RUSSIAN JOURNAL OF EARTH SCIENCES, VOL. 21, ES1003, doi:10.2205/2020ES000723, 2021
M. Ulyanova1, T. Malakhova2, D. Evtushenko2, Yu. Artemov2, V. Egorov2
1Shirshov Institute of Oceanology, Russian Academy of Sciences, Moscow, Russian Federation
2A. O. Kovalevsky Institute of Biology of the Southern Seas, Russian Academy of Sciences, Sevastopol, Russian Federation
[1] In 2015, geoacoustic and gas-geochemical studies of bottom sediments of the Vistula Lagoon of the Baltic Sea and the Sevastopol Bay of the Black Sea were carried out. A comparative analysis showed differences in the particle size distribution and methane content in the sediments of the studied basins. Maximum methane concentrations were observed in the sediments of the central part of the Sevastopol Bay (1856 $\mu$mol dm$^{3}$), characterized by the highest content of aleuropelite fraction. The diffusion flux of methane at the water-bottom interface was directed from the sediment to water and varied from 0.004 to 0.132 mmol/(m$^{2}$ day) in the Sevastopol Bay, and from 0.005 to 0.030 mmol/(m$^{2}$ day) in the Vistula Lagoon. The value of methane solubility in pore waters for the Vistula Lagoon, calculated for the average depth of the basin, is significantly lower compared to the same value for the Sevastopol Bay. Despite this, bubble outgassing was not recorded in the Vistula Lagoon, unlike the Sevastopol Bay.
The problem of global climate change is now recognized as one of the most acute facing the world community. Methane is a greenhouse gas and its distribution requires a detailed quantitative assessment. Globally, methane from the ocean to the atmosphere is negligible, despite the rapid gas formation in the sediments of the continental shelf. High rates of sediment accumulation in shallow coastal areas [Thang et al., 2013] usually lead to a decrease in the lifetime of organic matter in the sulfate reduction zone, which increases the methane formation zone to deeper sediment layers. Besides, methanotrophs are less active under conditions of intense sedimentation [Dale et al., 2008]. As a result, it is coastal shallow water areas that are the main sources of ocean methane release into the atmosphere, and the anthropogenic eutrophication, which affects both studied reservoirs, leads to an increase in methane flux from bottom sediments [Egger et al., 2016].
Methane, as part of the organic carbon cycle, is involved in biogeochemical processes taking place in silts. A large proportion of anaerobic oxidation of methane occurs in sediments of passive continental margins, where dissolved methane in pore waters is transported mainly due to molecular diffusion. Estimates of the exchange of chemical elements at geochemical barriers, both horizontal (coast-sea, river-sea, upwelling, etc.) and vertical (surface water microlayer, water-sediment surface, upper "active" layer of sediments (up to 1–5 cm) and others), serve as the basis for understanding the cycle of substances [Vershinin and Rozanov, 2002].
The Baltic Sea, including the two largest and most productive lagoons – the Curonian and Kaliningrad (Vistula) lagoons, is an area of high anthropogenic pressure. This also applies to the Sevastopol Bay of the Black Sea. These shallow basins are buffer zones between rivers and the open sea and they accumulate organic matter and pollutants from the land. The presence of increased concentrations of methane creates a special geochemical background. The study of the lagoons reveals differences in the behavior of methane in water basins with various hydrochemical characteristics.
The research aims to compare the distribution of methane in the bottom sediments of shallow lagoons of the Baltic and Black Seas, which differ in geo- and hydrochemical conditions.
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| Table 1 |
The circulation of water in the lagoon is one of the most important factors affecting the physical and biochemical processes. Shallow depths, mixing in the water column, river runoff, and exchange with the open sea – the main factors affecting the hydrochemical regime – differ between the Vistula Lagoon of the Baltic Sea and the Sevastopol Bay of the Black Sea (Table 1).
The water of the lagoon is usually saturated with oxygen, however, during the period of intensive "blooming" (peaks in April and July–August) oxygen completely disappears, being replaced by hydrogen sulfide [Alexandrov, 2010]. Increased nutrient loading in recent decades and limited water exchange have led to the eutrophication of the lagoon, as the shallow basins near densely populated coastal areas are primarily subject to anthropogenic stress [Wulff et al., 1990]. The consumption of oxygen in combination with the enrichment of bottom sediments with organic matter increases the occurrence of anaerobic conditions and promotes methanogenesis.
