Northwestern Aegean Sea Research Articles

Population Characteristics of the Upper Infralittoral Sea Urchin Arbacia lixula (Linnaeus, 1758) in Eastern Mediterranean (Central Greece): An Indicator Species for Coastal Water Quality

The black sea urchin (Arbacia lixula, Linnaeus, 1758) is a non-edible marine echinoderm of high ecological importance with the potential to affect marine ecological communities. A. lixula were sampled monthly for one year from the supralittoral fringe at two locations in the Pagasitikos Gulf, in the north-western Aegean Sea. Morphometric characteristics exhibited significant spatiotemporal variation. The population in closer proximity to treated sewage effluent outflow exhibited significantly higher biometric relationships resulting in possible improved physiological conditions. Spatial distribution exhibited a clumped pattern of dispersion, consisting of predominantly six age classes. The dominant cohort was the four-year age class, comprising 31.2% of the total population. Significant negative allometric relationships were exhibited between all morphometric characteristics. The maximum approximate age of the total A. lixula population was estimated at 15.27 years. The von Bertalanffy growth equation for the entire population was estimated as: test diameter = 62.881×1−e−0.196×Age+1.147. The gonadosomatic index indicated a seasonal cycle with a peak in late spring. The approximate age of sexual maturity was estimated at 4.45 years. We observed a significantly higher number of females than expected at the site in closer proximity to the treated sewage effluents (32% of total female number).

Ecosystem trophic structure and fishing effort simulations of a major fishing ground in the northeastern Mediterranean Sea (Thermaikos Gulf)

Ecosystem modelling constitutes a useful tool for the ecosystem approach to fisheries management, which demands a shift to more complex models that include multi-species trophic interactions, environmental and anthropogenic forcing. The Thermaikos Gulf is a shallow gulf in the northwestern Aegean Sea (Greece) and one of the major fishing grounds of the northeastern Mediterranean concentrating high fishing effort of trawlers and purse-seiners and producing more than 20% of the total Greek catches. In the present work, we developed an Ecopath base model and ran Ecosim simulations for 26 years (2000–2025), including the calibration period (2000–2016), aiming to describe the food web structure and function of the Thermaikos Gulf, identify main components and interactions among the 33 functional groups, assess the ecosystem impacts of fishing over time, and compare ecosystem properties with other Mediterranean areas. Overall ecosystem degradation with biomass and catch decline was observed at the end of the calibration period due to the impact of environmental factors and fishing activities. The ecosystem seemed to stabilize in an intermediate state by the end of the projection years, but with an overall biomass and catch decline. Fishing effort reduction after 2016 resulted in higher biomass and catches compared to 2014–2015, that could not however reach the 2000 levels in most cases. The examined fishing effort reduction scenarios clearly showed that the more the fishing effort is reduced, the higher the biomass in the ecosystem and the lower the catches obtained compared to the baseline scenario. In a nutshell, since environmental drivers may be harder to predict or control, lowering the exploitation levels is an important step towards the rebuilding of overfished marine resources and more resilient ecosystems.

Seasonal living coccolithophore distribution in the enclosed coastal environments of the Thessaloniki Bay (Thermaikos Gulf, NW Aegean Sea)

Living coccolithophores collected from a yearly time interval, January to December 2016, at the SP1 station of the Thessaloniki Bay, an enclosed coastal environment located in the inner part of the Thermaikos Gulf (Northwestern Aegean Sea) were analyzed to determine their seasonal variation and evaluate the effects of environmental factors on their density, species composition and distribution. Our results showed that coccolithophores constituted a significant component of the phytoplankton community and were characterized by the dominance of few opportunistic species, as an impact of eutrophic environmental conditions. Temperature seasonal cycle and the variations in nutrient inputs from riverine and/or anthropogenic sources seem to control the seasonal pattern of coccolithophores. Emiliania huxleyi with densities as high as ∼4.0 × 105 cells l−1 predominated during the late spring–early summer, whereas a strong dominance of Gephyrocapsa oceanica with densities ∼2.0 × 105 cells l−1 was observed during the late summer and the autumn. Emiliania huxleyi appeared to dominate the coccolithophore assemblages once more during the winter; while Helicosphaera carteri was continuously present during the wintertime (exceeding ∼2.7 × 103 cells l−1). Emiliania huxleyi was entirely represented by morphotype type A in the studied samples. Morphometric analysis of E. huxleyi coccoliths indicated that the population is dominated by lightly calcified forms, although an increase of more heavily calcified coccoliths was observed during late autumn-early spring in line with the well-documented seasonal succession pattern of E. huxleyi type A forms in the Aegean Sea.

