On the distribution of dissolved methane in Davis Strait, North Atlantic Ocean

Marine Chemistry 161 (2014) 20–25

Contents lists available at ScienceDirect

Marine Chemistry journal homepage: www.elsevier.com/locate/marchem

On the distribution of dissolved methane in Davis Strait, North Atlantic Ocean Stephen Punshon a,⁎, Kumiko Azetsu-Scott a, Craig M. Lee b a b

Oceanography and Climate Section, Bedford Institute of Oceanography, Dartmouth, Nova Scotia B2Y4A2, Canada Applied Physics Laboratory, University of Washington, Seattle, WA, USA

a r t i c l e

i n f o

Article history: Received 5 September 2013 Received in revised form 20 January 2014 Accepted 18 February 2014 Available online 28 February 2014 Keywords: Arctic zone Methane Vertical distribution Gas seepage

a b s t r a c t Depth profiles of dissolved methane were measured along three transects of Davis Strait and the northern Labrador Sea in October 2011. Concentrations ranged from 0.2 nmol L−1 (6% saturation) in the remarkably methane depleted Baffin Bay Deep Water to 38.8 nmol L−1 (1057% saturation) in localised subsurface anomalies near the Baffin Island Shelf. These anomalies may be the result of natural gas seepage and this hypothesis is supported by the distribution of potential sea surface oil slicks detected by satellite radar backscatter. In contrast, methane concentrations within the Baffin Island Current 200 km to the south of these anomalies were only slightly above atmospheric equilibrium. Methane was moderately supersaturated in West Greenland Shelf Water (b200%) with a distribution consistent with a sediment source. These measurements represent the first detailed baseline study of the vertical distribution of dissolved methane in an important Canadian Arctic Archipelago outflow region. © 2014 Elsevier B.V. All rights reserved.

1. Introduction The Arctic is identified as being particularly sensitive to radiative warming with the loss of sea ice occurring at a seemingly unprecedented rate (Kinnard et al., 2011). There has been speculation that reserves of methane hydrate stored in Arctic shelf sediments and permafrost may be destabilised over a relatively short time scale by climate warming, with the result that huge quantities of the greenhouse gas methane (CH4) are released to the water column and hence the atmosphere (Kennett et al., 2002; Schaefer et al., 2011). In addition to the potential contribution to radiative climate forcing, large scale methane release from Arctic sediments could contribute to ocean acidification and regional oxygen depletion through the microbial oxidation of dissolved CH4 to carbon dioxide (Boetius and Wenzhöfer, 2013). The process of hydrate erosion may already be underway in shallow Siberian Shelf waters (Shakhova and Semiletov, 2007; Shakhova et al., 2010) and along the West Spitsbergen continental margin where numerous bubble plumes have been observed (Westbrook et al., 2009). In Canadian waters, Majorowicz and Osadetz (2001) estimated that the potential volume of gas stored in hydrates is 0.19–6.2 × 1014 m3 in the Canadian Arctic Archipelago and 1.9–7.8 ×1013 m3 on the Canadian Atlantic Margin. Very few studies of dissolved methane have been made in the eastern Canadian Arctic and most published data are for surface waters (e.g. Kitidis et al.,

⁎ Corresponding author. Tel.: +1 9024264147. E-mail address: [email protected] (S. Punshon).

http://dx.doi.org/10.1016/j.marchem.2014.02.004 0304-4203/© 2014 Elsevier B.V. All rights reserved.

2010). Investigators face the challenge of distinguishing natural post glacial variability in Arctic methane fluxes from changes due to anthropogenic climate change without the benefit of long term observations. Hence there is a need for more detailed study of dissolved methane in Canadian Arctic waters in order to establish a baseline against which future changes, including the effects of hydrate decomposition, can be gauged. This study presents 470 measurements of dissolved CH4 made during three transects of Davis Strait between Greenland and Baffin Island by the R.V. Knorr in October 2011. The objectives of this work were to map the distribution of dissolved methane in order to identify sources and sinks in this region and to provide a high spatial resolution baseline study of high latitude methane dynamics.

