Western Black Sea Shelf Research Articles

Long-term Variations in the Black Sea Population of Smooth Scallop, Flexopecten glaber (Linnaeus, 1758) (Bivalvia: Pectinidae): A Review

Data on the presence and characteristics of smooth scallop Flexopecten glaber (Linnaeus, 1758) aggregations in various parts of the Black Sea shelf (BSS) have been subjected to a retrospective analysis. The waters of the northern (Crimean coast), eastern (Central and North Caucasian), and Anatolian sectors of BSS can be considered the most favorable habitats for smooth scallop, where massive beds of this species were found in the first half of the 20th century. However, no such data are available for the northwestern and western BSS, where freshwater discharges from the Danube, Dniester, and Dnieper form a permanent zone of lower water salinity. The period of decline in the Black Sea scallop population recorded in the 1970s–1990s generally coincided with the well known ecological crisis due to the anthropogenic eutrophication of the Black Sea in the second half of the 20th century. In the late 2000s and the 2010s, the scallop beds in the Crimean BSS showed a tendency to recover, which may be associated with the general improvement of the ecological condition of the Black Sea basin. A similar recovery of scallop beds is expected to occur in the eastern (North and Central Caucasian) and southern BSS, but has not yet been recorded. The results obtained suggest a possibility of downgrading the conservation status of this species in the Red Data Books of the City of Sevastopol and the Republic of Crimea from the `species declining in abundance` to the `species recovering in abundance and distribution`.

Temporal changes of phytoplankton biomass in the western Black Sea shelf waters: Evaluation by satellite data (1998–2018)

This study investigated temporal changes of phytoplankton biomass with particular focus on blooms and their underlying mechanisms within different shelf regions in the western part of the Black Sea for the poorly studied post-eutrophication period. We combined satellite chlorophyll with carbon-to-Chl a ratios to obtain a better proxy of phytoplankton biomass, and applied an algorithm to partition these data into a base seasonal variation and blooms. The base phytoplankton biomass was characterized by a seasonal maximum in October–November, highest values in the Romanian inner shelf region that receives inputs directly from the Danube River, and downward time trends in all shelf regions, coinciding with reduced nitrate inputs from the Danube. The majority of phytoplankton blooms were short-lived events (lasting up to 5 days), which accounted for over 60% of total bloom events. The initiation of spring blooms coincided with the peak riverine input for the Romanian inner shelf and with the formation of the thermocline in other regions. Autumn blooms were observed with the breakdown of the thermocline in October–November. These were most likely sustained by nutrient accumulation in bottom waters originating from riverine inputs in spring and regenerated nutrients from sedimenting organic matter and sediments. Large diatoms may contribute to autumn blooms in the outer shelf regions through harvesting “new” nitrate from deeper waters and hosting endosymbiotic cyanobacteria to extract “new” nitrogen from the atmosphere. Finally, high intensity of upwelling processes could support winter blooms in the Bulgarian outer shelf region.

USE OF X-RAY MICRO-COMPUTED TOMOGRAPHY ON SELECTED UPPER TRIASSIC (RHAETIAN) FORAMINIFERA FROM THE WESTERN BLACK SEA SHELF, OFFSHORE ROMANIA

Upper Triassic (Rhaetian) foraminifera belonging to the species Glomospira charoides (Jones & Parker), Gaudryinopsis kelleri (Tappan), G. triadica (Kristan-Tollmann), Ammobaculites zlambachensis Kristan-Tollmann, Verneuilinoides racema (Trifonova), and Trochammina cf. jaunensis Brönnimann & Page were investigated using X-ray micro-computed tomography. Foraminifera were recovered from the drill core CM31 of the 817 Lebăda Vest borehole, located off the coast of Romania on the western Black Sea shelf, from depths of 2623 m to 2625 m. Tomographic data was used to generate digital models, which were then virtually sectioned in desirable ways. The acquired transects can be used for comparison with specimens viewed in thin sections, providing a better connection between specimens recovered from residues and those observed in thin sections using transmitted light microscopy.