Vertical distribution of 137Cs and natural radioactivity in core sediments of Thermaikos Gulf, northwestern Aegean Sea

Sub-superficial sediments have been collected from the Thermaikos Gulf, located in the NW Aegean Sea, in order to determine sedimentation rates using vertical distributions of 137Cs and 210Pb. In addition measurements of the characteristic radionuclides of the 238U and 232Th natural series has also been performed to better understand the transport and the sedimentation process in a marine environment, which is strongly influenced by the main North Greece rivers. The sedimentation rates of the five selected cores using the two methods were consistent and varied from 0.20 ± 0.01 till 0.30 ± 0.02 cm y-1. The mean sedimentation rates derived with both methods are enhanced during the last ten years compared to previous values at the studied region by one order of magnitude, due to human implications. The activity concentration of the 238U decay products does not exhibit variation along the depth. The activities of 226Ra are enhanced (~30 %) compared to the activities of radon daughters (214Pb, 214Bi) especially at the surface layer. The results of the correlation between 232Th decay products and 40K indicate that there is positive correlation between their averaged activities. The good proportionality between 232Th and 40K concentrations may be attributed to the fact that thorium is a particularly insoluble element in water.

Mapping Mediterranean seagrasses with Sentinel-2 imagery

Mediterranean seagrasses have been hailed for their numerous ecosystem services, yet they are undergoing a decline in their coverage. The major complication with resolving this tendency is the sparsity of data on their overall distribution. This study addresses the suitability of the recently launched Sentinel-2 satellite for mapping the distribution of Mediterranean seagrass meadows. A comprehensive methodology is presented which applies atmospheric and analytical water column corrections and compares the performance of three different supervised classifiers. Remote sensing of the Thermaikos Gulf, northwestern Aegean Sea (Greece, eastern Mediterranean Sea) reveals that the utilization of Support Vector Machines on water column corrected reflectances yields best accuracies. Two Mediterranean seagrasses, Posidonia oceanica and Cymodocea nodosa, cover a total submerged area of 1.48km2 between depths of 1.4–16.5m. With its 10-m spatial resolution and 5-day revisit frequency, Sentinel-2 imagery can mitigate the Mediterranean seagrass distribution data gap and allow better management and conservation in the future in a retrospective, time- and cost-effective fashion.

Air and seawater pollution and air-sea gas exchange of persistent toxic substances in the Aegean Sea: spatial trends of PAHs, PCBs, OCPs and PBDEs.

Near-ground air (26 substances) and surface seawater (55 substances) concentrations of persistent toxic substances (PTS) were determined in July 2012 in a coordinated and coherent way around the Aegean Sea based on passive air (10 sites in 5 areas) and water (4 sites in 2 areas) sampling. The direction of air-sea exchange was determined for 18 PTS. Identical samplers were deployed at all sites and were analysed at one laboratory. hexachlorobenzene (HCB), hexachlorocyclohexanes (HCHs) as well as dichlorodiphenyltrichloroethane (DDT) and its degradation products are evenly distributed in the air of the whole region. Air concentrations of p,p'-dichlorodiphenyldichloroethylene (p,p'-DDE) and o,p'-DDT and seawater concentrations of p,p'-DDE and p,p'-DDD were elevated in Thermaikos Gulf, northwestern Aegean Sea. The polychlorinated biphenyl (PCB) congener pattern in air is identical throughout the region, while polybrominated diphenylether (PBDE)patterns are obviously dissimilar between Greece and Turkey. Various pollutants, polycyclic aromatic hydrocarbons (PAHs), PCBs, DDE, and penta- and hexachlorobenzene are found close to phase equilibrium or net-volatilisational (upward flux), similarly at a remote site (on Crete) and in the more polluted Thermaikos Gulf. The results suggest that effective passive air sampling volumes may not be representative across sites when PAHs significantly partitioning to the particulate phase are included.

