Fine hydrochemical structure of the redox zone in the black sea according to the results of measurements with an open oxygen sensor and with bottle samplers

This study is based on the data obtained on July 14–17, 2004 over a section across the coastal anticyclonic eddy near the town of Tuapse. The data on the oxygen distribution obtained by the Winkler procedure and with an open (membrane-free) sensor with a 15-cm spatial resolution were compared. The divergence of the results in the lower part of the oxycline was 3 μmol or lower; the sensor error was estimated as 1 μmol. The previous data on the absence of the layer of oxygen and hydrogen sulfide coexistence were confirmed. It was shown that the lower boundary of oxygen permeation (where its concentration appeared to be below 1.0 μmol) closely coincided with the upper boundary of the bivalent manganese occurrence, because their coexistence layer was 1 ± 1 m in thickness. Within the same layer or 1–2 m above it, the upper boundary of the deep-sea ammonium and the phosphate minimum were situated. These data may be treated in the following way: oxygen is completely utilized for the oxidation of ammonia and bivalent manganese, and the oxidized suspended manganese formed sinks and is utilized in the reactions of hydrogen sulfide oxidation. The role of the near-slope downwelling (the supply of oxygen-enriched waters to the layers close to the redox zone), which was also observed in the cross section, is discussed.     

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.     

Study of the influence of mud volcanoes on the contents of hydrogen sulfide and silicic acid in the Black Sea

We consider data on the contents of hydrogen sulfide and silicic acid in waters of the Black Sea obtained in the course of field investigations during Cruise 4 of the R/VKiev. The special attention is given to the results of studying the influence of recently opened mud volcano manifestations on the chemical composition of benthic waters and, first of all, on the content of hydrogen sulfide. The data concerning the contents of hydrogen sulfide and silicic acid and the analysis of the relative chemical composition indicate the absence of any direct influence of mud volcanoes on the chemical composition of waters. At the same time, the investigations carried out confirmed the possibility of presence of distributed sources of hydrogen sulfide, which is a product of bacterial utilization of incoming hydrocarbons.     

Water circulation and hydrological structure in the active layer of the 50-mile near-shore zone of the Russian sector of the black sea in August 2004

The water dynamics and hydrological structure in the active (oxygen-containing) layer are considered on the basis of the hydrological survey carried out in the 50-mile near-shore zone of the Russian sector of the Black Sea in August 2004 and over the permanent section from Gelendzhik to the central part of the sea. Five mesoscale eddy structures of different signs were observed in the Main Black Sea Current between Sochi and the Kerch Strait. Such a dynamic situation contributed to the intensive horizontal water exchange between the near-shore and open sea waters as well as to the redistribution of water masses over the vertical in the active sea layer, which is indicated by the deepening of the top boundary of the hydrogen sulfide zone in the Russian sector of the sea by 15–20 m.     

The distribution of oxygen and hydrogen sulfide in Black Sea waters during winter-spring period

Regularities of oxygen and hydrogen sulfide are examined, using the method of spatial isopycnic analysis. The contribution that ventilation of winter-time surface waters over the domes of cyclonic gyres makes to the transfer of O₂ towards the upper boundary of the constant pycnocline and to the oxicline layer is demonstrated. The paper provides spatial scales of this phenomenon and indicates the areas where the upper boundary of the anoxic layer in the Black Sea, relative to the conventional density, is located much higher compared with the rest of the sea. The suboxic zone is shown to be a specific feature of the O₂/H₂S distribution in the Black Sea waters, typical, at least, of the northern part of the basin. Analysis of the suboxic spatial variability in the vertical has been conducted. Translated by Vladimir A. Puchkin.     

