Vedat Ediger

Distribution of surficial shelf sediments in the northeastern and southwestern parts of the Sea of Marmara: Strait and canyon regimes of the Dardanelles and Bosporus

Surficial sediment samples collected from the northeastern and southwestern shelf regions of the Sea of Marmara, together with data available from other sources, indicate marked variations in sediment compositions resulting from differences in topographical, hydrological and biological conditions. In the strait channels of the Dardanelles and Bosporus, where strong undercurrents prevail, the floor was covered mostly by coarse-grained sediments (rich in sand and gravel). However, in areas of relatively low energy conditions, sediments contained appreciable amounts of mud, with a tendency towards an increase in the amount of clay towards the open sea. The effects of the strong undercurrents on the bedforms was also apparent in the southern Strait of Bosporus where sidescan sonar surveys revealed the presence of asymmetrical sand ripples. Although terrigenous mud is the principal sediment type in the two canyons (Dardanelles and South Bosporus), the sediments, in particular on the floor of South Bosporus Canyon, show a distinct contrast between the inner N-S and outer E-W trending parts: along its axis, where depths are greatest, the outer part of the canyon appears to contain much more coarse sediment (in part derived from the benthic communities) than the inner canyon. In general, both the topography-related current regimes and the biological activities in the study areas mostly determine the types and modes of sediment distribution. Among the biogenic components, the calcareous corraline algae Rhodophycae (chiefly Lithothamnium calcareum and L. fruticolusum) and the molusc families Galeommatacae and Cerithiospidae are associated with the prevailing Mediterranean undercurrents, especially in the shallower waters of the junction of the Sea of Marmara and Strait of Bosporus. The relatively abundant mollusc species Mytilus galloprovincialis and Modiolus barbatus and the Trochidae in the Strait of Bosporus are largely affected by Black Sea waters. The wide range of carbonate concentrations in the sediments generally reflects the relative abundances of biogenic admixtures in the samples. High carbonate percentages in the sediments are usually associated with low mud contents, and vice versa. Interestingly, the organic carbon contents of the sediments generally increase with proximity to the Black Sea, suggesting influxes of appreciable amounts of organic matter from the Black Sea.

Source and depositional controls on heavy metal distribution in marine sediments of the Gulf of İskenderun, Eastern Mediterranean

The concentrations of major (Al, Mg, Fe, and Mn) and trace (Zn, Cr, Ni, Co, Cu, and Pb) elements as well as carbonate, organic carbon, and grain size distributions have been determined in surface sediments collected at 73 stations from the Gulf of Iskenderun. The shelf sediments of the Gulf of Iskenderun are largely muds but sediments with higher contents of sand and gravel also occur due to the presence of high biogenic CaCO3 contents (up to 80%) in the samples. The abundance and regional distributions of elements in the surface sediments of the Gulf are largely controlled by differences in grain size and in the level of dilution by skeletal/shell remains, and by the variation in composition of source materials. Mg, Ni, Cr, Co, and to a lesser extent Zn and Pb, are significantly enriched relative to their average crustal abundances. These metal enrichments are found especially in the eastern part along the SW-NE-trending areas of the Gulf and are mostly consistent with the chemical and mineralogical compositions of the major basic and ultrabasic source rocks present on the adjacent coast and hinterland. There is strong evidence that suggest that aluminosilicates, Fe and Mn compounds (possibly oxyhydroxides), and organic matter act as the most effective carriers for transfer of available metals (Zn, Cr, Ni, Co, and Cu) to the sediments.

Seismic evidence of shallow gas in the sediment on the shelf off Trabzon, southeastern Black Sea

Abstract High-resolution seismic surveys carried out on the shelf off Trabzon (southeastern Black Sea) show that the sub-bottom stratigraphy consists of two main depositional sequences (A and B) one of which contains zones of acoustic turbidity. Of these, the upper depositional sequence (A) is thought to comprise Holocene sediments, while the lower depositional sequence (B) is interpreted as approximating to the Pleistocene. The boundary between these sequences is defined by a reflector (R), which is interpreted as the pre-Holocene erosional surface. The acoustic turbidity observed on seismic profiles is interpreted as representing gas accumulations in the sediments of the upper depositional sequence. Previous geochemical investigations in the area indicate that acoustic turbidity implies biogenic methane gases in bubble form. On the basis of published hypotheses on the generation of the methane gases, it is suggested that the upper boundary of the zone of acoustic turbidity on seismic profiles in this study corresponds to the boundary between a biogenic sulfate-reducing zone and the underlying carbonate-reducing zone. Apart from acoustic turbidity, some bright spot anomalies which are also interpreted to arise from the presence of methane gases within the upper depositional sequence are observed on the seismic records collected from the shelf off Trabzon.

