Budget and Fluxes of Particulate Organic Carbon and Nitrogen According to the Data on Their Vertical Distribution in the Deep Part of the Black Sea

On the basis of the analysis of the many-year data on the vertical distributions of particulate organic carbon and nitrogen, we compute their annual average amounts for three typical layers of water in the deep part of the Black Sea: for a layer located above the oxycline and characterized by the formation of new portions of particulate organic matter in the course of photosynthesis, inside the oxycline, where the major part of oxygen is consumed and the major part of the flux of particulate organic matter is oxidized, and for the upper part of the anoxic zone characterized by the most active microbiological processes of oxidation of the organic substances and production of sulfides. The available literature data on sedimentation traps are used to study the downward annual average fluxes of particulate organic matter from the euphotic zone into the oxycline and into the anaerobic zone. The seasonal variability of the amounts and fluxes of particulate carbon and nitrogen is revealed.     

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.     

A physical–biochemical model of plankton productivity and nitrogen cycling in the Black Sea

A one-dimensional, vertically resolved, physical–biochemical upper ocean model is utilized to study plankton productivity and nitrogen cycling in the central Black Sea region characterized by cyclonic gyral circulation. The model is an extension of the one given by Oguz et al. (1996, J. Geophys. Res. 101, 16585–16599) with identical physical characteristics but incorporating a multi-component plankton structure in its biological module. Phytoplankton are represented by two groups, typifying diatoms and flagellates. Zooplankton are also separated into two groups: microzooplankton (nominally <200 μm) and mesozooplankton (0.2–2 mm). The other components of the biochemical model are detritus and nitrogen in the forms of nitrate and ammonium. The model incorporates, in addition to plankton productivity and organic matter generation, nitrogen remineralization (ammonification) and ammonium oxidation (nitrification) in the water column. Numerical simulations are described and compared with the available data from the central Black Sea. The main seasonal and vertical characteristics of phytoplankton and nutrient dynamics inferred from observations appear to be reasonably well represented by the model. Fractionation of the biotic community structure is shown to lead to increased plankton productivity during the summer period following the diatom-based early spring (March) bloom. The annual nitrogen budget for the euphotic zone reveals the substantial role of recycled nitrogen in the surface waters of the Black Sea.     

Mechanism of horizontal mass- and salt-exchange between the waters of continental slope and central part of the Black Sea

Estimation of horizontal salt transfer from the central part of the Black Sea to the continental slope is carried out based on numerical modeling. A box hydrodynamic model of the Black Sea based on the POM model is used. Horizontal transport caused by seasonal variability of vertical velocity in the central part of the Black Sea and horizontal turbulent diffusion is considered. In this work we demonstrate that, in the 0- to 30-m layer on average for the year, there is an outflow of salty water under the impact of both factors. In the surface layer, the transport caused by seasonal variability contributes significantly (30%). In the 80- to 150-m layer on average over the year, the waters from the continental slope arrive into the central part of the basin; they are characterized by higher salinity due to the inflow of the Marmara Sea waters across the Bosporus Strait.     

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.     

Modelling subsurface dynamics in the Black Sea

Dependency of major hydrophysical/chemical features of highly stratified basins on density surfaces in the vertical makes isopycnic models an attractive tool for simulating the dynamics of marginal marine environments such as the Black Sea because of the ability of these models to restrict vertical transport to some desirable degree. In the present work the seasonal variations of the subsurface dynamics of the Black Sea are investigated using an isopycnic model. Particular attention is given to the interfaces of the Cold intermediate layer and Suboxic layer and finally, the deep layer circulation in the basin is studied. It appears that although the depth range of the base of the Cold intermediate layer and the lower Soboxic layer interface do not change seasonally, their horizontal distribution is defined by the upper layer dynamics of the basin. Cyclonic surface circulation diminishes with increasing depth and the deep layer circulation is characterised by an anti-cyclonic rim current driven by density gradients created from river runoff and the influx of Mediterranean water.     

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.     

