Gravity-driven flow in a submarine channel bend: Direct field evidence of helical flow reversal

Submarine meandering channels are conduits that transport gravity-driven flows and sediments into the deep sea. Such channel systems form distributive networks across submarine fans, ultimately forming the largest sedimentary deposits on Earth. Despite this, our understanding of flow processes and the sedimentary evolution of sinuous submarine channel systems remains poor, primarily due to a lack of, thus far elusive, direct field observation and measurements of flows within submarine channel bends. In the absence of direct field measurements, our understanding of these systems has necessarily been speculative, relying until very recently on bend flow theory derived from research in subaerial river channel bends. Although recent measurements and results from scaled laboratory experiments and numerical modeling of submarine channel bends have advanced our understanding of some of the relations between flows, forms, and the implications for sedimentary deposits, key results from this recent research have been contradictory. Notably, several studies have indicated that the helical flow structures within submarine bend flows closely match those found ubiquitously in subaerial river channels, while conflicting research has reported a reversal of this helical flow structure, with associated far-reaching implications for deep-sea sedimentology. This paper presents the first direct three-dimensional measurements of the flow field in a natural submarine channel bend. The results, from a submarine channel bend on the Black Sea shelf, demonstrate for the first time that a reversed helical flow structure can occur in seafloor channel bends. Such findings have major implications for process sedimentology in these environments, because the direction and strength of helical flow fields are known to impart a significant influence on cross-stream sediment sorting in bends and thus the stratigraphy of the deposits produced by these channel systems.