Use of a problem-oriented database for statistical analysis of the hydrochemical characteristics of the redox layer of the Black Sea

A problem-oriented database (PODB) was developed for the statistical analysis of the hydrochemical characteristics of the redox layer of the Black Sea. The paper describes the features of the PODB, including the algorithm of interpolation and the location of the levels of the appearance and disappearance of hydrochemical parameters (the so-called onset levels) based on Akima’s spline. The application of the PODB allowed us to obtain a series of biogeochemical estimates, in particular, (1) to reveal the constancy of the vertical gradients of the hydrochemical parameters at selected density levels; (2) to calculate the seasonal variability of the degree of manifestation of the phosphate minimum in the near-shore and open sea areas; and (3) to calculate the interannual variability of the level of the disappearance of the hydrogen sulfide, ammonium, total manganese, and methane, as well as the oxygen content in the cold intermediate layer, and to find out their relations with the climatic variations.     

Determination of the reduced sulfur species in the anoxic zone of the Black Sea: A comparison of the spectrophotometry and iodometry techniques

The report presents the results of the studies of the reduced sulfur species in the water of the anoxic zone of the Black Sea. The content of hydrogen sulfide was determined by means of spectrophotometry using dilution with oxygen-free distilled water. The detection limit of the H₂S amounted to 0.3 μM with the method’s precision below 3%. The accuracy of the spectrophotometric determination was verified by iodometry after the fixation of the hydrogen sulfide in zinc acetate under the distillation with argon from the acidified seawater sample.     

Spectrophotometric and iodometric methods for the detection of hydrogen sulfide in the Black Sea: Comparison of the results of analysis

We discuss the results of analysis of published data and field and laboratory investigations aimed at the solution of the problem of comparabilityof the results of detection of hydrogen sulfide in the Black Sea by spectrophotometric and iodometric methods. We show that the reproducibility of the results of spectrophotometric analysis is higher than hat of the iodometric method only in the case where the content of sulfide in a sample is less than 10–30 μMl⁻¹. When larger concentrations are involved, the traditional iodometric analysis proves to be the most precise and reliable method for the detection of hydrogen sulfide in the Black-Sea waters. Translated by Peter V. Malyshev and Dmitry V. Malyshev     

A sulfur budget for the Black Sea anoxic zone

A budget for the sulfur cycle in the Black Sea is proposed which incorporates specific biogeochemical process rates. The average sulfide production in the water column is estimated to be 30–50 Tg yr⁻¹, occurring essentially in the layer between 500 and 2000 m. About 3.2–5.2 Tg sulfide yr⁻¹ form during sulfate reduction in surface sediments of the anoxic zone. Total sulfur burial in anoxic sediments of 1 Tg yr⁻¹ consists of 10–70% (ca. 40–50% is the average) water column formed (syngenetic) component, the rest being diagenetic pyrite. As a maximum, between 3 and 5 Tg yr⁻¹ contribute sulfide to the bottom water or diffuse downward in the sediment. About 20–50 Tg yr⁻¹ sulfide is oxidized mostly at the chemocline and about 10–20% of this amount (4.4–9.2 Tg yr⁻¹) below the chemocline by the oxygen of the Lower Bosphorus Current. A model simulating the vertical distribution of sulfide in the Black Sea water column shows net consumption in the upper layers down to ca. 500 m, essentially due to oxidation at the chemocline, and net production down to the bottom. On the basis of the calculated budget anoxic conditions in the Black Sea are sustained by the balance between sulfide production in the anoxic water column and oxidation at the chemocline. On average the residence time of sulfide in the anoxic zone is about 90–150 yr, comparable to the water exchange time between oxic and anoxic zones. Hydrophysical control on the sulfur cycle appears to be the main factor regulating the extent of anoxic conditions in the Black Sea water column, rather than rates of biogeochemical processes.     

Stable isotope composition of dissolved sulfate and hydrogen sulfide in the Black Sea

The stable isotope ratio of sulfur (³⁴S/³²S) in dissolved sulfate and hydrogen sulfide was measured for 20 water samples from two deep hydrocasts from the south-central Black Sea. The isotope ratio of total reduced sulfur was also measured for surface sediment collected below each hydrocast. The range in the δ³⁴S measurements for sulfate was +18.20 to $\text{+20.17‰}$ and for hydrogen sulfide ?38.71 to $\text{?4.85‰}$. The distribution pattern for δ ³⁴S in both sulfate and sulfide appears to be the result of in situ sulfate reduction.     

