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The paper analyses the sediment sink processes at the end of a large river sea system as the Danube - Black Sea is. It tries to evaluate the amount of sediments accumulated within the high stand depocentre, represented by the present-day Danube Delta, and the low stand one - the Danube deep sea fan complex. The deep-sea fan complex mobilised over 40,000 km3 of sediments with an accumulation rate that ranges between 88×106 t/a and 302×106 t/a (Wong et al., 1997; Winguth et al., 1997, 2000), while the amount of sediments accumulated in the present-day Danube Delta, including all the morphologic and depositional units, as Fluvial and Marine Delta Plains, the Delta-front unit and the Prodelta, is only of some 1,200 km3. The last, Holocene, progradational littoral sandy sheet is formed of about 22 km3 of sediments. The Danube River average annual sediment discharge during the Holocene was estimated to about 80×106 m3/a that is consistent with the Danube sediment discharge before the Iron Gate barrage was completed (about 70 to 80×106 m3/a). These evaluations are of large approximation but even in such conditions they give an apercu on the sediment sink processes at the end of a large river sea system. [ABSTRACT FROM AUTHOR]

The present article is in extenso version of the paper « Histoire Quaternaire du Delta du Danube. Essai d'interprétation des faciès des dépôts deltaïques » signed by N. Panin that was published in 1972 in Cercetari Marine No. 4, p. 5-15. The first author of the present paper decided to present the initial in extenso version of the article as an historical document representing a step forward in the knowledge of the Danube Delta geological structure and evolution. A sedimentological analysis at the knowledge level of the seventies of data from more than 100 drillings is presented and consequently an interpretation of the delta geological structure and development is given. This new interpretation was based on a modern at that time knowledge about the deltaic sedimentary processes and the Black Sea evolution during the Quaternary. It was proposed as an alternative to the older hypothesis of Liteanu et al. (1961). The new hypothesis stated that the Danube Delta represents a Holocene formation, corresponding to a highstand of the Black Sea. The Neoeuxinian (Würmian) lowstand determined a quite total removal of older deposits. At least two transgressive sequences have been recognised (dated as Old Black Sea phase and New Black Sea phase, respectively) separated by a regressive sequence, probably of Phanagorian age. Two main sedimentary environments were recognised: (1) the upstream part of the delta formed under river influenced sedimentary environment, situated from the apex to a line constituting the western boundary of the Jibrieni - Letea - Caraorman littoral accumulative formations and (2) the external, eastern part of the delta, from the above mentioned line eastward, where the sedimentary environment is predominantly influenced by the sea. [ABSTRACT FROM AUTHOR]

This study is a synthesis of gas-related features in recent sediments across the western Black Sea basin. The investigation is based on an extensive seismic dataset, and integrates published information from previous local studies. Our data reveal widespread occurrences of seismic facies indicating free gas in sediments and gas escape in the water column. The presence of gas hydrates is inferred from bottom-simulating reflections (BSRs). The distribution of the gas facies shows (1) major gas accumulations close to the seafloor in the coastal area and along the shelfbreak, (2) ubiquitous gas migration from the deeper subsurface on the shelf and (3) gas hydrate occurrences on the lower slope (below 750 m water depth). The coastal and shelfbreak shallow gas areas correspond to the highstand and lowstand depocentres, respectively. Gas in these areas most likely results from in situ degradation of biogenic methane, probably with a contribution of deep gas in the shelfbreak accumulation. On the western shelf, vertical gas migration appears to originate from a source of Eocene age or older and, in some cases, it is clearly related to known deep oil and gas fields. Gas release at the seafloor is abundant at water depths shallower than 725 m, which corresponds to the minimum theoretical depth for methane hydrate stability, but occurs only exceptionally at water depths where hydrates can form. As such, gas entering the hydrate stability field appears to form hydrates, acting as a buffer for gas migration towards the seafloor and subsequent escape. [ABSTRACT FROM AUTHOR]

