Distribution of carbonate and organic carbon contents in late quaternary sediments of the southern Marmara shelf

M. Namık Çağatay, Oya Algan, Nuray Balkıs, Mehmet Balkıs

Institute of Marine Sciences and Management, University of Istanbul, Istanbul, TURKEY


A total of 165 surface and 81 core samples from four gravity cores were analyzed for their total carbonate and organic carbon contents. The sediments are generally low in carbonate content (<10% CaCO3) along the inner shelf. Relatively higher values (>10% CaCO3) are found on the outer shelf to the north, along a belt extending from the Bozburun Peninsula to west of Mamara Island. Patches of >20% CaCOvalues occur between the lmralı Island and Bozburun Peninsula, NE of the Kapıdağ Peninsula, and in a belt extending west from the Kapıdağ Peninsula, through the Avşa Island to Karabiga. The high carbonate areas generally correspond to the sandy sediments with high contents of shell material. Total organic carbon content of the surface sediments ranges from 0.10-2.50%, with the high values being located in shallow areas along the coast and decreasing values occuring in the offshore direction. This distribution pattern suggests that the organic-carbon matter is mainly of terrestrial origin. Depth profiles of organic carbon values along the core samples indicate the presence of a sapropel layer in two cores (No.22 and 13) in the Gemlik Gulf. The highest carbonate (8-12% CaCO3) and organic carbon values (1.7-2.11%) are found in a phosphorescent green, plastic, clayey sapropelic mud horizon at a depth ranging from 1. 75 to 2. 15 m. This layer is enriched in planktonic foraminifera but depleted in benthic foraminifera species, suggesting both the high surface organic productivity and bottom unoxic conditions. It was probably deposited following the Holocene transgression during a pluival period when increased quantities of nutrients were supplied by rivers into the Sea of Marmara and when water stratification was established. Radiometric carbon, organic and isotope geochemical studies are in progress on this layer.

Keywords: Sea of Marmara, quaternary sediments, sapropels, organic carbon, total carbonate