Upper slope sediment waves in the Cilician Basin, northeastern Mediterranean

Large sediment waves have been observed within the upper slope deposits of the Cilician Basin (northeastern Mediterranean), at the interfluve between two submarine canyons present offshore of the Mersin Shelf. There are several generations of sediment waves stacked within the sedimentary sequence, with the most recent bedforms found on the seabed in an area consisting of fine-grained sediments. The surficial sediment wave field, estimated to cover an area of ~55 km2, is found at water depths between 250 and 310 m. The buried sediment wave fields have similar dimensions, but they are located further downslope. Wave dimensions increase with water depth and depth in the sedimentary sequence. The largest bedforms reach 40 m in height and 1.8 km in length. Most waves appear to have been migrating upslope, i.e. towards the north/northeast, and this migration direction is mostly consistent throughout the sedimentary sequence. This consistency indicates similar mechanisms of formation and maintenance over a considerable time interval. The morphology and migration pattern of the observed bedforms suggests that sedimentation in the Cilician Basin during wave formation has been controlled by near-bed flows resembling those generated by the present Asia Minor Current, although these flows may have been stronger in the past than they are at present

Seismic stratigraphy of the southeastern Black Sea shelf from high-resolution seismic records

Abstract High-resolution seismic reflection profiles obtained from the shelves off the Samsun and Trabzon coasts (southeastern Black Sea) have revealed the existence of two distinct reflectors (R and T). Reflector R forms the boundary between the upper depositional sequence A and the lower depositional sequence B. Seismo-stratigraphically, upper sequence A is characterized by stratified (simple, complex and lenticular) seismic reflection configurations which imply changing depositional systems and/or conditions, changes of source and supply of sediments due to the relative change of sea level. Additionally, the observed onlapping pattern within this sequence denotes rising of sea level during the last post-Glacial transgression. The mounded seismic facies beyond the steep gradients on the shelf area of Trabzon imply slump masses. The lower depositional sequence B on the Samsun and Trabzon shelves is commonly characterized by chaotic reflection configurations which are interpreted either as strata deposited in a variable, relatively high energy setting, or as initially continous strata that have been deformed. Additionally, the presence of some parallel-subparallel reflections within this sequence denotes its stratified character. The upper boundary of sequence B (reflector R) reflects a pre-Holocene unconformity, probably caused by falling sea level at the time of the Last Glacial Maximum. Additionally, toplap termination at the upper boundary of sequence B indicates a stillstand of sea level at 91 m depth during the Last Glacial Maximum. Based on borehole data, sequence B is composed of consolidated sand and gravel admixtures in the Samsun area, and basalt-agglomerate-tuff series in the Trabzon area. Reflector T forms the boundary of a zone of anomalous reflections (acoustic turbidity, AT) that are interpreted to represent gas in the sediments of the upper depositional sequence A. The gas accumulations tend to occur in “blanket” and “curtain” forms depending on their appearances on seismic records.

The experimental analysis on the Late Quaternary deposits of the Black Sea

Holocene sediments taken from the south-eastern and western Black Sea have been investigated in relation to their geochemical, sedimentological and mineralogical characteristics. Their textures are characterized by their low amount of sand, upward-increasing silt and downward-increasing clay contents. While the terrigenous materials transported from Anatolian volcanic-based sources and European alluvial sediments form the shore deposits, the deep-sea sediments mainly consist of the marine biological production. The highest amount of organic carbon was deposited following the formation of anoxic conditions at the bottom until the beginning of the still continuing carbonate-rich coccolith (Emiliania huxleyi) deposition. The high metal concentrations are associated with fine-grained sediments, some with organic material. The metal concentration is diluted by high organic carbon and carbonate contents within the depositional sequences. While the abundance of illite in the western Black Sea describes the deltaic depositions, the downward decreasing smectite/illite ratio along the core, off the south-eastern shelf, indicates the downward increasing precipitation during the deposition. The variation in the sedimentation pattern and sedimented material is believed to be the response of the biochemical environment in the sea to the changing geological, biological and chemical conditions in and around the Black Sea during the last climatic changes.

Recent surficial shelf sediments of the Cilician Basin (Turkey), northeastern Mediterranean

