EVIDENCE Of ATMOSPHERIC RESPONSE TO METHANE EMISSIONS fROM THE EAST SIBERIAN ARCTIC SHELf

Average atmospheric methane concentration (CH4) in the Arctic is generally higher than in other regions of the globe. Due to the lack of observations in the Arctic there is a deficiency of robust information about sources of the methane emissions. Measured concentrations of methane and its isotopic composition in ambient air can be used to discriminate sources of CH4. Here we present the results of measurements of the atmospheric methane concentration and its isotope composition (δCCH4) in the East Siberian Arctic Seas during the cruise in the autumn 2016. Local sections where the concentration of methane in the near-water layer of the atmosphere reaches 3.6 ppm are identified. The measurements indicated possibility of formation of high methane peaks in atmospheric surface air above the East Siberian Arctic Shelf (ESAS) where methane release from the bottom sediments has been assumed. KEY wORDS: atmospheric methane, methane emissions, Arctic, sub-sea permafrost, warming, shipborne measurements, atmospheric surface layer, East Siberian Arctic Shelf CITATION: Natalia Pankratova, Andrey Skorokhod, Igor Belikov, Nikolai Elansky, Vadim Rakitin, Yury Shtabkin, Elena Berezina (2018) Evidence Of Atmospheric Response To Methane Emissions From The East Siberian Arctic Shelf. Geography, Environment, Sustainability, Vol.11, No 1, p. 85-92 DOI-10.24057/2071-9388-2018-11-1-85-92


INTRODUCTION
Methane (CH 4 ) is a second greenhouse gas after carbon dioxide which atmospheric concentration has increased by 150% since pre-industrial times (IPCC, 2013).However, the CH 4 global warming potential is approximately 28 times higher than that of CO 2 over a 100-year frame (Myhre et al. 2013).It accounts for 20% of the global radiative forcing of well-mixed greenhouse gases (Quay et al. 1999;Dlugokencky et al. 2014).
As is thought, methane contributes greatly to warming in the Arctic region, which is characterized by abundant methane sources such as, for example, wetlands of the northern Eurasia, shelf areas of the Arctic seas (Shakhova et al. 2014), gas combustion (Stohl et al. 2005), and anthropogenic emissions.It is assumed that the influence of methane sources on the climate of the region should progressively increase with temperature growth in the Arctic (Shakhova et al. 2015).
Sub-sea permafrost and hydrates in the shelf regions of the seas of the Eastern Arctic are significant methane pool and potentially can be large source of atmospheric methane emissions (Berchet et al. 2015).A significant number of localized seeps of methane in offshore regions of the East Arctic seas have been found (Shakhova et al. 2015;Thornton et al. 2016), but the quantity and quality of the available experimental data is currently insufficient to obtain stable estimates of CH 4 emissions into the air above-sea layer, which are still very contradictory (Berchet et al. 2015;Shakhova et. al. 2014).The evidence of methane release from the ESAS bottom layers have been previously reported (Thornton et al. 2016;Shakhova et al. 2010), while the ability of benthic methane to penetrate into the atmosphere had not yet been proven.Satellite measurements of the surface methane concentration cover the whole Earth but do not have sufficient accuracy.Very little data are available for the isotope δ 13 C CH4 in methane, which provides information on sources of methane in the atmosphere (Warwick et al. 2016;Fisher et al. 2011).Thus, it is very important to expand the experimental studies of methane concentrations in the Arctic to check whether methane released from the seawater into the atmosphere.Present work is continuing the Arctic methane study started during previous observation campaign of 2015 described in (Skorokhod et al. 2016).