Characterization of Pliocene Biogenic Gas Reservoirs from the Western Black Sea Shelf (Romanian Offshore) by Integration of Well Logs and Core Data

The successful interpretation of open-hole well logging data relies on jointly using all available petrophysical and geological information. This paper presents relevant case studies related to the integration of well logs with core measurements for exploration wells drilled in the Romanian continental shelf area of the Western Black Sea basin. The analyzed wells targeted gas-bearing sands and silts complexes of Early Pliocene (Dacian) age, developed in a deltaic to shallow marine sedimentary environment in two distinct fields. The wireline logging programs included conventional formation evaluation logs, pressure surveys, nuclear magnetic resonance, and borehole electrical imaging logs. The core dataset comprised routine and special measurements (porosity, grain density, permeability, water saturation, and Archie parameters) carried out at quasi-reservoir confining pressure. The wireline logging suites were interpreted via a deterministic workflow, including core-derived interpretation parameters. Other core-derived parameters were used for constraining and validating the log interpretations. The results show that a problem related to the ambiguity of formation water resistivity can be overcome through resistivity–porosity dependencies constructed to include potential aquifer zones in the proximity of the Dacian gas-bearing reservoirs. This study also revealed and quantified uncertainties regarding the estimation of gas–water contacts from formation pressure surveys, which can be mitigated by the confirmation or correction of pressure-derived fluid contacts via the well log interpretation results. Lastly, we identified a probable resistivity logs suppression effect related both to high contents of capillary-bound water and also to the limited resolution of electrical logging tools in the presence of sand-shale thin bedding or laminations.

EXTENSIONAL OVERPRINTS IN THE LOWER CRUSTAL ARCHITECTURE, INSIGHTS FROM GRAVITY MODELLING OF THE ROMANIAN SHELF

The structure of the Romanian Western Black Sea shelf has been studied for the last 50 years. This paper aims to give a new interpretation, highlight a new vision regarding the lower crust architecture by combining the known local data with new measurements. In 2005, a new set of gravimetric data was collected by the Romanian National Institute of Marine Geology and Geo-ecology (GeoEcoMar), in the project Marine Geohazard, financed by the EU, which summed up to thousands of gravimetric profiles on the Romanian and Bulgarian offshore. By combining the new data with the known geological setting, a new vision emerged regarding the understanding of the complex structural dynamics of the deep shelf area and the geomorphology of the deep geological structure. For this investigation, a set of 3 profiles have been constructed as a starting point for the geological structure. In the process of inserting data, we had to take into consideration all the implicit information available from the local wells that are considered a secure source of upperstructure information. Once the model obtained, the image showed different areas where the Western Black Sea back-arc opening of the basin showed itsmarks, by presenting an unexpectedly complex architecture, considering all the constraints and local limitations.

Heavy Metalspatial Distribution and Pollution Assessment in the Surface Sediments of the North � Western Black Sea Shelf

22 surface sediment samples were collected in August 2018 from the Romanian inner shelf (Nord-Western Black Sea). Concentrations of some metals (Al, Cr, Cu, Ni, Zn, As, Pb, and Hg), TOC content, and grain size of sediment samples were determined by specific techniques. The order of accumulation of heavy metals was Zn]Cr]Ni]Cu]Pb]As]Hg. Multivariate analysis indicated that As, Ni, Cu, Zn, Pb, and Hg concentrations had similar behavior and they were positively correlated with the clay content, whereas Al and Cr concentrations presented close patterns and they were negatively correlated with the water depth. Sediment pollution assessment indices (enrichment factor, contamination factor, and geo-accumulation index) suggested no/low pollution for most of the metals analyzed, excepting for Pb and Hg (moderate pollution). Values of pollution indices highlighted a higher sediment pollution with Pb and Hg along the Danube�s plume direction, in the oil platform area (eastern edge of the Portita Bay), and partially in the Constanta and Mangalia area, suggesting the influence of port activities, tourism, urban wastewater discharges, oil and gas extraction.