1.1. Study area Davis Strait is an important corridor for Arctic Water (AW) from the Canadian Arctic Archipelago flowing south through Baffin Bay to the North Atlantic Ocean. This cold, fresh, Baffin Island Current (θ ≤ 1 °C, S ≤ 33.7) is largely confined to the upper 300 m of the water column along the Canadian continental margin. In contrast, the West Greenland Shelf/Slope is dominated by the northward-flowing warm West Greenland Shelf Water (θ b 7 °C, S ≤ 34.1) and West Greenland Irminger Water (θ N 2 °C, S N 34.1) (after Curry et al., 2011). The Davis Strait bathymetry takes the form of a sill with a mean depth around 600 m that restricts deep water exchange between the ocean basins of Baffin Bay and the Labrador Sea (Tang et al. 2004). Three principal

S. Punshon et al. / Marine Chemistry 161 (2014) 20–25

subsurface water masses have been indentified in Baffin Bay: Baffin Bay Intermediate Water (300–800 m), Baffin Bay Deep Water (800–1200 m), and Baffin Bay Bottom Water (N1200 m). To the south of Davis Strait, Labrador Sea Water is found from the surface down to around 2200 m depth. Samples for dissolved methane were collected and analysed during three transects of Davis Strait shown in Fig. 1: the Northern Line (NL) which crossed southern Baffin Bay with 15 sampling stations; the Mooring Line (ML) with 18 stations mainly along the line of the sill; and the combined Northern Labrador Sea East (NLSE) and Northern Labrador Sea West (NLSW) sections with a total of 17 stations.

2. Methods Seawater samples were collected using a rosette of 10 L Niskin bottles. Samples were then drawn into 160 mL volume borosilicate glass serum bottles (Wheaton Scientific, Millville, New Jersey), without the inclusion of bubbles, using a 30 cm length of flexible tubing and allowing each sample bottle to overflow by a minimum of two volumes. The samples were then immediately stabilised against further microbial activity by the addition of 50 μL of saturated of mercuric chloride solution, crimp-sealed with Teflon-faced butyl rubber septa, then stored at 4 °C prior to analysis, usually within 12 h. Concentrations of dissolved methane were determined by a batch static headspace equilibrium/gas chromatography method after Neill et al. (1997). In brief, batches of sample bottles were thermally equilibrated to 22 °C in a water bath whereupon a 10 mL headspace of ultra-high-purity nitrogen was introduced into each bottle and the samples were shaken vigorously for 10 min to achieve phase equilibration. Aliquots of headspace gas were transferred by displacement through a short drying tube packed with magnesium perchlorate to the sample loop of a gas chromatograph (SRI Instruments, California, USA)

