A method for the calculation of anaerobic oxidation of methane rates across regional scales: an example from the Belt Seas and The Sound (North Sea–Baltic Sea transition)

Estimating the amount of methane in the seafloor globally as well as the flux of methane from sediments toward the ocean–atmosphere system are important considerations in both geological and climate sciences. Nevertheless, global estimates of methane inventories and rates of methane production and consumption through anaerobic oxidation in marine sediments are very poorly constrained. Tools for regionally assessing methane formation and consumption rates would greatly increase our understanding of the spatial heterogeneity of the methane cycle as well as help constrain the global methane budget. In this article, an algorithm for calculating methane consumption rates in the inner shelf is applied to the gas-rich sediments of the Belt Seas and The Sound (North Sea–Baltic Sea transition). It is based on the depth of free gas determined by hydroacoustic techniques and the local methane solubility concentration. Due to the continuous nature of shipboard hydroacoustic measurements, this algorithm captures spatial heterogeneities in methane fluxes better than geochemical analyses of point sources such as observational/sampling stations. The sensibility of the algorithm with respect to the resolution of the free gas depth measurements (2 m vs. 50 cm) is proven of minor importance (a discrepancy of <10%) for a small part of the study area. The algorithm-derived anaerobic methane oxidation rates compare well with previous measured and modeling studies. Finally, regional results reveal that contemporary anaerobic methane oxidation in worldwide inner-shelf sediments may be an order of magnitude lower (ca. 0.24 Tmol year^–1) than previous estimates (4.6 Tmol year^–1). These algorithms ultimately help improve regional estimates of anaerobic oxidation of methane rates.     

Mathematical simulation of the distribution of hydrocarbons in the aerobic and anaerobic waters of the Black Sea

We improve and extend the set of parametrizations of chemical and biological processes governing the assimilation of the emulsion-dissolved fraction of hydrocarbons in marine media with regard for the oxidation-reduction conditions in the oxycline and in the aerobic, suboxic, and anaerobic zones of the Black Sea. The numerical experiments aimed the reconstruction of the vertical and spatial distributions of hydrocarbons are performed. The increase in the number of fractions of hydrocarbons from three to five, taking into account the influence of the concentration of oxygen, and the analysis of the processes of anaerobic and photochemical oxidation of hydrocarbons make it possible to construct a numerical model of distribution of hydrocarbons in the aerobic and anaerobic waters of the sea and get good quantitative agreement between the results of numerical experiments and the data of observations. __________ Translated from Morskoi Gidrofizicheskii Zhurnal, No. 6, pp. 53–67, November–December, 2006.     

冷泉系统甲烷气体渗漏的声学特征及潜在环境效应

The amount of methane leaked from deep sea cold seeps is enormous and potentially affects the global warming,ocean acidification and global carbon cycle. It is of great significance to study the methane bubble movement and dissolution process in the water column and its output to the atmosphere. Methane bubbles produce strong acoustic impedance in water bodies, and bubble strings released from deep sea cold seeps are called "gas flares"which expressed as flame-like strong backscatter in the water column. We characterized the morphology and movement of methane bubbles released into the water using multibeam water column data at two cold seeps. The result shows that methane at site I reached 920 m water depth without passing through the top of the gas hydrate stability zone(GHSZ, 850 m), while methane bubbles at site II passed through the top of the GHSZ(597 m) and entered the non-GHSZ(above 550 m). By applying two methods on the multibeam data, the bubble rising velocity in the water column at sites I and II were estimated to be 9.6 cm/s and 24 cm/s, respectively. Bubble velocity is positively associated with water depth which is inferred to be resulted from decrease of bubble size during methane ascending in the water. Combined with numerical simulation, we concluded that formation of gas hydrate shells plays an important role in helping methane bubbles entering the upper water bodies, while other factors, including water depth, bubble velocity, initial kinetic energy and bubble size, also influence the bubble residence time in the water and the possibility of methane entering the atmosphere. We estimate that methane gas flux at these two sites is 0.4×10~6–87.6×10~6 mol/a which is extremely small compared to the total amount of methane in the ocean body, however, methane leakage might exert significant impact on the ocean acidification considering the widespread distributed cold seeps. In addition, although methane entering the atmosphere is not observed, further research is still needed to understand its potential impact on increasing methane concentration in the surface seawater and gas-water interface methane exchange rate, which consequently increase the greenhouse effect.     