Formation evaluation challenges in Pliocene gas-bearing reservoirs from the Romanian Western Black Sea shelf

This paper addresses some formation evaluation challenges and petrophysical particularities regarding two gas fields of Early Pliocene age, belonging to the biogenic hydrocarbon system of Western Black Sea Basin - Romanian continental shelf. Although these structures are located at the same depth and only 15 km apart, the wells that intercepted the sands and silts gas-bearing reservoirs indicate an important lateral facies variation and different reservoir qualities. We analyzed and interpreted data from exploration and appraisal wells that targeted these reservoirs, showing that: (1) there is a limited radioactivity contrast between the reservoir and nonreservoir intervals, so a clay volume determination based solely on the gamma ray log is not practical; (2) the reservoirs are characterized by high capillary-bound water contents, leading sometimes to abnormally low resistivity readings; (3) an additional resistivity suppression might be caused by the limited vertical resolution of the electrical logging tools, in the presence of thinly laminated sand - shale intervals; (4) the identification of gas-water contacts based exclusively on pressure gradients may be inaccurate and should be checked against the results of conventional geophysical logs interpretation and of nuclear magnetic resonance logs, for delineating the intervals with bound water or with movable fluids.

Source potential and depositional environment of Oligocene and Miocene rocks offshore Bulgaria

Abstract Oligo-Miocene (‘Maikopian’) deposits are considered the main source rocks in the Black Sea area, although only a few source-rock data are available. Geochemical logs from nine wells are used together with age constraints provided by calcareous nannoplankton, well and seismic data to determine vertical and lateral changes of the source potential. Oligocene rocks overlie Eocene deposits with a major unconformity on the western Black Sea shelf in Bulgaria. A west–east-trending erosional structure (the Kaliakra canyon) developed during Lower Oligocene time and was filled with Oligo-Miocene deposits. Potential source rocks are present in different stratigraphic units, but the most prolific intervals accumulated during time intervals when the isolation of the Paratethys resulted in oxygen-depleted, brackish environments with high bioproductivity. These include Lower Solenovian rocks related to blooms of calcareous nannoplankton, which form an extensive layer outside the Kaliakra canyon. This unit hosts a good potential to generate oil and gas. Diatom-rich, very good oil-prone source rocks accumulated during a second isolation event in the Kozakhurian. Thick sections of these diatom-rich rocks occur within the canyon and are present in thin layers outside of it. High productivity of siliceous organisms is attributed to upwelling within the canyon. All studied units are thermally immature on the shelf.

Metal Pollution in Water, Mussel and Surface Sediments of the Western Black Sea Shelf, Turkey

