ISSN 1068-3739, Russian Meteorology and Hydrology, 2015, Vol. 40, No. 6, pp. 427–433. Ó Allerton Press, Inc., 2015. Original Russian Text Ó G.I. Mishukova, V.F. Mishukov, A.I. Obzhirov, N.L. Pestrikova, O.F. Vereshchagina, 2015, published in Meteorologiya i Gidrologiya, 2015, No. 6, pp. 89–96.

Peculiarities of the Distribution of Methane Concentration and Methane Fluxes at the Water–Air Interface in the Tatar Strait of the Sea of Japan G. I. Mishukova, V. F. Mishukov, A. I. Obzhirov, N. L. Pestrikova, and O. F. Vereshchagina Il’ichev Pacific Oceanological Institute, Far East Branch, Russian Academy of Sciences, ul. Baltiiskaya 43, Vladivostok, 690041 Russia, e-mail: [email protected] Received August 25, 2014

Abstract—The spatial distribution of the methane content in the sea water in the Tatar Strait of the Sea of Japan is investigated using the data of expeditionary studies in 2012–2013. The developed model enabled computing methane fluxes at the water–air interface. Identified are the water areas of active methane emission in the area of underwater gas blowouts. Revealed is the abnormal concentration of methane in the areas of emission of natural gas bubbles on the western shelf and on the slope of Sakhalin Island. These bubbles give rise to the upwelling of bottom water to the overlying layers. It is discovered that the formation of the cold subsurface layer (CSL) of sea water being especially active in the area of gas blowouts is caused by phase transitions as a result of the hydrophobic hydration of natural gas molecules.

DOI: 10.3103/S1068373915060096 Keywords: Methane distribution, methane flux, the Sea of Japan

INTRODUCTION The studies of the distribution of methane and gas hydrates in the Northwest Pacific have been actively carried out in the Il’ichev Pacific Oceanological Institute of the Far East Branch of Russian Academy of Sciences in the recent decade. Numerous papers and several monographs deal with the results of these studies [1, 3, 5, 6]. Besides the solution of energetic problems, the interest to this topic is caused by the need in the determination of the degree of the impact that methane produces on the greenhouse effect and formation of ozone holes [3]. The results of the computation of methane fluxes to the atmosphere in the area of the Sea of Japan presented in [2, 4, 7, 15] were obtained for methane concentration in the surface sea water and atmosphere. The analysis of the results demonstrated that in the northwestern part of the Sea of Japan (for example, the Vityaz’ Upland, the Gamov Canyon, the slope of the Peter the Great Gulf, and the La Perouse Strait) the areas can be singled out where the high values of methane concentration and increase in methane fluxes to the atmosphere are registered. The objective of the present paper is to analyze the integrated data obtained in the Tatar Strait for identifying the impact of meteorological, hydrological, and hydrochemical parameters of sea water on the distribution of methane and on the computation of the value of its fluxes at the water–air interface taking into account the variations of seismic activity in the Sea of Japan. OBJECTS AND METHODS OF RESEARCH The sampling of water from different horizons was carried out by Rozzet Niskin bottles in combination with the multiparameter CTD probe. The concentration of methane in the water was determined by the method of equilibrium paraphase analysis described in [15]. The analysis of the gas phase was carried out on board the vessel using the Kristall-Lyuks-4000m chromatograph (Yoshkar-Ola, Russia). 427

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Fig. 1. The distribution of methane concentration in the surface waters (nmol/l) and the methane flux (mol/(km2 day)) in the Sea of Japan. (1, 2) Water sampling stations during the expedition of the research vessel Akademik Lavrent’ev: voyage 59, August 2012 and voyage 62, June 2013, respectively; (3) earthquake epicenters; (4, 5) and numerals are the methane fluxes (mol/(km2 day)) computed from the data of the expeditions of the research vessel Akademik Lavrent’ev: voyage 59, August 2012 (profile 1) and voyage 62, June 2013 (profile 2).

