Structure of Late Pleistocene and Holocene Sediments in the Petrozavodsk Bay, Lake Onego (NW Russia)

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Article Structure of Late Pleistocene and Holocene Sediments in the Petrozavodsk Bay, Lake Onego (NW Russia)

Dmitry Subetto 1,2,* , Alexandr Rybalko 2,3,4, Vera Strakhovenko 2,5 , Natalia Belkina 2, Mikhail Tokarev 6,7, Maksim Potakhin 2 , Mikhail Aleshin 6 , Pavel Belyaev 2,8 , Nathalie Dubois 9,10, Vladislav Kuznetzov 1,3, Dmitry Korost 6,7, Andrei Loktev 7, Natalia Shalaeva 6, Alexandra Kiskina 2,3, Natalia Kostromina 3, Yuriy Kublitskiy 1,2 and Alexander Orlov 1,2 1 Faculty of Geography, Herzen State Pedagogical University of Russia, 191186 Saint Petersburg, Russia; [email protected] (V.K.); [email protected] (Y.K.); [email protected] (A.O.) 2 Laboratory of Paleolimnology, Northern Water Problems Institute, Federal Research Centre “Karelian Research Center of the Russian Academy of Sciences”, 185030 Petrozavodsk, Russia; [email protected] (A.R.); [email protected] (V.S.); [email protected] (N.B.); [email protected] (M.P.); [email protected] (P.B.); [email protected] (A.K.) 3 Institute of Earth Sciences, Saint Petersburg State University, 199034 Saint Petersburg, Russia; [email protected] 4 Seismic Data Analysis Centre (SDAC), Lomonosov Moscow State University, 119991 Moscow, Russia 5 V.S. Sobolev Institute of Geology and Mineralogy, Russian Academy of Sciences, 630090 Novosibirsk, Russia 6 Geological Faculty, Lomonosov Moscow State University, 119991 Moscow, Russia; [email protected] (M.T.); [email protected] (M.A.); [email protected] (D.K.); [email protected] (N.S.) 7 Marine Investigation Centre (MIC), Lomonosov Moscow State University, 119991 Moscow, Russia; [email protected] 8 FSBI VNII Oceangeologia, 190121 Saint Petersburg, Russia 9 Department of Earth Sciences, ETH Zurich, 8092 Zurich, Switzerland; [email protected] 10 Department of Surface Waters—Research and Management, EAWAG Swiss Federal Institute of Aquatic Sciences and Technology, 8600 Dübendorf, Switzerland * Correspondence: [email protected]

Received: 13 August 2020; Accepted: 26 October 2020; Published: 28 October 2020

Abstract: Here, we present new results from seismic, geological, and geochemical studies conducted in 2015–2019 in the Petrozavodsk Bay of Lake Onego, NW Russia. The aims of these investigations were to (i) to characterize the structure of Quaternary deposits and (ii) provide new evidence of modern geodynamic movements and gas-seepage in Holocene sediments. The structure of the recovered deposits was composed of lacustrine mud, silt and sands from the Holocene, limno-glacial clays (varved clays) from the Late Glacial–Interglacial Transition, and glacial deposits (till) from the Late Pleistocene. The thickness of these deposits varied in diﬀerent parts of the bay. Many pockmarks created by gases escaping and reaching sediment-water interface were observed in these deposits. Such pockmarks can play a signiﬁcant role in the geochemical and biological processes in the bottom sediment surface, and gases that escape might modify the physicochemical characteristics of the environment.

Keywords: Lake Onego; bottom sediments; Late Pleistocene; Holocene; seismoacoustic proﬁling; gas in sediments; pockmarks

1. Introduction Geological explorations of Lake Onego have been conducted since the 1960s, but research was carried out only with short gravity corers [1].The sediment cores obtained in these studies were no

Minerals 2020, 10, 964; doi:10.3390/min10110964 www.mdpi.com/journal/minerals Minerals 2020, 10, 964 2 of 20 longer than 1–1.5 m, thus only reaching the Upper and Middle Holocene. Only at the end of the last century did Finnish scientists and Karelian Research Center experts conduct geochronological and lithostratigraphic studies [2]. These studies have determined the lake basin formation about 15,000 years before present (yr. BP) [3]. As no seismoacoustic and long-core sediment studies existed in this area, paleogeography of Lake Onego in the Late Pleistocene–Holocene drew upon well-investigated geological sections of Quaternary deposits around the lake basin, including sediment cores from small lakes located near Lake Onego [4]. Demidov [5,6] used these data and the findings of Finnish researchers to present the first model of the development of Lake Onego in the Late Pleistocene and Holocene [2], which has been used until now in different variations. In recent years, the stratigraphy of Lake Onego’ sediments and the main stages of paleogeographic development of the lake basin during the Late Pleistocene–Holocene has become significantly more precise [4,7,8]. However, specific data on the structure of the bottom sediments in Lake Onego are currently not available. Given the limited knowledge on this issue, a joint research in 2014–2019 was conducted to investigate the stratigraphic features and mineral composition of bottom sediments in the Petrozavodsk Bay of Lake Onego using seismoacoustic, geological, geochemical, and mineralogical methods. In 2015, gas accumulations in the soft lacustrine sediments were discovered during geophysical surveys.

2. Regional Setting

Lake Onego (61◦420 N, 35◦250 E) is the second-largest lake in Europe, and is located at the Baltic (Fennoscandian) Crystalline Shield and Russian Plate boundary. The geological structure and development history of the lake and its drainage basin were explained in previous work [5,6,8,9]. The depression of Lake Onego is of tectonic origin [10]. Its drainage area is composed of Precambrian crystalline rocks and Vendian-Phanerozoic sedimentary rocks, which are resistant to abrasion and partly obscured by overlying Quaternary deposits [11,12]. The oligotrophic waters of Lake Onego are characterized by low mineralization (39–46 mg/L). The area of the water table is 9720 km2. The maximum depth is 127 m, the average depth is 30 m, the length (the longest distance from south to north) is 248 km, and the width is 83 km (Figure1a). The Petrozavodsk Bay is located in the northwestern part of Lake Onego, with the town of Petrozavodsk lying on its northwestern shore. The bay stretches from north-west to south-east. From the north-east, it is separated from the Bolshoe (Big) Onego Bay by the Ivanovskie Islands. The bay’s estuary opens into the central part of Lake Onego by a 2-km-wide and 23-m-deep strait. Residents of Petrozavodsk use the bay for industrial, transport, and recreational purposes and draw water for the city’s needs. The maximum depth of the bay is 29 m. The water area is 180 km2. Minerals 2020, 10, 964 3 of 20 Minerals 2020, 10, x FOR PEER REVIEW 3 of 21

FigureFigure 1. 1.Map Map of geophysical geophysical profiles profiles and geological and geological stations stationslocation. (a location.)—Location (ofa)—Location Lake (b)— of LakeLocation (b)—Location of the Petrozavodsk of the Petrozavodsk Bay, (c-1)—Coring Bay, locations (c-1)—Coring in the Petrozavodsk locations in Bay. the Numbers Petrozavodsk indicate Bay. core position, (c-2)—location of the geophysical profiles in the Petrozavodsk Bay. The digits indicate Numbers indicate core position, (c-2)—location of the geophysical profiles in the Petrozavodsk the position: 1—the seismic profile 1 (Figure 2); 2—the seismic profile 2 (Figure 3); 3—the seismic Bay. The digits indicate the position: 1—the seismic profile 1 (Figure 2); 2—the seismic profile 2 profile with gas accumulation (Figure 13); 4—the sonogram of a pockmark field (Figure 14). (Figure 3); 3—the seismic profile with gas accumulation (Figure 13); 4—the sonogram of a pockmark field3. Materials (Figure 14).and Methods

