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Spectrochimica Acta Part A 66 (2007) 788795

Review

Spatial distribution of methane over Lake Baikal surface

V.A. Kapitanov a,, I.S. Tyryshkin a, N.P. Krivolutskii a, Yu.N. Ponomarev a,M. De Batist b, R.Yu. Gnatovsky c

a Institute of Atmospheric Optics, SB RAS, 1 Akademicheskii ave, 634055, Tomsk, Russiab Renard Center of Marine Geology, Ghent University, Belgium

c Limnology Institute, SB RAS, Irkutsk, Russia

Received 15 October 2006; accepted 17 October 2006

Abstract

The results of application of a high sensitivity methane laser detector to investigations of the methane concentration in the atmosphere overBaikal lake are presented as well as methane flows from the water into the atmosphere. The measurements were conducted at a stationary stationand aboard the research vessel Vereschagin during two summer expeditions in 2003 and 2004. Mean background concentration was equal to(2.00 0.16) ppm in August 2003 and (1.90 0.07) ppm in June 2004. The areas of methane emission through the waters surface are found to bedistinctly localized and to have a characteristic size of about 150300 m in diameter. The methane concentration in the centers of these areas canreach approximately 27 ppm. Methane flows into the atmosphere in some Baikal regions were measured as well. 2006 Elsevier B.V. All rights reserved.

Keywords: Diode laser detector; Lake Baikal; Atmospheric methane; Concentration measurement

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7882. Laser gas-analyzer of methane and measurement technique . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7893. Detector specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7904. Measurement procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7905. Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7926. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 795

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 795References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 795

1. Introduction

Methane is the most important representative of organic sub-stances in the atmosphere [1] and its concentration significantlyexceeds the concentration of other organic compounds. In recentyears, the amount of methane in the atmosphere increased ata rate about 1% a year, and this increase favors intensificationof the greenhouse effect, because methane efficiently absorbsthe Earths thermal radiation in the IR spectral region. The

Corresponding author. Tel.: +7 3822 492645; fax: +7 3822 492086.E-mail address: kvan@asd.iao.ru (V.A. Kapitanov).

contribution of methane to the greenhouse effect is about 30%of that due to carbon dioxide.

By estimates, the methane emission from natural andanthropogenic sources into atmosphere is approximately500600 Mt/year; and approximately one-third of this amount isdue to the natural sources: wetlands, termites, forest fires, GlobalOcean, and fresh water bodies. Although the general balance ofmethane in the atmosphere is well known, the contribution fromindividual sources remains poorly studied.

The discovery in 1969 of methane gas-hydrates in the Earthsinterior made by Vasilev et al. [2] has aroused an increasedinterest in gas-hydrates and stimulated a series of investigations,which found a huge fuel reserve in the form of gas-hydrates in the

1386-1425/$ see front matter 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.saa.2006.10.036

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bottom of the Global Ocean. According to the estimates avail-able, the reserves of the hydrocarbon fuel (mostly, methane)in the gas-hydrate form markedly exceed the combined fuelreserves in all other forms on the Earth. Gas-hydrates attractincreased attention not only because they can be used as fuel andchemical raw materials, but they may also be connected to pos-sible methane emissions into the atmosphere both in future fielddevelopments and due to relatively small changes in thermody-namic (climatic) conditions leading to a violation of the stabilityof the gas-hydrates, which may cause additional methane emis-sions into the atmosphere leading to additional ecological andclimatic problems.

Lake Baikal is the largest fresh water body on the Earth. Thearea of its water surface is about 31,722 km2. Quite recentlygas hydrates were found in its bottom [3,4]. Lake Baikal is aconvenient object for investigation of such formations, as wellas processes of methane emission from gas-hydrates. As wasshown in ref. [5], despite the fact that the first direct or indirectdata on gas emissions into water and atmosphere were obtainedas long as 200 years ago, the methane concentrations in the waterand atmosphere have not yet been measured.

The goal of our work was to study methane concentrationfields and flows, as well as to find anomalies in the methanedistribution in the atmosphere over the Lake Baikal surface usinga high-sensitive laser methane detector.

2. Laser gas-analyzer of methane and measurementtechnique

To measure methane concentrations and flows, we developedan improved version of the detector originally designed at the

Institute of General Physics RAS based on a multifrequencynear-IR diode laser and a multi-pass cell [6]. The detector isshown schematically in Fig. 1.