To compute the content of methane dissolved in the sea water the equation presented in [16] was used. The methane sensitivity amounted to 1 ´ 10–6%. The relative error of determination of the volume of gas dissolved in the sea water did not exceed 0.6% and the coefficient of variation in the case of the repeated determination of methane concentration at one point under natural conditions did not exceed 3.2%. The peculiarities of the methane distribution at the water–air interface were studied in the Tatar Strait of the Sea of Japan. The special attention was paid to the testing areas westwards of Sakhalin Island including the shelf areas where the depth reached 200 m, the slope, and the deep areas (the depth is up to 1200 m) where underwater gas blowouts were detected by echo sounding using the backscattering. The data obtained during two expeditions of the research vessel Akademik Lavrent’ev (voyage 59 in August 2012 and voyage 62 in June 2013) were used for the analysis. The data on the position of epicenters and on the time and magnitude of earthquakes for 2011–2013 were obtained from the website of the United States Geological Survey [14]. These data were needed for considering the impact of seismic conditions in the region at the moment of experimental studies. Methane fluxes were computed using the method described in [3, 12] and meteorological measurements were carried out at the onboard weather station during the drifting. RESULTS AND DISCUSSION Figure 1 presents the scheme of the location of water sampling stations, the distribution of methane concentration in the surface layer of sea water, methane fluxes in the Tatar Strait, and the position of earthquake epicenters in the area of Sakhalin Island in 2011–2013. According to the data of expeditions, the horizontal distribution of methane concentration in the surface layer of sea water in the area of the Sea of Japan under study is inhomogeneous. The mean equilibria (with the atmosphere) concentration of methane in the sea water corresponds to the approximate value C * » 2.6 nmol/l at its salinity and temperature at the moment of measurement, and the methane emission is observed if C > 2.6 nmol/l (see the table). The data presented in Fig. 1 demonstrate that the maximum methane emission was observed on the western shelf of Sakhalin Island in the area of underwater gas blowouts. RUSSIAN METEOROLOGY AND HYDROLOGY

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Characteristics of variations in methane concentration and in the parameters of the sea water surface layer in the Sea of Japan Characteristic

C

C*

DC K, %

T, °C

U, m/s

S, ‰

nmol/l

F, mol/(km2 day)

Research vessel Akademik Lavrent’ev, voyage 59, August 2012 Maximum Minimum Mean

11.5 3.4 5.6

2.9 2.2 2.5

8.6 0.9 3.1

393 134 221

20.0 10.0 17.6

7 2 5

33.42 33.10 33.24

14.1 1.6 5.1

Research vessel Akademik Lavrent’ev, voyage 62, June 2013 Maximum Minimum Mean

10.4 3.3 5.0

3.1 2.6 2.7

7.7 0.1 2.3

383 102 184

13.8 10.5 12.3

15 9 12

33.74 32.51 32.97

38.1 0.5 11.8

Note: C is the measured methane concentration; C* is the equilibrial concentration that atmospheric methane of the sea water could have at the given temperature T, salinity S of the sea water in the three-meter surface layer, and air pressure; DC is the difference between the measured and equilibrial values of methane concentration; K is the parameter of water enrichment with methane; F is the methane flux at the water–air interface; U is the wind speed for the water areas under study.

The comparison of the schemes of methane distribution in the surface water and of earthquakes on Sakhalin Island and in the coastal water of the Sea of Japan indicates that the activation of methane emission from the lithosphere to the sea water takes place under the influence of earthquakes. Maximum methane fluxes were observed between 47.5° and 48.5° N where the earthquake epicenter was registered near the water sampling stations on May 8, 2013. Another earthquake occurred after the sampling near Moneron Island on July 4, 2013. It should be noted that no earthquakes were observed in the region under study in 2011 and 2012. The distribution of methane concentration, temperature, and salinity of sea water in the Tatar Strait in profiles 1 and 2 (see Fig. 1) is presented in Fig. 2. The vertical distribution of methane in the region is of clear inhomogeneous nature that is evidently caused both by the complex hydrological structure of currents in the area under study and by the effects that seasonal variations of the hydrochemical parameters of sea water have on methane distribution. Thus, the active emission of methane from underwater sources on the western shelf of Sakhalin Island causes the formation of the subsurface layer of sea water (the depth is 280–380 m). This layer had maximum values of methane concentration in summer 2012 in the water area with the length of about 150 km (Fig. 2a). The formation of local zones with the maximum methane concentration in the sea water at the depth of 430–470 m, 550 m, and in the bottom layer can also be noted. Profile 2 differs significantly from profile 1 in the vertical distribution of all parameters (Figs. 2c and 2d) in June 2013. First of all, this is caused by its closer location to the coastline. Besides, profile 2 was obtained for the area where the active methane emission in the form of numerous “bubble blowouts” was detected. Three zones with high methane content in the sea water can be separated in this area: at the depth of 40, 130, and 260 m (and, perhaps, at 450 m). The vertical extent of these zones is from ten to one hundred meters and the horizontal size is from 50 to 120 km. The formation of the zones with the maximum methane concentration is accompanied by decrease in the temperature and salinity of sea water at the corresponding depth. RESULTS OF MODELING To explain the above peculiarities of the distribution of methane concentration as well as of other characteristics of sea water, the simulation of some natural processes was carried out on the basis of the model of thermogas divergence. This molel is based on the model of gas condensate transport [3] and on the body of mathematics of the models which were worked out before and were described in [8, 17]. Unlike the model used in [3], the Princeton Oceanic Model whose description is open for public in the Internet, was RUSSIAN METEOROLOGY AND HYDROLOGY

