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 (~1250 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‰ 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 cmbsf 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 (~ ‐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.

Plain Language Summary

Bulk carbonates in two sediment cores from the Western High and the Çınarcık Basin in the Sea of Marmara (SoM) are investigated to determine their origin and their paleoceanographic significance. The 87Sr/86Sr and δ18O results of the bulk carbonates from the Western High reveal an incursion of the Mediterranean seawater after the Last Glacial Maximum, while the comparatively uniform geochemical values of the bulk carbonates in the core from the Çınarcık Basin imply that this core did not experience the lacustrine/marine transition. In the Western High, the acidic environment in the sediments was likely caused by migration of petroleum that biodegraded anaerobically close to its migration depth. The acidic environment first led to the dissolution of primary carbonates, after which authigenic carbonates precipitated. The mixing model based on element ratios of Sr/Ca and Mg/Ca suggests that the major components of bulk carbonates are authigenic aragonite and high Mg‐calcite in the Western High, while the major components of bulk carbonates are biogenic and detrital calcite in the Çınarcık Basin.