It consists of:

(1) Optical block including a diode laser (DL) with aFabriPerrot cavity, two optical cells: a multi-pass cell witha tunable optical way length (analytical channel) and a ref-erence one; two photoreceivers (PR);

(2) personal computer (Pentium);(3) many-functional Input/Output board AT-MIO-16E1 (or

AT-MIO-16E4), produced by National Instruments Inc.company, NI-DAQ board installed into the PCI bus;

(4) interface module located in the computer front panel;(5) programs controlling the detector.

The detector employs a GaInPAs diode laser as a sourceof radiation at a working temperature of 0 to +50 C. Chang-ing the magnitude of current and temperature, it is possible totune the laser frequency in the range from 6000 to 6080 cm1(1.6451.666 m), where rather strong absorption lines ofmethane are observed. The diode laser emits several (510)longitudinal modes. The radiation power in one of them isabout 70% of the total laser power (3 mW); the width of anindividual mode is 103 cm1. The laser is mounted on aPeltier element. Radiation frequency tuning of the highest-power mode to the frequency of methane absorption line centeris performed by changing the DL temperature. The temper-ature is measured by a thermal sensor (thermistor), situatednear the laser. The DL long-term temperature stability is 102degree.

Fig. 1. Methane detector block diagram.

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The diode laser operates in a repetitively pulsed modewith a period of 4.5 ms and a pulse duration of 4 ms. Currentpulses supplying the laser have a trapezoid shape. This allowsscanning the laser frequency in the range within 1 cm1 duringa single pulse and recording the transmission spectrum of themethane individual line. The diode laser emits in two oppositedirections. The principal laser beam comes into the multipassanalytical optical cell with a photodetector at the exit. Atthis time atmospheric air is continuously blown through thecell.

The laser radiation emitted in the opposite direction passesthrough the reference cell filled with a mixture of methaneand nitrogen at a preset methane concentration and is incidenton another photodetector. The method of methane concentra-tion determination is based on a calculation of the correlationfunction of the signals shape (absorption spectra of themethanenitrogen mixture and the atmospheric air) in both chan-nels. This allows us to achieve a high selectivity of the devicewith respect to other gases.

The multipass analytical cell is a Chernin matrix system [7]with a base length of 0.75 m, an optical path length of 157.5 m,a mirror reflection coefficient of 0.998, and a cell capacity of14 l. The atmospheric air was continuously blown through thecell by means of the membrane pump at a rate of 0.2 l/s.

The DL block was controlled by the NI-DAQ board andthe Interface Module (see Fig. 1). The latter provided controlcommunication between the NI-DAQ board and optical block ofthe device. The multifunctional board NI-DAQ (AT-MIO-16E4)includes two output channels (DAC), an input channel (ADC)able to operate with 16 inputs using a multiplexer, a timer and abuffer memory. Two DACs in the NI-DAQ board were used forthe current supply of the laser (Output 1) and Peltier element(Output 2). The ADC in the NI-DAQ board received signalsfrom the reference channel photoreceiver (Input 1), analyticalchannel photoreceiver (Input 2), and the temperature sensor(Input 3).

In order to power the DL power supply, the control programinitiates the generation of current pulse trains of a trapezoidshape. The pulse parameters are selected in such a way as toprovide an optimal mode of DL signal generation (i.e. tuning tothe methane absorption line) and a sufficient number of points(2001000) in each pulse. Such a generation mode (from 20to 170 pulses) in a train gives the possibility to process sig-nals, averaged over each train, during the time lapse betweenthe trains.

Reaching the preset DL temperature and stabilizing of thetemperature are provided by the controlling program moduletaking into account the proportional (P), integral (I) and differen-tial (D) components of the signal so that the preset temperaturecan be reached in a systematic way. In addition, the programmodule allows the frequency stabilization relative to the positionof the methane absorption line center in the reference channel.

The processing of photoreceiver signals in both channels iscarried out in the control program synchronously with respect tolaser pulse generation resulting in the calculation of the methaneconcentration in the analytical channel. First, signals from refer-ence and analytical channels (Inputs 1 and 2) are averaged by the

pulse train. The number of pulses can vary from 1 pulse to 2720pulses (at averaging time between 0.004 and 12.25 s). Second,the interval containing the methane absorption line is separated.The shape of the methane absorption line is determined con-sidering the laser instrumental function (current dependence ofincident radiation power). Then the signals from the separatedinterval are differentiated in three steps by the averaging ofhigh frequencies. Finally, the correlation coefficient of differ-entiated signals is evaluated, which is proportional to methaneconcentration in the analytical channel. The electronic and sig-nal processing procedure ensures measuring the lines with anoptical density of more than 103.

The controlling program for the methane detector is producedin the LabView 5.0 medium and operates under Windows-95 (or-98). It consists of two program modules controlling the laserdetector and the information processing.