3. Materials3.1. Fieldworks and Methods

3.1. Fieldworks3.1.1. Seismoacoustic and Sonar Profiling 3.1.1. SeismoacousticSeismoacoustic, and high-resolution Sonar Profiling acoustic (by means of a profile recorder), and sonar profiling was carried out in the Petrozavodsk Bay using a detailed seismic grid (Figure 1). Profiles were 7 km Seismoacoustic,long, the distance high-resolution between them was acoustic 250 m.(by Along means the south-western of a profile recorder), coast, near and Petrozavodsk, sonar profiling the was carriedspacing out in between the Petrozavodsk the profiles was Bay reduced using a to detailed 50 m. In seismic total, 96 grid km of (Figure geophysical1). Profiles profiles were have 7 been km long, the distancecovered. between them was 250 m. Along the south-western coast, near Petrozavodsk, the spacing between theBased profiles on the wasinterpretation reduced toof seismoacoustic 50 m. In total, and 96 kmsonar of profiling, geophysical locations profiles for five have stations been were covered. Basedchosen onto retrieve the interpretation sediment samples. of seismoacoustic At each station, and four sonarsediment profiling, cores were locations taken for for (1) five sample stations wereselection chosen toand retrieve a description sediment of the samples. lithology, At each(2) stratigraphic station, four studies, sediment (3) radiography cores were with taken the for (1) subsequent determination of geotechnical properties of the bottom sediments, and (4) sampling for sample selection and a description of the lithology, (2) stratigraphic studies, (3) radiography with the gas-geochemical surveys. Navigational referencing of the observations was done by means of GPS subsequent determination of geotechnical properties of the bottom sediments, and (4) sampling for with a differential mode, using the reference station installed on the shore. gas-geochemicalSeismoacoustic surveys. profiling Navigational was carried referencing out at a vessel of the speed observations of 3–4.5 knots was (two done research by means vessels of GPS with a‘Professor differential Zenkevich’ mode, usingand ‘Ecolog’ the reference were used). station Excitation installed of onelastic the modes shore. was conducted using a SeismoacousticBoomer source. This profiling allowed was a signal carried at the out center at a frequency vessel speed of at least of 3–4.5 1000 knotsHz to be (two generated. research Data vessels ‘Professorlogging Zenkevich’ was performed and ‘Ecolog’using a were14-bit used).seismic Excitationstation, with of a elastic resolution modes of wasat least conducted 0.5 ms and using a Boomerrecording source. duration This allowedof at least a200 signal ms in at SEG-Y the center format. frequency For more detailed of at least images 1000 of the Hz upper to be part generated. of Datathe logging section, was a sub-bottom performed profiler using SES-2000 a 14-bit Light seismic was station, used. with a resolution of at least 0.5 ms and recordingSonar duration profiling of at was least carried 200 ms out inby SEG-Y means of format. the backscattering For more detailed technique, images whichof allowed the upper images part of of the lake bottom in a stripe of a specified width along the profile line to be captured. A side-scan the section, a sub-bottom profiler SES-2000 Light was used. sonar was applied for the survey, towed on a rigid bar on the starboard side of the vessel. The antenna Sonar profiling was carried out by means of the backscattering technique, which allowed images of the lake bottom in a stripe of a specified width along the profile line to be captured. A side-scan sonar was applied for the survey, towed on a rigid bar on the starboard side of the vessel. The antenna of the tool was placed at a depth of 3 m. The applied frequency was 450 kHz, enabling us to explore Minerals 2020, 10, 964 4 of 20 the surface of the seabed in detail, with a swath width of 100 m and a resolution of 0.5 m. The survey was undertaken at a vessel speed of 4–5 knots, and the signal was emitted six times per second, i.e., every 40–50 cm. Upon the completion of each profile, data were loaded into a processing system based on the RadExPro software package to perform quality control and preprocessing.

3.1.2. Sediment Sampling Sediment sampling on the ‘Professor Zenkevich’ R/V was carried out by means of a 3-m-long and 127-mm-diameter gravity core sampler with a total weight of 300 kg (Table1).

Table 1. Coordinates, water depth, and core lengths of the stations.

Coordinates No. Station Lake Depth (m) Core Length (m) Longitude Latitude

1 ONG 1 21 2.80 61◦46.120 34◦29.320

2 ONG 2 * 22 3.20 61◦47.720 34◦24.770

3 ONG 3 ** 21.1 0 61◦47.840 34◦23.860

4 ONG 4 * 21.6 2.45 61◦47.500 34◦24.860

5 ONG 5 23.3 3.04 61◦46.620 34◦28.860

6 ONG 6 24.8 3.10 61◦46.100 34◦29.970

7 ONG 7 * 22.5 3.20 61◦46.250 34◦29.240 * The stations where lithological–geochemical studies were conducted.** The core was empty.

The sediment cores from stations ONG-2, ONG-4, and ONG-7 were placed in plastic tubes of an appropriate diameter, hermetically sealed and sent to the laboratories in Petrozavodsk, Moscow, and Saint Petersburg for analyses. Core ONG-5 was squeezed into trays. Next step, the core samples were collected for gas measurements, placed in a shaker, and sent ashore to the laboratory for degassing on the same day.

3.2. Laboratory Analysis

3.2.1. CT Imaging CT imaging was performed using an RKT-180 (Geologika, Novosibirsk, Russia) scanner to study textural features of the bottom sediments. The cores were scanned vertically with a resolution of 230 µm at 160 keV voltages and a current of 3 A using the oversize spiral mode. The total dataset includes several subscans connected on the vertical axis. Data processing was done with the software TOMO (reconstruction, by Geologika) and Dataviewer/CTan/CTVol (3D model producing and imaging, by SkyScan). The research ﬁndings allowed the textural features of the bottom sediments throughout the section to be studied.

3.2.2. Radiocarbon Analyses Tree bulk sediment samples from core ONG-2 and one bulk sediment sample from core ONG-5 were 14C dated at the K˝oppen-Lab(Laboratory of Geomorphological and Paleogeographical Studies of Polar Regions and the World Ocean) of the Institute of Earth Sciences (Saint Petersburg State University, Saint Petersburg, Russia). A liquid scintillation method for measuring the 14C activity in benzene synthesized from carbon-containing samples was used. The details of the laboratory procedures were described by [13,14]. The calibration of 14C dating was conducted using CalPal2007_HULU (http://www.calpal-online.de). The Table2 represents radiocarbon dates for selected intervals. The table includes information concerning the sample preparation step and results. Total organic carbon amount was obtained by loss-on-ignition method (LOI). Samples for radiocarbon dating were chosen based on Minerals 2020, 10, 964 5 of 20 maximum organic carbon content in intervals. Charcoal weight is the weight of pure carbon that is represented by charcoal after sample pretreatment stage.

Table 2. Results of radiocarbon dating.

