Black Sea Margin Research Articles

Morphology and potential geohazards using seismo-acoustic data in Sakarya Canyon, Western Black Sea Margin

Multi-channel seismic, 3.5-kHz Chirp seismic, and multibeam bathymetric data were collected along the western Black Sea margin, offshore Sakarya River, to investigate the morphology and to evaluate the potential geologic hazards. The multibeam bathymetric data show that the morphology of the margin is controlled by the Sakarya Canyon consisting of three distinct canyon heads, all incising the southern continental shelf. Deep-water sediment erosion along the canyon walls and scour marks along the distal canyon floors indicate that both Sakarya and Kefken Canyons may be active in terms of sediment erosion and turbidity currents. We identify the depositional and erosional features in the area by means of echo-character mapping. The distribution of different echo-types is mainly controlled by the morphology of the margin, as well as the shape, location, and structure of the major canyon systems. Erosional features, constituting 47% of the total surficial area, are classified as slides, erosional truncations, gravitational mass wasting, gullies, and outcropping seafloor while depositional features, constituting 53% of the total surficial area, comprise shelf sediments, turbidites, pelagic/hemi-pelagic sediments, and sediment waves. Different types of geohazards coexist along the Sakarya Canyon, which are classified as hazards linked to (1) local and/or regional tectonism, (2) morphology of the continental margin (turbidity currents, slope overstepeening), and (3) prevailing sedimentary processes (mass transports, submarine fluid flow, loss of support due to the truncation scarps and bedforms).

Controls on Gas Emission Distribution on the Continental Slope of the Western Black Sea

The continental slopes of the Black Sea show abundant manifestations of gas seepage in water depth of <720 m, but underlying controls are still not fully understood. Here, we investigate gas seepage along the Bulgarian and Romanian Black Sea margin using acoustic multibeam water column, bathymetry, backscatter, and sub-bottom profiler data to determine linkages between sub-seafloor structures, seafloor gas seeps, and gas discharge into the water column. More than 10,000 seepage sites over an area of ∼3,000 km2were identified. The maximum water depth of gas seepage is controlled by the onset of the structure I gas hydrate stability zone in ∼720 m depth. However, gas seepage is not randomly distributed elsewhere. We classify three factors controlling on gas seepage locations into depositional, erosional, and tectonic factors. Depositional factors are associated with regionally occurring sediment waves forming focusing effects and mass-transport deposits (MTDs) with limited sediment drape. Elongated seafloor depressions linked to faulting and gas seepage develop at the base between adjacent sediment waves. The elongated depressions become progressively wider and deeper toward shallow water depths and culminate in some locations into clusters of pockmarks. MTDs cover larger regions and level out paleo-topography. Their surface morphology results in fault-like deformation patterns of the sediment drape on top of the MTDs that is locally utilized for gas migration. Erosional factors are seen along channels and canyons as well as slope failures, where gas discharge occurs along head-scarps and ridges. Sediment that was removed by slope failures cover larger regions down-slope. Those regions are devoid of gas seepage either by forming impermeable barriers to gas migration or by removal of the formerly gas-rich sediments. Deep-rooted tectonic control on gas migration is seen in the eastern study region with wide-spread normal faulting promoting gas migration. Overall, gas seepage is widespread along the margin. Gas migration appears more vigorous in shallow waters below ∼160 m water depth, but the number of flare sites is not necessarily an indicator of the total volume of gas released.

Linking Danube River activity to Alpine Ice-Sheet fluctuations during the last glacial (ca. 33–17 ka BP): Insights into the continental signature of Heinrich Stadials