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| Figure 1 |
Vistula Lagoon is a typical lagoon with simplified morphology. In the coastal part of the lagoon, to a depth of 1.5–2 m, sediments are represented by sand and sandy loam (Figure 1A). Their thickness does not exceed several meters. In the deeper parts of the lagoon the silts are located, the thickness of which reaches 10 m. These deposits formed in the Litorina and Post-Litorina stages of the Baltic Sea development.
The structure of the sedimentary cover of the Kaliningrad Lagoon has not been sufficiently studied [Otmas and Kokhanova, 2015]. The study area is located within the boundaries of the Mamonovskaya depression, which stands out as a large negative structure of the submeridional direction, occupying mainly the Gdansk Deep, forming a generally depressed structural bay [Otmas et al., 2017]. Mostly in the system of discontinuous dislocations, the Mamonovskaya depression linkage with neighboring second-order tectonic elements [Otmas et al., 2006].
A significant part (25%) of organic matter comes to the Vistula Lagoon from the open sea. Current conditions of sedimentation in this area are characterized by the outflow of the sediment into the open sea [Pustelnikovas, 1998], resulting in a low sedimentation rate (0.4 mm/year) [Chechko, 2006]. The distribution of bottom sediments in the lagoon belongs to the "circumcontinental" type [Emelyanov, 2002], when fine sediments are confined to the central, deeper regions, and sands are distributed along the periphery.
The Sevastopol Bay is located along the fault line, stretching from the city of Sevastopol to the city of Simeiz [Ivanov et al., 2009] and it is a rather deeply cut in sublatitudinal elongated trough-shaped depression about 6.5 km long, with a width of 1.4 km and an existing depth of the sea of up to 19 m. Its steep slopes and parent bottom are composed of Neogene-Paleogene limestones, marls and clays, and the seabed filled with unconsolidated Quaternary sediments. Their thickness ranges from 28 in the bay apex to 40 m in the estuary, where they are mainly represented by detritus-shell sand. To the east, they are replaced by sandy loam and clayey silts, contributing to the accumulation of organic matter (Figure 1B). The sublatitudinal tectonic breaks that formed the graben of the Sevastopol Bay discontinue lithified Neogene–Paleogene rocks, and the time of movement along them refers to Quaternary activation [Bondarev et al., 2015]. At the same time, orthogonal and diagonal breaking faults were activated, which are traced along modern draws and bays adjacent the Sevastopol Bay, the largest of which passes through the Yuzhnaya Bay. Along the same submeridional faults, the other largest bays of the Heracles Peninsula are laid.
The hydrological and hydrochemical parameters of the Sevastopol Bay are characterized by significant spatial and temporal variability, which is determined by the intensity of its water exchange with the adjacent waters of the Black Sea, the peculiarities of the water circulation within the bay, and the flow regime of the Chernaya River and waste water [Ivanov et al., 2006]. In the water of the bay, there is an excess of nutrients, which is an order of magnitude higher than in the open areas of the Black Sea, a lack of oxygen in the bottom layer, increased values of the total suspended matter, pH and total alkalinity [Minkovskaya et al., 2007]. This hydrochemical regime contributed to the eutrophication of the waters of the Sevastopol Bay in the 90s of the last century. In recent years, the problem of improving the quality of the waters of the Sevastopol coast and classifying them as transitional from oligotrophic to mesotrophic has been discussed [Gubanov et al., 2002].
Geoacoustic studies in the Sevastopol Bay were carried out by a portable single-beam Simrad EA 400SP sonar echo sounder (frequency 38 and 200 kHz). In the Vistula Lagoon, geoacoustic profiles were obtained with a dual-beam profilograph Knudsen Chirp 3212 (frequency from 3.5 to 210 kHz).
The sampling of bottom sediments and near-bottom water was carried out by a hermetic Niemistø corer. Immediately after corer appearing on board, the samples were taken for gas analysis.
Water from the corer were drained through a silicone tube in penicillin vials of 30 ml volume, with pre-placed in them solid potassium hydroxide to inhibit microbial processes. The same volume of water was squeezed out by special organic glass dispensers of and closed a gas-tight stopper is made of butyl rubber. Samples from sediments were taken with 2 ml of a plastic syringe with a trimmed tip. Then the sediments were placed in a methane bottle with clamp, filled with degassed water and has a standard volume of water, closed with a butyl rubber stopper.
