Internal waves, an under-explored source of turbulence events in the sedimentary record

The impact of internal waves in the sedimentary record has been largely unrecognized, even though their occurrence is ubiquitous in oceans and lakes. Tempestites (products of surface storm waves) and turbidites (sediment remobilization on the shelf margin and redeposition along the slope and into the basin) are the most frequently recognized event deposits (eventites) in the stratigraphic record. They share, as product of turbulence events, a first erosional phase and a subsequent depositional phase during the waning of the turbulence. Their differences, resulting in distinct sedimentary structures, are related to the different nature of the turbulence process. In contrast to tempestites and turbidites internal waves (IWs) create episodic turbulence events in mid-shelf settings, where the seasonal pycnocline forms, or on the continental slope and submarine canyons under the influence of the permanent pycnocline at the base of the mixed layer. Internal waves propagate between two density-stratified fluids. Any perturbation of the pycnocline (storms, wind-stress fluctuations, tsunamis, tidal currents, etc.) can propagate as an internal wave. IWs behave similarly to surface waves but amplitudes can be larger than 100 m and can occur at lower frequencies. Large internal solitary waves (LIWs or solitons) are widespread and ubiquitous wherever water currents and stratification occur in the neighborhood of irregular topography. The propagation of LIWs forces short-period, strong bottom-current pulses and may trigger upslope-surging vortex cores of dense fluid (boluses), which can induce remobilization of bottom sediments on the shelf. Internal tides are internal waves at tidal frequency; they are very common on the continental shelf, slope and canyons, and are generated as the surface tides move stratified water up and down a sloping surface. The impact of IWs in carbonate systems is two fold: sediment remobilization and impact on the carbonate-producing biota. IWs are fundamental in carrying nutrients, in influencing plankton and larvae distribution, and also in inducing thermal variations associated with vertical displacements of the thermocline. In carbonate systems, the differentiation between IWs and surface storm waves becomes more reliable because the bathymetric dependence of many skeletal- and some non-skeletal grains provides information of the original bathymetric position of the grains impinged by IWs. Additionally, a sharp gradient in nutrients and a maximum of chlorophyll are often co-located with the base of the seasonal pycnocline, commonly in the lower part of the photic zone where nutrient-rich (deeper) water enters the nutrient-depleted photic zone through the mechanism of upwelling or internal waves. Since substantial habitats are at middle to outer shelf or ramp settings, the depth at which the base of the mixed layer/top of the thermocline commonly occurs (i.e., ~30 to ~130 m), internal waves may also have been a mechanism to force diversification, evolution and extinction with changes in ocean stratification.