Offshore archives retrieved from marine/lacustrine environments receiving sediment from large river systems are valuable Quaternary continental records. In the present study, we reconstruct the Danube River activity at the end of the last glacial period based on sedimentological, mineralogical and geochemical analyses performed on long-piston cores from the north-west Black Sea margin. Our data suggest that the Danube River produced hyperpycnal floods throughout the ca. 33–17 ka period. Four main periods of enhanced Danube flood frequency, each of 1.5–3 kyr duration, are recorded at ca. 32.5–30.5 ka (equivalent to the first part of Heinrich Stadial –HS– 3), at ca. 29–27.5 ka (equivalent to Greenland Stadial 4), at ca. 25.3–23.8 ka (equivalent to HS 2) and at ca. 22.3–19 ka. Based on mineralogical and geochemical data, we relate these events to enhanced surface melting of the Alpine Ice Sheet (AIS) that covered ∼50,000 km2 of the Danube watershed at the Last Glacial Maximum (LGM). Our results suggest that (i) the AIS growth from the inner Alps to its LGM position in the northern Alpine foreland started from ca. 30.5 ka, ended no later than ca. 25.3 ka, and was interrupted by a melting episode ca. 29–27.5 ka; (ii) the AIS volume drastically decreased from ca. 22.3 to 19 ka, as soon as summer insolation energy at the AIS latitude increased; and (iii) HSs strongly impacted the AIS mass balance through enhanced summer surface melt. This, together with evidence of severely cool winters and the rapid expansion of sea ice in the North Atlantic, implies strong seasonality in continental Europe during stadials.

Evolution of the Danube Deep-Sea Fan since the Last Glacial Maximum: new insights into Black Sea water-level fluctuations

The Danube Deep-Sea Fan (NW Black Sea) is one of the most developed deep-sea sediment depositional systems in Europe. Although the morphology and the architecture have been widely described in the past years, little is known about the stratigraphy of this depositional system. For the late Quaternary, this results from the lack of significant stratigraphic markers, the scarcity of radiocarbon ages and the difficulty in constraining reservoir ages. Recent robust quantification of reservoir ages has allowed the construction of a new stratigraphic framework for the Black Sea from the end of the last glacial period to the Holocene, thus giving the opportunity to correlate sedimentological and geochemical features previously described on the NW Black Sea margin with climatic events identified in the Northern Hemisphere. Based on this approach, we propose an improved chrono-lithostratigraphic framework for the Danube Deep-Sea Fan channel–levees since the Last Glacial Maximum. We show that the Danube Deep-Sea Fan was active during the Last Glacial Maximum until the Younger Dryas–Early Holocene transition ca. 11,700 cal a. BP, when the turbidite activity abruptly terminated in the whole system. Throughout this period, the Danube River was the main source of the deep depositional system, except between ca. 17,200 cal a. BP and 15,700 ± 300 cal a. BP. At that time, the deposition of ‘red turbidites’ in the deep basin, concomitant with the deposition of the so-called ‘Red Layers’ onto the continental shelf and the upper slope, emphasises the direct impact of the increased meltwater runoff of the Dnieper River as far as the Danube Deep-Sea Fan. Some significant changes in the location of the depocentre of the Danube Deep-Sea Fan occurred through time. The main change in depocentre location occurred at ca. 28,000 cal a. BP, with the northward avulsion leading to the formation of the Middle Channel. Although the Middle and Northern Channel became the preferential turbidite depocentres after this event, the deposition of turbidites has persisted in the Southern channel–levee until 14,700 cal a. BP, indicating that the Southern Channel was definitely not abandoned after the upstream northward avulsion. Unsurprisingly, water-level fluctuations and river sediment flux acted as the main forcings on the evolution of the Danube Deep-Sea Fan. Based on these results, and on the morphology of the Danube Canyon, we propose that (i) lowstand conditions (≤ − 110 m) prevailed during the Last Glacial Maximum, and possibly between 15,700 ± 300 cal a. BP and 14,700 cal a. BP, (ii) water-level ranged from ≥ − 110 and ≤ − 70 m between 14,700 and 11,700 cal a. BP, and (iii) water-level ranged from ≥ − 70 and ≤ − 30 m from 11,700 cal a. BP and until the reconnection of the Black Sea ‘Lake’ with the global ocean at ca. 9000 cal a. BP.

Cross-sectional anatomy and geodynamic evolution of the Central Pontide orogenic belt (northern Turkey)

