Widespread overspill from a saline density-current channel and its interaction with topography on the south-west Black Sea shelf



Seaward of the Bosphorus Strait, the south-west Black Sea shelf is dominated by the world's largest channel network maintained by a quasi-continuous saline (ca 35 → 31 psu) underflow. Calculations indicate that >85% of the initial discharge of ca 104 m3s−1 spills overbank before the shelf edge. This paper documents interaction of the overspill with sea bed topography using multibeam bathymetry, echo-sounder images of the water column, conductivity–temperature–depth profiles and sediment cores. Overbank spill is widespread, particularly through crevasse channels and on the middle shelf where confinement by channel banks is negligible. Towards the outer shelf, the wind-driven Rim Current advects mud along the shelf, contributing to levée successions and deposition on stoss sides of elongate transverse ridges. Echo-sounder profiles reveal metre-scale eddies over megaflutes, and breaking lee waves and internal hydraulic jumps over ridges. Megaflutes reach 600 m long and 7 m deep, yet form where the underflow, outside the flute, is no thicker than ca 2 to 5 m. Two types of elongate seaward-facing ridges are recognized. Type 1 ridges, 2 to 5 m high, consist of bivalve-rich muddy sand in low-angle (3·5° to 6°) cross-sets created by the underflow. Type 2 ridges, ca 5 m high, have crests up to 2 km long and a buried wedge-shaped foundation (the ‘ridge-core’) comprised of facies similar to Type 1 ridges. These ridge-cores are blanketed on the landward side by stratified muds, and are capped by obliquely oriented ribs supporting a diverse benthic community. This facies distribution is interpreted to result from stoss-side and lee-side velocity and turbulence fluctuations induced by internal hydraulic jumps and breaking lee waves in overspilling portions of the underflow. Experimental results published by W.H. Snyder and co-workers effectively explain ridge evolution and flow across the ridges, and therefore can be applied with confidence to less easily studied deep-marine settings swept by turbidity currents.