The methane concentration in the water and bottom sediments was measured by the method of phase-equilibrium degassing [Sakagami et al., 2012], known in the literature as "headspace analysis". The methane concentration in the gas phase of water samples and bottom sediments of the Vistula Lagoon was determined using a Crystal 2000 gas chromatograph (Meta-Chrom, Russia) with a flame ionization detector and helium as a carrier gas. The relative error in the determination of independent measurements was about 8%. Determination of methane concentrations in water and bottom sediments of the Sevastopol Bay was carried out on an chromatograph HP 5890, the relative error of 7%.
[2] Determination of sulphate content in silt waters. Silt water was obtained by centrifuging sediments at 8000 rpm for 10 min in a TsUM-1 centrifuge (Russia). Quantitative determination of the sulfate ion in the pore sediment water was carried out using a Stayer ion chromatograph (Russia).
[3] The methane diffuse flux ($J$) at the water-bottom interface was calculated from the concentrations of methane ($C$) in pore waters according to Fick's first law:
\begin{eqnarray*} J = \phi \cdot D_{s} \cdot dC/dz \end{eqnarray*}where $J$ is the diffuse methane flux, mmol/(m$^{2}$ day); $D_{s}$ is the coefficient of molecular diffusion of methane in the sediment, cm$^{2}$/day; $\phi$ is the sediment porosity, $dC/dz$ is the methane concentration gradient in the upper centimeters of the sediment layer ($C$ is the concentration of dissolved methane in pore water, $z$ is the depth below the sediment surface), mmol/cm [Frenzel et al., 1992].
The solubility of methane in pore water was estimated from the calculation of the solubility coefficient of Bunsen [Yamamoto et al., 1976], which depends on the temperature and salinity of the environment:
\begin{eqnarray*} \ln \beta = A_{1} + A_{2} \frac{100}{T} + A_{3} \ln \frac{T}{100} + + S \left[B_{1} + B_{2} \frac{T}{100} + B_{3} \bigl(\frac{T}{100}\bigr)^{2}\right] \end{eqnarray*}where $\beta$ is the Bunsen coefficient, ml $\cdot$ ml$^{-1}$; $T$ – water temperature, K; $S$ – salinity of water, ‰.
To calculate the solubility of methane, an in situ pressure correction was also introduced in accordance with Henry's first law, according to [Albert et al., 1998]. The salinity and temperature of pore water over the entire depth of the studied columns were taken equal to the values for bottom water at the corresponding stations.
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| Figure 2 |
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| Table 2 |
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| Figure 3 |
Bottom sediments in the Vistula Lagoon (the Baltic Sea) were sampled at 5 stations (see Table 2), two of which (1 and 2) had already been studied earlier (July 2011, September 2012 and 2013), while the remaining three (GNO, GNO2 and Per) were studied for the first time in July 2015 based on the results of the interpretation of geoacoustic profiles (Figure 3).
Geoacoustic profiles obtained in December 2014 visually showed signs of gas saturation of sediments (Figure 3), similar to those observed in the open part of the Baltic Sea (Gdansk Deep) [Ulyanova et al., 2013]. Darkness directly below the bottom surface, caused by increased reflection of the sound wave in near-surface sediments, below which acoustic windows are traced, appearing as white spots of intense absorption of the acoustic signal, may indicate the presence of increased gas concentrations.
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| Figure 4 |
The studied sediments were represented by silty sands (Figure 4). The fragments of shells were found in all samples.
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| Figure 5 |
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| Table 3 |
In the sediments of the remaining points of the Vistula Lagoon, methane concentrations did not exceed 16 $\mu$mol/dm$^{3}$. Values of methane content obtained in September 2013 at a horizon of up to 20 cm of point 2 were maximum for the entire observation period 2011–2015 (see Table 3). For comparison, it should be noted that for the southern part of the Vistula Lagoon (the water area of Poland), the maximum concentration of methane was recorded in the southwestern part and amounted to 6450 $\mu$mol/dm$^{3}$, the minimum in the southeastern part, 7.1 $\mu$mol/dm$^{3}$ [Reindl and Bolałek, 2017]. The rest of the Polish water area is characterized by values of 7–37 $\mu$mol/dm$^{3}$, which is slightly higher than the values obtained for the Russian part of the Vistula Lagoon in 2015. The maximum concentration obtained for the Polish water area is identical to the values in Eckernf\"{o}rde Bay, the Baltic Sea, and is explained by weak hydrodynamics and low salinity area. In the Curonian Lagoon (southern part) of the Baltic Sea, the methane concentration in the surface layer of sediments varies in the range 1.6–1000 $\mu$mol/dm$^{3}$ [Pimenov et al., 2013b; Ulyanova et al., 2013].