Radiocarbon Calibrated Lab Depth, Charcoal, Benzene, Age, yr. BP yr. cal. BP Core Lithology TOC % Index cm g ml (Scintillator (Scintillator Spectrometer) Spectrometer) LU-8394 ONG-2 45–50 Clayey silt 6.43 1.22 0.52 1420 100 1340 100 ± ± LU-8395 ONG-2 165–170 Clayey silt 8.55 1.06 0.38 2420 100 2510 130 ± ± LU-8396 ONG-2 230–235 Clayey silt 7.88 1.11 0.41 4910 150 5660 180 ± ± LU-8397 ONG-5 49–51 Sandy silt 8.50 1.09 0.39 4150 180 4680 260 ± ±

3.2.3. Gas Analyses in Bottom Sediments Headspace sampling (the vapor phase calibration method) under field conditions was used to identify gaseous hydrocarbons (GHC) composed of C1-C5 in the gas phase of the sediments. A 100 mL sample of the sediments was taken from the core with a syringe and placed into a 270 mL vial containing 120 mL of 5% NaCl solution. Intervals are shown in Table3. Once closed, this vial was shaken for 3 h on a special rotating platform. The gas was transferred from the headspace bottle to the receiving vial by displacing it with an equal volume of water in a syringe. Compositional analysis of gaseous hydrocarbons (GHC) in the equilibrated gas phase of the bottom sediments was based on the gas chromatography method, using a Shimadzu GC 2014 chromatograph equipped with a flame ionization 13 12 detector (FID). The carbon isotope composition (C /C ) in CH4 was measured with an Agilent 6890 N GC (Agilent Technologies, Santa Clara, CA, USA) interfaced to a Finigan Delta S IRMS (Finigan is now part of Thermo Scientific, Bremen, Germany) using a Finigan GC-C II interface. The GC was equipped with a Molsieve column (12 m, 0.32 mm i.d.) and an injection valve. Samples were calibrated regularly against a certified standard, and the resulting isotopic signatures were reported in δ notation and per mil (%) values vs. VPDB.

3.2.4. Mineralogical Analyses Mineralogical analyses were carried out for the upper 40 cm at intervals of 1–2 cm at the Analytical Center of the Institute of Geology and Mineralogy, Siberian Branch of Russian Academy of Sciences. The mineral composition of the bottom sediments was determined by powder X-ray diffraction and IR spectroscopy. The samples were studied with an ARL X’TRA diffractometer (Thermo Electron Corporation, Waltham, MA, USA, CuKα radiation). The morphology and phase and chemical compositions of some minerals of the sediments were studied using a MIRA 3 Tescan SEM (Tescan, Brno-Kohoutovice, Czech Republic). The current modification of the equipment uses an Si (Li) energetic detector (OXFORD mark, Oxford 5 of 23 Instruments, Abingdon, UK). Minerals 2020, 10, 964 6 of 20

Table 3. Composition of gases taken from the cores ONG-2, ONG-4 and ONG-7 of the Petrozavodsk Bay.

Sample Methane Ethane Ethylene Propane Propylene i-Butane n-Butane i-Butene i-Butylene i-Pentane n-Pentane δ13C, % Sample Depth ΣC2-C5 CH4/ΣC2-C5 C2H6/C2H4 Number CH4 C2H6 C2H4 C3H8 C3H6 i-C4H10 n-C4H10 C4H8 i-C4H8 i-C5H12 n-C5H12 CH4 (cm) Concentration, ppm, in Vapor Phase VPDB ONG-2 10–13 10732.059 0.512 0.465 0.368 0.522 b.d.l. * b.d.l. b.d.l. 0.534 1.859 b.d.l. 4.259 2520 1.10 - ONG-2 20–23 10710.900 0.678 b.d.l. 0.231 0.505 b.d.l. b.d.l. b.d.l. 0.598 3.603 b.d.l. 5.616 1907 - - ONG-2 40–43 17399.170 0.410 0.411 0.418 0.484 b.d.l. b.d.l. b.d.l. 0.478 2.404 b.d.l. 4.604 3779 1.00 73.70 − ONG-2 89–92 21386.316 0.907 1.192 0.451 0.551 b.d.l. b.d.l. b.d.l. 0.734 3.928 b.d.l. 7.762 2755 0.76 72.30 − ONG-2 170–174 42787.588 0.911 0.778 0.547 0.474 0.025 0.104 b.d.l. 0.823 3.639 b.d.l. 7.300 5862 1.17 70.40 − ONG-2 248–250 34558.356 1.222 1.036 0.667 0.463 0.051 0.141 b.d.l. 0.786 2.888 b.d.l. 7.254 4764 1.18 - ONG-4 30–33 34873.493 0.229 b.d.l. 0.530 0.517 b.d.l. b.d.l. b.d.l. 0.839 3.488 b.d.l. 5.603 6224 - - ONG-4 40–43 39497.419 0.819 0.000 0.473 0.706 0.034 0.037 b.d.l. 1.274 3.699 b.d.l. 7.041 5609 - 69.30 − ONG-4 90–93 39976.843 0.278 0.815 0.559 0.844 0.085 0.057 0.101 0.763 3.034 b.d.l. 6.537 6115 0.34 - ONG-4 143–146 30363.287 0.316 0.925 0.600 0.659 0.065 0.098 b.d.l. 1.337 5.530 b.d.l. 9.532 3186 0.34 68.50 − ONG-4 163–166 28176.536 0.311 0.856 0.660 0.702 0.048 0.076 b.d.l. 1.267 3.711 b.d.l. 7.631 3692 0.36 - ONG-4 204–206 27135.585 0.232 1.004 0.619 0.637 0.063 0.201 b.d.l. 0.760 4.468 b.d.l. 7.984 3399 0.23 68.40 − ONG-4 225–228 23719.020 0.261 0.969 0.619 0.852 0.161 0.000 b.d.l. 1.450 4.678 b.d.l. 8.988 2639 0.27 - ONG-7 41–45 3068.731 b.d.l. 0.744 0.281 0.325 0.165 0.013 b.d.l. 2.157 8.269 b.d.l. 11.953 257 - - ONG-7 84–89 2990.461 0.149 1.141 0.214 0.485 b.d.l. b.d.l. b.d.l. 0.446 4.804 b.d.l. 7.239 413 0.13 - * b.d.l.: below the detection limit; -: no data. Minerals 2020, 10, 964 7 of 20

4. Results and Discussion

4.1. Seismic Proﬁles Seismoacoustic proﬁling allows to characterize the full section of unconsolidated Quaternary sediments overlapping the bedrock of the Archaean and Proterozoic ages. At present, for all (marine and lake) large water basins of the NW Russia, a typical section of the Quaternary deposits, begins with the moraine of the Valday (Weichselian) glaciation (from the bottom to the top), which is overlain by limno-glacial and lacustrine (marine) sediments. The seismic horizons, which we segregated in seismograms, were compared by analogy with earlier studies on Lake Ladoga [7] with the Minerals 2020, 10, x FOR PEER REVIEW 6 of 21 seismostratigraphicMinerals 2020, 10, x FOR horizons PEER REVIEW indicated above (Figure2). It has been established that although6 of 21 the lake bottom topography of the Petrozavodsk Bay is typically subdued, the surface of the Pleistocene lake bottom topography of the Petrozavodsk Bay is typically subdued, the surface of the Pleistocene limno-glaciallake bottom clays topography is uneven of andthe Petrozavodsk covered by HoloceneBay is typically deposits subdued, with distinctthe surface cross-bedding of the Pleistocene (Figure 3). limno-glaciallimno-glacial clays clays is is uneven uneven and and co coveredvered by by Holocene Holocene deposits deposits with with distinct distinct cross-bedding cross-bedding (Figure (Figure 3). 3).