Sediment Flux of Particulate Organic Phosphorus in the Open Black Sea

The interannual variation of the monthly average (weighted average) concentrations of particulate organic phosphorus (P_POM) in the photosynthetic layer, oxycline, redox zone, and H₂S zone in the open Black Sea is estimated based on long-term observation data. The suspension sedimentation rates from the studied layers are assessed using model calculations and published data. The annual variation of P_POM sediment fluxes from the photosynthetic layer, oxycline, redox zone, and upper H₂S zone to the anaerobic zone of the sea and the correspondingly annual average values are estimated for the first time. A regular decrease in the P_POM annual average flux with depth in the upper active layer is demonstrated. A correlation between the annual average values of P_POM sediment flux from the photosynthetic layer and ascending phosphate flux to this layer is shown, which suggests their balance in the open sea. The results are discussed in terms of the phosphorus biogeochemical cycle and the concept of new and regenerative primary production in the open Black Sea.     

Ventilation of the Black Sea pycnocline. Parameterization of convection, numerical simulations and validations against observed chlorofluorocarbon data

Data from field observations and numerical model simulations are used to understand and quantify the pathways by which passive tracers penetrate into the Black Sea intermediate and deep layers. Chlorofluorocarbon (CFC) concentrations measured during the1988 R.V. Knorr cruise show strong decrease with increasing density in the Black Sea and illustrate the very slow rate of ventilation of deep water in this basin. We develop a 3D numerical model based on the Modular Ocean Model (MOM), and calibrate it in a way to produce consistent simulations of observed temperature, salinity and CFCs. One important feature is the implementation of a special parameterization for convection, which is an alternative of the convective adjustment in MOM and handles the penetration of the Bosporus plume into the halocline. The model forcing includes interannually variable wind, heat and water fluxes constructed from Comprehensive Ocean–Atmosphere Data Set and ECMWF atmospheric analysis data and river runoff data. The analysis of observations and simulated data are focused on correlations between thermohaline and tracer fields, dynamic control of ventilation, and the relative contributions of sources at the sea surface and outflow from the Bosporus Strait in the formation of intermediate and deep waters. A simple theory is developed which incorporates the outflow from the strait along with the vertical circulation (vertical turbulent mixing and Ekman upwelling) and reveals their mutual adjustment. The analyses of simulated and observed CFCs demonstrate that most of the CFC penetrating the deep layers has its source at the sea surface within the Black Sea rather than from the Marmara Sea via the Bosporus undercurrent. Under present-day conditions, the surface CFC signals have reached only the upper halocline. Intrusions below 600 m are not simulated. The major pathways of penetration of CFCs are associated with cold-water mass formation sites, Bosporus effluent, as well as with the diapycnal mixing in the area of Rim Current. Future CFC sampling strategies coherent with the unique conditions in the Black Sea are discussed.     

Reactive iron in Black Sea Sediments: implications for iron cycling

The distribution of reactive iron in sediments of the northwestern shelf, the shelf edge and the abyssal part of the Black Sea has been studied. In the euxinic Black Sea, iron sulfides (pyrite and iron monosulfide) are formed in the upper part of the anoxic water column and sink to the deep-sea floor where they are buried in the sediment. This flux of iron sulfides from the water column is reflected in enhanced concentrations of highly reactive iron and a high degree of pyritization (0.57–0.80) for the deep-water sediments of the Black Sea. The iron enrichment of deep-water sediments is balanced by a loss of highly reactive iron from the oxic continental shelf. Calculations from a numerical diagenetic model and reported in situ flux measurements indicate that the dissolved iron flux out of the shelf sediments is more than sufficient to balance the enrichment in reactive iron in deep-sea sediments, and that the majority of the dissolved iron efflux is redeposited on the continental shelf. This iron mobilization mechanism likely operates in most shelf areas, but its net effect becomes only apparent when reactive iron is trapped in sulfidic water bodies as iron sulfides or when iron is incompletely oxidized in low oxygen zones of the ocean and transported over long distances.     