Geophysical data allowed the construction of a ~250-km-long lithospheric-scale balanced cross section of the southern Black Sea margin (Espurt et al. in Lithosphere 6:26–34, 2014). In this paper, we combine structural field data, stratigraphic data, and fault kinematics analyses with the 70-km-long onshore part of the section to reconstruct the geodynamic evolution of the Central Pontide orogen. These data reveal new aspects of the structural evolution of the Pontides since the Early Cretaceous. The Central Pontides is a doubly vergent orogenic wedge that results from the inversion of normal faults. Extensional subsidence occurred with an ENE-trend from Aptian to Paleocene. We infer that the Black Sea back-arc basin also opened during this period, which was also the period of subduction of the Tethys Ocean below the Pontides. As in the Western Pontides, the Cretaceous–Paleocene subsidence was interrupted from Latest Albian to Coniacian time by uplift and erosion that was probably related to a block collision and accretion in the subduction zone. The restoration of the section to its pre-shortening state (Paleocene) shows that fault-related subsidence locally reached 3600 m within the forearc basin. Structural inversion occurred from Early Eocene to Mid-Miocene as a result of collision and indentation of the Pontides by the Kirsehir continental block to the south, with 27.5 km (~28 %) shortening along the section studied. The inversion was characterized by NNE-trending shortening that predated the Late Neogene dextral escape of Anatolia along the North Anatolian Fault and the modern stress field characterized by NW-trending compression within the Eocene Boyabat basin.

Black Sea desiccation during the Messinian Salinity Crisis: Fact or fiction?

The late Miocene Messinian Salinity Crisis (MSC) was an extraordinary geologic event in the Mediterranean Basin marked by massive salt accumulation and presumably basin desiccation as a consequence of the reduced water exchange with the Atlantic Ocean. The discovery of a desiccation deposit in the Black Sea, the so-called Pebbly Breccia unit, was used to claim that the Black Sea also became desiccated during the MSC. Erosional features interpreted from seismic profiles of the Black Sea margin, correlated by some to the Pebbly Breccia unit, were used to support this hypothesis. However, the age of the Pebbly Breccia is poorly constrained, and its origin and relevance to the MSC subject to controversy. Here we present new biostratigraphic (dinoflagellate cyst) data from two key sedimentary successions located in a deep and a marginal setting of the Black Sea Basin. These records demonstrate that the Pebbly Breccia predates the Mediterranean water-level drop during the MSC. We argue that the presumed erosional features in the Black Sea Basin are not related to the MSC and likely represent an older Miocene event.

Quaternary mass wasting on the western Black Sea margin, offshore of Amasra

In recent years, the western Black Sea margin has become well-studied due to its potential for petroleum plays in relatively deeper waters. In 2010, multi‐channel seismic, multibeam bathymetry and Chirp high resolution seismic data were collected in order to define the existing geohazards along the margin, to identify the seabed morphology and to determine mass movement types and their run‐out distances.Seismic data indicate that the western Black Sea margin is an unstable region with sediment erosion. Particularly, an unstable area offshore of Amasra in the NW consisting of four slides and four buried debris lobes is named the Amasra mass failure zone. Different types of sliding with varying sizes and different mechanisms are observed. These include sliding in the steep slope zones where block‐type sliding occurs, smaller‐scale slides on the canyon walls, and relatively larger slides in the Amasra mass failure zone. Block‐type sliding is observed on the upper continental slope to the south as well as on the canyon walls. They are formed along the rotational faults and occur due to the gravitational loading on the steep slope zones possibly triggered by local seismic activity. In addition, seven large debris lobes identified in the northern toe of the slope buried in the Quaternary sediments triggered by excess pore pressures due to high sediment input and submarine fluid flow.We suggest that earthquake activity may be an important agent for all kind of mass movements in the area. In addition, we propose that the slides in the Amasra mass failure zone are triggered by excess pore pressures in shallow sediments due to the submarine fluid flow possibly produced from gas hydrate dissociation. Warmer Mediterranean seawater input during the rapid transgression period after the Last Glacial Maximum in the Black Sea together with the rapid sedimentation resulted in destabilization of gas hydrates, which caused excess pore pressures in shallow sediments leading to massive sediment failures. Small‐scale normal faults around the scarps may be a secondary factor promoting the failures providing the suitable pathways for the fluid flow as well as the suitable weak surfaces for the sliding.

