Deltas south of the Bosphorus Strait record persistent Black Sea outflow to the Marmara Sea since ∼10 ka

Abstract

At the southern exit of the Bosphorus Strait in the northeastern Marmara Sea, high-resolution seismic profiles reveal two lobate, progradational delta lobes in modern water depths of ∼40–65 m. The younger delta was active from ∼10 to 9 ka based on radiocarbon dates of equivalent prodelta deposits and the elevation of its topset-to-foreset transition. The topset-to-foreset transition climbs in the seaward direction because the delta prograded into a rising sea. Low abundances of marine fauna and flora in the 10–9-ka interval support a deltaic interpretation. There are no rivers in the area that could have fed the delta; instead, all evidence points to the strait itself as the source of sediment and water. When this outflow was strongest (∼10.6–6.0 ka), sapropels accumulated in basinal areas of both the Aegean and Marmara seas. Benthic foraminiferal and dinoflagellate cyst data from contemporary deposits elsewhere in the Marmara Sea point to the continual presence through the Holocene of a surface layer of brackish water that we ascribe to this same outflow from the Black Sea through the Bosphorus Strait. By ∼9.1–8.5 ka, two-layer flow developed in the Bosphorus Strait as global sea level continued to rise, and the sediment supply to the younger delta was cut off because the outflowing Black Sea water ceased to be in contact with the floor of the strait. The older delta lobe lies below a prominent lowstand unconformity and is tentatively interpreted to have formed from ∼29.5 to 23.5 ka (oxygen-isotopic stage 3) when the Marmara Sea stood at ∼−55 m and a second sapropel accumulated in deep basinal areas.