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Fig. 2. The vertical distribution of (a, c) methane concentration (nmol/l and (b, d) temperature (°C) in the Sea of Japan during the expeditions of the research vessel Akademik Lavrent’ev. (a, b) Voyage 59, August 2012 (profile 1); (c, d) voyage 62, June 2013 (profile 2).

applied as the basic model for computing the field of currents [11]. This is the three-dimensional nonstationary nonlinear numerical model taking into account the density uden(x, y, t) and wind currents uw(x, y, t). Besides, the computations took account of tidal utide(x, y, t) and turbulent ud(x, y, t) currents. The general scheme of the computation of the horizontal components of the currents is expressed by the following formula: (1)

u total = {u den ( x, y, t ) + u w ( x, y, t ) + u tide ( x, y, t ) + u d ( x, y, t )}.

The computation of the fields of currents for the Northwest Pacific including the Sea of Japan and Sea of Okhotsk was carried out at the grid of 15 ´ 20 km. The time step was equal to 10 minutes. Figure 3 presents the map of the computational domain with the depth distribution and location of weather stations where basic meteorological parameters were measured such as the wind speed, wind direction, air pressure, air humidity, and air temperature. New experimental values of meteorological parameters were introduced into the computation every six hours; they were interpolated to the grid points for the Sea of Japan and Sea of Okhotsk and were extrapolated for the Northwest Pacific. The initial values of the distribution of temperature and salinity of sea water were taken from electronic atlases [10]. The presented model is solved numerically. The results of the computation of the fields of currents at the grid points are averaged for 6 hours and are recorded to the dataset on the velocity and direction of currents; the dataset is used afterwards for computing the rate of propagation of markers in the water area. Methane dissolved in the sea water was considered as a marker. It does not affect the physical and chemical properties of sea water at the prescribed values of concentration and is transported together with the body of water. The computation of the movement of markers in the Cartesian coordinate system was carried out using the following formula: RUSSIAN METEOROLOGY AND HYDROLOGY

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Fig. 3. The computational domain of the fields of currents and distribution of impurities. The white circles are weather stations; the squares with numerals are the selected points for constructing the temporal variations of computed parameters; the black lines are the cross sections constructed during the voyage of the research vessel Professor Gagarinskii in June 2012.

dx = {u den ( x, y, t ) + u w ( x, y, t ) + u tide ( x, y, t ) + u d ( x, y, d )}. dt

(2)

The presented model was tested during the voyage of the research vessel Professor Gagarinskii in June 2012 (Fig. 3); the testing took into account the study of the distribution of artificial radionuclides in the sea water after the Fukushima Daiichi nuclear disaster. The analysis demonstrated that the spatial distribution of methane concentration in the sea water and its other hydrochemical characteristics change in the presence of gas bubble blowouts. The entrainment of water into the jet of gas bubbles is observed in the areas of gas blowout. As a result, the sea water transport to the overlying layers takes place (upwelling). During the emerging of bubbles, methane contained in them is actively dissolved in the sea water that is accompanied by the transition of oxygen and nitrogen dissolved in the sea water to the gas phase of a bubble. Thus, the sea water in the upwelling zone is enriched with methane and the content of the dissolved oxygen and nitrogen in water decreases. It was revealed that single gas bubbles with the diameter of less than 1 mm are fully dissolved in the water if they move vertically for 60–100 m. If gas blowouts are located at the depth of less than 60–100 m, the direct transport of gas from the bottom to the surface waters is possible with the subsequent emission of methane to the atmosphere. If gas blowouts are located at the depth of more than 100 m, methane is concentrated in the water column at the depth of the complete dissolution of gas bubbles. If the usual for summer distribution of temperature in the sea takes places, that is, it drops with depth, the entrainment of cold bottom waters to the gas blowout and their vertical transport forms not only the cold water layer at overlying horizons but also the water layer with high methane concentration. The formation of horizontal layers of sea water enriched with dissolved methane is described in detail for the Sea of Okhotsk in [3]. The high velocity of tidal currents on the western shelf of Sakhalin Island (up to 1 m/s) at the mean velocity of the transport of the Kuroshio western branch to the north, determines the rapid horizontal movement of the formed intermediate layer of cold water enriched with methane to the large distances from the area of its formation. This explains the observed pattern of inhomogeneous layer-by-layer distribution of methane in the sea water and of its temperature. The obtained maximum values of methane concentration at several stations located at the significant distance from each other and at the certain depth indicate that they are not of random nature (Fig. 2). RUSSIAN METEOROLOGY AND HYDROLOGY