Dating of the Black Sea Basin: new nannoplankton ages from its inverted margin in the Central Pontides (Turkey)

Abstract The Eocene uplift and inversion of a part of the Black Sea margin in the Central Pontides, allows us to study the stratigraphic sequence of the Western Black Sea Basin (WBS). The revision of this sequence, with 164 nannoplankton ages, indicates that subsidence and rifting started in the Upper Barremian and accelerated during the Aptian. The rifting of the western Black Sea Basin lasted about 40 Ma (from late Barremian to Coniacian). In the inner, inverted, Black Sea margin, the syn-rift sequence ends up with shallow marine sands. The uppermost Albian to Turonian was a period of erosion or non deposition. This regional mid-Cretaceous stratigraphical gap might result from rift flank uplift, as expected in the case of a thick and cold pre-rift lithosphere. However, coeval collision of the Kargi Block, along the North Tethyan subduction zone at the southern margin of the Pontides, might also have contributed to this uplift. A rapid thermal post-rift subsidence of the margin occurred during the Coniacian–Santonian. Collision of the Kirşehir continental block commenced in Early Eocene time (zone NP12) giving rise to compressional deformation and sedimentation in piggyback basins in the Central Pontides, whereas the eastern Black Sea was still opening.

Fluvial sequences as evidence for landscape and climatic evolution in the Late Cenozoic: A synthesis of data from IGCP 518

This editorial synthesis introduces a collection of papers derived from International Geoscience Programme (IGCP) Project 518, entitled ‘Fluvial Sequences as Evidence for Landscape and Climatic Evolution in the Late Cenozoic’. Building on information collected during an earlier project (IGCP 449: ‘Global Correlation of Late Cenozoic Fluvial Deposits’), this has examined the data accumulated on fluvial records, particularly river terrace sequences, for patterns that contribute to the interpretation of Late Cenozoic landscape and climatic evolution. This introductory paper reviews the baseline evidence, noting that there are patterns (from terrace sequences in different regions) of differing amounts of fluvial incision, indicating differing uplift rates, that appear to be related to crustal province. There seems to be no general role for plate tectonics; instead the patterns are of regional uplift, probably an isostatic response to erosion, enhanced by positive feedback effects, arguably due to lower-crustal flow. As well as depocentres, which are subsiding due to loading by accumulating sediment, cratonic areas are also exceptions to the rule of widespread uplift; these show minimal Late Cenozoic uplift, presumably because they lack mobile lower crust. The ten papers that follow are reviewed briefly in this context, these being contributions concerning Turkey, the Black Sea margin of Ukraine, Morocco (×2), the Czech Republic, Britain (×2), the Netherlands, New Zealand, and China.

Manganese deposits in Turkey: Distribution, types and tectonic setting

Manganese deposits in Turkey are divided into four main groups on the basis of their age, host rocks and processes of formation. The first is hydrothermal–hydrogenetic and rarely diagenetic-type manganese deposits which are chiefly associated with radiolarian chert series. These deposits occur as lenses and small ore beds within ophiolitic melanges zones of Paleo-Tethyan, Karakaya and Neo-Tethyan (North Anatolian and South Anatolian) suture belts. The second is black shale-hosted manganese deposits that is found in black shale, calcareous shale and calciturbiditic limestone horizons of the passive margin sediments of Late Cretaceous age in the western Taurides. These deposits generally are metamorphosed to greenschist facies and include Mn carbonate–silicate and oxide minerals. The third is volcanic arc-hosted ores along the Black Sea margin which occur in different types of metasomatic, hydrothermal and stratabound settings. Rock associations include Upper Cretaceous dacitic tuff, reddish limestone, marl and hemipelagic claystone. The fourth is Oligocene-hosted ores in the Thrace Basin. These deposits are similar to those of the Paratethyan Oligocene belt which occur on the eastern and northern margin of Black Sea and includes the Chiatura (Georgia), Nikopol (Ukraine) and Varna (Bulgaria), respectively. The Oligocene Mn deposits of the Thrace Basin were formed by diagenetic replacement processes during a marine regression.

Numerical modelling of regional neotectonic movements in the northern Black Sea

The paper examines the manifestation of tectonic processes related to the Neogene and recent subsidence of the Black Sea margin and uplift of the Crimea mountains as well as current seismicity in the southern Crimea. A 2-D finite-element model, based on a seismic line across the northern Black Sea and the Crimean mountains, that predicts the present-day tectonic velocity field, using the available neotectonic data as boundary conditions, has been developed. The model results indicate that the observed structure and tectonic motions cannot be explained by forces imposed on the external side, boundaries of the model only. Rather, movements consisting of upwelling beneath the Black Sea and subsidence under the Crimea, interpreted as the effects of mantle processes, are necessary to explain the observed topographic evolution of the area. Zones where the ratio of the calculated shear stress to calculated pressure is greatest correspond to the position of the Southern Coast fault that controls the seismicity of the northern Black Sea margin.