21

equipped with a flame ionisation detector. Methane was separated on a 0.5 m pre-column packed with 80/100 mesh Porasil A and a 2 m main column packed with 80/100 mesh HayeSep A. The columns were held at 50 °C and configured to backflush the pre-column to vent once the methane analyte peak had reached the detector. Each sample run was referenced to automated 1 mL injections of 2.044 ppmv (parts per million by volume) and 8.181 ppmv CH4 primary standards (Air Liquide). Additional measurements of a 101.62 ppmv CH4 primary standard were used to construct calibration curves bracketing the full range of sample headspace mixing ratios encountered. Methane concentrations in the water phase were calculated using the Bunsen solubility coefficients of Wiesenburg and Guinasso (1979). The analytical precision of repeated injections of standard gas was better than 0.5% standard error, while the overall analytical precision, determined from measurements of triplicate samples drawn from selected depths, was around 2%. Air samples for methane analysis were collected daily from the upwind side of the ship in a glass syringe equipped with a 3-way stopcock and analysed in the same manner as the headspace samples. Temperature and salinity profiles were measured using Seabird SBE3T and SBE4C sensors while dissolved oxygen concentrations were measured with a pair of Seabird SBE43 oxygen sensors mounted on the CTD. These sensors are routinely calibrated and maintained by the Woods Hole Oceanographic Institute Calibration Laboratory, however, the oxygen sensor data should be interpreted with caution, as no Winkler titration measurements were available for calibration. 3. Results and discussion 3.1. Air and surface water CH4 A total of 23 air samples drawn over the study area returned a mean CH4 mixing ratio of 1.80 ppmv (range: 1.78–1.84) and this figure, together with in situ temperature and salinity data, was used to calculate dissolved CH4 saturation for the seawater samples. Plots of surface (b 5 m) methane saturation along the NL, ML and NLS cruise tracks are shown in Fig. 2. Mean CH4 surface concentration for the entire study area was 3.79 nmol L−1 (112% saturation) with a range of 2.82– 5.58 nmol L−1 (89–174% saturation). For the central NL, CH4 was mostly very close to atmospheric equilibrium, rising to modest supersaturations at the inshore stations. In the case of the ML and NLSW/NLSE, elevated levels of CH4 were most prominent near the Greenland Shelf. A notable exception was at the most inshore ML station where surface CH4 saturation was only 89%. Methane undersaturation in highlatitude surface water has been attributed to seasonal ice melt (Kitidis et al., 2010) but in this case the shallow water column was relatively well mixed (δS/δz b 0.001) and salinity cannot account for the change between undersaturation in the near surface water to supersaturation in the bottom water (see following section) even if dissolved methane was entirely absent in the glacial meltwater input. Although no sea ice was observed during this study, the influence of glacial melt water on regional dissolved methane distributions cannot be ruled out. Indeed, glacial melt water is the predominant source of fresh water to the eastern Davis Strait (Azetsu-Scott et al., 2012). 3.2. Sea/air CH4 fluxes The flux F of CH4 across the air/sea interface was calculated for each station using the equation F ¼ kw ðC w −C a Þ

Fig. 1. The study area showing station positions (black dots) along the three transects: Northern Line (NL), Mooring Line (ML) and the Northern Labrador Sea East and West Lines (NLSE, NLSW). The general surface circulation is shown by arrows representing the Baffin Island Current (BIC) and the West Greenland Current (WGC), CS is Cumberland Sound. Open circles represent dark radar backscatter features identified on RADARSATimages, potentially surface oil slicks (from Budkewitsch et al. 2013). Note that the satellite coverage does not extend as far north as the Northern Line.

where kw is the gas transfer velocity for methane, Cw is the measured methane concentration in the surface water and Ca is the equilibrium concentration of methane at surface water temperature and salinity calculated using the measured CH4 atmospheric mixing ratio and the Bunsen solubility coefficient of Wiesenburg and Guinasso (1979).

22

S. Punshon et al. / Marine Chemistry 161 (2014) 20–25

5 4 3 2 100

1 0

80 0

CH4 saturation (%)

b

50

100

150

200

250

300

350

400

180

10 9 8 7 6 5 4 3 2 1 0 -1 350

160 140 120 100 80 0

CH4 saturation (%)

c

50

100

150

200

250

-1 450

300

8 7 6 5 4 3 2 1 0 -1

160 140 120 100 80 0

100

200

300

400

500

600

CH4 flux (µmol m-2 d-1)

120

CH4 flux (µ µmol m-2 d-1)

6

140

CH4 flux (µmol m-2 d-1)

CH4 saturation (%)

a

700

Distance along transect (km) Fig. 2. Surface water methane saturation (solid triangles and solid line) and estimated sea/air fluxes (empty triangles and dashed line) for (a) NL, (b) ML and (c) NLSW/NLSE lines. The vertical black line on the bottom plot represents the junction between the NLSE and NLSW sections.