Recent evolution of a Mediterranean deltaic coastal zone: human impacts on the Inner Thermaikos Gulf, NW Aegean Sea

The Inner Thermaikos Gulf is located in the northwestern Aegean Sea, receiving water and sediment fluxes from the Axios, Aliakmon, Gallikos and Loudias Rivers. The geomorphological and sedimentological evolution of the system is reconstructed for the last 150 years (1850–2000), on the basis of detailed analysis of historical bathymetric charts. Late Holocene history is considered within the context of changing riverine sediment supply and human activities. Three evolutionary stages are identified. Stage I (1850–1916) corresponds to a natural phase of rapid deltaic progradation and sea-floor deposition, with an average sediment accumulation rate of 6.5 × 10 6 m 3 a −1 . During Stage II (1916–1956), human interference (e.g. artificial changes in river delta plains, realignment of channels and land reclamation schemes) to the deltaic system increased sediment delivery to the coastal waters by a factor of three; this, in turn, enhanced the progradation of the active river mouth areas. In contrast, Stage III (1956–2000) is characterized by significant coastline (deltaic) retreat and erosion of the adjacent sea floor (net loss of 2.5 × 10 6 m 3 a −1 ); this was as a result of extensive river damming, which trapped a significant part of the sediment moving seaward. Furthermore, these human impacts have affected the character of the surficial sea-bed sediments of the Gulf, by reducing the proportion of mud. The response of the deltaic margin of the Inner Thermaikos Gulf to various anthropogenic interventions seems to be analogous to that of other deltas in the Mediterranean region where large drainage projects, the development of irrigation networks and dam construction have taken place within their river basins.

234Th: 238U disequilibrium as an indicator of sediment resuspension in Thermaikos Gulf, northwestern Aegean Sea

234Th: 238U disequilibria have been used extensively as tracers of particle dynamics in marine environments. 234Th ( t 1/2=24.1 days) can be used as “proxy” for particle reactive pollutants, due to their similar rapid rate of scavenging onto particles and subsequent removal from the water column, to the sediments. Radioactive disequilibrium can be exploited to determine the rates and time-scales of processes occurring over days to months; in this instance the residence times of dissolved and particulate species with the benthic nepheloid layer (BNL). Three sampling cruises were undertaken in Thermaikos Gulf (NW Aegean Sea) during contrasting periods, to examine the impact of natural and anthropogenic activity on sediment resuspension. September and October 2001 represented background and trawling periods, respectively; January 2002 represented a mixed period, of trawling and storms. Dissolved 234Th is scavenged actively at the BNL, in the presence of suspended particulate material (SPM), with a mean residence time of 16 days. There is a weak inverse correlation between dissolved 234Th residence time and SPM concentration in the BNL, with the shortest residence times occurring during October 2001. No relationship was observed between particulate 234Th activities and SPM concentrations, indicating that particles are rapidly removed from the system, either by sinking or advection. The mean particulate 234Th residence time is 5 days.

Effects of bottom trawling on the quantity and biochemical composition of organic matter in coastal marine sediments (Thermaikos Gulf, northwestern Aegean Sea)

The effects of bottom trawling on quantity and the biochemical composition of sediment organic matter were assessed, along a gradient of trophic state in a coastal area of the Gulf of Thermaikos (Aegean Sea). Total organic carbon concentrations and other organic variables (such as biopolymeric C concentrations) displayed a significant increase immediately after the initiation of trawling activities, reaching unexpectedly high values when compared to a control (i.e. samples collected before trawling events). The lack of significant changes in phytopigment content of the sediments, observed after trawling events, indicates that primary production processes did not change markedly. Since also sediment properties did not vary significantly during the period of investigation, the observed changes in sediment organic content can be related with organic matter uplift from deeper sediment layers, caused by reworking (bottom up transfer) and/or reduced C and N consumption by benthic biota. Trawling determined also significant changes in the biochemical composition of sediment organic matter. Indeed, sediment resuspension increased the hydrolysable fractions of protein and carbohydrate pools, likely to be the result of increased degradation rates under oxic conditions. Such an effect indicates that trawling may increase the quality and bioavailability of organic C to consumers; this, in turn, could modify the energetic and trophic state of the benthic systems. We conclude that bottom trawling might have important trophodynamic consequences, for benthic microbial and meiofaunal assemblages.