Investigation of the fields of concentration of the suspension on the northwest shelf of the Black Sea in the case of roiling of the bottom sediments by a moving cyclone

The processes of roiling of the bottom sediments, diffusion transport of the suspension, and its repeated sedimentation on the northwest shelf of the Black Sea caused by a moving cyclone are studied by using a numerical sigma-coordinate model. It is supposed that bottom sediments are formed by particles of the same type. We determine the regions of the most intense rise of the suspension, directions of its transport by the flows of water, and the vertical profiles of concentration of suspended bottom sediments in different regions. In particular, it is shown that the distribution of suspended substances at a distance of 1 m from the bottom reflects the location of the active and inactive centers of erosion. __________ Translated from Morskoi Gidrofizicheskii Zhurnal, No. 1, pp. 3–20, January–February, 2007.     

Characteristics of trapped topographic waves in the northwest part of the Black Sea

We study trapped baroclinic topographic waves in the northwest shelf of the Black Sea for the actual slopes of the bottom and stratification. The time scales of trapped waves are determined. The space scales of the amplitude of oscillations are computed. It is shown that the vertical distribution of the amplitude of oscillations is in qualitative agreement with the experimentally observed distribution. The energy of topographic waves trapped by the sloping bottom is concentrated in the bottom layer, which agrees with the data of measurements. __________ Translated from Morskoi Gidrofizicheskii Zhurnal, No. 5, pp. 44–52, September–October, 2006.     

Methane in the Sevastopol coastal area, Black Sea

Concentrations of dissolved methane in seawater and bottom sediments, as well as of methane emanating from gas seeps were measured at 18 stations including several small bays in the Sevastopol coastal area (Black Sea) during 2007–2008. Methane concentrations in surface waters ranged from 10 to 2,970 nmol l^?1, and correlated well with values recorded for sediments. Methane concentrations in the water column were influenced by water depth, as well as by air and water temperatures. In the spring and summer of 2008, in situ CH₄ saturation relative to air was in the range of 970–71,900%. Maximum saturation was in summer. CH₄ fluxes to the atmosphere from the Sevastopol coastal area were estimated to vary from 190 to 1,550 μmol m^?2 day^?1. Gas bubbles escaping from the seepages contained about 57 vol% methane. Radiocarbon dating of the methane revealed an age not exceeding 150 years, implying a biogenic origin.     

Breakup of deep-water methane bubbles

During the Russian Academy of Sciences “MIRI na Baikale. 2008–2009” expedition, deep-water experiments on methane bubbles emerging from the lake bottom at depth of 1400 and 860 meters were carried out. Bubbles escaping the seabed were caught by a trap, which was an inverted glass. Entering in the trap, bubbles became covered by a gas hydrate envelope and then after a time period collapsed into a number of gas hydrate solid fragments. Due to positive buoyancy, fragments remained in the top part of a trap, exhibiting properties of a powder. The glass’s bottom was replaced with a 1 mm mesh grid, allowing the finest gas hydrate particles to sift through the grid, rising upwards. It is proposed that bubble collapse into fragments is related to the pressure drop in the bubble in the course of formation of the gas hydrate envelope. No visible changes in the gas hydrate powder were observed in the course of lifting it to a depth of 380 meters. Shallower than 380 meters, i.e., outside the zone of gas hydrate stability, decomposition of the gas hydrate powder into methane gas was observed.     

On the rates of sulfide formation and oxidation in the Black Sea during the cold season

The rate of the hydrogen sulfide oxidation in the redox zone of the Black Sea and the rate of the hydrogen sulfide formation due to bacterial sulfate reduction in the upper layer of the anaerobic waters were measured during the period of February–April 1991. The measurements were made using a sulfur radioisotope under conditions close to those in situ. It was established that the hydrogen sulfide is oxidized in the layer where oxygen and hydrogen sulfide coexist, which is under the upper boundary of the hydrogen sulfide layer. The maximum rate of the hydrogen sulfide oxidation was recorded within the limits of the density values δτ of 16.20–16.30, while varying in the layer from 2 to 4.5 μM/day. The average rate of the hydrogen sulfide oxidation was 1.5–3 times higher than that during the warm season. Sulfide formation was not observed at most of the stations in the examined lower portion of the pycnocline layer (140 to 400 m depths). Noticeable sulfate reduction was detected only at one station on the northwestern shelf. A probable reason for such noticeable changes in the sulfur dynamics in the water mass of the Black Sea may be the intensified hydrodynamics in the upper layers of the water mass during the cold season. The data suggesting that hydrogen sulfide oxidation proceeds under the hydrogen sulfide boundary indicate the absence of the so called “suboxic zone” in this basin.     