3. Detector specifications

Based on laboratory tests and field measurements [8], thedetectors main specifications were determined:

(1) Threshold sensitivity: 0.03 ppm.(2) Range of measurable methane concentrations:

0.0310,000 ppm.(3) Duration of a measurement: 4.5 ms12 s.(4) Time constant for the detector on the whole after accounting

for the air flow rate and the cell volume: 99 s.(5) Total mass (without computer): 15 kg.(6) Sizes: 0.50 m 0.50 m 0.95 m.

4. Measurement procedure

Measurements of methane concentrations in atmosphere areconducted through continuous sampling of air at heights of 10and 2 m above the Lake Baikal water surface at a quantiza-tion step of 12 s. Coordinates of the measurement points weredetermined by the GPS vessel system. The methane detectorcalibration testing was made in the laboratory conditions usingstandard methanenitrogen mixtures of 2 and 6 ppm. The deter-mination of the gas-analyzer signal zero level was carried outevery 3 h. To eliminate zero drift, the analytical cell was purgedby spectrally pure nitrogen.

A simple procedure is used to estimate the methane flowsthrough the surface [9]. The surface under study (some area ofthe ground or water surface) is enclosed by a chamber of a vol-ume V (m3) and an area base Sb (m2). After some accumulationtime t (s), the gas concentration C (kg m3) in a sample takenfrom the chamber is measured. Providing that the condition ofequilibrium in the system object-atmosphere is not violatedduring the accumulation time t and the sample volume Vs ismuch less than V, the flow (gas emission rate) into atmosphereF (kg s1 m2) can be calculated as

F = (C C0) V(tSb)

(1)

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Fig. 2. Scheme of air probe capture from the floating chamber.

where C0 (kg m3) is the initial gas concentration (gas concen-tration in the atmosphere).

It follows from Eq. (1) that to measure weak flows, high-sensitive detectors must be used, as well as small-volumeaccumulation chambers V with big open base Sb and large accu-mulation times t.

At present, a highly sensitive chromatographic method is usedto measure the atmospheric methane concentration [9,10]]. Thevolume of the air sample Vs required to the chromatographicanalysis is about (24)E6 m3, and accumulation chambers of asmall volume (0.21)E3 m3 are used to measure weak methaneflows. The volume of the analytical cell of highly sensitivemethane laser detectors is (120)E3 m3, and it is necessaryto use the accumulation chambers of a big size.

We used the procedure, which allows sampling independentof the accumulation chamber size.

The process of sampling from the accumulation chamber isschematically shown in Fig. 2.

The procedure is very simple. Total measurement time isdivided into two periods: the accumulation time ta and themeasurement time tm. During the accumulation time ta,atmospheric air continuously runs through the analytical celland the gas concentration C0(t) in the atmosphere is being mea-sured. After the accumulation time has elapsed, the air flow isdirected by a valve to the accumulation chamber and the analyt-ical cell of the detector. The air passes through the accumulationchamber and the analytical cell during the measurement timetm and the gas concentration C(t) in analytical cell is beingmeasured. When the measurement time has elapsed, the valveturns and air runs again through the analytical cell. An exampleof the time dependent methane concentration is shown in Fig. 3.

Let us evaluate the methane amount budget at the input andoutput of the methane detector during the measurement timetm. If the methane emission rate is constant, the methaneamount M+ that will come to the detector input through theaccumulation chamber during the measurement time tm is:

M+ = FSb(ta +tm) + S0tm

C0(t)dt, (2)

where S0 (m3/s) is the gas volume at the atmospheric pressurethat passes through the analytical cell for the unit of time.

At the same time, the methane amount M, which haspassed through the detector analytical cell during the measure-ment time tm, is:

M = S0tm

C(t)dt. (3)

Fig. 3. The typical time dependent methane concentration under flow measure-ments. The accumulation time ta is 20 min, the measurement time tm is7 min.

Setting (2) equal to (3), we obtain the emission rate F:

F = S0tm

(C(t) C0(t))dtSb(ta +tm) (4)

Since a simultaneous measurement of the concentrationC0(t)during tm is impossible, and its variations are lower thanthe threshold sensitivity, the magnitude of C0(t) in (4) can bereplaced by its mean value determined for the accumulationtime:

F = S0tm

(C(t) C0m)dtSb(ta +tm) , (5)

where

C0m = 1ta

ta

C(t)dt. (6)

The methane emission from the lake surface was measuredusing a floating chamber (Fig. 4) of 0.035 m3 volume and 0.2 m2

base area.

Fig. 4. Floating chamber for flow measurements.

