Recent Canyon Heads at the Bosphorus Outlet

The Black and Marmara Seas have witnessed increased scientific interest in last decade due to improved cooperation between the riparian countries and western scientific institutions but also due to the controversy existing about the origin of the reconnection of the Black Sea and Mediterranean seas after the last Glacial Maximum and its ensuing sea level rise. The Black Sea is linked to the global ocean only through the Bosphorus-Dardanelles system of straits. The Bosphorus is narrow (0.76 to 3.6 km wide) and shallow (32 m) at the sill, restricting the two-way water exchange between the brackish Black Sea and the very saline Mediterranean Sea. The Bosphorus sill was responsible for the behaviour of the Black Sea during the global glaciations and deglaciations, during which the Black Sea level followed the global sea level changes as long as they were higher than the sill. When global sea level was lower than the Bosphorus sill the variations of the Black Sea level reflected specific regional climate conditions without being coupled to the ocean changes. Recent studies suggest that a rapid flooding event may have occurred in the Black Sea during the Holocene. In 1998, a French-Romanian survey collected 4500 km of high-resolution seismic profiles, multibeam bathymetry, and sediment cores on the northern margin of the Black Sea where the shelf is sufficiently wide to preserve ancient shorelines in the vicinity of the shelf edge. If rapid flooding occurred through the Bosphorus Strait to drown these shorelines, it should have created a cataract. In August 2002, the French research vessel "Le Suroit" equipped with a EM 300 multibeam echosounder and a TritonElics Chirp Sonar mapped the Bosphorus outlet at the shelf edge. The results show a large retrogressive canyon deeply incised into the shelf which can be followed landward towards the Bosphorus outlet. Coring on the shelf and in the canyon revealed mega-ripples of shell debris of recent origin.