Ozet Bati Karadeniz Selfi'nde Agustos 2003 ve Haziran 2004 olmak uzere iki donemde 14 istasyonda su, midye ve yuzey sedimenti orneklemeleri yapilmistir. Bu orneklerde secilen bazi metallerin (Al, Fe, Mn, Pb, Cu, Zn, Ni, Cd, Cr ve Hg ) icerikleri incelenmistir. Yuzey sedimentlerinde Pb, Cd, Hg ve Cr icerikleri seyl ortalamasinin uzerinde bulunmustur. Bu metallerin degisim araligi Pb icin 10-56 μg/g, Cd icin 0,91-2,7 μg/g, Hg icin 69-362 ng/g ve Cr icin 61-110 μg/g olarak belirlenmistir. Yuzey sedimentlerindeki yuksek toplam organik karbon (TOK) ve cozunmus hidrojen sulfur (CHS) icerikleri de bu sonuclari desteklemektedir. TOK degerlerinin degisim araligi % 1,11-4,11 olarak belirlenirken, CHS iceriklerinin degisim araligi 31,7-419 μg/g olarak bulunmustur. Fe, Cu, Zn ve Ni icerikleri ise seyl ortalamasinin altindadir. Dusuk degerler onceki yillarda yapilan calismalarla uyum gostermektedir. Bu calismalarda Karadeniz guncel cokellerinin derin deniz cokellerine gore Cu, Ni, Co, Mn, Ba ve V yonunden cok daha fakir oldugu belirtilmistir. Midye orneklerinde ise Cd, Pb, Cr ve Zn icerikleri Su Urunleri Kontrol Yonetmeligi'nde belirtilen degerlerin oldukca uzerindedir. Degisim araliklari Cd icin 2,9-6,4 μg/g, Pb icin <0,01-28 μg/g, Cr icin 1,97-7,5 μg/g ve Zn icin 115-234 μg/g olarak bulunmustur. Yuksek degerler ozellikle Tuna nehri ile olan antropojenik (evsel ve endustriyel) tasinimlara isaret etmektedir. Bunun yani sira TBK2 (28,3048 ve 41,9565) TBK14 (28,158 ve 41,6841) nolu istasyonlardaki yuksek Mn icerikleri (867-1616 μg/g), Trakya havzasinda bulunan Binkilic manganez yatagindan kiyi boyunca olusan asinimlarla denizel ortama olan karasal kaynakli manganez girdilerini gostermektedir. Anahtar Kelimeler: Karadeniz, kirlenme, metal, midye, sediment. Metal Pollution in Water, Mussel and Surface Sediments of the Western Black Sea Shelf, Turkey Abstract Water, mytilus and surface sediment samples from the Western Black Sea Shelf have been taken during August 2003 and June 2004 periods. Total metal (Al, Fe, Mn, Pb, Cu, Zn, Ni, Cd, Cr and Hg) contents have been investigated in these samples. Pb, Cd, Hg and Cr concentrations of the surface sediments have been found higher than the shale average. Pb, Cd, Hg and Cr values range from 10 and 56 μg/g, 0.91 and 2.7 μg/g, 69 and 362 ng/g, and 61 and 110 μg/g, respectively. The high total organic carbon (TOC) and dissolved hydrogen sulphure (DHS) contents of the surface sediments are support these results. While TOC values range from 1.11-4.11 %, DHS concentrations vary between 31.7 and 419 μg/g in the surface sediments. In contrast, Fe, Cu, Zn and Ni contents of surface sediments are lower than the shale average. The low metal values are harmonious with the previous study results. In these researches, it was seen that the Black Sea recent sediments was much poorer in therms of Cu, Ni, Co, Mn, Ba and V according to deeper marine sediments. In contrast, Cd, Pb, Cr and Zn contents of mytilus from the Western Black Sea Shelf are considerably higher than the critical limits set by the Turkish Ministry of Environment for Aquatic Products. Cd, Pb, Cr and Zn values vary between 2.9 and 6.4 μg/g, <0.01 and 28 μg/g, 1.97 and 7.5 μg/g, and 115 and 234 μg/g, respectively. The high values are indicate the anthropogenic (domestic + industrial) inputs via especially the Danube River. Besides, high Mn contents at stations TBK2 and TBK14 (867 and 1616 μg/g, respectively) are point out the terrestrial manganese inputs originated from Binkilic mining zone in the Thrace Basin.

Seasonal and inter-annual temperature variability in the bottom waters over the western Black Sea shelf