Inter-annual variability of the hydrological-chemical characteristics in the north-western Black Sea shelf area over recent decades (1960–1990)

A number of significant hydrologicla and chemical parameters of the Black Sea shelf area have been identified and investigated: the inter-annual variability of the mean annual and seasonal (winter and summer) values of the air and seawater temperatures and salinity in the vicinity of Odessa; silicon and phosphate concentrations averaged over the north-western Black Sea shelf area (NWBS); discharges of the Danube and Dnieper rivers; mean surface and near-bottom oxygen concentrations in summer acquired through the analysis of the summarized data collected at three oceanographic stations occupied between Bolshoi Fontan and the Tendra isthmus from the early 1950s up to date. From these data, the variability of the areas subject to oxygen deficit in the near-bottom layer of the NWBS shelf during the summers of 1973–1994 has been assessed. Predominant oscillation periods and trends in the inter-annual variability of the studied parameters have been revealed and their relationship and climatic dependence has been demonstrated. Translated by Vladimir A. Puchkin.     

Suboxic and Anoxic Conditions of Deep Waters in the Gdansk Basin of the Baltic Sea

Oceanology - Long-term variability of the concentrations of oxygen and hydrogen sulfide in deep waters of the Gdansk Basin of the Baltic Sea for the last 25 years was considered. The dependence of...     

Vertical distribution of the hydrogen sulphide sources in the Black Sea

Data on the diffusion coefficientK _zand the concentration of H₂S in the Black Sea are used to compute the depth distribution of the vertical flux, and the intensity of the H₂S sources and sinks. On average, the total production of H₂S in the Black Sea reaches 37×10⁶ t/year. The main bulk of H₂S is produced not at the bottom, but in the layer of 450–1300 m. Destruction of H₂S prevails above the 400 m layer. Dissolved oxygen penetrating the H₂S zone can oxidize only half of the hydrogen sulphide produced in the sea.Translated by Mikhail M. Trufanov.     

Seasonal and long-term dynamics of the chlorophyll concentration in the Black Sea according to satellite observations

The spatial and temporal variability of the chlorophyll (Chl) concentration in the surface water layer of the Black Sea in 1998–2008 has been analyzed using the data obtained by the SeaWiFS satellite sensor. In the deep-sea areas, the seasonal pattern of the Chl concentration is represented by a U-shape curve. The maximal concentrations are observed in the winter-spring and autumn periods, while the minimal, in the summertime. In the northwestern Black Sea, the maximal concentrations are registered in mostly the summer and autumn periods. Pronounced interannual variability is found for the summer concentrations of Chl observed for an 11-year period. After a cold winter, the concentration of Chl in the spring period is 3–5 times higher compared to the mild-winter years. In December–March, a negative correlation between the water temperature and the average Chl concentration is registered.     

Transformations in the Caspian Sea ecosystem under the fall and rise of the sea level

By the data of the surveys performed in 1976, 1983, and annually from 1995 to 2006, the successive transformations of the ecosystem were traced in the Central and Southern Caspian Sea: from the anoxic conditions in 1933–1934 to the oxic state during the low level period in 1978, then the hypoxia increase and accumulation of nutrients registered anew after 1995, and to the hydrogen sulfide appearance in the near-bottom layers of the Southern Caspian Sea. Some irreversible changes in the silicon distribution in the Central Caspian Sea were revealed (the concentration increase to 217 μM at the depth of 780 m, which considerably exceeded the absolute values for the World Ocean).     

Simulation of the impact of internal waves on the oxygen/hydrogen sulphide co-existence zone in the open part of the Black Sea

This paper focuses on the impact of periodic internal waves on the oxygen/hydrogen sulphide co-existence zone in the open Black Sea. The numerical model is based on a set of transport/diffusion equations governing the evolution of oxygen/hydrogen sulphide concentrations and considering the reaction between them. The wave velocity field is determined by solving a spectrum problem for preinertial period internal waves, using the characteristic vertical density profile for summertime. Via analysis of the model data, the influence of wave characteristics on the parameters of the O₂/H₂S co-existence zone has been assessed.Translated by Vladimir A. Puchkin.     