Effects of fine grain size on distribution of Mn in shallow and deep water Black Sea sediments: A comparison between oxic and anoxic depositional environments

The uppermost 5–6 cm of the sediments (between 8 and 2248 m water depths) were studied to understand the effects of varying redox conditions on the Mn distribution in the recent sediments of the Black Sea. It was found that most Mn concentrations are consistent with the average abundance in crustal and/or sedimentary rocks. There exist no important differences between Mn concentrations in oxic (shallower water; <70 m) and anoxic (deeper water; 120 m) sediments. Previously reported Mn-enrichment above the Black Sea oxic/anoxic interface, due to the peculiar redox cycling, shows no significant contribution of Mn to the bottom sediments. A marked relationship between total Mn concentrations and clay/mud contents at shelf depth along the southern Black Sea margin indicates increased accumulation of Mn in association with the fine-grained particles and eastward water circulation.

Provenance and diagenesis of organic matter in Late Cretaceous and Tertiary sediments from the southern Black Sea margin

The organic matter in the Late Cretaceous and Tertiary sediments from the southern Black Sea margin is assigned to the terrestrial-marine/terrestrial range of organic facies. Within this range, the stratigraphic section yields different organic facies types in response to different accumulation and preservation controlling processes. During the Late Companian-Maastrichtian, organic material from the shelf and slope was re-deposited in the deeper oxic parts of the basin. Rapid transport and sedimentation resulted in a higher degree of preservation of lipid-rich, terrestrial components (sporinite, cutinite, resinite) in comparison to the autochthonous sediments. The increase in organic carbon with increasing silt/clay content together with low carbon concentrations in the allochthonous sediments suggest that the accumulation of organic matter in the source areas was controlled by terrigenous influx and that the accumulation conditions were not favorable.

Source mechanism of the Bartin earthquake of September 3, 1968 in northwestern Turkey: Evidence for active thrust faulting at the southern Black Sea margin

The Bartin earthquake of September 3, 1968 is the strongest instrumentally recorded earthquake to occur along the Black Sea margin in northwestern Turkey. The source mechanism of the Bartin earthquake, as determined in this study from long period P wave first motions and from the inversion of P and SH waves, indicates thrust faulting with a strike of 28°, dip of 38° and rake of 80°. The average (centroid) depth is 4 km and the seismic moment is 3.9 × 10 25 dyn cm. The Bartin earthquake provides the first evidence for active thrust faulting at the southern margin of the Black Sea. The Black Sea has probably evolved as a back-arc basin resulting from the northward subduction of Tethys. Seismicity, fault plane solutions, and other geophysical data suggest that at the present time the Black Sea is closing under north-south compression.

Biological reconstruction of late ancient populations from Bulgarian lands

Manganese deposits in Turkey are divided into four main groups on the basis of their age, host rocks and processes of formation. The first is hydrothermal–hydrogenetic and rarely diagenetic-type manganese deposits which are chiefly associated with radiolarian chert series. These deposits occur as lenses and small ore beds within ophiolitic melanges zones of Paleo-Tethyan, Karakaya and Neo-Tethyan (North Anatolian and South Anatolian) suture belts. The second is black shale-hosted manganese deposits that is found in black shale, calcareous shale and calciturbiditic limestone horizons of the passive margin sediments of Late Cretaceous age in the western Taurides. These deposits generally are metamorphosed to greenschist facies and include Mn carbonate–silicate and oxide minerals. The third is volcanic arc-hosted ores along the Black Sea margin which occur in different types of metasomatic, hydrothermal and stratabound settings. Rock associations include Upper Cretaceous dacitic tuff, reddish limestone, marl and hemipelagic claystone. The fourth is Oligocene-hosted ores in the Thrace Basin. These deposits are similar to those of the Paratethyan Oligocene belt which occur on the eastern and northern margin of Black Sea and includes the Chiatura (Georgia), Nikopol (Ukraine) and Varna (Bulgaria), respectively. The Oligocene Mn deposits of the Thrace Basin were formed by diagenetic replacement processes during a marine regression.