Abstract. Long-term changes in the state of the Bottom Shelf Water (BSW) on the Western shelf of the Black Sea are assessed using analysis of intra-seasonal and inter-annual temperature variations. For the purpose of this study the BSW is defined as such shelf water mass between the seabed and the upper mixed layer (bounded by the σθ = 14.2 isopycnal) which has limited ability to mix vertically with oxygen-rich surface waters during the warm season due to formation of a seasonal pycnocline. A long-term time series of temperature anomalies in the BSW is constructed from in-situ observations taken over the 2nd half of the 20th century. The BSW is shown to occupy nearly half of the shelf area during the summer stratification period (May–November).The results reveal a warm phase in the 1960s/70s, followed by a cold phase between 1985 and 1995 and a further warming after 1995. The transition between the warm and cold periods coincides with a regime shift in the Black Sea ecosystem. While it was confirmed that the memory of winter convection is well preserved over the following months in the deep sea, the signal of winter cooling in the BSW significantly reduces during the warm season. The potential of the BSW to ventilate horizontally during the warm season with the deep-sea waters is assessed using isopycnic analysis of temperature variations. It is shown that temperature in the BSW is stronger correlated with the temperature of Cold Intermediate Waters (CIW) in the deep sea than with the severity of the previous winters, thus indicating that the isopycnal exchanges with the deep sea are more important for inter-annual/inter-decadal variability of the BSW on the western Black Sea shelf than effects of winter convection on the shelf itself.

Possible atmospheric origin of the 7 May 2007 western Black Sea shelf tsunami event

The paper examines the possibility that tsunami event recorded in the western Black Sea on 7 May 2007 was triggered by a traveling atmospheric disturbance. As meteotsunamis are favored by specific synoptic conditions, we inspected available ground and sounding observations, European Centre for Medium‐Range Weather Forecasts reanalysis fields, and satellite‐based products and compared them to the documented Mediterranean meteotsunamis. We found an atmospheric disturbance traveling toward 30° (NNE) with amplitude of 2–3 hPa and propagation speed of about 16 m/s, passing through few tens of kilometers wide pathway over the region affected by the tsunami. This disturbance occurred in the lower troposphere, but it was capped by instable convective cell that preserved gravity disturbance's coherence over a region at least 150 km long. An ocean modeling study showed that such a disturbance is capable of generating large tsunami waves and strong currents over the shallow regions, following the observations over the region where maximum sea level oscillations have been documented. Therefore, this event has a potential to be classified as a meteotsunami, the first of such kind in the Black Sea.

Abrupt transition of the northwestern Black Sea shelf ecosystem from a eutrophic to an alternative pristine state

Long-term (1960-2005) ecological data were used to identify a regime shift in the north- western Black Sea shelf and to assess whether it has recovered from its former eutrophic state. Fol- lowing the collapse of fish stocks and the population explosion of the ctenophore Mnemiopsis leidyi between 1988 and 1991, anthropogenic P-PO4 loads from the River Danube dropped strongly in 1992-1993. This decline in P-PO4 levels was caused by reductions in fertilizer use and emissions from land-based point sources during the economic recession of former Eastern Bloc countries, as well as by the low discharge rate of the River Danube. Commencing in 1993, the phosphate limitation apparently maintained a low-energy, inefficient food web dominated by the dinoflagellate Noctiluca scintillans and jellyfish, and relatively low levels of phytoplankton, bacterioplankton, mesozoo- plankton and fish. This 'post-eutrophication' regime was markedly different from the classical phytoplankton-mesozooplankton-fish chain of the similarly low nutrient 'pre-eutrophication' regime prior to 1970. Therefore it appears that the food web can attain 2 alternative regimes during periods of low productivity of the ecosystem. The post-eutrophication state cannot be considered as a major improvement or restoration of the northwestern coastal ecosystem.