MATERIALS AND METHODS
Atmospheric CH 4 mixing ratio and changes in the 13 C: 12 C ratio in CH 4 (reported a changes relative to a reference ratio and denoted as δ 13 C CH4 ) were measured from aboard the research vessel (R/V) Akademik M.A. Lavrentiev from 23 September to 3 November 2016 in the Laptev, East Siberian and Chukchi Seas and as well as the North Pacific and the Sea of Japan (see Fig. 1).The measurements were performed using a Cavity-Ring-Down Spectrometer (CRDS) from Picarro™ (model G2132-i).Together with methane concentrations of other trace gases (CO 2 , NO, NO 2 , O 3 ) were measured.CRDS (G2132i) allows to measure the methane concentration in the range from 1800 to 12000 ppb with an error of less than 5 ppb, and the value of δ 13 C CH4 with an error of less than 1 ‰.The cameral experiments showed that the intrinsic noise of the device did not exceed the indicated error values.

ENVIRONMENT
Calibrations of the CRDS (G2132i) were carried out according to the secondary standard, which was a 1-liter compressed air cylinder provided by the Norwegian Institute for Air Research (NILU) with known values of methane and δ 13 C CH4 concentrations.
The relative error of this measurement did not exceed 0.03% for methane and 0.1% for δ 13 C CH4 .The secondary standard was calibrated by primary standard known as NOAA04 (Dlugokencky et al. 2005) for CH 4 , while calibration for δ 13 C CH4 was made by method described in (Fisher et al. 2006).
Calibrations of the G2132i were carried out with a period of 1-2 months, immediately before and after the ship campaign.The scheme of the experimental setup and the calibration results are shown in Fig. 2. All obtained values differ within error of the CRDS, indicated in its technical specification (5 ppb).A special study of the short-period drift of the instrument readings showed that the root-mean-square deviation of the instrument readings did not exceed 0.05% within a time period of 10-20 minutes.All the observational data were recalculated in accordance with the obtained calibration coefficients, and a series of data were averaged over the intervals of 1 min and 10 min.
During the ship campaign, the air was sampled from the inlet at the front of the deck at height of 11 meters above sea level.The length of the pipeline was 30 m, and the inner diameter was 10 mm with air flow rate of 3 litres min -1 .Such arrangement of the air intake allows minimizing the perturbations of the airflow by the vessel during air sampling (Edson et al. 1998).
The diesel fuel used by ship engine does not contain methane, though it contains hydrocarbons, such as, for example, cetane and alpha-methylnaphthalene-an aromatic hydrocarbon.If smoke from ship chimney occurs into the G2132-i air intake, hydrocarbons may distort the methane concentration value.Thus, to exclude the influence of the ship itself, data on the CO 2 concentration were analyzed.CH 4 data, which were corresponding to a high concentration of CO 2 , were excluded from analyses.
The measurements were carried out in the autumn period, when advection of cold air occurred on the coast of the Arctic seas, and a snow cover began to form.During the cruise the temperature was lower than 0ºC coastal zone was under the snow cover.Under these conditions, the mainland natural sources of methane were not active.It was confirmed by the wind direction analysis.The frequency of wind direction were calculated for four sectors (316-45, 46-135, 136-225, 226-315 degrees) for the Laptev, the East-Siberian and the Chukchi seas and the mean CH 4 concentration for each sector was obtained.There is no significant dependence of the methane concentration on the direction of the wind in the seas of the Eastern Arctic -difference between the mean methane concentrations calculated for the each sector is less than 1 %.

