial
resolution of about 0.0002 × 0.0009° (about 120 × 500 m in the LS).
Because the speed of Oden during the cruise varied greatly and included many stops for sampling (especially
around known seafloor CH4 seeps, biasing the raw data toward seep areas), a spatially normalized data set
THORNTON ET AL.
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Table 1. Spatially Corrected Air and Water CH4 Concentrations (ppm) by Region
Arctic Ocean
Shelf breaks and upper continental slope
Shelf seas (LS + ESS), ice-free regions
LS (all)
LS without seep areas
ESS, ice-covered/melt regions
c
ESS, ice-free regions
Min
Air
Max
Air
Mean
Air
Median
Air
Min
Water
Max
Water
Mean
Water
Median
Water
Total CH4
in Water
Observations (%)
Average
Sea Depth (m)
1.856
1.865
1.872
1.872
1.872
1.823
1.873
1.906
1.902
b
1.941
b
1.941
1.907
2.049
1.897
1.882
1.880
1.878
1.877
1.877
1.893
1.882
1.882
1.878
1.876
1.876
1.876
1.891
1.882
1.792
2.192
1.976
1.976
1.976
2.337
2.132
4.286
9.140
a
102.8
a
102.8
32.97
a
212.5
5.672
2.087
3.989
5.762
6.601
4.889
24.23
3.736
1.984
3.369
2.634
2.331
2.290
11.74
3.777
22,278 (24.7%)
8,079 (9.0%)
15,089 (16.8%)
10,672 (11.9%)
9,152 (10.2%)
20,330 (22.6%)
4,417 (4.9%)
2,170
250
53.6
56.5
54.3
49.3
46.5
a
Data outside the manufacturer’s specification for the analyzer. However, tests with calibration gases showed that the order of magnitude of these measurements
can be trusted (better than ±5 ppm).
b
Highest CH4 in air measured in LS excluding measurements at 4 m height while the ship was at anchor. If measurements at 4 m height are included, five 70 s
measurements exceeded 2 ppm, with a maximum of 2.172 ppm.
c
139–148.5°E.
was created. The spatially normalized data set was created by rounding all observations’ GPS coordinates to
0.001° then binning and averaging air and water data points which shared the same rounded coordinates.
We calculated the expected diffusive
water-air CH4 flux via the Wanninkhof
model, which takes as inputs wind
speed, water temperature, and water
and air concentrations [Wanninkhof,
2014]. This diffusive flux model is especially suited for regions of long fetches,
such as the open sea.
3. Results
A portion of the SWERUS-C3 cruise was
designed to sample the middle-to-outer
portions of the ESAS which had been
relatively undersampled in previous
regional studies. Therefore, the cruise
spent considerable time in relatively
deeper water areas of the continental
shelf but still in waters <60 m deep
(Table 1). Weather conditions during
SWERUS-C3 were relatively calm much
of the time, often with a very shallow
inversion layer, especially during the
ESS portion of the cruise [Tjernström
et al., 2015], which would tend to trap
gases, including CH4, in the planetary
boundary layer, causing local concentrations to rise.
3.1. Seafloor Gas Seeps and Surface
CH4 Enhancements
Figure 1. SWERUS-C3 Oden cruise track, 11 July to 17 August 2014, with
study regions indicated for (a) CH4 in air observations, (b) CH4 in seawater
observations, and (c) ice coverage at time of Oden passage (%). Dotted purple
curve indicates approximate limits of Arctic Ocean proper. Yellow dotted
lines indicate approximate top of continental slope when crossed by Oden.
Magenta dotted line indicates division between Laptev and East Siberian Seas.
THORNTON ET AL.
CH4 FLUXES FROM SIBERIAN SHELF SEAS
Relatively rare enhancements in air and
water CH4 observations above background levels (Figure S1 and Text S3 in
the supporting information) were generally spatially correlated and occurred
in regions with identified active seafloor
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gas seeps. Seafloor seeps were located
as gas bubble trains in the water column
observed with the ship’s sonar systems;
sonar measurements are otherwise
beyond the scope of this paper. By
design, the first half of the SWERUS-C3
cruise spent extended time sampling
around two regions of known seep activity in the LS and ESS. Methane in air was
above background levels in the region
around the first-encounted subsea seep
area in the LS, in waters ~65 m deep,
centered at 76.894°N, 127.798°E (the
seep center is defined here by the peak
in surface water CH4 measurements).