Nutrient and phytoplankton trends on the western Black Sea shelf in response to cultural eutrophication and climate changes

Long-term trends (1960s–1990s) in surface winter nutrients and summer oxygen concentration of the euphotic zone, as well as seasonal and interannual variability in surface chlorophyll a (chl a) and Secchi depth ( Z d) were investigated for different shelf regions (depth <50 m) of the western Black Sea. Increasing phosphate and nitrate levels and changes in the summer oxygen concentration on the shelf before the mid-1980s corresponded well with the increase in riverine nutrient inputs. At the same time, decreasing silicate levels resulted almost equally from enhanced diatom stripping and trapping of silicate in the numerous dams constructed on the Danube River. The associated decrease in the Si:N ratio caused a shift towards more non-siliceous phytoplankton blooms. A decoupling of winter nutrient levels and summer oxygen concentration on the shelf after the mid-1980s suggests that other sources of inputs, such as regenerated nutrients from shelf sediments and/or upwelling, may have increased substantially. Large variations in the regional climate during the 1980s and 1990s could potentially account for increases from either or both of these sources and the resulting high summer primary production despite decreasing winter nutrients. The seasonal pattern in chl a within the Ukrainian NW shelf is similar to the open Black Sea, with low chl a in summer and high concentrations in cold months. The seasonal chl a variations on the Romanian and Bulgarian shelves also show high concentrations in May/June, most likely related to the Danube River maximum discharge during spring. As a result, chl a annual means in these two regions are significantly higher than – in the Ukrainian NW shelf.

Coastal/deep ocean interactions in the Black Sea and their ecological/environmental impacts

Satellite (CZCS, AVHRR) and in-situ (CTD, ADCP) data are utilized to characterize the impact of meso-scale motions and boundary currents on transport and productivity along the wide Western Black Sea Shelf. Various forms of isolated transient features including filaments, coherent dipole and monopole eddies and jets transport materials and momentum across the continental shelf. Unstable meandering motions generated at topographic irregularities propagate along the continental slope. As a result of the meandering motions, the material transported spreads into an area several times wider than the continental shelf/slope region. The spring-time surface productivity in the southern Black Sea is modulated by transient dynamics. Species differentiation and competition are evident along the boundary current system. Early summer plankton blooms coincide with peak flood discharges from major rivers, and influence the spread of eutrophication in the basin. Upwelling patches are evident along the west Anatolian coastline, in response to transient surface divergence. The upwelling locally has an adverse effect on fish eggs and larvae. In winter, cold water is formed on the western continental shelf. As the band of cold water follows the coast and interacts with coastal geometry, it leads to winter plankton blooms, particularly along the southern coast. Coherent meso-scale/synoptic eddies and turbulent features are shown to evolve rapidly in the system, indicating rapid conversions between different forms of energy in the upper layers of the ocean.

Statistical characteristics and the variability of currents over the western Black Sea shelf

Statistical characteristics and the parameters of the temporal variability of currents are analysed using data on current measurements conducted by five moored autonomous buoy stations over Bulgaria's shelf. Estimates of the specific contribution to the variability of synoptic (60%) and mesoscale (40%) oscillations are derived. The qualitative spectrum of the currents is defined, constituted by synoptic oscillations, short-period synoptic oscillations, inertial oscillations, and internal waves. Estimates of periods and oscillation amplitudes are also derived. The kinetic energy of the currents is calculated and the specific contribution of the energy of different-scale oscillations and their interrelations are determined.

Taxonomy and biostratigraphy of the Pleistocene ostracods of the Western Black Sea Shelf

The Pleistocene ostracod fauna in the studied region of the Black Sea shelf is subsaline, represented mainly by species of the genera Candona, Amnicythere, Euxinocythere, Loxoconcha,Tyrrhenocythere. 53 taxa of the species group are described, 12 of them being new for the science. On the basis of this fauna three assemblage zones are divided: 1. Candona Zone (Early PleistoceneTchaudinian); 2. Cryptocyprideis bogatschovi Zone (Middle Pleistocene - Oldeuxinian); 3. Loxoconcha lepida Zone (Late Pleistocene- Neoeuxinian). The first zone is characterized dominantly by subgeneric and species variability of g. Candona. The second is divided on the basis of the presence of species of g. g. Amnicythere, Euxinocythere and Loxoconcha. The third, uppermost zone is distinguished by an association of transite species with marked predominance in quantitative respect of Loxoconcha lepida or Loxoconcha gibboides.