Measurements of shoreline change between 1962 and 1987 were used to construct a sediment budget and to derive net longshore sediment transport rates for the Romanian Danube delta coast. The resulting pattern was in good agreement with the potential longshore transport pattern predicted by the wave energy flux based on wave measurements between 1972 and 1981. Due to the general coast orientation, the longshore sand transport is almost unidirectional southwardly all year long. Hence estimates for the net longshore transport were as high as high 2,350,000 m3/year. Because of this high intensity longshore transport and also due to a reduction of Danube sediment and the interruption of longshore transport by shore-perpendicular structures, much of the coast is undergoing erosion. Near the Sfantu Gheorghe mouth, a rapidly migrating barrier island acts as a substantial sink for littoral sand producing a downdrift deficit in the sand budget. [ABSTRACT FROM AUTHOR]

The Danube Canyon is a large shelf-indenting canyon that has developed seaward of the late Pleistocene paleo–Danube valley. Mechanisms of canyon evolution and factors that controlled it are revealed by analyzing the morphology and the sedimentary structure of the canyon, as well as the main features of the continental margin around the canyon. This is based on investigation by swath bathymetry in the canyon area combined with different types of seismic data.The canyon is a major erosional trough with a flat bottom cut by an entrenched axial thalweg. The thalweg path varies from highly meandering to fairly straight in relation to the local gradient. Segments of the canyon are characterized by specific morphology, orientation and gradient along the axial thalweg. We interpret these segments in terms of canyon maturity. The sedimentary structure of the canyon documents an older phase of erosion followed by partial infilling, and thus attests for repeated cycles of canyon development.Canyon morphology is interpreted as a result of erosive sediment flows along the entrenched axial thalweg that caused downcutting into the canyon bottom and instability of the canyon walls, and hence enlargement of the canyon and expansion by headward erosion. During the last lowstand level of the Black Sea the canyon was located in an area of high sediment supply close to the paleo-Danube River mouths. This is indicated by buried fluvial channels on the shelf and by a wave-cut terrace associated with a water level situated about -90 m below the present level. We infer that erosive flows in the canyon resulted from hyperpycnal currents at the river mouths, probably favored by the low salinity environment that characterized the Black Sea during lowstand times. Other mechanisms could have contributed to trigger sediment failure along the canyon, such as instability related to the presence of shallow gas, or the effect of a deep fault. [Copyright &y& Elsevier]

Suspended matter and sediments from the Danube River and the adjacent north-western Black Sea were analysed for organic carbon, nitrogen and biogenic carbonate and silica, as well as for different fractions of organic matter such as amino acids and carbohydrates. In addition, the nitrogen isotopic composition of surficial sediments of the NW Black Sea was examined. Particulate organic carbon (POC) contents decrease with total suspended solid (TSS) concentration due to dilution by mineral matter derived from upstream and riverbed erosion. The contribution of labile constituents such as amino acids and carbohydrates to POC remains uniform across the measured TSS contents. This suggests that any possible decrease in labile POC (LPOC) from dilution with refractory organic matter is masked by inputs of fresh organic matter from tributaries downstream. Comparison of TSS, POC and LPOC leaving the Iron Gate I barrier at the head of the Danube delta shows that the bulk of material currently entering the Black Sea is derived from inputs from tributaries and riverbed erosion, downstream of the Iron Gate I. The nature and distribution of organic matter in the surface sediments of the Black Sea are influenced by mixing between Danube-derived material and freshly produced organic matter on the shelf and in the open sea, on the one hand, and by oxic degradation during transport from the shelf to the abyssal Black sea, on the other. Preliminary data on the stable isotopic composition of nitrogen in surface sediments of the Black Sea further suggest that organic matter production on the shelf and the adjacent north-western Black Sea is fuelled by different nitrogen sources. [Copyright &y& Elsevier]

Between 1994 and 1998, the European Union EROS project has focused its research on the biogeochemistry and ecological quality of the Danube–north-western Black Sea continuum. This area was chosen as a pilot site to understand the complex interactions between human activities and the quality of the coastal environment. Project methodology included analysis of historical data, field expeditions, process-level studies and mathematical modelling. Main results achieved are shortly described highlighting the rapid response of the coastal Black Sea ecosystem to positive and negative effects of human changes. [Copyright &y& Elsevier]