The windspeed parameterisation of Wanninkhof (1992) for an instantaneous windspeed measurement u was used to calculate kw where 2

−0:5

kw ¼ 0:31 u ðSc=660Þ

:

Windspeeds recorded by the ship's anemometer at 1 min intervals were averaged over the time that the ship occupied each station to give u, which ranged from 3.1 to 14.3 m s−1 during the study. The appropriate Schmidt number Sc for methane was calculated using the temperature dependent polynomial relationship from Wanninkhof (1992). The estimated fluxes for each station are shown in Fig. 2 and range from − 0.3 to 9.0 μmol CH4 m−2 d−1. The average flux for the entire Davis Strait study area was 1.6 μmol CH4 m−2 d−1 which is central in the range of flux densities, calculated using the windspeed relationship of Wanninkhof (1992), reported for the mid-latitude North Atlantic (Forster et al., 2009). 3.3. Sub-surface methane distribution The sub-surface distributions of dissolved methane and temperature along the three transect lines are shown in Fig. 3 while the distribution of temperature, salinity and dissolved oxygen along these transects is shown in Fig. 4. CH4 distribution can be conveniently categorised into three regions: Greenland Shelf, Baffin Island Shelf/Slope, and central basin. Fig. 5 shows the distribution of dissolved methane in terms of

potential temperature and salinity, with water mass definitions from Curry et al. (2011) and Tang et al. (2004). 3.3.1. Greenland Shelf All three transects showed elevated levels of dissolved methane in Greenland Shelf waters. For stations on the NL, methane ranged from 178 to 224% saturation in near bottom water with concentrations declining to around atmospheric equilibrium at the near surface (98– 127%). The situation was very similar for ML, with subsurface CH4 saturations around 150% in the five most inshore stations, although less vertical structure was apparent compared with the NL. The elevated levels of methane in both these shelf regions occurred in the range of temperature and salinity (2.2–3.8 °C, θ b 34) characteristic of seasonally warmed West Greenland Current (Tang et al., 2004). Excess dissolved methane was also seen to a more limited extent in the eastern NLSE section with a near-bottom maximum value of 5.23 nmol L−1 (153% saturation), found at the most inshore station. Potential sources of methane to Arctic Shelf waters include riverine input (Shakhova and Semiletov, 2007), contemporary methanogenesis in anoxic organic rich environments such as sediments and animal guts (Kvenvolden et al., 1993; DeAngelis and Lee, 1994), and gas seeps (Westbrook et al., 2009). The Qinnguata Kuussua, a principal river of western Greenland, discharges glacial melt water into Davis Strait and this was evident at the three inshore shelf/slope stations of the NLSE Line as a b50 m layer of cold, fresh water (b 2 °C, ~ 32 S) overlying the warmer, more saline West Greenland Current (~5 °C, ~34 S). No significant correlation between CH4 saturation and salinity was seen within

S. Punshon et al. / Marine Chemistry 161 (2014) 20–25

a

32 30 28 26 24 22 20 18 16 14 12 10 8 6 4 2 0

-200

Depth (m)

-400 -600 -800 -1000 -1200 -1400 -1600 50

b

100

150

200

250

300

350

0

32 30 28 26 24 22 20 18 16 14 12 10 8 6 4 2 0

Depth (m)

-200 -400 -600 -800 -1000 50

100

150

200

250

300

c 6

-500

Depth (m)

5

-1000 4

-1500 3

-2000

2

-2500

1

100

200

300

400

500

600

700

Distance (km) Fig. 3. Distribution of dissolved methane (nmol L−1) along a) NL, b) ML and c) NLSE/NLSW lines with the junction indicated by a vertical line. Note the differing colour scale on c) to enhance details.

the upper 200 m at these stations (R2 = 0.12), nor in data from all inshore Greenland Shelf stations (R2 = 0.007) indicating that river input was not an important factor influencing dissolved methane in the Greenland Shelf region. The vertical gradients and ranges of methane concentration in these well-oxygenated Greenland Shelf waters are similar to those seen in other high-latitude shallow seas and are in keeping with sedimentary methanogenesis as the primary source of CH4 (Cline et al., 1986; Tilbrook and Karl, 1994) although no stable isotope data are available to support this hypothesis. It is also possible that seepage may contribute to the excess methane observed in these Greenland Shelf profiles although no seeps have so far been reported in this region. 3.3.2. Baffin Island Shelf The most notable features of the NL and ML profiles were shallow subsurface CH4 anomalies extending eastwards from the Baffin Island Shelf. In the case of the NL, this zone was centred at around 100 m depth and confined within Arctic Water. The highest measured CH4 concentration along the NL was 27.14 nmol L−1 (722% saturation) just above the shelf break. A second methane anomaly observed in the western ML section had a wider distribution, extending down to around 400 m and centred about 20 km offshore. Methane levels here reached