FigureFigureFigure 2. Seismic 2.2. SeismicSeismic stratigraphic stratigraphicstratigraphic structure structurestructure of sediments ofof sedimentssediments in the inin Petrozavodskthethe PetrozavodskPetrozavodsk Bay BayBay (the (the(the position, position,position, see Figureseesee 1). Figure 1). Legend: (1) lacustrine sediments of the Middle and Upper Holocene (lH2–3); (2) lacustrine Legend:Figure (1) 1). lacustrine Legend: (1) sediments lacustrine of sediments the Middle of the and Middle Upper and Holocene Upper (lHHolocene2–3); (2) (lH lacustrine2–3); (2) lacustrine sediments of sediments of the Lower Holocene (lH1); (3) limno-glacial sediments (varved clays) of the Late the Lowersediments Holocene of the (lHLower1); (3) Holocene limno-glacial (lH1); sediments(3) limno-glacial (varved sediments clays) of the(varved Late clays) Pleistocene of the (the Late Valday Pleistocene (the Valday (Weichselian) Glacial stage) (lgIII); (4) glacial sediments (till) of the Late (Weichselian)Pleistocene Glacial(the Valday stage) (Weichselian) (lgIII); (4) glacial Glacial sediments stage) (lgIII); (till) (4) of glacial the Late sediments Pleistocene (till) of (Valday the Late Glacial Pleistocene (Valday Glacial stage) (gIII); 5: bedrock. The arrow indicates the location of a pockmark. stage)Pleistocene (gIII); 5: (Valday bedrock. Glacial The arrowstage) (gIII); indicates 5: bedrock. the location The arrow of a indicates pockmark. the location of a pockmark.

Figure 3. The seismic profile (the position, see Figure 1) with a sharp shift of the roof of limno-glacial FigureFigure 3. The 3. The seismic seismic proﬁle profile (the (the position,position, see Figure Figure 1)1 )with with a asharp sharp shift shift of the of theroof roof of limno-glacial of limno-glacial (varved) clays caused by tectonic dislocation (thick dashed line). The figures in the seismic record (varved)(varved) clays clays caused caused by by tectonic tectonic dislocation dislocation (thi (thickck dashed dashed line). line). The The figures ﬁgures in the in seismic the seismic record record showshow thethe following:following: 1:1: HoloceneHolocene lacustrinelacustrine muds;muds; 2:2: limno-glaciallimno-glacial clays;clays; 3:3: tilltill ofof thethe ValdayValday show the following: 1: Holocene lacustrine muds; 2: limno-glacial clays; 3: till of the Valday (Weichselian)(Weichselian) Glacial Glacial stage. stage. Dashed Dashed arrows arrows indicate indicate large large areas areas of of gas gas accumu accumulationlation and and solid solid arrows arrows (Weichselian) Glacial stage. Dashed arrows indicate large areas of gas accumulation and solid arrows indicateindicate pockmarks. pockmarks. indicate pockmarks. GasGas concentrations concentrations were were also also detected detected in in Holocene Holocene sediments sediments characterized characterized by by a a great great thickness. thickness. Often,Often, nearnear thesethese clusters,clusters, steepsteep ledgesledges werewere founfoundd inin thethe thicknessthickness ofof thethe pre-Holocenepre-Holocene sedimentssediments (Figure(Figure 3).3). ThisThis maymay indicateindicate thatthat gasgas fluidsfluids cancan bebe suppliedsupplied viavia thesethese currentlycurrently activatedactivated tectonictectonic disturbances.disturbances. This This is is confirmed confirmed by by the the presence presence of of pockmarks pockmarks at at the the bottom bottom of of the the lake lake [15].f [15].f

4.2.4.2. Stratigraphic Stratigraphic Features Features Minerals 2020, 10, 964 8 of 20

Gas concentrations were also detected in Holocene sediments characterized by a great thickness. Often, near these clusters, steep ledges were found in the thickness of the pre-Holocene sediments (Figure3). This may indicate that gas ﬂuids can be supplied via these currently activated tectonic disturbances. This is conﬁrmed by the presence of pockmarks at the bottom of the lake [15].

4.2. Stratigraphic Features Information about stratigraphic features of the Quaternary deposits of the Petrozavodsk Bay was obtained as a result of geological sampling and bottom drilling. Currently, the official stratigraphic scheme of NW Russia [16] includes the following units, which builds up the bottom of the lakes (top to bottom):

1. Holocene lacustrine deposits—silt and sand (lH, where l—lacustrine) Minerals 2020, 10, x FOR PEER REVIEW 7 of 21 2. Upper Pleistocene deposits of local Ice Lakes—varved clays (lgIII, where lg—limno-glacial) 3. Tills and fluvioglacialInformation about deposits stratigraphic of Upper features of Pleistocene—coarse the Quaternary deposits of sand the Petrozavodsk with pebbles, Bay clays with was obtained as a result of geological sampling and bottom drilling. Currently, the official boulders, boulderstratigraphic loam scheme (gIII; of NW fIII, Russia where [16] includes g—glacial, the following f—fluvio-glacial) units, which builds up the bottom of the lakes (top to bottom): According1. to coringHolocene andlacustrine geological deposits—silt sampling and sand (lH, in where the Petrozavodsk l—lacustrine) Bay, recovered deposits were subdivided to 52. lithological Upper Pleistocene units: deposits of local Ice Lakes—varved clays (lgIII, where lg—limno-glacial) Lithological3. unitTills and 1. fluvioglacial Liquid silty-clay deposits of Upper gyttja Plei withstocene—coarse organic sand remains, with pebbles, greenish-gray clays with (Figure4). boulders, boulder loam (gIII; fIII, where g—glacial, f—fluvio-glacial) Thickness of this unit—0.3–0.35 m. There is a zone of oxidation at the top of this unit, <1 cm thick. According to coring and geological sampling in the Petrozavodsk Bay, recovered deposits were Sometimes, thicknesssubdivided of to this 5 lithological unit may units: be less due to an underwater erosion. In the interval from 20 to 30 cm, there areLithological thin layers unit of1. Liquid black, silty-clay green, gyttja and with fawn-colored organic remains, mineralsgreenish-gray (rhodochrosite, (Figure 4). siderite, Thickness of this unit—0.3–0.35 m. There is a zone of oxidation at the top of this unit, <1 cm thick. Mn-Fe hydroxides).Sometimes, The thickness number ofand this unit distribution may be less due of microlayersto an underwater in erosion. the cores In the areinterval diff fromerent. 20 Further down this unit, silts andto 30 clays cm, there become are thin denser layers of and black, the green, layering and fawn-colored becomes minerals thinner (rhodochrosite, and more distinct.siderite, The number Mn-Fe hydroxides). The number and distribution of microlayers in the cores are different. Further of black bandsdown in the this sediment unit, silts and increases clays become towards denser and bottomthe layering of becomes this unit,thinner whichand more is distinct. correlated with an increase of the organicThe number matter of black [bands17]. in the sediment increases towards bottom of this unit, which is correlated with an increase of the organic matter [17].

Figure 4. Holocene lacustrine sediments: (a) upper part of Holocene sediments. (b) middle part of Figure 4. HoloceneHolocene lacustrine sediments. Diagenetic sediments: banding (a) can upper be seen part at the of bottom Holocene of the unit. sediments. Lithological unit (b )1, middle part of Holocene sediments.cоre ONG-2. Diagenetic banding can be seen at the bottom of the unit. Lithological unit 1, core ONG-2. Minerals 2020, 10, 964 9 of 20

Minerals 2020, 10, x FOR PEER REVIEW 8 of 21 Lithological unit 2. Gray and brown-gray silty-clay (Figure5). The thickness of it varies from 0.2 Minerals 2020, 10, x FOR PEER REVIEW 8 of 21 to 1.1 m. ThisLithological unit is unit denser 2. Gray than and the brown-gray overlying silty-clay one. Authigenous (Figure 5). The minerals thickness become of it varies more from crystalized. 0.2 Sedimentsto 1.1 m.Lithological are This microlaminated. unit isunit denser 2. Gray than This and the canbrown-gray overlying be seen one. insilty-clay the Authigenous tomographic (Figure minerals5). The image thickness become (Figure ofmore it 13). varies crystalized. Lamination from 0.2 is relatedSedimentsto to 1.1 intercalation m. Thisare microlaminated. unit is of denser bright than andThis the darkcan overlying be microlayers. seen inone. the Authigenous tomographic The color minerals image of the (Figure layersbecome 13). on more Lamination the crystalized. tomogram is is relatedrelatedSediments to the to iron intercalation are content microlaminated. and of bright sediment Thisand density.darkcan be microlayers. seen Towards in the tomographicThe the bottomcolor of sediments imagethe layers (Figure on of this the13). tomogram unit,Lamination the contrast is is of therelated layersrelated to onto the intercalation the iron tomogram content of andbright becomes sediment and dark higher, density. microlayers. and Towards its thickness The the color bottom increases.of the sediments layers The on of lowerthe this tomogram unit, boundary the is is contrast of the layers on the tomogram becomes higher, and its thickness increases. The lower often regular,related to sharp, the iron and content sometimes and sediment it is eroded. density. Towards the bottom sediments of this unit, the boundarycontrast isof often the layersregular, on shar thep, tomogram and sometimes become it iss eroded.higher, and its thickness increases. The lower boundary is often regular, sharp, and sometimes it is eroded.