Abstract: The Danube River Basin–Black Sea area represents a unique natural laboratory for studying the interplay between lithosphere and surface as well as source to sink relationships and their impact on global change. This paper addresses some information on the “active sink” of the system; i.e. the Danube deep sea fan and the Black Sea basin. The present study focuses on the distal sedimentary processes and the evolution of sedimentation since the Last Glacial Maximum. This is investigated through recently acquired long piston coring and shallow seismic data recovered at the boundary of influence of the distal part of the Danube turbidite system (to the north-west) and the Turkish margin (to the south). This dataset provides a good record of the recent changes in the sedimentary supply and climato-eustasy in the Black Sea region during the last 25ka. This study demonstrates that the deep basin deposits bear the record of the Late Quaternary paleoenvironmental changes and that the western Black Sea constitutes an asymmetric subsident basin bordered by a northern passive margin with confined, mid-size, mud-rich turbidite systems mainly controlled by sea-level, and a southern turbidite ramp margin, tectonically active. [Copyright &y& Elsevier]

Abstract: A previously unknown pattern of multiple bottom-simulating reflections (BSRs) occurs on high-resolution reflection seismic data in the Danube deep-sea fan, associated with acoustic features indicating free gas. Our study provides evidence that this pattern is developed in relation with the architecture of distinct channel–levee systems of the Danube fan. Channel–levee systems hosting multiple BSRs act as relatively sealed gas-bearing systems whose top is situated above the base of the gas hydrate stability zone (BGHSZ). Inside these systems, free gas accumulates below the BGHSZ under a combined lithological, structural and stratigraphical control. The uppermost BSR marks the current equilibrium BGHSZ, for a gas composition of more than 99% methane. Model-derived depths of the BGHSZ for different gas compositions and pressure–temperature conditions show that multiple BSRs would correspond to the BGHSZ either for (1) layers of gas hydrates with high contents of heavy hydrocarbons or hydrogen sulphide, or (2) stable climatic episodes with temperatures between glacial values and the present-day conditions. As the gas hydrate compositions required by hypothesis (1) are in sharp contradiction with the general background of the gas composition in the study area, we suggest that multiple BSRs are most probably relics of former positions of the BGHSZ, corresponding to successive steps of climate warming. In this case, they can provide sea-bottom paleotemperature values for these episodes, and hence they are potential new proxies for deciphering past climate conditions. [Copyright &y& Elsevier]

Four of the 38 sediment cores, collected from the northwestern part of the Black Sea (French–Romanian BLASON Programme, 1998), were analysed in order to study the major hydrological and environmental change of the sea during the Holocene. This change was caused by the inflow of the marine Mediterranean waters into the freshwater basin of the Black Sea. Two cores from the northwestern shelf and two from the deep part of the sea, representing distinct sedimentary environments along the same profile, were chosen. Lacustrine sediments from deep-water cores and uppermost marine sediments in all studied cores are characterised by the presence of pyrite (FeS2), which indicates anoxic, sulphate reducing conditions in sediment and in water column (as it is observed now in the Black Sea). Pyrite formation in this anoxic environment takes place below the oxic/anoxic limit, by reduction and sulphidation of iron. The transition from freshwater to marine conditions is marked in all studied cores by the presence of ferrimagnetic iron sulphide––greigite (Fe3S4). Greigite is an intermediate mineral in the pyritisation reactions during early diagenesis, which can be preserved in the sediment if pyritisation cannot be accomplished, due to a high Fe/S ratio. In shallow water cores from the shelf, where marine transgression is marked by a discontinuity, greigite is present below and above it. This suggests that the anoxic, sulphate reducing conditions, allowing preservation of greigite, started in the sediment after marine transgression. In cores from the deep part of the sea, the transgression is represented by a level of sediments with high organic matter content (sapropel). The greigite is present only in the lower part of this level and its formation is related to the arrival of marine water. The presence of pyrite in the upper part of the lacustrine sediments indicates that anoxic conditions probably prevailed here during marine transgression. These results show the sensitivity of magnetic parameters to major environmental change in the Black Sea, and allow a correlation between cores from different depths, by recognition of greigite in the sediments marking the marine transgression. [Copyright &y& Elsevier]