We compared data from the R/V Helmer Hanssen to those from the Zeppelin Observatory for the period from 20 June to 1 August. The CH 4 mixing ratio measured aboard the ship during the measurements off Prins Karls Forland agrees well with those recorded by the Zeppelin Observatory, as does the isotopic ratio (see supporting information Figure S3 ). Our measurements above the flares were not influenced by long‐range transport of methane‐enhanced air masses from lower latitudes, as this would have produced noticeable transient enhancements in CH 4 , as exemplified in Figure S3 .

This is in agreement with changes reported by Steinle et al . [ 2015 ] for bottom water and sea surface water. This change in concentration can be explained either by slower advection during the later observations or that the water was previously CH 4 enriched by an emission burst from one or several nearby seep sites. Gas bubble dissolution modeling from a previous study in the deeper area to the west of our study area estimated that 80% of the bubble‐released CH 4 is dissolved below the summer pycnocline, and the remaining CH 4 is transported northward where it is most likely oxidized by methanotrophic bacteria [ Gentz et al ., 2014 ; Steinle et al ., 2015 ]. A similar conclusion came from a box modeling result of dissolved CH 4 indicating that ~60% of CH 4 released at the seafloor becomes already oxidized before it reaches the overlying surface waters [ Graves et al ., 2015 ]. Although our single beam echo sounder studies show bubbles reaching the sea surface, very little CH 4 remains in such bubbles by the time they reach the surface [ Greinert and McGinnis , 2009 ].

A 6 km transect was sampled twice in 1 week by the R/V Helmer Hanssen to monitor rapid variations of oceanographic conditions and their effects on the dissolved CH 4 distribution. The maximum bottom water CH 4 concentration doubled in 1 week from 200 to 400 nmol L −1 (see Figures 4 d and S2 ), while bottom water temperatures remained relatively stable. At the same time, the concentrations above the pycnocline and at sea surface remained relatively stable and low (4–11 nmol L −1 and ~10 nmol L −1 in the surface water on 24 June and 1 July, respectively).

Comparison of CHvariations in the ocean and atmosphere west of Svalbard and corresponding CHflux to the atmosphere. (a) Contour plot of near‐surface CHconcentration (color scale) at ~10 m depth in the water column. CHwas measured by oceanographic conductivity‐temperature‐depth (CTD) stations (crosses) west of Prins Karls Forland (PKF). Observed flares are shown by pink markers. (b) Contour plot of atmospheric CHmixing ratio in parts per billion measured aboard R/V(color scale). Ship track is shown by black line; flares are shown by pink markers. (c) CHmeasured by the FAAM aircraft; flares are shown by pink markers. (d) CHconcentration in the water column along a transect of CTD stations taken on 1 July 2014 showing a clear stratification of water masses with the pycnocline near 50 m water depth. Density is shown as black contours. (The transect location offshore of Prins Karls Forland is shown in Figure S2b ). (e) CHflux to the atmosphere at each CTD location as a function of ocean CHconcentration according to a diffusive model (green points). Flux previously modeled off Northern Siberia during stormy weather [.,] is given by the grey point. Dashed lines show the model flux at different isotachs (lines of constant wind speed), assuming constant salinity and temperature (averaged over the sampling period used). Horizontal lines show the maximum possible flux constrained by the atmospheric measurements from the ship, according to FLEXPART and Oslo CTM3 models. FLEXPART and CTM constraints are for the atmospheric sampling period 20 June to 1 August and will vary with weather patterns.

The sea surface CH 4 ocean concentrations (Figure 4 a) and the atmospheric mixing ratio measured by both the ship (Figure 4 b) and the aircraft (Figure 4 c) show very similar patterns. In the surface water CH 4 was generally <8 nmol L −1 (Figure 4 a) with a median of 4.8 nmol L −1 . A maximum of 26 nmol L −1 was found near the shore, where no gas flares are found in the vicinity (Figures 4 a and 4 b). The elevated surface water CH 4 concentrations coincide with a small increase (<2 ppb) of atmospheric CH 4 mixing ratio detected by the ship. This slightly elevated CH 4 close to the shore is probably not due to CH 4 released from the seafloor/seeps. Figure 3 and Figure 4 show that the bottom CH 4 concentrations are low in this coastal area. A simultaneous decrease in salinity suggests the intrusion of methane‐enriched fresher water [ Damm et al ., 2005 ] near the surface increasing the dissolved CH 4 concentrations in this particular area.

CH 4 concentrations from a hydrocast survey offshore of Prins Karls Forland. The first three bottles were taken 5, 15, and 30 m above the seafloor, and the last three bottles were taken 10, 20, and 30 m below the sea surface. The rest of the samples were spread equally in the water column depending on the bottom depth. CH 4 concentrations in the ocean are illustrated by colored dots (scale on the bottom left in nmol L −1 ). Black dots indicate the location of the gas flares. Isobaths are from International Bathymetric Chart of the Arctic Ocean version 3 grid and the superimposed higher resolution bathymetry is from the multibeam survey performed during the R/V Helmer Hanssen cruise; data were recorded over the period 25 June to 1 July 2014.