Mixing ratios as high as 2.172 ppm
(Tables 1 and S3) were observed though
not specifically collocated with the peak
in water CH4 enhancements of up to
~250 ppm (Figure S2 and Table 1). The
second seep region was centered at
74.957°E, 161.091°E in the ESS in icecovered seas (Figure 1c). Enhanced CH4
in the water (up to ~200 ppm) was less
geographically confined in the ESS than
in the LS (Figures 1 and 2), which may
be due to the sea ice cover. The ESS seep
region in ice-covered waters was confined within a few tens of kilometers of
Figure 2. Spatially corrected histograms of CH4 in (a) air (1 ppb bins) CH4
the peak-observed water concentrations
and (b) seawater (1 ppm bins) CH4. Red vertical dotted lines indicate
but with some apparent spreading of
where 70%, 80%, and 90% of the measured data, respectively, are less
the surface water CH4 out to ~300 km
than each line.
around the region of the most active
ebullition (Figure S3). Waters with sea ice concentrations below 90% were 200 km or more from the studied
seep region in the ESS (Figure 1c).
The seep region in the LS had no sea ice cover to impede the passage of CH4 from seawater to the atmosphere. Within approximately 100 m at the sea surface of the peak-observed water CH4, our estimated CH4
flux was 83 mg m2 d 1. The spatial extent of the high flux was limited; within 1 km, the flux was 58 mg m2 d 1
but within 100 km around the seep area; the average flux was estimated at 14 mg m2 d 1.
Generally, there was no significant difference (1σ values overlap) between atmospheric CH4 measured
between the lowest (9 m) and highest (35 m) inlets (Figure S4). At four anchor stations, we were able to lower
one air sampling inlet to 2–4 m above the sea surface, a strategy to better collect locally emitted CH4. Even
under these conditions, average gradient enhancements were minimal (Figure S4 and Table S3). At all four
anchor stations the ship rode the anchor downwind of the seep areas so the surface flow was across the seep
areas to our profile mast air intake array. In the LS on Julian day 199 while anchored near the seep area, CH4 in
air measurements did show an apparent gradient. Values reached 2.172 ppm CH4 at the temporary lowest
(4 m) air inlet, while the highest (35 m) air inlet showed values up to 1.90 ppm during the same 4 h period.
The mean concentrations during this same 4 h period were 1.95 ppm and 1.88 ppm at 4 and 35 m above
sea level, respectively, consistent with our flux calculations in this active seep area.
3.2. Regional Observations
On average, we observed slightly more CH4 in the atmosphere while traversing ice-covered regions in the ESS
and in the AO regions (Table 1). In the atmosphere, the most frequent CH4 concentration bin was 1.875 ppm
THORNTON ET AL.
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a
Table 2. Calculated Mean Sea-Air CH4 Fluxes by Region
Temporal (Cruise Track) Fluxes
(ng m
b
Arctic Ocean
Shelf breaks and upper continental slope
Shelf seas (LS + ESS), ice-free regions
LS (all)
LS without seep areas
b
ESS, ice-covered/melt regions
ESS, ice-free regions
2
s
18.4
148
190
24.2
287
41.2
1
)
(mg m
2
1.6
12.8
16.5
2.1
24.8
3.6
d
1
Average Observed Wind Speed
1
(Cruise Max 15.2 m s )
Spatially Corrected Fluxes
)
(ng m
2
s
1
)
16
44
45
17
190
43
(mg m
2
1.4
3.8
3.9
1.5
16.4
3.7
d
1
)
ms
1
± 1 SD
5.6 ± 1.3
5.1 ± 1.8
4.5 ± 3.3
2.9 ± 1.9
2.6 ± 1.8
5.1 ± 2.0
8.5 ± 2.2
a
Wind
b
measurements were collected at 35 m above sea surface. The numbers presented are corrected down to a 10 m level.