The sulfate-methane transition zone was recorded at two points: at point 2 at a horizon of 5 cm and at a GNO points at a horizon of 11 cm. The amount of methane in bottom water varied slightly (0.07 to 0.23 $\mu$mol/l). Thus, the areas selected based on the results of geoacoustic profiling did not confirm the presence of increased gas concentrations in bottom sediments. However, it must be borne in mind that sampling was performed several months after profiling. Changing seasons and, consequently, hydrological parameters could contribute to the release of gas.
For the first time, the solubility of methane in pore waters was calculated for the Vistula Lagoon: 2.12 mmol/l for point 1, 2.04 mmol/l for point 2 and 2.00 mmol/l for other points. It is known that with a change in temperature, the rate of molecular motion changes and, consequently, the solubility of various substances in liquids also changes. The solubility of gases decreases with increasing temperature. A significant effect on the solubility of gases in water is exerted by pressure. The amount of gas dissolving in water increases in direct proportion to the increase in its partial pressure, i.e. gases obey Henry's law. The pressure of the gas phase at any point in the bottom sediments will depend on changes in atmospheric and hydrostatic pressure. Therefore, under slightly changing conditions in the water area, the solubility values can be considered comparable. In general, in the Vistula Lagoon under the observed conditions, there was no excess of methane concentration in pore waters over its solubility.
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| Figure 6 |
The diffusion flux of methane at the water-bottom interface varied from 0.005 at point 1 to 0.030 mmol/(m$^{2}$ day) at point Per (Figure 6). At point 2, the value was 0.008 mmol/(m$^{2}$ day), at the points of GNO and GNO2 – 0.017 and 0.021 mmol/(m$^{2}$ day), respectively.
When comparing the magnitude of the diffusion flux at points 1 and 2 with the previously obtained data (2011–2013), we can conclude that in July 2015, there was a minimal methane flux from the surface sediment layer to the water of the Vistula Lagoon.
There were no reliable signs of the presence of gas-saturated layers of bottom sediments in the Sevastopol Bay. Sites visually similar to the areas of gas-saturated sediment distribution were studied, but insignificant methane contents confirmed the assumption that there were no elevated gas concentrations. It should be noted that stratification of sediments also was not recorded, although the study area is partially characterized by the predominance of aleuritic-pelitic sediments on the bottom surface, and according to the technical characteristics of the echo-profiler, several upper thin layers of sediments should be expected to be displayed. At the same time, at points 9 and 2, acoustic anomalies in the water column were observed, similar to bubble outgassing (see Figure 2A, Figure 2B). Stream gas emissions were repeatedly recorded earlier in the coastal waters of the Heracles Peninsula, including the Sevastopol Bay [Egorov et al., 2011; Malakhova et al., 2015; Pimenov et al., 2013a]. It has been established that the mechanism of formation of biogenic methane seeps can be different. Bubble gas can form directly in the upper layers of bottom sediments due to intense microbial processes, such as in Kazachya Bay; and in deeper layers of the sedimentary layer, as in the case of the estuarine part of the Sevastopol Bay [Malakhova et al., 2015].
In Figure 2B, the track from the bubbles was recorded after the contact of the corer with the bottom at point 9. Most likely, the mechanical effect on the sediments released a certain amount of gas in bubble form. The grain-size composition of the sediment at point 9 ensures that microbubbles are collected on the surface of the bounding layer, consisting of a sediment with small-diameter pores (Figure 4). Whereas in the sandy sediments of the Vistula Lagoon, dissolved gas and microbubbles pass unhindered at the water-bottom sediment interface without forming sufficiently large accumulations.
[4] Gasgeochemical studies. The investigated sediments of the Sevastopol Bay were represented by clayey aleurite and aleuritic-pelitic clayey silts and siltstones (see Figure 4).
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| Table 4 |
The methane content in the bottom sediments of the Sevastopol Bay differed within the studied area (Table 4, see Figure 5). In October 2015, the maximum methane values were recorded for point 9 at a horizon of 17 cm. The maximum methane concentration in the near-bottom water (0.38 $\mu$mol/l) was also observed here. The entire sediment core, samples at point 9, located in the central part of the bay, was characterized by elevated methane concentrations ranging from 19 to 1856 $\mu$mol/dm$^{3}$. At other points, the methane concentration fluctuated slightly (from 0.58 to 17.61 $\mu$mol/dm$^{3}$).