23

38.82 nmol L−1 (1057% saturation). Although the excess methane in the ML anomaly was largely contained within Arctic Water, the temperature and salinity data show high methane concentrations extending into Baffin Bay Intermediate Water (Fig. 5). Near-surface methane concentrations in these regions were close to atmospheric equilibrium, hence there was no appreciable localised release of CH4 to the atmosphere. Further south, a weak subsurface CH4 maximum at about 50 m depth (range: 4.88–5.62 nmol L−1, 130–155% saturation) occurring in the western shelf waters of the NLSW transect was also associated with Arctic Water. Near-surface aerobic CH4 maxima in this concentration range resulting from in situ sources are not unusual (Forster et al., 2009; Damm et al., 2010), but it is tempting to conclude that this excess methane originated from the high concentration anomalies seen in Arctic Water on the NL and ML, with the bulk of the dissolved CH4 being lost during the journey south in the Baffin Island Current, presumably by a combination of diffusive mixing, ventilation and in situ microbial consumption. As so little information is available on microbial methane oxidation rates in cold Arctic Water it is worth estimating the first-order microbial uptake rate constant that would alone be required to reduce the concentration of dissolved methane from ~40 nmol L−1 on the western ML to ~5 nmol L−1 in the NLSW. The distance between these two sites is approximately 200 km, while the Baffin Island Current velocity, as determined by current meters deployed on the ML, is around 0.2 m s − 1 (Curry et al., 2011). The time therefore required for a water parcel to traverse this distance is about 12 days, giving an apparent CH 4 uptake rate constant of 7.2 × 10− 3 h− 1. Although this is an upper range estimate not taking into account other potential CH4 loss mechanisms, or indeed the occurrence of other seep sites between ML and NLSW, it is not far removed from the figure of 3.8 × 10 − 3 h− 1 obtained from incubations of near-surface Arctic Water by Kitidis et al. (2010). A plausable explanation for the source of the strong methane anomalies on the western NL and ML sections is gas seepage from the Baffin Island Shelf/Slope. Oil slicks and ebullition at the sea surface have been observed off the north-east coast of Baffin Island near Scott Inlet and Buchan Gulf (MacLean et al., 1981). Further south, the Saglek Basin in the eastern Hudson Strait holds proven reserves of natural gas, and structures resembling fluid escape chimneys have been identified (Jauer and Budkewitsch, 2010). A study of Synthetic Aperture Radar backscatter data from the RADARSAT satellite, collected in 2003, assessed the extent of wave-dampening surface oil slicks in Davis Strait and Baffin Bay (Budkewitsch et al., 2013). Although the spatial coverage of that study does not include the NL transect, a dense aggregation of large radar backscatter features coincides almost exactly with the position of the subsurface methane anomaly on our ML transect (Fig. 1). Conversely, while numerous backscatter features occur inshore around Cumberland Sound to the west, none lie along the NLSW/NLSE line where only modest CH4 supersaturations were observed. The superposition of dark radar targets and the subsurface CH4 anomaly on the western ML supports the presence of hydrocarbon seepage in this vicinity. We hypothesise that this CH4 anomaly is of thermogenic origin rather than the result of decomposing gas hydrate of biogenic origin. 3.3.3. Central basin regions Away from the influence of eastern and western shelf/slope regions, the upper part of water column, down to around 200 m on the NL and ML sections and ~ 500–800 m in the case of the NLS, was generally close to atmospheric equilibrium, although a modest subsurface CH4 maximum was frequently seen at 50 m the base of the mixed layer. These shallow CH4 maxima are commonly observed features of the surface ocean and have been attributed to methanogenesis in reducing environments such as nepheloid layers and zooplankton guts (DeAngelis and Lee, 1994). The microbial utilisation of methylated compounds, including methylphosphonate and dimethylsulphoniopropionate, has also been suggested to produce methane in oxygenated surface waters (Karl et al., 2008; Damm et al., 2010). In deeper water, dissolved

24

S. Punshon et al. / Marine Chemistry 161 (2014) 20–25

Fig. 4. Distributions of a) temperature (°C), b) salinity (psu), and c) dissolved oxygen (mL/L) along the NL, ML and NLSE and NLSW lines.