Study of long-term variations in the Black Sea fields using an interdisciplinary physical and biogeochemical model

Using an interdisciplinary three-dimensional physical and biogeochemical model developed for the Black Sea, the long-term evolution of marine dynamics and ecosystem is investigated. The hydrophysical fields were calculated from a model of Black Sea circulation with assimilation of hydrographic survey and satellite measurement data from 1971 to 2001. The circulation model reproduces well processes of various scales in both space and time (particularly the seasonal course and interannual variability of main hydrophysical fields). The resulting flow fields are then used to calculate the long-term evolution of the components of the lower level of the food chain in the Black Sea ecosystem. The biogeochemical model used in the calculations is based on the nitrogen cycle and includes a parameterization of the main biological and chemical interactions and processes in the upper layer of the Black Sea. The numerical experiments indicated that the biogeochemical component of the model rather successfully reproduces the main features and evolution trends in the Black Sea ecosystem for the period under consideration: the growth in the phytoplankton biomass during eutrophication and changes in seasonal cycles of the main ecosystem components. Also, the hydrophysical processes were shown to be important for a reliable reproduction of long-term changes in the ecosystem.     

Interannual variations of the renewal of waters of the cold intermediate layer in the Black Sea for the last decades

On the basis of the archival hydrological data, recently accumulated data of buoy profilometers, and the array of satellite data on sea-surface temperature, we estimate the interannual variability of the renewal of waters of the cold intermediate layer in the Black Sea for the last two decades. We propose a procedure of estimation of the intensity of renewal of waters for this layer according to the data on sea-surface temperature in the winter period without taking into account the vertical profiles of temperature. It is indicated that the last 50 yr are characterized by the alternation of periods with different intensities of ventilation of the upper layer of the sea. We analyze the correlations between the temperature of waters of the cold intermediate layer and the winter temperature of air (according to the data of meteorological stations located on the north coast of the Black Sea) and the level of salinity in the surface layer of the sea. The role played by the last factor is emphasized.     

Anoxia in waters of the Maikop paleobasin (Tethys Ocean,Azeri sector), with implications for the modern Caspian Sea

Anoxic conditions and contamination of the water column by hydrogen sulfide can be observed in several areas of the world ocean, particularly in intracontinental basins like the Black Sea. Study of the geologic, geochemical and paleontological features of the Tertiary Maikop paleobasin of the Tethys Ocean, which at its maximum development covered a large area from the Carpathians to western Turkmenistan and included part of the modern territory of Azerbaijan, shows that the processes taking place currently in some intracontinental basins like the Black Sea were also typical for some areas of this paleo-ocean. The development of anoxic environments and contamination by H₂S in this basin depended upon many parameters, including basin closure, the quantity and type of organic matter deposited, complexities of the water cycle, and a strongly reducing environment. Characteristics of Maikop deposits indicate some similarities between anoxic conditions and mass mortalities in the Maikop paleobasin and in parts of the modern Caspian Sea, suggesting potential dangers for the latter.     

On the problem of a turbid layer occurring near the upper boundary of the hydrogen sulphide zone in the Black Sea

This paper considers the possibility of aldehyde reduction by Black Sea hydrogen sulphide accompanied by the formation of nitrogen-containing organic substances The fine structure of nitrates and ammonium profiles close to the boundary of the O₂/H₂S zone is studied. Using data collected during Cruise 18 of the R/VProfessor Kolesnikov, it is shown that the largest ammonium concentration gradient is centred 20 m below the maximum of nitrate concentration. It has been deduced that an appreciable part of the so-called chemosynthesis production may be a result of the non-biotic synthesis with involvement of hydrogen sulphide.Translated by V. Puchkin.