Temporal and spatial variations in the geochemistry of major and minor particulate and selected dissolved elements of Thermaikos Gulf, Northwestern Aegean Sea

The objectives of this study were to examine both spatial and temporal changes of particulate major elements and minor metals, as well as dissolved Mn and Cd, in the waters of Thermaikos Gulf. Collections of water and suspended particulate matter (SPM), as depth profiles (5–8 depths), were undertaken at 10 principal stations, essentially on a N–S traverse of the western side of the Gulf. One of the principal aims of the study was to observe if there was any change in the patterning of the elements between the three occupations of the stations: (a) in September 2001, immediately before the commencement of trawling; (b) in October 2001, whilst fishing was active; and (c) in winter/early spring conditions (February 2002), when fishing was still active, but after a change of river/atmospheric conditions. Bottom (∼20 m) waters were dominated by sediment resuspension; this was identified by concentration changes in the aluminosilicate elements (e.g. Al, Ti, K, Fe) of the SPM. A two- to three-fold increase occurred between September and October, caused probably by trawling; this was sustained at the offshore stations, in February. During February, the western inshore stations showed little sediment resuspension, caused by extreme winter cooling and the sinking of water. Consequently, a N–S density discontinuity existed at all water depths, which prevented the thermohaline cyclonic circulation from penetrating into the western seaboard of the Gulf. The distribution of dissolved and particulate Mn in the lower waters was due to redox cycling of the element at the benthic boundary; this was more intense in the north, where the organic supply was higher. Biogenic element concentrations and Ca/Al, Si/Al ratios showed no evidence that trawling activity promoted higher biological production. Strong correlations of Co, Cr, Ni and V, with Al and K, showed that these elements were associated strongly with detrital aluminosilicates. However, the variable association of Cd, Pb and Zn, with K (and Al), especially in the upper waters, implied an anthropogenic source derived from the rivers and the city of Thessaloniki. Examination of the Kd's of Cd showed a two-order of magnitude decrease with depth, caused by resuspension and possible advection of relatively unpolluted sediments, into the western Gulf.

Modelling studies on the processes that influence matter transfer on the Gulf of Thermaikos (NW Aegean Sea)

The development and application of a three-dimensional hydrodynamic and sediment transport model for the Gulf of Thermaikos, in the Northwestern Aegean Sea is presented. The processes that control the transfer of river-borne waters and fine-grained sediments are examined. These processes are: the overall basin circulation, the wind-driven flow, the buoyancy-driven flow that is associated with river plume dynamics and the sedimentation processes (flocculation, settling, deposition and erosion). We perform individual experiments, where only some of the processes are included, in order to study the different components of the transport mechanism separately. We also perform comprehensive experiments, where all processes are taken into account. It is found that a large percentage of the suspended particulate matter is deposited on the seabed of the Thermaikos Gulf and mainly near the river discharge sites. Although the sedimentation processes are of primary importance, the transport rates and pathways are greatly affected by the hydrodynamic processes that control circulation. Furthermore, the basin topography has a considerable impact on the transport, as the main rivers discharge near the narrow basin head and near areas of strong coastline curvature that act as sediment trapping zones.

A Note On Three Abnormal Samples Of Benthic Foraminifers From The Dikili Bay (TURKEY) In The Northeastrn Aegean Sea: Peneroplis planatus (Fichtel and Moll), Rosalina sp., and Elphidium crispum (Linne)

Three of nine foraminifer samples collected from the Dikili Bay in the northwestern Aegean Sea yielded three species demonstrating aberrance in development: Peneroplis planatus (Fichtel and Moll), Rosalina sp. and Elphidium crispum (Linne). We suggest that the cause for such abnormalities may well be thermal springs, which are known to exist in the regions of the Dikili Bay and the Island of Lesbos to the west, and correspondingly high proportion of certain elements in the waters.

Toxicity and heavy metal contamination of surficial sediments from the Bay of Thessaloniki (Northwestern Aegean Sea) Greece.

Surficial sediments (the fraction < 2000 microm) from the Bay of Thessaloniki, in the Northwestern Aegean Sea, Greece were examined for heavy metal and organic carbon contents, as well as for acute LUMIStox toxicity of pore waters (PWs), wet sediment elutriates (WSEs) and dry sediment elutriates (DSEs) obtained from the solid material remaining after PW extraction. WSEs where not toxic. EC20/50 values could be measured in some DSE and PW samples. In all sediment samples, the DSE toxicity was greater than the corresponding PW toxicity. Sediment concentrations of total and labile Cd, Pb, Cu, Cr, Zn, Mn, Ni and Fe were determined and evaluated in relation to sediment quality guidelines. Pollution levels ranged from low to high for certain metals. A misfit between sediment toxicities calculated from heavy metal concentrations and those biologically measured was observed. Toxicity values were in general poorly correlated with sediment's heavy metal or organic carbon content. As significant correlation was however found between the DSE toxicity with total Ni and labile Fe, as well as for the PW toxicity with total Ni, labile Fe and labile Cr.