Includes ice-covered regions with sea ice where flux calculations represent theoretical fluxes for ice-free state and thus exceeding actual fluxes. No fluxes are
calculated for Arctic Ocean region due to high sea ice coverage.
(Figure 2a), essentially background levels. Enhancements were regarded as atmospheric CH4 concentrations
above a background level of 1.875 ppm, which was observed during the early AO section of the cruise, north
of Novaya Zemlya. For comparison, the NOAA flask measurements of CH4 for July 2014 were 1.882 ppm at
Zeppelin Mountain, Ny Ålesund, Svalbard, and 1.887 ppm at Barrow, Alaska (July was the minimum for
2014; in August, 1.882 ppm was measured at Zeppelin and 1.898 ppm was measured at Barrow). In the
seawater, the most commonly observed value is 2.5 ppm with 57% of the data being <5 ppm (more than
72% were below 10 ppm) (Table S1). A few rare observations (0.7% of total) exceeded 200 ppm in
seawater (max = 240 ppm).
In the spatially normalized data set, more than 70% of the surface water data points were <5 ppm. Eighty
percent of the surface water observations were <8.8 ppm, and 0.1% were >200 ppm (Figure 2b). The spatially
normalized data set (Table 1) better represents the true geographic distribution of CH4 in the cruise sampling
region. Figure 2 contains histograms of all discrete CH4 observations in the air and water along the cruise
track marked in Figure 1.
We calculated sea-air CH4 fluxes for the different study regions, using both the temporal and spatially
corrected data sets (Table 2). No fluxes were calculated for the AO region, due to continuous ice cover. We
calculated fluxes for the ESS, even though there was considerable ice cover (Figure 1c) in this region, thus
examining a scenario of no sea ice cover. The much higher surface water concentrations (Table 1) in the
ESS contribute greatly to the larger, hypothetical, 16.4 mg m2 d 1 fluxes in that region. In open waters
including seeps, fluxes averaged 3.8 mg m2 d 1.
4. Discussion
The methodological approach in this paper (high-resolution simultaneous continuous measurements in
surface waters and in the air) allows us to test two recently proposed hypotheses about sea-air exchange
of CH4 across the ESAS. The proposed extensive bubble flux from subsea CH4 seeps to the atmosphere could
not be validated for the middle and outer ESAS waters >35 m depth. Neither widespread strong enhancements of ambient CH4 nor strong near-surface gradients could be identified. Our high-resolution data set
rather suggests a spatially limited effect of gas seep bubbles on atmospheric CH4 mixing ratios. Instead,
diffusive fluxes alone can explain observed atmospheric mixing ratios that are slightly elevated in some areas
but much less than those reported for shallow inshore areas.
The average spatially normalized CH4 flux we derived for the entire studied open water ESAS region was
3.8 mg m2 d 1, is considerably higher than has been reported for many shelf seas, such as the North Sea
(0.04 mg m 2 d 1) [Bange et al., 1994] and subarctic Alaskan bays (0.008–0.013 mg m 2 d 1) [Kodovska
et al., 2016]. Although this study does not achieve the spatial coverage closer to shore as some previous work
[Shakhova et al., 2005, 2010a, 2010b, 2014], the cruise track provides us with a vast data set of synchronized
surface water and air measurements across a variety of ESAS environs near the average depth of the ESS
and LS; therefore, we have applied our average-calculated fluxes across the continental shelf areas of the
Laptev, East Siberian, and Chukchi Seas (2,105,000 km2); this would yield an annual flux of 2.9 Tg CH4 yr 1.
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However, important caveats of our limited spatial coverage must be applied to this estimate, and sea ice
covers these areas for about 70% of the year [Proshutinsky et al., 1999], as discussed below. Our annual flux,
not accounting for impermeable ice cover periods, is in rough agreement with previously reported regional
fluxes based on measurements in open waters of the LS and ESS, 1–4.5 Tg yr 1 [Shakhova et al., 2005], though
later work noted similar fluxes with additional contribution from subsea CH4 seeps [Salyuk and Semiletov,
2010]. A more recent study focused on shallower portions of the LS and ESS (6–24 m depth) and found an
average flux of 287 mg m 2 d 1, based on bubble fluxes derived from sonar [Shakhova et al., 2014]. This same
study also extrapolated shelf-wide fluxes as well, reporting a flux from subsea CH4 seeps to the atmosphere of
9 Tg CH4 yr 1, and a total flux (including diffusive fluxes) of 17 Tg CH4 yr 1 for the LS and ESS, by estimating
that their study accounted for 10% of extant ESAS seeps. Finally, we note that our in situ results (along with all
previous in situ results) are higher than a recent model of CH4 release (bubbling + diffusion) from the Siberian
continental shelves of 0.42 Tg yr 1 [Archer, 2015] but are similar to a recent modeling estimate based on
Pan-Arctic CH4 measurements from long-term monitoring stations on land [Berchet et al., 2016].
