Aspects of carbon isotope biogeochemistry of late Quaternary sediments from the Marmara Sea and Black Sea

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The Marmara Sea is situated between the world’s largest permanently anoxic basin, the Black Sea, and an enclosed marginal sea, the Aegean Sea, which experienced quasi-periodic sapropel deposition since Miocene time. It is connected to the Black Sea and the Aegean Sea through the Straits of Bosphorus and Dardanelles, respectively. Sapropel M1, which contains from 1 to 2% total organic carbon, was deposited in the Marmara Sea between 6 and 10.5 ka. We inferred the carbon isotopic composition of dissolved inorganic carbon (DIC) in the water column of the Marmara Sea before, during and after the deposition of sapropel M1 from the carbon isotopic composition of a planktonic foraminifera ( Turborotalita quinqueloba) and individual hexadecanoic and octadecanoic fatty acids. The period of sapropel deposition is marked by a depletion of 13C in the water column DIC, contrary to what may be expected if sapropel deposition represents a period of enhanced primary productivity. Instead, we propose that both the relative 13C depletion of DIC during sapropel deposition and the absolute values we estimated (δ 13C=−13 to −14‰) are consistent with enhancement of the relative contribution of 13C-depleted respired carbon to the water column DIC pool. Such enhancement possibly resulted from density stratification that existed during sapropel M1 deposition. The existence of density stratification is also argued from palynological and other lines of evidence (other papers in this issue), and is believed to have resulted from the encroachment of Black Sea water into the Sea of Marmara before 10 000 yr ago. Thus, sapropel M1 appears to have formed during periods of enhanced runoff and preservation and not enhanced primary productivity.