About transformation of the deep-water methane bubbles into hydrate powder and hydrate foam

During the Russian Academy of Sciences “MIRI na Baikale, 2008–2010” expedition, deep-water experiments with the bubbles of methane seeping from the bottom at depths 405, 860 and 1400 meters were carried out. These depths correspond to gas hydrate stability zone. Bubbles were caught by the trap which was looked like an inverted glass. It was found that the behavior of bubbles in a trap depends on the depth. At depth of 405 meters formation of hydrates was not observed. Having got to a trap at the depth of 860 meters, bubbles became covered by solid hydrate envelope, kept the initial form, and after a time period collapsed in a number of hydrate fragments which showed all properties of a granular matter. No visible changes in the hydrate granular matter were observed in the course of lifting it to a depth of 380 meters. Shallower, the decomposition of the hydrate granular matter into methane gas was observed. In the experiments at depth of 1400 meters the caught bubbles, becoming covered by hydrate envelope formed solid hydrate foam in the trap. At lifting this foam structure was deformed slightly but simultaneously a free gas left the foam and filled the trap. The volume of free gas in the trap at lifting varied according to the Boyle-Mariotte law.     

Intensification of Deep-Water Circulation in a Mesoscale Basin under the Influence of the Bottom Topography

It is generally accepted that the stationary wind circulation in the lower layer of a two-layer sea is negligibly weak even if the bottom topography is taken into account. This opinion is true for the oceanic scales where the bottom topography has no global slope along the meridian. However, in the case of circulation in closed and semiclosed seas, the trenchlike structure of the bottom topography together with the beta-effect can significantly affect the intensity of currents in the bottom layer of the sea. In the present work, we use a simple model to consider an example of intensification of deep-water wind-induced circulation under the influence of the meridional slope of the bottom. The discovered effect enables us, in particular, to make an assumption that the deep-water circulation in the Black Sea is much more intense than it was supposed earlier. __________ Translated from Morskoi Gidrofizicheskii Zhurnal, No. 2, pp. 3–10, March–April, 2005.     

Peculiarities of oceanographic processes in the north-western Black Sea

The paper examines how various factors affect the intensity of photosynthesis and oxidation reaction in the north-western part of the Black Sea. In the course of the evolution of the basin, natural aspects were widely changing; therefore their role is evaluated by the content of organic matter and diatoms in the bottom sediments. The influence of anthropogenic factors is determined by the current tendencies in the variation of physico-chemical and biological characteristics of the basin. The main factor upsetting steady functioning of the north-western Black Sea ecosystem is the large biogenic influx to the sea. The possible changing of the ecosystem during the nearest few decades is assessed. Translated by Vladimir A. Puchkin.     

Investigation of Transformations of the Local Regions of Pollution in the Northwest Part of the Black Sea Caused by the Motion of a Cyclone

By the method of mathematical simulation, we study the evolution of local discharges of pollutants on the northwest shelf of the Black Sea induced by the motion of a cyclone. We use the nonlinear equations of motion of a homogeneous viscous fluid in the hydrostatic approximation and the equation of turbulent diffusion. A cyclonic formation is represented by a moving axisymmetric area of low pressures. The discharges of pollutants are caused by the action of instantaneous sources located on the sea surface. We perform the analysis of dependences of the paths of motion of polluted regions, the periods of their dissipation, and the depths of penetration of pollutants on the bottom topography and the intensity of diffusion processes. __________ Translated from Morskoi Gidrofizicheskii Zhurnal, No. 6, pp. 17–27, November–December, 2005.     

Mud volcanoes and gas hydrates in the Black Sea: new data from Dvurechenskii and Odessa mud volcanoes

Meteor cruise M52/1 documented the presence of gas hydrates in sediments from mud volcanoes in the Sorokin Trough of the Black Sea. In a mud flow on the Odessa mud volcano, a carbonate crust currently forms in association with anaerobic methane oxidation. Dvurechenskii mud volcano (DMV), a flat-topped mud pie-type structure, appeared to be very active. Pore water in sediments of DMV is enriched in several constituents, such as ammonium and chloride, which seem to originate at depth. High sediment temperatures of up to 16.5 °C in close contact to the ambient bottom water of 9 °C also suggest strong advective transport of material from greater depth. Steep temperature gradients indicate a high fluid and/or mud flux within DMV, which is confirmed by the shape of the pore water profiles. Active fluid expulsion sites are evidenced by direct seafloor observation, and a potential flux of methane from the sediment to the bottom water is indicated by water-column methane measurements.     