Fig. 5. The distribution of dissolved methane for the entire study area shown as a function of potential temperature and salinity where the black circle diameter is proportional to CH4 saturation in the range 6 to 1057%. Coloured ellipses represent the temperature and salinity range of water masses where WGSW: Western Greenland Shelf Water, AW: Arctic Water, BBDW: Baffin Bay Deep Water, BBIW: Baffin Bay Intermediate Water, LSW: Labrador Sea Water and DSOW: Denmark Strait Overflow Water, after Curry et al. (2011), Tang et al. (2004) and Azetsu-Scott et al. (2005).

S. Punshon et al. / Marine Chemistry 161 (2014) 20–25

methane became increasingly undersaturated with increasing depth. Baffin Bay Deep Water was remarkable because of uniform depletion of CH4, with a minimum of only 0.20 nmol L−1 (6% saturation) seen at the deepest point of the NL, ranking Baffin Bay alongside the most methane-depleted ocean basins (Scranton and Brewer, 1978; Berner et al., 2003). Dissolved oxygen concentration also declined steadily with depth in Baffin Bay, reaching a minimum of 32% saturation (Fig. 4). There were similar declines in methane and oxygen concentration with depth in the central section of the ML. In the case of the NLSE/ NLSW deep station, the decrease in dissolved methane with depth commenced at around 800 m and reached a minimum of 1.30 nmol L− 1 (39% saturation) at 2400 m but with an increase of 0.51 nmol L−1 between 2400 and the near-bottom sample at 2627 m. This near-bottom increase in CH4 was accompanied by a slight increase in dissolved oxygen suggesting the presence of a more recently ventilated water mass, most likely Denmark Strait Overflow Water which persists at this location (Azetsu-Scott et al., 2005), rather than a sedimentary source of methane, in which case there would be no accompanying increase in oxygen (Fig. 5). The very low levels of methane observed in Baffin Bay Deep Water undoubtedly reflect net microbial oxidation. Circulation and exchange with the Labrador Sea are increasingly restricted below 600 m by the Davis Strait sill resulting in a relatively long residence time; the ventilation age of the underlying Baffin Bay Bottom Water has been estimated in the range of 200–750 years (Top et al., 1980; Bourke and Paquette, 1991). It seems reasonable to conclude that the methane dynamics of Baffin Bay Deep Water are in steady state, with very low rates of microbial CH4 oxidation in balance with meagre atmospheric and sediment sources. Methane turnover times of decades have been estimated for other low CH4 regions (Valentine et al., 2001; Rehder et al., 1999; Scranton and Brewer, 1978). On the other hand, it is worth noting that recent studies of isolated marine basins with restricted circulation have found extremely high methane oxidation potential (Heintz et al., 2012; Mau et al., 2013) indicating that very active communities of methanogens can be maintained by periodic CH4 input. With the limited information available it is not yet possible to determine whether this latter situation could exist in the deep Baffin Bay water. 4. Conclusions This first detailed study of dissolved methane in Davis Strait reveals the presence of net CH4 sources in both the Baffin Island and Greenland Shelf/Slope regions. The reported distribution of potential oil slicks detected by radar backscatter suggests that the area of highest CH4 concentration found in this study may be due to hydrocarbon seepage. The deep waters of Baffin Bay were remarkably depleted in dissolved CH4, presumably owing to net microbial consumption, and it would be of great interest to further investigate methane turnover rates in this basin. Future studies in this region should include estimates of methane flux from seeps along the Baffin Island Shelf, examine it's origin using stable isotope analysis and also investigate the potential for methane as a localised carbon source for endemic ecosystems. Acknowledgements This work was supported by the U.S. National Science Foundation Freshwater Initiative. Additional funding was provided by the National Centre for Arctic Aquatic Research Excellence (NCAARE), Fisheries and Oceans Canada. We would like to thank the captain and crew of RV Knorr for their assistance. References Azetsu-Scott, K., Jones, E.P., Gershey, R.M., 2005. Distribution and ventilation of water masses in the Labrador Sea inferred from CFCs and carbon tetrachloride. Mar. Chem. 94, 55–66.

