Like river canyons, steep-sided submarine channels are effective transportation systems capable of carrying billions of tons of sediment across distances of hundreds of kilometers. Although previous studies have shown that helical (spiraling) flow around meander bends plays an important role in transporting sediment in rivers, a lack of field measurements from deep-ocean turbidity currents has led to competing models describing their motion around curves.
To settle this controversy, Azpiroz-Zabala et al. present the first deep-ocean measurements of turbidity currents around a submarine channel bend. Using an acoustic Doppler current profiler anchored downstream of a meander at a depth of 2,000 meters in Congo Canyon, the team acquired the velocity-depth profiles of 10 flows that occurred between December 2009 and March 2010.
Surprisingly, despite having variable thicknesses ranging from 16 to 75 meters and durations lasting from 8 hours to 10 days, nearly all of the turbidity currents displayed the same helical flow structure. It consisted of two stacked cells rotating in opposite directions, with the bottom cell revolving in the direction opposite to helical flows observed in rivers. These results are consistent with models of other types of stratified flows and support the hypothesis that the same mechanism that forms circulation cells in other geophysical flows (such as rivers and saline flows)—the interaction of competing pressure gradients—also applies to turbidity currents.
These difficult-to-obtain measurements show that the type of circulation a large-scale flow will exhibit depends upon the extent to which the current is stratified. The resulting helical flow causes the sediment to slosh from side to side, to gather at the inner bend, or to be continuously overturned. In combination with fluid turbulence, these processes keep sediment in suspension across long distances and thus play a crucial role in the ability of turbidity currents to transport enormous amounts of sediment from the continental shelf all the way to the deep-ocean floor. (Geophysical Research Letters, https://doi.org/10.1002/2017GL075721, 2017)
—Terri Cook, Freelance Writer