RESULTS AND DISCUSSION
Table 1 gives the statistical characteristics of the results of observations for the Laptev and East Siberian seas, calculated with averaging of 1 minute.As follows from these data, in general, the methane content in the near-surface air of the seas of the Eastern Arctic and in the northern Pacific regions is very uniform in October (the standard deviation of the series is about 0.02 ppm) and stably exceeds the average global value characteristic for this period.
At the same time, in the Laptev and the East-Siberian seas, localized areas with a high concentrations of methane in the above-sea air have been identified (Fig. 3).Of greatest interest are the so-called methane-emission polygons described in (Shakhova et al. 2014).One of these polygons was located approximately at the coordinates of 75° N and 160° E (Fig. 4).The vessel was there from October 11 to October 13, 2016.Figure 3 shows the graph of the dependence of the methane concentrations on time according to the measurements by the G2132i instrument with averaging of 10 seconds.As can be seen from the graph, the concentration of methane above the water surface is characterized by a large number of peaks by the value of 2.0-2.2ppm and more.From the board of the vessel raising of methane bubbles from the water was visually detected.According to sonar data from the vessel, methane bubbles came directly from the bottom, as the depth in the observation area reached 45-50 meters.
Information on δ 13 C CH4 is important for identifying sources of atmospheric methane in the Arctic (Quay et al. 1999;Fisher et al. 2011).The average value of δ 13 C CH4 for well-mixed atmospheric air is about -47.1 ‰, but it strongly depends on the season and latitude (Rigby et al. 2012).Our results show the average δ 13 C CH4 value -49.86 ‰ for Laptev and East Siberian seas.
The presence of methane concentration peaks generated by the release of methane bubbles to the surface causes a slight increase in CH 4 average concentration in the surface air above the seeps.

ENVIRONMENT
Thus, the average daily methane concentration level on the seep polygon (October 12) is by few percent higher than the average daily methane concentration outside of this polygon (October 10 and 14).It is comparable with regional variations in the average daily concentration (for example, on October 9, when the vessel was near of Kolyma delta).However, the AIRS data Level 2 show the area with the excess total CH 4 content which can be connected with releasing of the methane from the sea water to the surface air (see Fig. 4).One can notice relatively slight decrease of δ 13 C CH4 opposite the highest methane peaks (see Fig. 3).This can be explained by similar isotopic signatures of ambient air and air from hydrates (Fisher et al. 2011) that makes application of isotopic analyses methods (like Keeling plot) for seeping regions not clear (Skorokhod et al. 2016).

CONCLUSIONS
Our measurements of CH4 in the atmosphere across ESAS during September and October 2016 in general show stable CH 4 and δ 13 C CH4 concentrations in the surface air.However, the possibility of CH 4 high peak (up to 3.54 ppm according to our measurements) ENVIRONMENT 90 formation in the atmospheric air above the ESAS in the areas of methane seeps was indicated.These enhancements cannot be associated with air pollution (including influence ship emissions) and terrestrial methane sources, which were likely to be inactive during the ship campaign.Therefore, the performed measurements were likely to be the first direct evidence of atmospheric response of benthic methane escape into the atmosphere in the Arctic.
On base of these data, it is difficult to assess real amount of methane released from the Arctic seas into the atmosphere.Meteorological conditions were not favorable to methane accumulation within the boundary layer.On the contrary, strong winds and unstable stratification led to fast dissipation of released methane.One can assume nevertheless that those methane emissions are significant enough to make quite stable footprint on maps built from satellite data.Thus, satellite data of AIRS (Atmospheric Infrared Sounder) steadily show an increased total methane content in the vertical column of the atmosphere in the area corresponding to the seep area.For instance, in October, 2016 CH4 total column exceeded 3.90x10*19 mol/cm2 inside the polygon comparing to 3.85x10*19 mol/cm2 out of it (see Fig. 4).
The local peaks of atmospheric methane in this region is a strong indication in favor of the hypothesis that the ESAS shelf is a potential significant source of atmospheric methane.But further studies are needed to clarify the quantitative characteristics of this source nowadays and in the future.

Fig. 2
Fig. 2. a -Schematic diagram of the experimental setup for calibration of the CRDS (G2132i) with different humidity of the analyzed air, b -Calibration results for the CRDS (G2132i) before and after the campaign (a) (b)

Fig. 3 .
Fig. 3. Observed CH4 and CO2 concentration and δ13CCH4 during the ship campaign of R/V 'Akademik M.A. Lavrentiev' (24 Sept-02 Nov 2016).Scaling up CH4 peak for methane emission polygon (of 75° N and 160° E) is shown in the frame on the right