25

Azetsu-Scott, K., Petrie, B., Yeats, P., Lee, C., 2012. Composition and fluxes of freshwater through Davis Strait using multiple chemical tracers. J. Geophys. Res. 117. http:// dx.doi.org/10.1029/2012JC008172 (C12011). Berner, U., Poggenburg, J., Faber, E., Quadfasel, D., Frische, A., 2003. Methane in ocean waters of the Bay of Bengal: its sources and exchange with the atmosphere. Deep-Sea Res. 50, 925–950. Boetius, A., Wenzhöfer, F., 2013. Seafloor oxygen consumption fuelled by methane from cold seeps. Nat. Geosci. 6, 725–734. Bourke, R.H., Paquette, R.G., 1991. Formation of Baffin Bay bottom and deep waters. Elsevier Oceanogr. Ser. 57, 135–155. Budkewitsch, P., Pavlic, G., Oakey, G., Jauer, C., Decker, V., 2013. Reconnaissance mapping of suspect oil seep occurrences in Baffin Bay and Davis Strait using satellite radar: preliminary results. Geol. Surv. Can. 7068. http://dx.doi.org/10.4095/292280 (Open File). Cline, J.D., Katz, C.N., Kelly-Hansen, K., 1986. Seasonal cycles of dissolved methane in the Southeastern Bering Sea. In: Church, T.M. (Ed.), Marine Chemistry in the Coastal Environment. ACS Symp Ser, 18, pp. 273–296 (Washington DC). Curry, B., Lee, C.M., Petrie, B., 2011. Volume, freshwater, and heat fluxes through Davis Strait, 2004–5. J. Phys. Oceanogr. 41, 429–436. Damm, E., Helmke, E., Thoms, S., Schauer, U., Nöthig, E., Bakker, K., Kiene, R.P., 2010. Methane production in aerobic oligotrophic surface water in the central Arctic Ocean. Biogeosciences 7, 1099–1108. DeAngelis, M.A., Lee, C., 1994. Methane production during zooplankton grazing on marine phytoplankton. Limnol. Oceanogr. 39, 1298–1308. Forster, G., Upstill-Goddard, R.C., Gist, N., Robinson, C., Uher, G., Woodward, E.M.S., 2009. Nitrous oxide and methane in the Atlantic Ocean between 50°N and 52°S: latitudinal distribution and sea-to-air flux. Deep-Sea Res. 56, 964–976. Heintz, M.B., Mau, S., Valentine, D.L., 2012. Physical control on methanotrophic potential in waters of the Santa Monica Basin, Southern California. Limnol. Oceanogr. 57, 420–432. Jauer, C.D., Budkewitsch, P., 2010. Old marine seismic and new satellite radar data: petroleum exploration of north west Labrador Sea, Canada. Mar. Pet. Geol. 27, 1379–1394. Karl, D.M., Beversdorf, L., Björkman, K., Church, M.J., Martinez, A., DeLong, E.F., 2008. Aerobic production of methane in the sea. Nat. Geosci. 1, 473–478. http://dx.doi.org/ 10.1038/ngeo234. Kennett, J.P., Cannariato, K.G., Hendy, L.L., Behl, R.J., 2002. Methane hydrates in quaternary climate change: the clathrate gun hypothesis. AGU Spec. Publ. Ser. 43 (Washington). Kinnard, C., Zdanowicz, C.M., Fisher, D.A., Isaksson, E., de Vernal, A., Thompson, L.G., 2011. Reconstructed changes in Arctic sea ice over the past 1450 years. Nature 479, 509–512. Kitidis, V., Upstill-Goddard, R.C., Anderson, L.G., 2010. Methane and nitrous oxide in surface water along the North-West Passage. Mar. Chem. 121, 80–86. Kvenvolden, K.A., Lilley, M.D., Lorenson, T.D., Barnes, P.W., McLaughlin, E., 1993. The Beaufort Sea