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Table 1Comparison of methane concentrations in atmosphere of Lake Baikal, measuredwith laser methane detector and gas chromatograph

Date of measurements Method ofmeasurements

Methane concentration(ppm)

21.06.04 GC (five air samples) 1.89 (0.056)Methane detector 1.94 (0.054)

22.06.04 GC (two air samples) 1.95 (0.056)Methane detector 2.02 (0.09)

The methane was being accumulated in the chamber onlyduring the vessel drift. In our measurements, the vessel drifttime (accumulation time) was between 1200 and 12000 s, andthe measurement time was about 600 s. The flow was estimatedby Eqs. (5) and (6).

5. Results

Investigation of time variability and spatial fields of themethane concentration was conducted in 2003 and 2004 bothunder stationary conditions and aboard the research vesselVereschagin.

To verify measurement data, in 2004 the methane back-ground concentration was simultaneously measured with a lasergas-analyzer and a SRI 8610c gases chromatograph (SRI Instru-ments, USA). Samples for the chromatographic analysis weretaken immediately from the analytical cell of the laser gas-analyzer. The intercalibration results are given in Table 1 andagree within error limits of the both methods.

In stationary conditions (background station of the Limnol-ogy Institute SB RAS, Bolshie Koty, sampling point at a heightof 1.5 m above the surface and at a distance of 1015 m offshore) the methane concentration, averaged for 4 days, was1.887 ppm (0.049). Measurements were conducted every 12 sand the averaging time was 3 h (Fig. 5).

Fig. 5. Weekly variations of methane concentration in the near surface air at afixed-site station (Bolshie Koty, June 916, 2004).

Fig. 6. Methane concentration in near-surface air along the vessel tracks: (a)August 2003 and (b) June 2004.

Fig. 7. Methane allocation for seep area near Selenga mouth, showing sharp andlimited in space elevation in air methane concentrations.

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Fig. 8. Methane concentration distribution at seep areas (six seep areas were observed).

Fig. 9. Vessel tracks near Selenga mouth (point 4, Fig. 8), The magnitude of the methane concentration is proportional to the size of the plotted point (left-handdiagram); the time dependent methane concentration corresponding to vessel track (right-hand diagram), y-scalemethane concentration, ppm.

For the measurement time, the methane concentrationvariations in stationary conditions were within 0.2 ppm(1.822.02 ppm), which agrees well with the data of sta-tionary background stations WDCGG [10]. Further, data we

obtained allowed us to find zones with methane concentrationsexceeding the mean value by plus 0.4 ppm (double variation),i.e. zones with increased methane emissions from the lakesurface.

Fig. 10. Vessel tracks near Mishikha (point 6, Fig. 8), The magnitude of the methane concentration is proportional to the size of the plotted point (left-hand diagram);the time dependent methane concentration corresponding to vessel track (right-hand diagram), y-scalemethane concentration, ppm.

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Table 2Methane flux evaluation in seep and non-seep areas in South and Middle Baikal (June, 1624 2004)

Date, start time Start position,latitude/longitude

End position,latitude/longitude

Storagetime (s)

C0air (ppm)(track average)

Ccham (ppm)(max)

Flux 1012(kg/s m2)

17.06.0415:30:03 51.6848/103.9560 51.6860/103.9550 7599 1.826(0.068) 1.788(0.03) 0.5(1)

19.06.0306:20:02 51.7502/105.8379 51.7502/105.8412 2411 1.915(0.066) 47.62(0.42) 2120(18)07:30:00 51.7506/105.8312 51.7538/105.8477 3244 1.918(0.069) 5.95(0.045) 139(1)08:58:08 51.7493/105.8323 51.7515/105.8796 7670 1.878(0.078) 3.786(0.1) 27.8(1.5)

20.06.0416:12:03 52.9360/107.2435 52.9389/107.2497 3525 1.866(0.05) 1.92(0.02) 1.7(1)18:42:48 52.9127/107.5270 52.9204/107.5378 8948 1.815(0.05) 1.915(0.015) 1.25(0.63)

21.06.0416:08:06 53.0623/106.8517 53.0623/106.8513 1273 1.882(0.025) 2.086(0.02) 17.9(2)21:22:04 52.6246/106.6612 52.6229/106.6601 5884 1.904(0.04) 1.949(0.06) 0.8(0.9)

22.06.0406:43:05 52.4905/106.7180 52.4899/106.7184 1910 2.097(0.05) 2.187(0.044) 5.3(3)

23.06.0413:02:03 51.8580/105.6893 51.8571/105.6830 6465 1.876(0.063) 2.688(0.049) 14.1(1)17:32:12 51.7527/105.8381 51.7521/105.8456 4787 2.009(0.069) 13.93(0.16) 279(3)

24.06.0401:37:08 51.6112/104.7263 51.6098/104.7247 12223 1.939(0.076) 2.27(0.036) 3(0.7)

Fig. 6 (a and b) presents results from the measurement ofmethane concentration fields in the atmosphere along the vesselcourse obtained during the research expedition in August 2003and June 2004.