As noted above, our seawater CH4 measurements do not exclude bubbles, and over several days of continuous measurements crossing the seep regions, our seawater CH4 values certainly incorporate some bubbledelivered CH4 in the surface waters. There is a large difference in transport success of bubble-contained CH4
in 6 m deep water and 40 m deep water [McGinnis et al., 2006]; rapid exchange and loss of CH4 to the generally deeper water column in our study alone may account for some of this difference between our and
previous studies closer to shore [Sergienko et al., 2012; Shakhova et al., 2014], in shallower depths than studied
during SWERUS-C3 (>35 m). Assuming an average CH4 concentration for the entire water column, we calculated the necessary sustained fluxes to raise the atmospheric concentrations by various amounts. We
assumed a very shallow 200 m mixing height (very low inversions were observed during SWERUS-C3)
[Tjernström et al., 2015], a 35 m water column, and an 80 ppb atmospheric CH4 enhancement, raising atmospheric CH4 from apparent background levels of about 1.87 ppm to 1.95 ppm. This requires a sustained
CH4 flux of about ~12 mg m 2 d 1, similar to the fluxes we observed during SWERUS in areas near subsea
gas seeps on the ESAS (Table 2). An average flux near ESAS subsea seeps of 13 mg m 2 d 1 was reported
previously [Sergienko et al., 2012]. Sustaining higher atmospheric CH4 concentrations is even more difficult:
to sustain 2.1 ppm CH4 in the atmosphere (35 m water column, 200 m mixing height) would require a flux
similar to a subarctic wetland (~36 mg m 2 d 1) [Bartlett and Harriss, 1993] or an order of magnitude above
the average fluxes we observed in the ice-free ESAS. (Various mixing scenarios’ effects on atmospheric CH4
are shown in the supporting information Table S2.)
Surveys conducted in shallower waters, closer inshore, have reported substantially higher atmospheric CH4,
2.97 and 2.66 ppm average in the LS and ESS, respectively, with spikes to 8.2 ppm [Semiletov et al., 2012;
Shakhova et al., 2010a, 2010b, 2007]. Such average atmospheric mixing ratios of CH4 across large expanses
of the LS and ESS require sustained regional CH4 fluxes of roughly 75–200 mg m 2 d 1, depending on local
winds and atmospheric mixing. Such atmospheric enhancements are not sustainable by local diffusive CH4
fluxes alone but require a substantial bubble contribution, due to mixing limitations and the strong salinity
gradient which acts as formidable barrier to mixing and transport in the ESAS, especially in the LS
[Wåhlström et al., 2012]. Using an entrainment velocity model (supporting information), we note that even
with sustained gale force winds of 20 m s 1, entrainment velocities in the LS are only 10 cm h 1, suggesting
that a week is needed to mix from 15 m depth. Storms with winds >15 m s 1 occur on average only once or
twice each summer in the ESAS [Proshutinsky et al., 1999], even less close to shore [Günther et al., 2015],
though rare large storms have occurred [Simmonds and Rudeva, 2012]. Thus, during the ice-free sea season,
generally, only the upper 10 m of water is subject to surface layer turbulent mixing. This stratification likely
contributes to limiting the diffusive transport of CH4 from deeper waters to the surface.