Numerical modeling of hydrophysical fields of the Black Sea under the conditions of alternation of atmospheric circulation processes

The hydrological regime of the Black Sea in the conditions of permanent alternation of atmospheric circulation processes was investigated on the basis of a baroclinic prognostic model of the sea dynamics. In the model, variations in the wind action were expressed as permanent alternation of 24 wind types characteristic of the Black Sea basin throughout the year. Thermohaline impact of the atmosphere was taken into account by specifying the annual trends of temperature and salinity at the sea surface, which was established from multiyear means of these parameters. The problem was solved numerically on the basis of the method of two-cycle splitting with the use of the grid with a horizontal spacing of 5 km. Results of the numerical experiment showed that, under the influence of a strong nonstationarity of atmospheric processes, the water circulation in the upper layer of the Black Sea changes qualitatively and quantitatively. The upper 20–30-m layer of the sea is particularly sensitive to atmospheric circulation variations. For any character of atmospheric circulation, the Black Sea circulation below this layer is nearly always cyclonic with internal cyclonic rotations.     

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.     

Bottom boundary layer in the Black Sea: Experimental data,turbulent diffusion,and fluxes

Measurements of 14 vertical profiles of currents and hydrological parameters in the near-bottom layer with a depth resolution of 0.1 m were carried out in several regions of the Black Sea shelf, at five points over the continental slope, and in three deep water regions. The upper boundary of the benthic boundary layer (BBL) was reliably determined at a point at a distance from 5–7 to 35–40 m from the bottom where the gradients of the density and the velocity of the currents changed. The experimental data obtained were used to determine the coefficient of the bottom friction, the friction velocity, the coefficients of the vertical diffusion of momentum and density, and the vertical fluxes of temperature and salinity in the BBL.     

西北冰洋中太平洋入流水营养盐的变化特征

利用1999,2003和2008年夏季(7-9月)三次中国北极科学考察数据资料,分析和讨论太平洋入流水营养盐的分布和楚科奇海关键生物地球化学过程对太平洋水化学性质的改造.结果表明,2003和2008年在白令海峡南部64.3°N纬向断面(BS断面)由于水团性质差异显著,营养盐呈西高、东低的分布趋势.2003年BS断面水柱...     

On the nature of short-period oscillations of the main Black Sea pycnocline, submesoscale eddies, and response of the marine environment to the catastrophic shower of 2012

Field studies performed at the Shirshov Institute of Oceanology, Russian Academy of Sciences (SIO RAS), Black Sea hydrophysical polygon in 2012 are illustrated. The variations in the vertical distribution of the hydrophysical characteristics (water temperature, salinity, and density, as well as current velocity) in the upper 200-m layer of the Black Sea above the continental slope in the cold season, obtained using an Aqualog autonomous profiler on a moored buoy station, have been analyzed. It has been established that the position of the permanent pycno-halocline and the hydrosulphuric zone upper boundary intensively oscillate with a characteristic period of 5–10 days. These oscillations cause short-period variations in the thickness of the oxigenated layer by 20–40 m, which reaches one-third of the total thickness of the layer. Measurements performed with autonomous stations (bottom ADCP, thermochain) at the experimental subsatellite polygon in the Gelendzhik coastal zone, as well as meteorological, ship, and satellite data obtained during the catastrophic rains and flooding on July 6–7, 2012, and afterward, have been simultaneously analyzed. It has been established that a catastrophic flow of turbid fresh water into the sea caused the formation of a belt of freshened (by 1.0–2.7 psu) less dense water with a high suspension concentration on the shelf and the upper continental slope. This water formed a quasi-geostrophic northwestward along-shore current, the velocity of which reached 40–50 cm/s. Therefore, the freshened and turbid water mostly escaped from the Gelendzhik region northwestward for two days after the flood, and the remaining water became free of suspension owing to its settlement during approximately the same period. The fields of the current velocity and suspension concentration in a submesoscale cyclonic eddy, identified on the satellite image, were measured at the hydrophysical polygon. It has been established that a high (when compared to the background values) suspension concentration in the surface-water layer in an eddy is related to intense upwelling at the eddy center and the rising of suspension (apparently phytoplankton) from the thermocline layer, where the suspension concentration is maximal.     

