Multi-proxy analyses and lithology of two cores, MRS-CS18 and MRS-CS27, from the İmralı Basin of the Sea of Marmara (SoM) provide important new information on environmental conditions, relative sea level, and sill depths of the straits of Bosporus and Dardanelles during the marine isotope stages (MIS) 5 and 6. The fossil and multi-proxy geochemical records show that lacustrine conditions prevailed in the SoM during most of MIS-6, from 173 to 134 ka BP, and that the transition to marine conditions during Termination II took place at~134.06±1.10 ka BP. MIS5 interstadials a, c, and e witnessed the formation of three sapropels (MSAP-4, MSAP-3 and MSAP-2) under suboxic to anoxic marine conditions, whereas during stadials MIS5b (~94-86) and MIS5d (~112-105 ka BP), lacustrine and marine conditions with deposition of sediments having relatively low TOC contents (<2 %) prevailed, respectively. Consideration of the global sea level, together with the timing of the marine reconnection of the SoM during Termination II and persistence of the marine conditions during MIS5, except for the MIS5b, suggests that the Dardanelles sill depth was at~-75±5 m during the reconnection at Termination II and at-55±5 m during most of MIS5. On similar considerations of the Black Sea marine reconnections and disruptions during the MIS5, a sill depth of-35 to-40 m (similar to the present day depth) is indicated for the Bosporus Strait. The SoM geochemical proxy records correlate well with the regional terrestrial and marine records and the NGRIP oxygen isotope record with its glacial and interglacial (GI) phases, showing the common effect of the North Atlantic climatic events triggered by the perturbations in the Atlantic Meridional Overturning Circulation (AMOC). However, the amplitude of the oscillations recorded in the SoM during MIS6 (Penultimate Glacial Period) is relatively smaller compared to the MIS4 to MIS2 (Last Glacial Period).

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Isotopic, mineralogical, and elemental analyses have been conducted for the geochemical characteristics of the bulk carbonates in the sediment cores from the Western High and Çınarcık Basin in the Sea of Marmara to investigate the authigenic, biogenic, and detrital components and their possible use in paleoceanographic studies. The Western High is a relatively shallow (−500 to −800 m) compressional area characterized by relatively low sedimentation rates (30–40 cm/Kyr), whereas the Çnarcık Basin is a deep (~1,250 m) transtentional area represented by high sedimentation rates (>1 m/Kyr). Both 87Sr/86Sr and δ18O of bulk carbonates from the Western High exhibit significant variations, increasing steeply from 87Sr/86Sr of 0.708437 to 0.708916 and δ18O of −3.2‰ Vienna Pee Dee Belemnite (VPDB) to 0.1‰ VPDB, indicating the incursion of the Mediterranean seawater after the last glacial and reflected in the change from lacustrine to marine environment. However, bulk carbonates in the core from the Çınarcık Basin have comparatively uniform values of 87Sr/86Sr (~0.708845) and δ18O (~0.0‰ VPDB), implying that this core did not experience the lacustrine/marine transition. In the Western High, δ13C values up to +24.3‰ VPDB at 400 cm below seafloor reveal the mineralization of heavy CO2, providing independent evidence for the subsurface biodegradation of petroleum. While in the Çınarcık Basin, δ13C values of bulk carbonates were relatively constant (approximately −2.94‰ VPDB). The high δ13C values of bulk carbonates from the Western High reflect the dissolution of primary carbonates as a result of the local acidic environment and precipitation of authigenic carbonates later, supported by mineralogical and elemental results.

This study represents the lithological correlation of multi-cores taken from the various parts of the current Lake Bafa basin, BAF35, - 37 - 39 - 41, - 42, - 46. Concerning the main depositional characteristics, we reconstructed fundamental characteristics of local abrupt and gradual environmental fluctuations. The gradual changes reflect four main environmental phases are lacustrine stage (last 0.8 ky), lagoon stage (0.8–1.75 ky BP), marine-river interaction stage (1.75–2.7 ky BP) and the earliest marine-dominated stage (>2.7 ky BP).

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