We present the results following the methane migration path from the seafloor through the water column to the lowermost atmosphere close to the sea surface (ship) and higher up using flight data covering a larger area. Figure 3 illustrates the dissolved CH 4 concentrations sampled over the investigated area. Elevated concentrations were found around the most extended cluster of flares, and the CH 4 distribution shows a rapid change at about ~50 m water depth, with the highest dissolved CH 4 concentrations near the seafloor ~150 m depth. Little CH 4 is found above the pycnocline (boundary where the density gradient is greatest, affected by temperature and salinity), but sea surface CH 4 concentrations are still oversaturated with respect to atmospheric concentrations in a few places eastward, close to the shore

3.2 Flux Estimates From Ocean to Atmosphere in the Svalbard Region During Summer

We estimate the median ocean‐atmosphere CH 4 flux based on observations in the ocean, in addition to three top‐down constrains of the flux employing three independent models and the atmospheric measurements.

We estimate a median ocean‐atmosphere CH 4 flux of 0.04 nmol m−2 s−1 (σ = 0.13) from data at each conductivity‐temperature‐depth (CTD) station using an ocean‐atmosphere gas exchange function [Wanninkhof et al., 2009] (Figure 4e). The maximum flux at the CTD stations is 0.8 nmol m−2 s−1 which occurred when both dissolved CH 4 concentrations and wind speeds were high, 25 nmol L−1 and 9 m s−1 respectively. This model only considers air‐sea exchange via diffusion of dissolved CH 4 and not the contribution of bubbles of gas reaching the surface. Figure 4e) shows the estimated flux at different wind speeds, assuming constant salinity and temperature (average from the campaign). Wind speed has a large effect: an increase from 5 to 10 m s−1 increases the modeled flux by almost an order of magnitude. The atmospheric CH 4 air mixing ratios aboard the R/V Helmer Hanssen and at Zeppelin before, during, and after the ship‐based measurements off Prins Karls Forland were very similar, with small variations (Figure S3). Hence, the CH 4 air mixing ratios above active seep areas were representative of wider regional atmospheric concentrations, with no elevated levels or transient large increases.

To complement our observational‐based flux estimates of dissolved CH 4 , we employed three independent atmospheric models to provide top‐down constraints of the ocean‐atmosphere flux, given the atmospheric concentrations sampled by the aircraft and the ship. This approach also takes potential CH 4 from bubbles into account. We only detected a weak increase of 2 ppb in the atmospheric mixing ratio at the ship location close to bubbles, reflecting the potential enhancement from both dissolved CH 4 and CH 4 from bubbles. We calculated, using a Lagrangian transport model (FLEXPART), the CH 4 enhancements at the ship for all locations that would result from a 1 nmol m−2 s−1 flux from the area, encompassing the identified and the potential CH 4 seep sites around Svalbard [Sahling et al., 2014] (Figure 2). Running FLEXPART backward in time for all ship positions over the period 20 June to 1 August, the modeled CH 4 enhancement is shown as the yellow line in the supporting information section, Figure S4; compared to the observations, no correlation (r2 = 0.003) is evident. The most sensitive days are the highest 20% modeled peaks (bold yellow line). Using the most sensitive days from this period, we estimate a top‐down constraint on the flux from the seep areas of <2.4 ± 1.3 nmol m−2 s−1. This estimation assumes that all of the measured 2 ppb variation in the atmosphere is solely due to a flux from the modeled seep areas around Svalbard (Figure 2). Similarly, using a forward chemistry transport model (Oslo CTM3) [Søvde et al., 2012], a flux of 3.8 ± 0.7 nmol m−2 s−1 was necessary to reproduce the 2 ppb increase in CH 4 at the ship, assuming the same emission region shown in Figure 2. This is equivalent to an annual emission of only 0.06 Tg for a constant flux throughout the year, very small compared to the total global annual emission of ~600 Tg of CH 4 [Kirschke et al., 2013]. In addition, we used the aircraft measurements to provide another independent constrain on the maximum possible CH 4 flux in the region. The aircraft flew transects below 100 m altitude upwind and downwind of the potential seep sites but observed no statistically significant change in CH 4 during these low‐level flights; see Figure 1d for altitudes. A Lagrangian mass balance calculation (similar to that employed by O'Shea et al. [2014] leads to an estimated flux of −3.0 ± 17.1 nmol m−2 s−1. An estimated upper limit on the ocean‐to‐atmosphere CH 4 flux averaged over the grey shaded area shown in Figure 1d can then be quantified by the mean + 1σ value of 14.1 nmol m−2 s−1. This represents the maximum possible flux for this area consistent with the aircraft CH 4 measurements and associated uncertainties.

FAAM aircraft measurements were also made in the same location off Prins Karls Forland in a previous Methane in the Arctic Measurement and Modelling (MAMM) campaign in summer 2012 as part of the UK Methane in the Arctic Measurement and Modelling (MAMM) project (see Allen et al. [2014] for details). Similarly, any emission from the seep areas was not detectable among the other signals in the aircraft data. Forward calculations, with a different dispersion model, led to very similar conclusions to those of 2014: that an emission flux of a few tens of nmol m−2 s−1 would have been required to detect the emission in the aircraft data [M. Cain, personal communication, 2016].

In sharp contrast to the flux calculations from the measurement‐led approaches discussed here (Table 1), the flux reported by Shakhova et al. [2014] from the East Siberian Arctic Shelf is more than 2 orders of magnitude larger, 70–450 nmol m−2 s−1 under windy conditions than our measurement‐derived maximum for the period. Figure 4e includes a comparison. Part of this large difference can be explained by both higher dissolved CH 4 concentrations in surface waters reported in the Siberian area (up to ~400 nmol L−1) and the higher wind speeds reported by Shakhova et al. [2014]. Table 1 compiles our estimates of the spatially averaged maximal flux in the region, as constrained by the atmospheric observations.