Isotope geochemistry of bulk carbonates from the Sea of Marmara: Implications for paleo-oceanographic event in Late-Pleistocene to Holocene

Bulk carbonates of sediments from the Sea of Marmara provide evidences for major palaeo-oceanographic events from the Late-Pleistocene to the Holocene. Two sediment cores taken from the Western High and the Cinarcik Basin in the Sea of Marmara were geochemically investigated. Both ⁸⁷Sr/⁸⁶Sr, δ¹⁸O, and Sr/Caratios exhibit significant variations in the core MRS-CS-05 form the Western High, rising steeply from the freshwater stage (⁸⁷Sr/⁸⁶Sr = 0.708437, δ¹⁸O = -3.2‰, Sr/Ca = 0.95mmol/mol) to the marine water stage (⁸⁷Sr/⁸⁶Sr = 0.708916, δ¹⁸O = 0.1‰, Sr/Ca = 1.55mmol/mol), reflecting the change in water sources and recording the first incursion of the Mediterranean seawater after the Last Glacial Maximum. The comparatively constant values of most of proxies such as ⁸⁷Sr/⁸⁶Sr ( 0.708845), δ¹⁸O ( 0.0‰), Sr/Ca and ( 1.63mmol/mol) indicate that the Cinarcik Basin experienced a period of stable marine stage after the transgression.

In the Western High, the δ¹³C value of +24.3‰ VPDB in the sediments at the depth of 400 cmbsf is almost identical to that of CO₂ (δ¹³C of 27.15 23.65‰ VPDB) collected in the Western High (Bourry et al., 2009), indicative of the mineralization of heavy CO₂, while the ¹³C-riched CO₂ found in the Western High of the Sea of Marmara are originated from the biodegradation of oil which is migrated from Thrace Basin. Both the absence of carbonate peaks on PXRD patterns and the depletion of carbonate-bound Mg content at the depths of 100, 200 and 400 cmbsf support the hypothesis that primary carbonates had been dissolved in the acidic environment resulted from oil anaerobic biodegradation in diagenetic process. On the other hand, the formation of authigenic carbonates was identified by an apparently higher Mg/Ca ratio at the depth of 100, 200, and 400 cmbsf of core MRS-CS-05. Especially at the depth of 400 cmbsf, X-ray data reveal that the oligonite is the dominant carbonate mineral, which is evidenced by a significantly high contents of Fe and Mn in the carbonate phase in selective extraction experiments.

References

Bourry, C., Chazallon, B., Charlou, J.L., Donval, J.P., Ruffine, L., Henry, P., Geli, L., Çagatay, M.N., İnan, S., Moreau, M., 2009. Free gas and gas hydrates from the Sea of Marmara, Turkey : Chemical and structural characterization. Chemical Geology 264, 197-206.