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

Internal waves occur nearly ubiquitously in lakes and oceans yet their sedimentary records remain largely unrecognized. Waves propagate at the interface between fluids of different densities. Surface waves propagate at the interface between air and water, which is a strong density gradient. Internal waves propagate along weaker gradients (pycnoclines) within density-stratified fluids, behaving similarly to surface waves but typically at lower frequencies and larger amplitudes. Internal waves that occur at tidal frequencies are called internal tides; they are very common on the outer continental shelf and slope, and are generated as the surface tides move stratified water up and down a sloping surface. Large internal solitary waves known as solitons are ubiquitous wherever strong currents and stratification occur in the vicinity of irregular topography. These waves can force short-period, strong bottom-current pulses and may trigger upslope-surging vortex cores of dense fluid (boluses) that can induce mobilization of bottom sediments. Internal-wave deposits (internalites) are highly variable and definitive criteria for recognition are still to be developed. In terrigenous-clastic systems and shallow-water settings, internalites can be seen as “out-of-context” tempestites, detached from shore-related deposits and lacking thickening/coarsening upward sequences. In contrast to surface storm waves, the impact of internal waves is usually strongest in mid-outer-shelf regions and weaker in shallow water. Internal waves also provide a plausible mechanism to explain the origin of hummocky cross-stratification, especially their occurrence in different depositional environments. In deeper settings (continental slopes and canyons), internalites may have sedimentary structures indicating tidal currents and may coexist with turbidites. In carbonate systems, internal waves influence both sediment remobilization and the carbonate-producing biota. Differentiation between internal waves and surface storm waves is more reliable because many skeletal constituents have specific bathymetric distributions. Moreover, internal waves influence nutrient, plankton and larval distributions while inducing thermal variations by vertical displacements of the thermocline. The sharp gradient in nutrients and the chlorophyll-maximum zone typically correspond with the base of the seasonal pycnocline, which is commonly in the lower part of the photic zone. Suspension-feeding metazoans can thrive near the pycnocline, which explains the common occurrence of Phanerozoic metazoan buildups at mid- and outer ramp settings. During paleoceanographic changes that have influenced ocean stratification, internal waves may also have been a mechanism influencing diversification and extinction of these mid- and outer-ramp benthic communities.