The mean value of the gas background concentrationwas found to be (2.00 0.16) ppm in August 2003 and(1.91 0.07) ppm in June 2004.

Somewhat higher background concentrations in 2003throughout Lake Baikal, as well as a region with methane con-centration of the order of 2.32.4 ppm in the north part of the lakeclose to shore line are, probably, connected with extensive firesin the Irkutsk region at that time (Fig. 6a). In places of notice-able methane emission into the atmosphere near the mouth ofthe Selenga river (a depth of 30 m), increased methane concen-trations reaching 5.5 ppm were recorded in a clearly localizedarea of about 150300 m in diameter (Fig. 7).

In 2004, methane concentrations significantly exceeding themean value (>2.4 ppm) were recorded already in six regions ofcentral Lake Baikal (Fig. 8).

In points 1, 2, 3, 6 the increased methane concentrations wereobserved in clearly localized areas. Near the Selenga mouth(point 4) and the Babushkin settlement (point 5), the methaneflows from the bottom to the lake surface were visible as gasbubbles, forming already fields of individual sources (Fig. 9).

The depth in points 1, 2, 3, 4, 5 was about 25100 m. A char-acteristic and significant increase of the methane concentrationwas recorded on June 18, near the Mishikha settlement (Fig. 10),where the depth is about 1000 m.

It should be noted that the bottom echo-sensing data show apresence here of a mud volcano.

Methane flows were measured with a floating chamber in 12areas of Lake Baikal. The emission estimate calculated by the

above method is shown in Table 2, which also presents date, time,start and end coordinates of the vessel drift, period of methaneaccumulation in the cell, and the averaged over trajectory valueof the methane concentration in air. In areas, where methaneconcentrations in chamber Ccham significantly exceed the meanvalue in air C0air, the flows are marked in bold type

Simultaneously with our measurements, our colleagues fromthe Pacific-Ocean Oceanology Institute FEB RAS (Vladivos-tok) measured the methane concentration in the near-surfacewater layer by the gas chromatography method. In areas,where the concentration of water-dissolved methane was within100200 nl/l (equilibrium with atmospheric one), the methaneflows did not exceed values of 3 1012 kg/s m2. In some

Fig. 11. Areas and vessel tracks of the methane flux measurements, the cor-responding methane concentrations in near-surface water (nl/l) are numerated(data of POI).

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Baikal regions, the flow magnitude averaged over the vesseldrift reached values exceeding 2000 1012 kg/s m2. Themethane concentrations in the near-surface water layer therealso exceeded the equilibrium one by two orders of magnitude(Fig. 11).

6. Conclusions

We presented results from the application of a methanelaser detector with a multipass optical cell to investigationsof the methane concentration in atmosphere over the LakeBaikal surface, as well as methane flows from the waterinto the atmosphere. The laser methane detector is character-ized by a threshold concentration sensitivity of 0.03 ppm andprovides for real-time methane concentration measurementswithin 0.0310000 ppm range. The measurements were con-ducted at the stationary station of the Limnology Institute SBRAS and during two summer expeditions of 2003 and 2004aboard the research vessel Vereschagin all over the LakeBaikal surface. Coordinates of each measurement point weredetermined with the vessel GPS system. Mean background con-centration was equal to (2.00 0.16) ppm in August 2003 and(1.91 0.07) ppm in June 2004. The areas of methane emissionthrough the water surface are found to be distinctly localizedand to have a characteristic size of about 150300 m in diam-eter. The methane concentration in centers of these areas canreach approximately 27 ppm. Methane flows into atmosphere insome Baikal regions were measured as well.

The measurement results obtained with the laser detectoragree well with parallel measurements by the gas chromatog-raphy method. The results of the expedition investigationsdemonstrate a reliability and efficient real capability of the diodelaser gas-analyzers for real-time in situ detection of methane

concentration fields and methane emissions through water andground surfaces.

Acknowledgements

This research was supported by the INTAS (project 01-2309) and Division of Physical Sciences of RAS (project Laserspectroscopy and standards of frequency). The authors thankNadezhdinsky A.I. for his help in the development of the exper-imental techniques and permanent support, Berezin A.G. fortechnical consultations, Granin N.G. for the organization ofthe expedition, Obzhirov A.I. and Vereschagina O.V. for mass-chromatography data presentation.

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