The spatial model of methane distribution in the near-surface bottom sediments of estuaries is based on the fact that the concentration of methane in pore water decreases in the sea direction. Nevertheless, there are several exceptions, including for highly eutrophied coastal bays with sufficiently high salinity and extremely high sedimentation rates of organic material. For example, in Eckernförde Bay in Germany and Cape Lookout in North Carolina, such conditions lead to sulphate depletion in the upper layers of bottom sediments and the methane concentration reaches several mmol/dm$^{3}$ at a depth of less than 1 m below the bottom [Abegg, 1997]. The formation of bubble methane in these areas is already possible at a depth of several tens of centimeters, where the methane concentration becomes higher than the gas solubility in pore water.
The presence of increased methane concentrations in the sediments of the central part of the bay compared with the peripheral parts can be explained by the peculiarities of sedimentation processes [Malakhova et al., 2018]. The sedimentary matter of the central part of the bay is characterized by a finer granulometric composition of sediments (60–65 % of aleuropelites), and its supply is associated with allochthonous matter [Malakhova et al., 2018], such as rainfall drains, emergency releases of untreated water, which can be a significant source of organic matter. In this regard, increased concentrations of CH$_{4}$ at point 9 are due to both a high content of organic carbon in the sediments and an increased dispersion, and, consequently, to the adsorption capacity of the muds.
Above the seep field (point 12), the values both in bottom sediments and in bottom water were among the lowest, however, a sharp increase in methane concentration at a horizon of 4–6 cm (up to 3.63 $\mu$mol/dm$^{3}$) occurred in comparison with the overlying and underlying layers (0.96 and 0.58 $\mu$mol/dm$^{3}$, respectively). A similar profile of methane with a maximum in subsurface horizons was previously noted both in this research area [Egorov et al., 2012] and in the study of methane seeps on the Black Sea shelf [Ivanov et al., 2002].
The solubility of methane in pore waters calculated for the Sevastopol Bay varied from 1.77 mmol/l for point 3 (the shallowest sampling depth is 3 m) to 3.81 mmol/l for point 12 (maximum depth 18 m), on average for the water area 2.5 mmol/l. Despite the rather high methane concentrations at point 9, even at it the concentration did not exceed the solubility of methane.
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| Figure 7 |
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| Figure 8 |
After the gas phase is formed, the gas movement between the gas and water phases is governed by Henry's law, according to which the concentration of dissolved gas is equivalent to the partial pressure of the gas multiplied by its solubility (Henry's constant), which is inversely dependent on temperature [Slabaugh and Parsons, 1976]. Thus, with increasing temperature throughout the summer, the solubility of the gas will decrease, which will lead to the transition of gas from the dissolved to the gaseous phase.
The diffusion flux of methane at the water-bottom boundary in the Sevastopol Bay varied from 0.004 at point 2 to 0.132 mmol/(m$^{2}$ day) at point 9 (see Table 4). The results are comparable with studies in 2011, when the methane flow in the open part of the Sevastopol Bay varied from 0.001 to 0.544 mmol/(m$^{2}$ day) [Malakhova et al., 2012]. Moreover, the maximum values both in 2011 and in 2015 were observed in the central part of the bay, characterized by the finest surface sediments. With the exception of sedimentation at point 9, we can say that the methane diffusion flux in the Sevastopol Bay was comparable with the data for the Vistula Lagoon. However, it should be taken into account that there are bubble emissions of methane from the bottom surface in the Sevastopol Bay [Egorov et al., 2012; Malakhova et al., 2013; 2015]. Their formation can be associated with fault systems of the studied water area that promote gas migration [Bondarev et al., 2015; Kravchenko, 2008].
The geoacoustic and gas-geochemical studies in the Vistula Lagoon of the Baltic Sea and Sevastopol Bay of the Black Sea revealed the following results:
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Received 23 January 2020; accepted 28 July 2020; published 26 January 2021.
Citation: Ulyanova M., T. Malakhova, D. Evtushenko, Yu. Artemov, V. Egorov (2021), Comparison of methane distribution in bottom sediments of shallow lagoons of the Baltic and Black Seas, Russ. J. Earth Sci., 21, ES1003, doi:10.2205/2020ES000723.
Copyright 2021 by the Geophysical Center RAS.