Acoustic investigations of mud volcanoes in the Sorokin Trough, Black Sea

The Sorokin Trough (Black Sea) is characterized by diapiric structures formed in a compressional tectonic regime that facilitate fluid migration to the seafloor. We present acoustic data in order to image details of mud volcanoes associated with the diapirs. Three types of mud volcanoes were distinguished: cone-shaped, flat-topped, and collapsed structures. All mud volcanoes, except for the Kazakov mud volcano, are located above shallow mud diapirs and diapiric ridges. Beyond the known near-surface occurrence of gas hydrates, bottom simulating reflectors are not seen on our seismic records, but pronounced lateral amplitude variations and bright spots may indicate the presence of gas hydrates and free gas.     

Upwelling in the north-western Black Sea during the period of summer-time warming

A numerical multilevel model based on primitive equations of sea thermohydrodynamics is applied to investigate the development of near-shore upwelling zones in the north-western Black Sea. Analysed are the results of eight numerical experiments on mapping the areas of upwelling and downwelling, depending on wind direction. The acquired data are matched up with the observations by the NOAA-11 satellite designed to measure sea surface radiation temperature twice a day. The numerical calculations are shown to agree with the remotely-sensed data. Translated by Vladimir A. Puchkin.     

Application of the Altimetry Data to the Analysis of Water Balance of the Black Sea

We compare the estimates of the flow rates for the Upper and Lower Bosporus Currents obtained by using both the altimetry data on the Black-Sea level and the data of the coastal stations. Earlier, in analyzing the water exchange, it was customary to use the mean sea level computed according to the data of coastal stations located along the coast of the former Soviet Union. It is shown that the seasonal and annual variations of the increments of the mean sea level are qualitatively similar for the data of both types. At the same time, the values of the amplitudes of seasonal variations are noticeably different. The use of the increments of the mean sea level computed according to the altimetry data leads to insignificant changes in the annual water exchange through Bosporus as compared with the values obtained by using the coastal data. However, the seasonal variations of these data are somewhat different. We propose an algorithm for the evaluation of increments of the sea level, which, in authors' opinion, decreases the error in the analysis of water balance.     

A two-dimensional numerical model of tidal motions on the shelf of Guinea

The general statement of a non-linear problem in modelling a barotropic tide for the limited part of a real shelf and the problems related to artificial division of the computation area in the ocean by a liquid boundary are considered. Linear combinations of various types of boundary conditions and analytical expressions are used to describe the tidal wave structure at the liquid boundaries. The results of test computations for basins with the simplest outlines are given. The model is used to reconstruct the spatial distribution of characteristics of theM ₂ tide on the shelf of Guinea. The methods of computation and specification of unambiguously determined model parameters are described. Joint analysis of the modelling results and the field data has been carried out. The tidal map of theM ₂ wave is plotted for the area under investigation.Translated by Mikhail M. Trufanov.     

Characterization of natural hydrocarbon seepage in the South Caspian Sea off Iran using satellite SAR and geological data

Abstract

Two groups of oil slicks have been repeatedly detected in the Southern Caspian Sea, off Cape Sefidrud and close to the Iranian coast of the sea on the synthetic aperture radar (SAR) images acquired by the Sentinel-1A and Sentinel-1B satellites in April–June 2018. Analysis of the SAR images together with bathymetry, geological and geophysical data in geographic information system showed that these manifestations (oil slicks) visible on the sea surface have natural origin, related to the bottom seepage phenomenon and are associated with existing hydrocarbon system. Oil slicks are concentrated over the continental slope and related to local geological formations of the sedimentary cover having oil deposits related to paleo-delta of Sefidrud River. Our evidence shows that two seeps in this area are active and repeating. It is concluded that the SAR is an excellent tool for monitoring of seepage phenomenon. Moreover, results indicate that the use of remote methods to identify offshore natural oil seeps related to existing active petroleum system can be considered as a well-accepted approach to support oil and gas exploration in frontier areas such as the South Caspian Basin.     

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.     

Cascades of dense water around the world ocean

Dense water overflow off continental shelves (cascading) is one of the contributing processes of shelf-deep ocean exchange, and of topical interest to climate studies and nutrient fluxes. Dense water originating from cooling, evaporation, freezing and salinization on a shallow shelf spills over the shelf edge and may develop as near-bottom gravity current or an intermediate-depth intrusion. It is difficult to observe in nature due to its intermittent character.This paper provides an extensive inventory of observed cases of water cascades around the World Ocean, summarises their locations and individual properties, and provides statistics of the identified cases. The search for cascading was carried out using oceanographic databases and a literature review. This study identified 61 confirmed cases world-wide, including 25 cases in the Arctic seas, 12 at mid-latitudes, seven in sub-tropical and tropical regions, and 17 off the Antarctic shelves. Eighteen cascades had not been reported before. We analyze a set of numerical parameters of dense water cascades, allowing us to quantify, compare and contrast the properties of water cascades. The overall average density contrast between the confirmed cascades and ambient water is 0.37 (kg/m³); it can be as much as 2 (kg/m³) on some Arctic shelves. Frequently initiated by strong cooling at the surface, cascades often remain colder through the descent, thus supplying the deep ocean with colder and fresher water. In non-dimensional variables, the data from all climate zones fit well to a unique curve, which represents a relationship between a cascade’s internal structure and the parameters describing its forcing. On average, the down-slope volumetric flux provided by dense water cascades is estimated as 0.05 to 0.08 Sv per 100 km of shelf edge.Regional terms: Arctic, Antarctic, North Atlantic Ocean, Barents Sea, Mediterranean Sea, Skagerrak, Tasman Sea, Sea of Okhotsk     

Dynamics of the Crimea shelf waters in summer

In this paper, the main results of field research into the seawater dynamics on the shelf of the Crimea in the summer of 1991 are reported. It is shown that in the weather frequency band, the oscillations with periods of 11–12 days have the greatest amplitude. These represent coastal-trapped waves with a spatial scale of the order of the length of the Black Sea coastline. These waves are generated by distant winds and significantly disturb the response of the Crimea shelf waters to local wind forcing. A transient upwelling-downwelling circulation having a period of 5–7 days is induced by local winds and is not accompanied by the generation of trapped waves at the frequency of forcing. Since the average circulation on the shelf of the Crimea is of cyclonic character, downwelling events are found more frequently and have longer time periods than the upwelling events. The wind-generated trapped waves on the shelves of the Crimea and Kerch Peninsula have an average period of 27 h. The inertial oscillations were well expressed during an experiment resembling long-wave motions.Translated by Mikhail M. Trufanov.     

On the variability of the high-frequency part of the spectrum of wind-forced sea waves

Data on the temporal variability of sea wave spectral components in the frequency range 1–8 Hz, collected by a drifting vessel in the Pacific ocean (wind speed 1–10 m/s), are discussed in this paper. For the frequency range 3–6 Hz (wind speed 5–8 m/s), a weak variability of the ripples is observed, synchronous with long waves; in the remaining part of the spectral range studied the fluctuations are fortuitous. It is concluded that the wind plays a crucial role in forming the ripples' fluctuation characteristics in the high-frequency part of the spectrum.Translated by Vladimir A. Puchkin.