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A b s t r a c tHigh-resolution seismic survey was conducted to investigate acoustic characteristics of gassy sediments along the southern shelf of the Sea of Marmara. The acoustic turbidity zones outlined within the study area are generally below 2-9 m (2-10 ms TWT) the seafloor whilst this vertical distance varies between 9 and 21 m (10-25 ms TWT) for acoustic blanket type reflections. The gassy sediments cover an area of sea floor of about 45, 110, and 75 km 2 in front of Gönen River, Kocasu River, and Gemlik Bay, respectively. The gassy sediments in the center of Gemlik Bay exhibited an elliptical geometry similar to its basin while the others have deltaic forms in front of the rivers. The sea bottom and near surface sedimentary units are made-up of organic-rich sediments, mostly transported by the southern rivers. The gas observed in sediments is thought to be of biogenic origin, which may be caused by degradation of organic matter in the sediment.

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High-resolution seismic data shed light on latest Pleistocene and Holocene sedimentation beneath the Büyükçekmece Bay, northern shelf area of the Marmara Sea, Turkey. Discontinuous fluvio-marine and marine deposits overlying the erosional truncation surface of Oligocene-Lower Miocene deposits are as thick as 30 m and preserved preferentially within the incised valleys that were controlled by some old faults. A series of prograding shoreline, laterally passing to the latest Pleistocene-Holocene valley-fill deposits, are thought to have accumulated mainly during times of shoreline transgression and sealevel rise. The overall morphology and stratigraphic setting observed in the Büyükçekmece Bay and at the southern outlet of the Bosphorus Strait have nearly same characteristics, implying that similar hydrodynamic conditions, erosional and depositional processes were mainly under the control of strong northerly flows during the Late Quaternary. These flows were less powerful in the Büyükçekmece region with decreased sediment input and smaller accommodation space.

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Recent sedimentation and Plio-Quaternary stratigraphy of northern Marmara shelf has been investigated by high-resolution seismic datasets. Seismic data indicate that a thin cover of Plio-Quaternary unit (termed Unit 1) overlies the Miocene/Oligocene age older sediments (termed Unit 2). The erosional surface between Unit 1 and Unit 2 is the regional unconformity for the shelf formed during the last sea level low stand. Unit 1 is subdivided into two sub-units as Unit 1a for Holocene deposits accumulated after the last glacial maximum (LGM) and Unit 1b for fluvial sediments deposited during the last low stand of the Marmara Sea. The thickness of the Holocene sediments is maximum at SW of Bosphorus outlet, in the Büyükçekmece Bay and along the coastal area between Silivri and Büyükçekmece reaching approximately 32 m. Mean sedimentation rate of the entire northern shelf is calculated as 0.4 m/1000 yr for the last 12,000 yr based on the Holocene sediment thickness. There are four depressions in the western part of the shelf, which correspond to palaeolakes during the LGM filled by Plio-Quaternary sediments. Transition from lacustrine to marine conditions in the palaeolakes occurred when the Marmara Sea level exceeded −62 m threshold depth during the sea level rise following the LGM at approximately 12,500-13,000 yr before present (BP).

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The concentrations of 12 polycyclic aromatic hydrocarbons (PAHs) were determined from 28 sediment samples taken from the Lake Iznik located in the north-west area in Turkey. Total concentration of the PAHs was observed as in the range of 17-835 ng g⁻¹ dry weight, with the highest values recorded offshore the cities of Iznik and Orhangazi, and the Sölöz creek. According to the molecular indices, contamination of the PAHs in the lake was a mixture of the atmospheric input of high temperature pyrolytic processes and the petrogenic sources transported by the creeks. Further, the higher proportion of high molecular-weight PAHs (> 85%) suggests the domination of combustion-related sources. Compared to the consensus-based sediment quality guidelines for PAHs, there are no harmful biological effects on the short term to aquatic life.