FigureFigure 5. The 5. The lowermost lowermost part part of of the the Holocene Holocene lacustrine sediments sediments with with clear clear black black diagenetic diagenetic layers. layers. LithologicalLithologicalFigureunit 5. The unit 2, lowermost core 2, core ONG-7. ONG-7. part of the Holocene lacustrine sediments with clear black diagenetic layers. Lithological unit 2, core ONG-7. Lithological unit 3. Typical unlaminated clays or thin-laminated limno-glacial (varved) clays Lithological unit 3. Typical unlaminated clays or thin-laminated limno-glacial (varved) clays (Figure 6). The thickness varies from 0.1 to 1.5 m. In the uppermost part the gradation layering is seen (Figure6). TheLithological thickness unit varies 3. Typical from 0.1unlaminated to 1.5 m. Inclays the or uppermost thin-laminated part thelimno-glacial gradation (varved) layering clays is seen clearly. Thickness of microvarves increases to the bottom. In some sediment sequences from central clearly.(Figure Thickness 6). The of thickness microvarves variesincreases from 0.1 to to 1.5 the m. bottom.In the uppermost In some part sediment the gradation sequences layering from is seen central partclearly. of Lake Thickness Onego ofthe microvarves thickness of increases varves varies to the from bottom. 1 to In 1.5 some cm. sedimentIn this unit, sequences there is froma marking central part of Lake Onego the thickness of varves varies from 1 to 1.5 cm. In this unit, there is a marking “pink” “pink”part of horizon Lake Onegowith a thicknessthe thickness of 0.3–0.4 of varves m. This varies micr olaminatedfrom 1 to 1.5 clays cm. are In called this unit, “pink there clay islayer” a marking [5,8]. horizon“pink” with horizon a thickness with a of thickness 0.3–0.4 of m. 0.3–0.4 This m. microlaminated This microlaminated clays clays are calledare called “pink “pink clay clay layer” layer” [5,8]. [5,8].

Figure 6. Limno-glacial sediments. Typical thin-laminated varved clays. Lithological unit 3, core Figure 6. Limno-glacial sediments. Typical thin-laminated varved clays. Lithological unit 3, core ONG-5. ONG-5.Figure 6. Limno-glacial sediments. Typical thin-laminated varved clays. Lithological unit 3, core ONG-5. Minerals 2020, 10, 964 10 of 20

Minerals 2020, 10, x FOR PEER REVIEW 9 of 21 Lithological unit 4.Typical varved clays with graded layering with thickness up to 3–4 m (Figure7). Lithological unit 4.Typical varved clays with graded layering with thickness up to 3–4 m (Figure 7). The thickness of a pair of layers reaches 7–10 mm (Figure7). These deposits were discovered during The thickness of a pair of layers reaches 7–10 mm (Figure 7). These deposits were discovered during sampling in the Central part of Lake Onego and in the Petrozavodsk Bay. sampling in the Central part of Lake Onego and in the Petrozavodsk Bay.

FigureFigure 7. 7.Typical Typical varved varved clays clays with with graded graded layering.layering. Lithological unit unit 4, 4, core core ONG-5. ONG-5.

LithologicalLithological unit unit 5. 5. Sandy Sandy clays clays or or clayeyclayey sandssands with thickness up up to to 1 1m. m. Gray, Gray, non-laminated, non-laminated, withwith coarse coarse sand sand and and pebbles. pebbles. This This unit unit was was identified identified onlyonly in the Petrozavodsk Bay. Bay. LithostratigraphyLithostratigraphy of of investigated investigated depositsdeposits shownshown at Figure 88,, and the correlation between between the the sedimentsediment cores cores shown shown at at Figure Figure9. 9. AccordingAccording to datato data of previous of previous studies studies [1,11 ],[1,11], we can we correlate can correlate all Lithological all Lithological Units with Units geological with horizonsgeological of NW horizons Russia. of Units NW 1Russia. and 2 areUnits related 1 and to 2 Upperare related (1) and to LowerUpper (2)(1) Holocene.and Lower Unit (2) Holocene. 3 correlates withUnit limno-glacial 3 correlates deposits with limno-glacial of the Upper deposits Pleistocene. of the UnitUpper 4 correspondsPleistocene. toUnit limnoglacial 4 corresponds deposits to (Valdaylimnoglacial horizon) deposits (Figure (Valday4). More horizon) accurate (Figure information 4). Moreabout accurate age information of Holocene about deposits age of was Holocene gained fromdeposits radiocarbon was gained dating. from radiocarbon dating. Lithological and seismo-stratigraphic units correlate well. Lithological unit 5 correlates with glacial horizon, which includes fluvio-glacial deposits. Lithological units 3 and 4 correlate with limno-glacial horizon, which was diagnosed by seismic (seismic-acoustic) profiling where the significant feature is seismic layering, which gets in line with layering of the deposits. Lithological units 2 and 1 are not expressed on seismic profiles (Figures2 and3). Minerals 2020, 10, 964 11 of 20 Minerals 2020, 10, x FOR PEER REVIEW 10 of 21 Minerals 2020, 10, x FOR PEER REVIEW 10 of 21

Figure 8. Lithostratigraphic section of Quaternary deposits in the Petrozavodsk Bay. Additionally, FigureFigure 8. 8.Lithostratigraphic Lithostratigraphic section section of of Quaternary Quaternary deposits deposits in in the the Petrozavodsk Petrozavodsk Bay. Bay. Additionally, Additionally, here here we place legend for Figures 9, 10, and 12. wehere place we legend place legend for Figures for Figures 9, 10, and9, 10, 12. and 12.

Figure 9. Correlation of Quaternary deposits from sediment cores in the Petrozavodsk Bay. Legend,

see Figure 8. The bold line demonstrates the Late Pleistocene-Holocene boundary. The core ONG-3 FigureFigurewas 9. empty9.Correlation Correlation during ofthe of Quaternary samplingQuaternary of depositsbottomdeposits sediments. from sediment The coring cores cores tube in in themay the Petrozavodsk have Petrozavodsk fallen on Bay. crystalline Bay. Legend, Legend, see Figure 8. The bold line demonstrates the Late Pleistocene-Holocene boundary. The core ONG-3 see Figurerocks 8or. boulders. The bold line demonstrates the Late Pleistocene-Holocene boundary. The core ONG-3 was empty during the sampling of bottom sediments. The coring tube may have fallen on crystalline was empty during the sampling of bottom sediments. The coring tube may have fallen on crystalline rocks or boulders. rocks or boulders. Minerals 2020, 10, x FOR PEER REVIEW 11 of 21

Lithological and seismo-stratigraphic units correlate well. Lithological unit 5 correlates with glacial horizon, which includes fluvio-glacial deposits. Lithological units 3 and 4 correlate with limno-glacial horizon, which was diagnosed by seismic (seismic-acoustic) profiling where the Mineralssignificant2020, 10feature, 964 is seismic layering, which gets in line with layering of the deposits. Lithological12 of 20 units 2 and 1 are not expressed on seismic profiles (Figures 2 and 3). 4.3. Radiocarbon Dating 4.3. Radiocarbon Dating Sediment cores ONG-2 and ONG-5 were radiocarbon dated (Figure 10). Radiocarbon absolute Sediment cores ONG-2 and ONG-5 were radiocarbon dated (Figure 10). Radiocarbon absolute dates were determined from bulk organic matter. It was only present in suﬃcient quantities in the part dates were determined from bulk organic matter. It was only present in sufficient quantities in the of the studied sediment cores (Table2). part of the studied sediment cores (Table 2).