continental-shelf as a seasonal source of atmospheric methane. Geophys. Res. Lett. 20, 2459–2462. MacLean, B., Falconer, R.K.H., Levy, E.M., 1981. Geological, geophysical and chemical evidence for natural seepage of petroleum off the northeast coast of Baffin Island. Bull. Can. Petrol. Geol. 29, 75–95. Majorowicz, J.A., Osadetz, K.G., 2001. Gas hydrate distribution and volume in Canada. Am. Assoc. Pet. Geol. Bull. 85, 1211–1230. Mau, S., Blees, J., Helmke, E., Niemann, H., Damm, E., 2013. Different methanotrophic potentials in stratified polar fjord waters (Storfjorden, Spitsbergen) identified by using a combination of methane oxidation techniques. Biogeosciences 10, 6267–6278. Neill, C., Johnson, K.M., Lewis, E., Wallace, D.W.R., 1997. Accurate headspace analysis of fCO2 in discrete water samples using batch equilibration. Limnol. Oceanogr. 42, 1774–1783. Rehder, G., Keir, R.S., Suess, E., Rhein, M., 1999. Methane in the northern Atlantic controlled by microbial oxidation and atmospheric history. Geophys. Res. Lett. 26, 587–590. Schaefer, K., Zhang, T., Bruhwiler, T.L., Barrett, A.P., 2011. Amount and timing of permafrost carbon release in response to climate warming. Tellus 63B, 165–180. Scranton, M.I., Brewer, P.G., 1978. Consumption of dissolved methane in the deep ocean. Limnol. Oceanogr. 23, 1207–1213. Shakhova, N., Semiletov, I., 2007. Methane release and coastal environment in the East Siberian Arctic shelf. J. Mar. Syst. 66, 227–243. Shakhova, N., Semiletov, I., Salyuk, A., Yusupov, V., Kosmach, D., Gustafsson, O., 2010. Extensive methane venting to the atmosphere from sediments of the East Siberian Arctic Shelf. Science 327, 1246–1250. Tang, C.C.L., Ross, C.K., Yao, T., Petrie, B., DeTracey, B.M., Dunlap, E., 2004. The circulation, water masses and sea-ice of Baffin Bay. Prog. Oceanogr. 63, 183–228. Tilbrook, B.D., Karl, D.M., 1994. Dissolved methane distributions, sources, and sinks in the western Bransfield Strait, Antarctica. J. Geophys. Res. 99, 16,383–16,393. Top, Z., Clarke, W.B., Eismont, W.C., Jones, E.P., 1980. Radiogenic helium in Baffin Bay bottom water. J. Mar. Res. 38, 435–451. Valentine, D.L., Blanton, D.C., Reeburgh, W.S., Kastner, M., 2001. Water column methane oxidation adjacent to an area of active hydrate dissociation. Geochim. Cosmochim. Acta 65, 2633–2640. Wanninkhof, R., 1992. Relationship between wind speed and gas exchange over the ocean. J. Geophys. Res. 97, 7373–7382. Westbrook, G.K., Thatcher, K.E., Rohling, E.J., Piotrowski, A.M., Pälike, H., Osborne, A.H., Nisbet, E.G., Minshull, T.A., Lanoisellé, M., James, R.H., Hühnerbach, V., Green, D., Fisher, R.E., Crocker, A.J., Chabert, A., Bolton, C., Beszczynska-Möller, A., Berndt, C., Aquilina, A., 2009. Escape of methane gas from the seabed along the West Spitsbergen continental margin. Geophys. Res. Lett. 36. http://dx.doi.org/10.1029/ 2009GL039191 (L15608). Wiesenburg, D.A., Guinasso, N.L., 1979. Equilibrium solubilities of methane, carbon monoxide, and hydrogen in water and seawater. J. Chem. Eng. Data 24, 356–360.