River plume development in semi-enclosed Mediterranean regions: North Adriatic Sea and Northwestern Aegean Sea

The Mediterranean Sea has many coastal regions with complex topography. The study concentrates on semi-enclosed regions that receive freshwater input from rivers. The development of the river plumes under the influence of the important circulation forcing mechanisms and under the guidance of the topographic controls is examined. The study rivers are the Po (North Adriatic) and the Axios (North Aegean). They both provide important inputs of low-salinity waters and land-drained materials for their respected shelf regimes. The rivers are parameterised as sources of zero salinity in the three-dimensional, primitive equation Princeton Ocean Model (POM). The river plumes are studied in the presence of realistic topography and idealised winds. The influence of the general basin circulation on the plume structure is also examined. The development and evolution of the two river plumes exhibit the following common characteristics. The buoyancy-driven flow is characterised by an anticyclonic bulge seaward of the river mouth and a coastal current south of the discharge site contained within a narrow strip along the coast. The coastal current has a southward direction, as both rivers discharge at the west side of the basin. The “meandering” along this dynamic region is due to a baroclinic instability process. Downwelling-favorable winds enhance the buoyancy-driven coastal current and restrict offshore removal of low-salinity waters, while the opposite holds for upwelling-favorable winds. The circulation is influenced by the presence of three coastal boundaries. Differences in the modeled phenomena for the two study regions arise from (a) interaction with ambient flow, which may substantially influence the behavior of the plumes, and (b) the topography and geometry of each basin. For instance, the presence of strong neighboring rivers influences the Po River plume. On the other hand, the narrowness of the basin near the discharge site of the Axios River allows for offshore expansion of the plume all the way to the east coast and the establishment of anticyclonic flow over a large part of the shelf, regardless of wind conditions.

Chemical cycles in Pliocene sapropel-bearing and sapropel-barren eastern Mediterranean sediments

High-resolution (3–4 cm) investigations of major and minor elements were carried out on sediment cores from ODP Leg 160 Sites 964 (Ionian Basin), 969 (Mediterranean Ridge), 967 and 966 (northern slope and top of Eratosthenes Seamount, south of Cyprus). At Site 967, element/Al ratios display pronounced cyclic records that are related to astronomically-forced climatic variations. Sediments from the top of Eratosthenes Seamount (Site 966) show a similar pattern to those from Site 967, but they are more disturbed by bioturbation and also strongly influenced by long-term variations in sediment dispersal. Sediments from Site 969 display similar trends to those from Site 964, i.e. they are characterized by higher Mg/Al, slightly higher K/Al and lower Ti/Al ratios during sapropel formation. Higher Ti/Al ratios at Site 969 compared with sediments from Site 964 may be explained by a westward transport of material from a source in the east, where Ti/Al ratios are highest (Sites 966 and 967). Elevated Mg/Al ratios at Site 969 during sapropel intervals are due to river-derived material, transported through the Kithira Strait between Peloponnesus and Crete from the northwestern Aegean Sea, which originates from ultramafic rocks. A change in intermediate water currents (current reversal?) during sapropel formation may explain the enhanced supply of Mg-rich and the relatively diminished supply of Ti-rich material. We propose that, during deposition of the reddish intervals at Site 969, a change in circulation did not occur, because a drastic decrease in Ti/Al ratios is not evident. In these sediments Ba peaks, indicative of former sapropels, are not present. Ba enrichments in the same sapropel-barren intervals at Site 967 (northern slope of Eratosthenes Seamount) suggest that, during such intervals, the ‘burn-down’ process was more effective than during the sapropel-bearing intervals. Mn contents in all investigated Pliocene sediments correlate with the carbonate contents. Higher Mn contents of the sapropel-barren sediments suggest that, during time intervals of sapropel formation, either Mn accumulation at the margins of the permanently oxic upper water column, or Mn export into the western Mediterranean, might have occured. The latter could only be explained by sub- or anoxic conditions in large parts of the water column.

Water circulation and eutrophication in the northwestern Aegean Sea: Thermaikos Gulf

Seasonal water property distributions and a combination of Lagrangian and Eulerian current data are used to investigate water circulation patterns, and associated transfer mechanisms of pollutants, in the northwestern Aegean Sea (Thermaikos Gulf). Nutrient concentrations within the various water masses of the study area are presented and comparisons are made with nutrient concentrations in other coastal waters of the Aegean Sea, to assess the extent of eutrophication in Thermaikos Gulf. Nutrient levels are attributed to the different nutrient sources, as well as to the morphology and water circulation patterns in the various areas.