FigureFigure 10.10.Results Results ofof thethe radiocarbonradiocarbon datingdating inin corescores ONGONG -2-2 andand ONG- ONG- 5. 5. LegendLegend seesee FigureFigure8 .8.

The sedimentation rates were based on radiocarbon dating of bottom sediments from ONG-2 core. At interval from 0.5 m down to 1.65 m, the estimated sedimentation rate is estimated ~1.1 mm/year. This speed is slightly higher than previously calculated for Lake Ladoga (0.4 mm per year) [4]. Such fast Minerals 2020, 10, x FOR PEER REVIEW 12 of 21

The sedimentation rates were based on radiocarbon dating of bottom sediments from ONG-2 Mineralscore.2020 At, 10interval, 964 from 0.5 m down to 1.65 m, the estimated sedimentation rate is estimated 13~1.1 of 20 mm/year. This speed is slightly higher than previously calculated for Lake Ladoga (0.4 mm per year) [4]. Such fast sedimentation is not peculiar for huge and deep lakes. High sedimentation rates in the sedimentationLadoga and isOnega not peculiarlakes occurred for huge during and the deep late lakes. glacial High period, sedimentation when large subglacial rates in the lakes Ladoga existed and Onegain their lakes place occurred and a huge during amount the late of glacialdetritus period, and suspended when large material subglacial came lakesfrom the existed glacier in and their from place andthe ahuge open amountareas around of detritus the lakes and that suspended were not material coveredcame with fromsoil and the vegetation glacier and [4]. from One the explanation open areas aroundfor the the high lakes rate that of weresedimentation not covered may with be soilthe fact and that vegetation the Petrozavodsk [4]. One explanation Bay is a receiver for the of high water rate of sedimentationfrom the River mayShuya. be the fact that the Petrozavodsk Bay is a receiver of water from the River Shuya. A fairlyA fairly old old date date (4680 (4680 ±260 260 yr. yr. cal. cal. BP)BP) fromfrom the upper horizon horizon in in column column ONG-5 ONG-5 may may be be ± associatedassociated with with either either erosion erosion of the of upper the upper horizons hori ofzons sediments of sediments underwater underwater current current or aging. or Possibly,aging. thePossibly, samples fromthe samples interval from of 45–50 interval cm (ONG-5)of 45–50 arecm contaminated(ONG-5) are contaminated by older carbon. by older This contaminationcarbon. This couldcontamination have occurred could during have the occurred core manipulations during the core in the manipulations laboratory or in by the natural laboratory isotope or fractionationby natural duringisotope increasing fractionation lake waterduring mixing. increasing lake water mixing. ConcerningConcerning ONG-5 ONG-5 core core the the date date of of 4680 4680 ±260 260 cal. cal. BP BP belongs belongs to to the theend end ofof AtlanticAtlantic periodperiod and it ± is conﬁrmedit is confirmed by published by published data. Filimonovadata. Filimonova and Lavrova and Lavrova (2017) [18(2017)] established [18] established the Atlantic-Subboreal the Atlantic- transitionSubboreal around transition 4700 around cal. BP 4700 in their cal. overview BP in their of overview data obtained of data for obtained Lake Onega. for Lake Onega.

4.4.4.4. Lamination Lamination Black-Black- (complex (complex oxides oxides/hydroxides/hydroxides ofof MnMn andand Fe),Fe), green-green- (vivianite), and and cream-colored cream-colored (rhodochrosite(rhodochrosite and and siderite) siderite) microlayers microlayers can can be observedbe observed in gray-green in gray-green mud mud (Figure (Figure 11, lithological11, lithological units units 1 and 2) [17]. The number of microlayers and their distribution in different cores varies. 1 and 2) [17]. The number of microlayers and their distribution in diﬀerent cores varies. Importantly, Importantly, the redox barrier zone in the upper parts of the sediment columns from the areas with the redox barrier zone in the upper parts of the sediment columns from the areas with an increased an increased gas concentration coincides with the sediment–water interface (a slightly oxidized layer gas concentration coincides with the sediment–water interface (a slightly oxidized layer <1 cm thick). <1 cm thick). We identified this upper horizon as belonging to the Late Holocene. Its thickness is 20– We identiﬁed this upper horizon as belonging to the Late Holocene. Its thickness is 20–35 cm. 35 cm.

FigureFigure 11. 11.Core Core ONG-4. ONG-4. Banded Banded diagenetic diagenetic structurestructure inin HoloceneHolocene silty-clay silty-clay (Litho (Lithologicallogical units units 1 and 1 and 2).2). BlackBlack bands bands are are composed composed of of manganese manganese oxides oxides and and vivianite,vivianite, rhodochrosite, siderite, siderite, and and formations formations withwith various various morphologies morphologies and and degrees degrees of of crystallinity. crystallinity.

TheThe underlying underlying horizons horizons usually usually contain contain thethe samesame sediments,sediments, but but with with more more numerous numerous thin thin layerslayers composed composed of of manganese manganese oxides oxides and and hydrous hydrous oxides,oxides, and vivianite, vivianite, rhod rhodochrosite,ochrosite, and and siderite siderite formationsformations with with various various morphologies morphologies and degreesand degree of crystallinity,s of crystallinity, forming forming banded banded diagenetic diagenetic textures (Figuretextures 11). (Figure In addition, 11). In sediments addition, changesediments their change consistency: their consistency: they become they more become viscous, more soft, viscous, and highlysoft, plastic.and highly Usually, plastic. this is Usually, observed this in is the observed cores at in a the depth cores of 20–30at a depth cm. of The 20–30 thickness cm. The is nothickness less than is no 1 m. less than 1 m. The contact between the Late Holocene and underlying the Middle Holocene sediments is often The contact between the Late Holocene and underlying the Middle Holocene sediments is often unconformable. This indicates a break in sedimentation at this time. Erosion at the base of the upper unconformable. This indicates a break in sedimentation at this time. Erosion at the base of the upper horizon is clearly visible in the tomogram (Figure 12). Due to organic matter enrichment, iron and horizon is clearly visible in the tomogram (Figure 12). Due to organic matter enrichment, iron and manganese minerals form in the sediments. These features indicate an increase in the supply of organic manganese minerals form in the sediments. These features indicate an increase in the supply of matterorganic to the matter lake to basin. the lake Our basin. previous Our studiesprevious on studies Lake Ladogaon Lake [ 4Ladoga] have shown[4] have that shown this that process this is usually associated with a climatic optimum of the Holocene. We can also assign the same age to the deposits of the second unit. Minerals 2020, 10, x FOR PEER REVIEW 13 of 21

Mineralsprocess2020 is, 10 usually, 964 associated with a climatic optimum of the Holocene. We can also assign the14 ofsame 20 age to the deposits of the second unit.