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Besides, the possibility of gas hydrate formation as a result of the dissolution of natural gas in the sea water in the process of gas bubble emerging was taken into account for the modeling. It is known [9] that the formation of hydrated molecules of the natural gas at the depth of 50–300 m at the sea water temperature of –1.8…0°C is observed as a result of the hydrophobic hydration, and intermediate compounds are generated and subsequently cause the formation of gas hydrates. Besides methane (88–94%), natural gas comprises ethane, propane, isobutane, normal butane, and isopentane [13]. The process of the formation of intermediate compounds and gas hydrate is accompanied by heat release. The size of molecules increases in the process of hydration. This dramatically reduces their translation properties and results in the concentration of these molecules in the water layer of low temperature and, hence, in the formation of the layer with the maximum concentration of methane; the latter is spatially distributed in the significant part of the Tatar Strait water area in the zone of gas blowouts (Fig. 2). After that, as hydrated molecules and gas hydrates emerge, their decomposition takes place at the depth of 50–250 m as a result of the significant heat absorption accompanied by the sea water temperature drop. Thus, the layer of sea water of low temperature favors increase in the concentration of methane that maintains low temperature in the cold subsurface layer of sea water in summer. CONCLUSIONS The presented experimental data on studying the distribution of methane and its fluxes at the water–air interface and the results of the modeling demonstrated the following: —the distribution of methane in the sea water in the Tatar Strait of the Sea of Japan is defined by the spatial distribution of underwater sources of methane; the active emission of methane to the marine environment and atmosphere takes place in the shelf zone of Sakhalin Island; —the abnormal concentration of methane in the sea water is observed in the areas of gas blowouts (on the shelf and slope of Sakhalin Island) which give rise to the upwelling of bottom water to the overlying horizons accompanied with synchronous decrease in its temperature; —the natural phenomenon of the formation of horizontal layers of sea water enriched with methane at different depths described in [3] was corroborated. The maximum extent of such layer is observed in the cold subsurface layer of the sea water whose existence is caused by phase transitions as a result of hydrophobic hydration of the molecules of natural gas. ACKNOWLEDGMENTS The research was carried out in the framework of the project “Computing the Fields of Currents, Transport, and Transformation of Pollutants and Environmental Hazards in the Far East Region of Russia” of the program of the Presidium of Russian Academy of Sciences “Fundamental Problems of Mathematical Modeling” and was supported by the Russian Foundation for Basic Research (grant 14-05-00294 “Studying the Background and Abnormal Gas-geochemical Fields and Their Interrelation with Active Geological Processes in the Sea of Okhotsk and Sea of Japan and in Their Surroundings”). REFERENCES 1. V. A. Akulichev, A. I. Obzhirov, R. B. Shakirov, et al., “Conditions of Gas Hydrate Formation in the Sea of Okhotsk,” Dokl. Akad. Nauk, No. 3, 454 (2014) [Dokl. Phys., No. 3, 454 (2014)]. 2. G. I. Mishukova, V. F. Mishukov, and A. K. Okulov, “Distribution of Methane and Its Fluxes at the Water–Air Interface in the Peter the Great Gulf (the Sea of Japan),” Vestnik DVO RAN, No. 6 (2013) [in Russian]. 3. G. I. Mishukova, A. I. Obzhirov, and V. F. Mishukov, Methane in Fresh and Sea Waters and Its Fluxes at the Water–Air Interface in the Far East Region (Dal’nauka, Vladivostok, 2007) [in Russian]. 4. G. I. Mishukova, N. L. Pestrikova, V. F. Mishukov, and O. S. Yanovskaya, “Distribution of Methane and Computation of Its Fluxes at the Water–Air Interface in the Northwestern Part of the Sea of Japan during the Warm Season,” Podvodnye Issledovaniya i Robototekhnika, No. 1 (2011) [in Russian]. 5. Methane Monitoring in the Sea of Okhotsk, Ed. by A. I. Obzhirov, A. N. Salyuk, and O. F. Vereshchagina (Dal’nauka, Vladivostok, 2002) [in Russian]. 6. A. I. Obzhirov, N. V. Astakhova, M. I. Lipkina, et al., Gas-geochemical Zoning and Bottom Mineral Associations in the Sea of Okhotsk (Dal’nauka, Vladivostok, 1999) [in Russian]. 7. A. I. Obzhirov, G. I. Mishukova, and V. F. Mishukov, “Gas-chemical Indicators of Underground and Surface Waters in Primorye and of Sea Water in the Peter the Great Gulf,” in Current Environmental Conditions in the Peter RUSSIAN METEOROLOGY AND HYDROLOGY

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