Water circulation and eutrophication in the northwestern Aegean Sea: Thermaikos Gulf

Abstract Seasonal water property distributions and a combination of Lagrangian and Eulerian current data are used to investigate water circulation patterns, and associated transfer mechanisms of pollutants, in the northwestern Aegean Sea (Thermaikos Gulf). Nutrient concentrations within the various water masses of the study area are presented and comparisons are made with nutrient concentrations in other coastal waters of the Aegean Sea to assess the extent of eutrophication in Thermaikos Gulf. Nutrient levels are attributed to the different nutrient sources, as well as to the morphology and water circulation patterns in the various areas.

Circulation and distribution of suspended matter in the Sporades basin (northwestern Aegean sea)

A hydrographic survey, comprising 85 stations with measurements of temperature, salinity and turbidity, was performed in the semi-enclosed margin of the Sporades Basin (northwestern Aegean Sea) in June 1987. The data have been used to investigate the shelf-slope suspended matter exchanges in relation to the hydrology. The circulation, derived from hydrographic data, produces complex eddy patterns. The suspended matter distribution, determined from light scattering measurements, indicate a relationship between the suspended matter and the density-related flow field. Transport of suspended matter, supplied by rivers, is traced through the bottom nepheloid layer, mostly over the inner shelf and along the western coastline. There is however some evidence of transfer of suspended matter from the shelf towards the upper slope. The mid-depth nepheloid layer along the slope is thought to relate to advection of terrigenous material by topographically-controlled currents, and its seaward extension is inhibited by a strong anticyclonic circulation in the deep basin.

Mechanisms of sediment transport and deposition: Sediment sequences and accumulation during the Holocene on the Thermaikos plateau, the continental slope, and basin (Sporadhes basin), northwestern Aegean Sea, Greece

Detailed 3.5-KHz seismic reflection profiling carried out on the Thermaikos plateau, the slope and the basin (Sporadhes basin), northwestern Aegean, has shown that during the Holocene, the area received fine-grained sediments discharged mainly by the rivers Axios, Aliakmon and Pinios. A well-developed oblique sigmoid progradation pattern has been observed on the seismic profiles taken across the deltaic Holocene sequences of these rivers. Three seismic facies have been identified within the oblique-sigmoid pattern: (1) Dark homogeneous facies that represent fine sand deposits along the delta front (topsets), (2) stratified facies that represent alternating muddier or sandier horizons (foresets), and (3) transparent facies in which the muddy sediments predominate (bottomsets). The base of the Holocene (reflector C) has been found beneath the oblique-sigmoid pattern. The thin homogeneous and transparent seismic facies throughout the slope and the basin has been interpreted as the Holocene muddy sediment sequence in the area. Bottom samples from the outer Thermaikos plateau have shown that it is floored by relict terrigenous sand at depths of 90–110 m. The sediment and clay mineral distribution and the presence of relict sand confirm that during the Holocene period, the fine-grained sediment dispersion and deposition was effected by the water-mass circulation. The majority (80–90%) of the sediments dispersed from the rivers during the Holocene has been captured by the extensive shelf (Thermaikos plateau), having deposited from the near-bottom nepheloid layer. Minor amounts (10–20%) of very fine-grained material have escaped and deposited from suspension (hemipelagic settling) in the slope and basin of the studied area.

Mass movements, geotechnical properties, and slope stability in the outer shelf—Upper slope, northwestern Aegean sea

Abstract Subbottom profiling (3.5 kHz) in the uppermost slope of the outer shelf of the northwestern Aegean Sea has shown downslope slumping and intense sediment deformation as well as a succession of microslumping within the Late Pleistocene delta sequences. Postdeposition bottom erosion by currents and the overconsolidated sediments (Cu/Po > 1) indicate the termination of deformations a long time ago. Deformation processes are estimated to have been active 18,000–10,000 B.P. Large‐scale active downslope deformations with clockwise sediment block rotation have affected the area as well. These longer‐period deformations are associated with regional neotectonics and older unconformities (i. e., Plio‐Pleistocene) as potential slide planes. Geotechnical properties reflect mainly textural variations and locally (within sapropelic layers) high organic matter content. Relatively high values of shear strength were measured (5–29 kPa) with intermediate sensitivities (2–5), whereas relatively low values of water c...