Abstract The continental shelf of Mersin Bay forms the northwestern margin of the Adana-Cilician Basin; it extends from the Goksu delta (in the southwest) to the Seyhan-Tarsus-Ceyhan delta (in the northeast) and is narrower and steeper in the southwest than in the northeast. Five depositional zones are recognised on the basis of the analysis of surficial sediments: (1) a “ Modern Terrigenous Prism ”; extending from the coast to 50 m water depth; (2) a “ Shelly Zone ” on the mid-shelf (50–100 m), characterised by an abundance of skeletal debris; (3 and 4) “ Prodeltaic Zones (I and II) ” in the northeastern and southwestern parts of the shelf, where silty sediment from the major Seyhan-Tarsus and Goksu rivers, respectively, has been deposited; and (5) the “ Outer Shelf Zone ”, extending from the 100-m contour to the shelf edge, characterised by clay-sized sediment and abundant remains of planktonic micro-organisms. Sediment appears to be reaching the shelf from two main sources: (i) a lateral supply from the coastal rivers between the two major deltas, which is being dispersed along the coast to form the “ Modern Terrigenous Prism ” and is dominated by near-shore wave activity and wave-generated currents; and (ii) fine-grained sediment from the Seyhan-Tarsus rivers, transported along the shelf under the action of the cyclonic Mediterranean circulation. Some of the latter sediments settle to form the distal tongue of the Seyhan-Tarsus delta (Prodeltaic Zone I), whilst the remainder settles over part of the shelf to the southwest. The sediment supplied by the Goksu river is generally swept away to the southwest, by the southwesterly-flowing cyclonic Mediterranean Current and it influences the shelf only within its immediate vicinity to form Prodeltaic Zone II. The mid-shelf area (Shelly Zone), lying at water depths of between 50 and 100 m, receives some fine silt and clay. However, it is cloaked in a thin layer of skeletal debris, which is difficult to explain but may be partly of anthropogenic origin although this requires further investigation.

Seismic stratigraphy of the fault-controlled submarine canyon/valley system on the shelf and upper slope of Anamur Bay, Northeastern Mediterranean Sea

Abstract High-resolution shallow seismic reflection (Uniboom) and echosounding profiles obtained on the shelf and upper slope areas of Anamur Bay (Turkey, northeastern Mediterranean) were studied together with previous data on the onshore geology of the area to investigate the origin and related seismic stratigraphy of the submarine Anamur Canyon. It was found that the main axial trend of the submarine Anamur Canyon is aligned with the offshore projection of the N-S orientated, onshore Anamur thrust fault of Late Cretaceous (Senonian) to Early Tertiary age. Interpretation of the distinct reflection configurations observed on the seismic profiles reveals the presence of various seismic facies comparable with the depositional sequences resulting from the relative sea-level changes during the Late Quaternary. The acoustic basement (probably Miocene in age) is overlain by a sedimentary column (mainly Plio-Quaternary) which, in turn, can be divided into two major seismic sequences and 12 seismic facies that show considerable variations in both their lateral and horizontal extent. A palaeo-deltaic environment (Sequence I), with its typical topset and foreset facies, was recognized and the corresponding deposits are interpreted to be indications of coastal-deltaic progradation during Late Pleistocene sea-level highstands, or during the early phases of the last major eustatic lowering. Reflections of Sequence II, which includes nine distinct facies, are marked by onlap configuration and are believed to be marine deposits of the Flandrian transgression. At least four transgressive cycles are evident from the seismic profiles, at 70–90, 60, 45 and 40 m water depths. The two sequences (I and II) are now covered with the most recent facies.

FRONTAL INSTABILITIES AND SUSPENDED SEDIMENT DISPERSAL OVER THE SHELF OF THE CILICIAN BASIN, SOUTHERN TURKEY

Abstract A narrow band of coastal waters with high sediment content, which develops along the southern Turkish coastline, is identified in satellite imagery. Such waters are present during periods of high freshwater runoff. The suspended sediment-laden waters extend from this narrow nearshore belt as a series of eddies, which are not normally seen on this scale in shallow waters. These eddies are produced by instability at the front, or density difference, between the nearshore waters and the offshore (more saline) continental shelf waters; they are analogous to similar phenomena produced in the laboratory. Flow within the eddies is initially perpendicular to the front, then across the general shore-parallel circulatory system over the area. The eddies extend over the whole width of the shelf and appear to be important agents for the seaward dispersal of fine-grained suspended sediments.

Recent sedimentation on the shelf and upper slope in the Bay of Anamur, southern coast of Turkey

Abstract Sedimentological studies, supported by Uniboom profiles and sonographs, of surface sediments from a part of the narrow southern shelf of Anatolia, indented by the head of a submarine canyon and partly covered by meadows of macrophytobenthos, revealed abrupt lateral and vertical variations in the Holocene sedimentary facies. These variations have essentially been controlled by the migration of depocentres, (palaeo-) topography of the shelf and an overall decrease in the rate of fluviatile sediment supply in the late Holocene. As sea level reached approximately its present position nearly 5 ka B.P., the bulk of the siliciclastic input began to be trapped in the inner shelf zone ( Most of the terrigenous mud bypasses the shelf to be trapped in the canyon head, but transport of modern detrital sand and gravel does not take place across the shelf. This is because of the relative weakness of the onshore-offshore currents, the prevailing microtidal conditions and a belt of phytobenthos between 10 and 30 m which traps the sediments. The plants also create the ecological conditions favourable for biological carbonate production. However, some gravel- and sand-size relict sediments have been transported from the outer shelf into the canyon head by gravity-induced or cross-canyon currents. The “mudline” occurs at about the 100 m isobath, marking the deepest limit of effective bottom turbulence for the resuspension of silt and clay on the shelf.