FigureFigure 12. 12.Radiographic Radiographic contrast contrast cross-section cross-section of of core core ONG-5 ONG-5 and and its its separate separate enlarged enlarged fragments. fragments. TheThe lower lower part part of of the the column column reveals revealsvarved varved claysclays ofof the Late Pleist Pleistoceneocene period. period. See See Figure Figure 88 for for the thelegend. legend. Minerals 2020, 10, 964 15 of 20

The lower horizon consists of gray and brownish-gray silty-clay of a homogeneous ﬂuid consistency, although slightly more consolidated compared with the overlying sediments. As is the case with sediments of Lake Onego, more carefully examined from a stratigraphic point of view [4], these deposits relate to the units of the Early Holocene (Preboreal and Boreal periods). In sediment core ONG-5 a microvarved clay unit was described at a depth of 1.74 m beneath the Holocene gyttja units. This layering is very clearly demonstrated in the tomogram (Figure 12). Microvarves are reﬂected in the alternation of more and less light and dark X-ray absorbing interlayers (due to the presence of iron minerals and/or a higher bulk density). As one moves further down the section, the contrast between the layers becomes more visible, while their thickness slightly increases. A decrease in the thickness of layers is probably related to the withdrawal of the ice margin from the varved clay accumulation area. The total thickness of exposed varved clays is about two meters.

4.5. Gas Accumulation in the Petrozavodsk Bay Bottom Sediments This study is the first to examine gas accumulations in the sediments of Lake Onego. The obtained seismograms contain background noise in the form of clusters of bubbles (Figure3) that are usually Minerals 2020, 10, x FOR PEER REVIEW 14 of 21 located in areas of thick bottom sediments and do not entirely block the record. A clearer picture of gas accumulationsThe lower could horizon be observedconsists of in gray seismic and profiles, brownish-gray where wholesilty-clay sections of a homogeneous of a seismic profilefluid are characterizedconsistency, by although a reduced slightly image more resolution. consolidated The seismiccompared lines with also the revealedoverlying distinctivesediments. structuresAs is the of gaseouscase fluid with emissionssediments of from Lake the Onego, sediment more deposits,carefully examined so-called from pockmarks a stratigraphic (Figures point 13 andof view 14). [4], Accordingthese deposits to therelate radiography to the units data,of the a Early large Holo numbercene of(Preboreal pores can and be Boreal seen inperiods). the sedimentary In sediment cores of gas-saturatedcore ONG-5 sedimentsa microvarved (Figure clay unit15). was Most described of these at newly a depth formed of 1.74 cavitiesm beneath have the Holocene the shape gyttja of lenses units. This layering is very clearly demonstrated in the tomogram (Figure 12). oriented along the vertical axis. In the upper part of the core, they acquire a rounded shape. This is a Microvarves are reflected in the alternation of more and less light and dark X-ray absorbing consequenceinterlayers of the(due extremely to the presence rapid of formation iron minerals of pores, and/or which a higher occurs bulk density). immediately As one after moves the further core leaves the waterdown due the tosection, the di thefference contrast in between pressures the inlayers the be bottomcomes andmore surface visible, partswhile oftheir the thickness water column. slightly Theincreases. concentrations A decrease of in gaseous the thickness hydrocarbons of layers is ofprobably a C1-C5 related composition to the withdrawal in the gas of phase the ice of margin sediments from thefrom Petrozavodsk the varved clay Bayaccumulation are shown area. in The Table total3. thickness of exposed varved clays is about two meters. All samples analyzed in this study are characterized by elevated methane concentrations exceeding 1000 ppm.4.5. Gas C2-C5Accumulation hydrocarbon in the Petrozavodsk gases are Bay characterized Bottom Sediments by low methane concentrations, which is manifestedThis by study the highis the rates first ofto CH4examine/ΣC2-C5 gas accumu (>500).lations The in highest the sediments methane of concentrationLake Onego. The in the sedimentsobtained was seismograms observed atcontain station background ONG-2 at noise the in depth the form interval of clusters of 170–174 of bubbles cm (Figure below 3) the that lake are floor (b.l.f.).usually The lowestlocated methanein areas of concentration thick bottom sediments was registered and do at not station entirely ONG-7 block the at the record. depth A clearer interval of 84–89picture cm b.l.f. of gas The accumulations peak value could of Σ C2-C5be observed was in recorded seismic profiles, in the sampleswhere whole from sections ONG-7 of a at seismic the depth intervalprofile of 41–45 are characterized cm b.l.f., which by a showed reduced high image concentrations resolution. The of seismic isobutylene lines also (2154 revealed ppm) anddistinctive isopentane structures of gaseous fluid emissions from the sediment deposits, so-called pockmarks (Figures 13 (8269 ppm). and 14).

FigureFigure 13. The 13. seismicThe seismic proﬁle profile with with the the gas gas accumulations accumulations (shown (shown in in red) red) in in the the layers layers ofof thethe Middle and Earlyand Holocene Early Holocene sediments sediments (the position, (the position, see Figure see1 ).Figure The arrows 1). The showarrows the show location the location of the pockmarks. of the pockmarks. m.s.—milliseconds. m.s.—milliseconds. Minerals 2020, 10, 964 16 of 20

Minerals 2020, 10, x FOR PEER REVIEW 15 of 21

Figure 14. Sonogram of a pockmarks field in the bottom of the Petrozavodsk Bay (the position, see FigureFigure 14. Sonogram 1). of a pockmarks ﬁeld in the bottom of the Petrozavodsk Bay (the position, see Figure1). Minerals 2020According, 10, x FOR to PEERthe radiography REVIEW data, a large number of pores can be seen in the sedimentary cores16 of 21 of gas-saturated sediments (Figure 15). Most of these newly formed cavities have the shape of lenses oriented along the vertical axis. In the upper part of the core, they acquire a rounded shape. This is a consequence of the extremely rapid formation of pores, which occurs immediately after the core leaves the water due to the difference in pressures in the bottom and surface parts of the water column.

(a) (b) Figure 15. (a) Vertical and separate horizontal radiographic contrast section views of the upper 50 cm in core ONG-1;Figure 15. (b ()a) three-dimensional Vertical and separate model horizo ofntal the radiographic cavity distribution contrast section (within views the upperof the upper 25 cm) 50 resulting cm fromin degassing core ONG-1; of the (b) sediments. three-dimensional model of the cavity distribution (within the upper 25 cm) resulting from degassing of the sediments.

The concentrations of gaseous hydrocarbons of a C1-C5 composition in the gas phase of sediments from the Petrozavodsk Bay are shown in Table 3. Minerals 2020, 10, 964 17 of 20

The distribution of values of CH4/ΣC2-C5 was determined by the concentration of methane, which is typical for hydrocarbon gases of microbial origin [19]. In all cases, isopentane prevails among the C2-C5 hydrocarbon gases. This feature is particularly interesting and requires further verification. Isopentane is an atypical component of the gas phase in sediments, both marine and lacustrine, and we have not found any references on its discovery and origin in previous research. Relatively low concentrations of ethane/ethylene may indicate active microbiological processes [20], possibly associated with the activities of methanogenic and methanotrophic microbiota. Unlike marine ecosystems, freshwater ecosystems have limited resources of electron acceptors (mainly in the form of sulfate), so the anaerobic oxidation of methane is of little importance in the carbon cycle, and methane actively flows into the water column and the atmosphere [20]. Therefore, the surveyed region and Lake Onego in particular, may substantially contribute to atmospheric methane emissions. The study outcomes are of particular importance due to the great interest in different sources of emissions and re-evaluations of their contribution to the global methane budget. For that reason, this region requires further research. 13 The isotopic composition δ C-CH4 was examined in cores ONG-2 and ONG-4. The values ranged from 68.4 to 69.3% VPDB and from 70.4 to 73.7% VPDB in cores ONG-2 and ONG-4, respectively. − − − − 13 These values clearly confirm the microbial (diagenetic) origin of methane. Notably, the δ C-CH4 values tend to decrease upwards in both cores. This usually occurs through the activation of methanogenesis processes, as during methane oxidation, the light isotope 12C decays first, and the remaining methane becomes enriched with the heavier isotope 13C[19]. Methanogenesis in the upper parts of the section may only prevail if pore waters do not contain sulfate ions, which is often the case with freshwater lakes.