The oxygen minimum zone in the Arabian Sea during 1995

This paper focuses on the characteristics of the oxygen minimum zone (OMZ) as observed in the Arabian Sea over the complete monsoon cycle of 1995. Dissolved oxygen, nitrite, nitrate and density values are used to delineate the OMZ, as well as identify regions where denitrification is observed. The suboxic conditions within the northern Arabian Sea are documented, as well as biological and chemical consequences of this phenomenon. Overall, the conditions found in the suboxic portion of the water column in the Arabian Sea were not greatly different from what has been reported in the literature with respect to oxygen, nitrate and nitrite distributions. Within the main thermocline, portions of the OMZ were found that were suboxic (oxygen less than ∼4.5 μM) and contained secondary nitrite maxima with concentrations that sometimes exceeded 6.0 μM, suggesting active nitrate reduction and denitrification. Although there may have been a reduction in the degree of suboxia during the Southwest monsoon, a dramatic seasonality was not observed, as has been suggested by some previous work. In particular, there was not much evidence for the occurrence of secondary nitrite maxima in waters with oxygen concentrations greater than 4.5 μM. Waters in the northern Arabian Sea appear to accumulate larger nitrate deficits due to longer residence times even though the denitrification rate might be lower, as evident in the reduced nitrite concentrations in the northern part of the basin. Organism distributions showed string relationships to the oxygen profiles, especially in locations where the OMZ was pronounced, but the biological responses to the OMZ varied with type of organism. The regional extent of intermediate nepheloid layers in our data corresponds well with the region of the secondary nitrite maximum. This is a region of denitrification, and the presence and activities of bacteria are assumed to cause the increase in particles. ADCP acoustic backscatter measurements show diel vertical migration of plankton or nekton and movement into the OMZ. Daytime acoustic returns from depth were strong, and the dawn sinking and dusk rise of the fauna were obvious. However, at night the biomass remaining in the suboxic zone was so low that no ADCP signal was detectable at these depths. There are at least two groups of organisms, one that stays in the upper mixed layer and another that makes daily excursions. A subsurface zooplankton peak in the lower OMZ (near the lower 4.5 μM oxycline) was also typically present; these animals occurred day and night and did not vertically migrate.     

Evidence for water exchange between the South China Sea and the Pacific Ocean through the Luzon Strait

An analysis of historical oxygen data provides evidence on the water exchange between theSouth China Sea (SCS) and the Pacific Ocean (PO). In the vicinity of the Luzon Strait (LS) , the dissolved oxygen concentration of sea water is found to be lower on the Pacific side than on the SCS side at depths between 700 and 1500 m (intermediate layer) , while the situation is reversed above 700 m (upper layer) and below 1 500 m (deep layer). The evidence suggests that water exits the SCS in the intermediate layer but enters it from the Pacific in both the upper and the deep layers, supporting the earlier speculation that the Luzon Strait transport has a sandwiched structure in the vertical. Within the SCS basin, the oxygen distribution indicates widespread vertical movement, including the upwelling in the intermediate layer and the downwelling in the deep layer.     

Using iron speciation in authigenic carbonates from hydrocarbon seeps to trace variable redox conditions

Seepage of hydrocarbon-rich fluids out of the marine sedimentary column is characterized by temporal changes of flow intensity and resultant spatially variable redox conditions. Authigenic carbonates at marine hydrocarbon seeps provide excellent geological and geochemical archives that serve to explore seepage dynamics over time. In this study, we investigated the potential of Mössbuaer spectroscopy and Fe contents of seep-related authigenic carbonates from the Congo Fan, the Gulf of Mexico, and the Black Sea for reconstructing past redox conditions and fluid seepage activity at cold seeps. The Fe speciation observed by Mössbauer spectroscopy and Fe contents suggest that (1) the Congo Fan carbonates precipitated in a sulfidic environment, (2) the formation conditions of seep carbonates were variable at the Gulf of Mexico seep site, ranging from oxic to suboxic and anoxic and even spanning into the methanogenic zone, and (3) the stratified water column of the Black Sea or suboxic condition resulted in low Fe contents of Black Sea carbonates. The study reveals that Fe speciation can provide constraints on the wide range of redox conditions that imprinted seep carbonates during the life span of seepage. Similarly, Mössbauer spectroscopy – particularly when used in combination with the analysis of redox-sensitive elements – is a promising tool to trace variable redox conditions in marine paleoenvironments other than seeps.