Winter studies in the Beaufort Sea showed no substantial sea-air CH4 flux during the ice-covered period
[Kvenvolden et al., 1993], except at ice leads [Kort et al., 2012]. If we assume conservatively that our sampled
waters are open a third of the year and fluxes are relatively constant in that period, then we would expect
0.87 Tg CH4 yr 1 to be released during the ice-free season. It is possible, even likely, that larger fluxes exist
at ice-out due to overwinter collection of CH4 in or beneath the ice. Below-ice seawater CH4 concentrations
in the ice-covered portions of the ESS appear to support the idea of storage of CH4 beneath the ice (we had
no measurement of CH4 within the sea ice during SWERUS-C3). In these ice-covered regions of the AO and
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ESS, we report hypothetical diffusive fluxes, we report hypothetical diffusive fluxes, which would occur if the
sea ice were removed (Table 2). The ESS ice-covered segment became ice-free approximately 1 week after
Oden’s passage. Diffusive CH4 fluxes at ice-out would have been substantially higher than those we calculated for the open water portion of the cruise, but such a degassing event could not be sustained for long.
If we include these high below-ice CH4 concentrations in our flux calculations, our whole-ESAS average flux
estimate would become 12.2 mg m2 d 1 (9.4 Tg yr 1 or 2.8 Tg yr 1 assuming that 30% of the year is ice free);
however, because this below-ice CH4 would be rapidly degassed after ice-out and not result in a sustained
flux, we find this estimate to be unrealistically high. It has been suggested that somewhat higher CH4
emissions may occur during the subsequent month due to convection induced as sea surface temperatures
again drop [Damm et al., 2015]. The ESAS begins freezing after the sea ice minimum occurs in September.
Subsequent CH4 releases may then be trapped under or in the sea ice lid until spring [Golden et al., 2007;
Shadwick et al., 2011] with some loss through wintertime polynyas [Damm et al., 2007]. Better quantifying
CH4 oxidation rates in ESAS waters [Bussmann, 2013] and CH4 behavior in ice [Damm et al., 2015] are critical
for understanding the efficiency of winter CH4 storage.
5. Conclusions
Our measurements of CH4 in the atmosphere and surface water across the middle and outer ESAS during
July and August 2014 show an average flux of CH4 from the Siberian shelf seas to the atmosphere (along the
ice-free portions of the cruise track) of 3.8 mg m 2 d 1. In a region of CH4 seeps in the LS, fluxes reached
14 mg m 2 d 1. Enhanced levels of CH4 were observed in below-ice waters of the ESS; such CH4 would have
to be stored for winter months and released with near-100% efficiency after late summer or early autumn
ice-out, providing a short-duration increase to the total flux, to reach annual fluxes of 2.9 Tg yr 1. Such shortduration fluxes at ice-out must be better quantified to constrain the total annual flux. We note that the
below-ice CH4 concentrations in the ESS were considerably below what would be expected if CH4 was collected
over the entire ice-covered months, suggesting overwinter loss processes and/or incorporation into sea ice.
This, combined with a lack of knowledge of fluxes through the full ice-free season, causes us to regard our
annual estimates of ESAS sea-air CH4 fluxes (2.9 Tg yr 1) as very likely high rather than low. Unquantified iceout fluxes could increase the annual ESAS CH4 flux above the estimated ice-free season flux of 0.87 Tg yr 1.
Although our estimated sea-air CH4 fluxes for the ESAS far exceed fluxes reported for other shelf seas, they
are roughly an order of magnitude below ESAS CH4 flux estimates reported previously. Reconciling these differences requires much better knowledge of the spatial extent of ESAS CH4 sources (including near-shore areas
not accessible during SWERUS-C3 where riverine and terrestrial CH4 sources may play a greater role), especially
the seemingly highly localized bubble sources, as well as quantification of stored CH4 released at ice-out.
Acknowledgments
We thank first the crew of I/B Oden who
made the SWERUS-C3 expedition
possible. SWERUS-C3 funding was
provided by the Knut and Alice
Wallenberg Foundation, Vetenskapsrådet
(Swedish Research Council), Stockholm
University, Swedish Polar Research
Secretariat, and the Bolin Centre for
Climate Research. Maps were produced
using Ocean Data View 4 by R. Schlitzer,
http://odv.awi.de. Atmospheric and
surface water CH4 data presented in this
manuscript will be archived and freely
available from the Bolin Centre database,
http://bolin.su.se/data
THORNTON ET AL.
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