The role of Internal Solitary Waves on deep-water sedimentary processes: the case of up-slope migrating sediment waves off the Messina Strait

Abstract

Subaqueous, asymmetric sand waves are typically observed in marine channel/canyon systems, tidal environments, and continental slopes exposed to strong currents, where they are formed by current shear resulting from a dominant unidirectional flow. However, sand-wave fields may be readily observed in marine environments where no such current exists; the physical processes driving their formation are enigmatic or not well understood. We propose that internal solitary waves (ISWs) induced by tides can produce an effective, unidirectional boundary “current” that forms asymmetric sand waves. We test this idea by examining a sand-wave field off the Messina Strait, where we hypothesize that ISWs formed at the interface between intermediate and surface waters are refracted by topography. Hence, we argue that the deflected pattern (i.e., the depth-dependent orientation) of the sand-wave field is due to refraction of such ISWs. Combining field observations and numerical modelling, we show that ISWs can account for three key features: ISWs produce fluid velocities capable of mobilizing bottom sediments; the predicted refraction pattern resulting from the interaction of ISWs with bottom topography matches the observed deflection of the sand waves; and predicted migration rates of sand waves match empirical estimates. This work shows how ISWs may contribute to sculpting the structure of continental margins and it represents a promising link between the geological and oceanographic communities.

Migrating Bedforms Generated by Solitary Waves

“…At experimental scale, it is difficult to distinguish whether our SSW‐induced bedforms are dunes or ripples. However, ISW‐induced forms that were found off Sicily are clearly dunes (Droghei et al, ). The analogy with what we observe off Messina thus suggests that our experimental bedforms are also dunes.…”

“…Droghei et al () linked the action of refracting, tidal‐induced ISWs to bedforms observed North of the Messina Strait (Alpers & Salusti, ). We set out to investigate if a nonsteady, nonhomogeneous, repeating flow, as induced by successive solitary waves, can generate bedforms from an initially flat, sandy bottom, and if so, whether they would resemble ordinary current‐induced dunes.…”

Bed failure induced by internal solitary waves

“…The depth considered for the baseline case (H = 50 m), in which ISWs propagate, is not unique. Nonlinear, ISW‐like, internal waves of depression propagate in water depths in excess of 50 m [ Reeder et al ., ; van Haren , ; Droghei et al ., ]. The results shown so far are limited to a small range of depths in which ISWs have been recorded.…”

“…Thus, for powerful resuspension events, such as the nepheloid layer observed by Reeder et al . [] or the formation of sand waves due to ISW passage [ Droghei et al ., ], the ISW‐induced pressure could drive a substantial contribution, and in the presence of shear stress, facilitate resuspension.…”

“…The depths across all these locations vary substantially and, consequently, so does the wave-bed interaction. Understanding this connection is of interest in terms of sediment resuspension which may influence the bed morphology [Reeder et al, 2011;Droghei et al, 2016], nutrient access for organisms living far away from the bed [Aghsaee and Boegman, 2015;Quaresma et al, 2007] and the integrity of bottom-lodged structures [Bjerrum, 1973;Nataraja and Gill, 1983;Gratiot and Mory, 2000;Sumer et al, 2006].…”