4.6. Mineralogy in Cores ONG-4 and ONG-7 The mineral composition of bottom sediments was studied directly on the sedimentary cores used in this study. According to previous studies conducted by the authors, it was found that the mineral composition of the bottom sediments of lake Onega is formed by minerals (quartz, feldspar, muscovite), which were introduced into the lake from river runoff and aerosols, as well as autigenic minerals. In contrast to wells from other areas of the lake, the mineral phases of Fe and Mn are represented by pyrolusite, vivianite, rhodochrosite, and siderite already in the upper first centimeters of sediments [17,21]. In addition to these minerals, there are opal (diatoms), Fe-Illite, and Fe-chlorite. [21]. The concentration of rhodochrosite, siderite, and vivianite increases in the zones of gas-saturated Silts (Figure 16). Measurements of microbial activity in the bottom sediments of the Petrozavodsk Bay showed the presence of sulfate-reducing and methane-forming bacteria in the recovery zone, although their number was quite low [22]. Numerous studies have shown that CH4 oxidation and microbial melanogenesis are widely developed when methane diffusion processes from deeper sediments to poorly lithiated layers are detected in modern bottom sediments. These processes were accompanied by the formation of autigenic carbonates. It is possible that the presence of methane leads to the formation of vivianite, rhodochrosite and siderite in large quantities. The geochemical evidence for this process consists in the marked enrichment of diagenetic carbonates with the 12C isotope [23]. To confirm this hypothesis, additional studies of the isotopic composition of carbonates should be conducted in the future. Minerals 2020, 10, 964 18 of 20 Minerals 2020, 10, x FOR PEER REVIEW 19 of 21

Figure 16. Electron micrograph of the minerals forming the bottom sediment layer in Lake Onego at a depth of 5–125–12 cmcm (ONG-4).(ONG-4). 1—rhodochrosite;1—rhodochrosite; 2—thin-plate2—thin-plate depositions of Fe-illiteFe-illite ++ Fe-chlorite + sections of diatoms, composed of opal; 3—feldspar debris; 4—biotite.4—biotite.

5. Conclusions For the firstfirst time, comprehensive geological and geophysical studies were conducted in the Petrozavodsk Bay of Lake Onego. It includes: includes: seismoacoustic seismoacoustic (in (in different different variations) variations) profiling, profiling, sonar profiling,profiling, and sediment coring with heavyheavy gravitygravity corers.corers. The The sediment cores were examined by tomography, revealingrevealing gasgas sevenseven distinctivedistinctive lithologicallithological unitsunits andand gas accumulation in pockmarks of Lake Onego sediments. The seismic profiles profiles showed that the surface of pre-Holocene pre-Holocene sediments is rugged rugged and and dissected, dissected, with somesome scarpsscarps probably probably caused caused by by Holocene Holocene geodynamic geodynamic movements. movements. These These scars scars sometimes sometimes occur occurnear areas near ofareas gas of release. gas release. This study is the first first provide evidence based on geophysical data for gas emission in lacustrine sediments in NW Russia. A significant significant gas accumulation in sediments must be taken into account in the ecological assessment of a lake. The The chemical composition of of gases in the bottom sediments of Lake Onego were elevated in methane concentrations exceedingexceeding 10001000 ppm.ppm. The formation of autigenic carbonates (rhodochrosi (rhodochrositete and siderite) in the surface layer of bottom sediments of the Petrozavodsk Bay of Lake Onego in significantsignificant quantities is associated with the processes ofof methane methane oxidation. oxidation. Confirmation Confirmation of this of hypothesisthis hypothesis for the formationfor the formation of Mn Fe carbonatesof Mn Fe carbonatesrequires isotopic requires studies isotopic in the studies future. in Based the future. on the studiedBased on sediment the studied cores sediment and their calibratedcores and dates,their wecalibrated have proposed dates, we a lithostratigraphichave proposed a lithostratigrap scheme that mighthic scheme be relevant that might for the be entire relevant Lake for Onego. the entire Lake Onego. Author Contributions: Conceptualization, D.S., V.S. and A.R.; methodology, A.R., V.S., D.S., N.B., N.D., M.P. and V.K.; fieldworks, A.R., N.B., M.P., N.D., A.L., Y.K., A.O.; software, M.T. and M.A.; validation, V.S., D.S. and A.R.;Author formal Contributions: analysis, V.K., Conceptualization, D.K. and A.K.; investigation,D.S., V.S. and D.S., A.R.; A.R., methodology, V.S., N.B., M.T.,A.R., M.P.,V.S., D.S., M.A., N.B., P.B., N.D., N.D., M.P. V.K., andD.K., V.K.; A.L., fieldworks, N.S., A.K., A.R., N.K., N.B., Y.K., M.P., A.O.; N.D., resources, A.L., V.S.,Y.K., D.S.,A.O.; V.K. software, and M.T.; M.T. data and curation,M.A.; validation, V.S. and V.S., A.R.; D.S. writing, and A.R.;original formal draft analysis, preparation, V.K., D.S.,D.K. A.R.,and A.K.; N.B., investigation, V.S., V.K., P.B.; D.S., writing, A.R., reviewV.S., N.B., and M.T., editing, M.P., D.S., M.A., A.R., P.B., V.S., N.D., V.K. V.K., and D.K.,P.B.; visualization, A.L., N.S., A.K., V.S., N.K., N.K. Y.K., and A.O.; P.B.; supervision,resources, V.S., V.S. D.S., and V.K. D.S.; and project M.T.; administration, data curation, V.S. D.S. and All A.R.; authors writing, have read and agreed to the published version of the manuscript. original draft preparation, D.S., A.R., N.B., V.S., V.K., P.B.; writing, review and editing, D.S., A.R., V.S., V.K. and P.B.; visualization, V.S., N.K. and P.B.; supervision, V.S. and D.S.; project administration, D.S. All authors have read and agreed to the published version of the manuscript.

Minerals 2020, 10, 964 19 of 20

Funding: The research was supported by Russian Science Foundation (RSF No. 18–17-00176), the Ministry of Science and Higher Education of the Russian Federation (project No. FSZN-2020-0016), by grants from Saint Petersburg State University (No. 18.42.1258.2014, 18.42.1488.2015, and 0.42.956.2016 for field research and No. 18.40.68.2017 for the purchasing of scientific equipment), by grants from the Russian Foundation for Basic Research (RFBR NN 18-05-00303& 19-05-50014), by the FEEL foundation (“Life Under the Ice project” http://wiki.epfl.ch/ladoga). A survey was conducted on the topic “Hardware and software systems development for the search, exploration, geophysical and geochemical monitoring of the exploitation of hydrocarbon reservoirs, including remote regions and challenging climate conditions”, with funding from the Ministry of Science and Higher Education of the Russian Federation, using equipment purchased under the Development Program of Lomonosov Moscow State University (Grant Agreement No. 14.607.21.0187 of 26 September 2017, RFMEFI60717 0187). × Conflicts of Interest: The authors declare no conflict of interest.

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