A spectral model of underwater irradiance in the Black Sea

We develop a semiempirical spectral model of penetrating irradiance taking into account the biooptical characteristics of the Black Sea. The evaluation of the contributions of the principal optically active components to the total absorption of light in the sea shows that, in the short-wave range (400–500 nm), light is mainly absorbed by the dissolved organic matter (41–77%). The contribution of phytoplankton to the total absorption attains its maximum values (26–37%) in the abyssal part of the sea in the period of spring blooming of diatoms. In the coastal waters, the absorption of light by suspended nonalgae particles in summer is almost twice as intense (20–30%) as in the open sea (8–13%). The analysis of the sensitivity of our model shows that the absorption of light by dissolved organic matter is more significant for the estimation of the photosynthetically active radiation in the Black Sea than the concentration of pigments and backscattering of light by suspended particles. The comparison of the results of model computations with the data of measurements of the underwater irradiance reveals high accuracy of the proposed model.     

Bottom boundary layer in the Black Sea: formation of the stationary state

We make an attempt to answer the following question: how a natural stationary system formed by a layer and an interface “selects” a unique set of governing parameters from a great number of possible collections under the conditions of double-diffusion layer convection (e.g., for the bottom boundary layer in the Black Sea)? As the “rule of selection,” we use the principle of minimum entropy production for systems close to the state of thermodynamic equilibrium. In the process of solution of the problem, the system is regarded as a heat engine. The proposed approach is reduced to a simple procedure of application of the principle of minimum entropy production to the analyzed case. The combined analysis of the theoretical results, the data of deepwater field measurements in the Black Sea, and the results of laboratory experiments leads us to the conclusion that, most likely, the stationary system “selects” its governing parameters according to the Prigogine–Glansdorff principle. The density ratio (approximately equal to three for the stationary case) proves to be the key parameter of the system. Translated from Morskoi Gidrofizicheskii Zhurnal, No. 1, pp. 16–25, January–February, 2009.     

Sea-floor methane blow-out and global firestorm at the K–T boundary

 A previously unsuspected source of fuel for the global firestorm recorded by soot in the Cretaceous–Tertiary impact layer may have resided in methane gas associated with gas hydrate in the end-Cretaceous seafloor. End-Cretaceous impact-generated shock and megawaves would have had the potential to initiate worldwide oceanic methane gas blow-outs from these deposits. The methane would likely have ignited and incompletely combusted. This large burst of methane would have been followed by longer-term methane release as a part of a positive thermal feedback in the disturbed ocean-atmosphere system. Received: 16 September 1998 / revision received: 11 January 1999     

Authigenic carbonates from methane seeps of the Congo deep-sea fan

Submersible investigations with the ROV Victor 6000 of some pockmark structures on the seafloor of the Congo deep-sea fan have shown that they are active venting sites of methane-rich fluids, associated with abundant fauna and carbonate crusts. Moreover, methane hydrates have been observed both outcropping and deep in the sediments in the centre of the “Regab” giant pockmark. Authigenic carbonates, mostly calcite sometimes mixed with aragonite, are cementing the sedimentary matrix components and fauna; diatoms are abundant but only as moulds, indicating that biogenic silica dissolution occurred in situ synchronous with carbonate precipitation. The occurrence of diagenetic barite and pyrite in some carbonate crusts demonstrates that they can be formed either within the sulphate/methane transition zone or deeper in sulphate-depleted sediments. The oxygen isotopic compositions of the diagenetic carbonates (3.17–6.01‰ V-PDB) indicate that precipitation occurred with bottom seawater mixed with a variable contribution of water from gas hydrate decomposition. The very low carbon isotopic compositions of the diagenetic carbonates (−57.1 to −27.75‰ V-PDB) demonstrate that carbon derives mostly from the microbial oxidation of methane.     

硫酸盐-甲烷界面与甲烷通量及下伏水合物赋存的关系

硫酸盐-甲烷界面在富甲烷和含天然气水合物的海洋沉积区已经成为一个重要的生物地球化学识别边界.在硫酸盐-甲烷界面之上,沉积物中的硫酸盐因参与分解有机质和甲烷厌氧氧化反应而被消耗,而界面之下沉积物中的甲烷则不断生成,含量逐渐增加.根据该界面附近硫酸盐浓度和甲烷浓度的变化特征,可以判断该区甲烷流体通量的大小,从而指示下伏天然气水合物的可能赋存状况.南海北部陆坡的柱状沉积物孔隙水数据的分析显示,其硫酸盐-甲烷界面埋深比较浅,表明该海域的甲烷通量较高.这种高甲烷通量很可能是由下伏的天然气水合物所引起的,并暗示着该区下伏海底可能有天然气水合物沉积层赋存.     

Influx of Streaming Methane into Anoxic Waters of the Black Sea Basin

Oceanology - Based on long-term data on the spatial distribution and fluxes of streaming (bubbling) methane in the Black Sea, the study considers various types of streaming gas emissions...