Abstract: A common belief about tidal sedimentation is that tides are always larger near the equator and negligible at high latitudes. This belief appears to be based on equilibrium tidal theory that predicts the existence of two ocean–surface bulges centered at low latitudes; however, it is a misconception because this theory is a poor model for real-world tides. Instead, the tide behaves as a set of shallow-water waves that are guided around the world by the continents. Tidal ranges and tidal-current speeds increase as the tidal wave propagates onto and across continental shelves; especially large ranges and fast currents can occur in coastal embayments and in straits that join two larger bodies of water. Models of real-world tides today demonstrate that tides in shallow water (<100 m) have amplitude peaks at 50° N to 70° N and 50° S to 60° S that are associated with especially wide continental shelves and coastal embayments in which the tidal wave is close to resonance. The small tides characterizing most polar areas today are the result of local geomorphic features: the Arctic Ocean is too small to have its own tide and has only a small connection to the Atlantic Ocean that prevents effective northward propagation of the tidal wave, and Antarctica has narrow and deep continental shelves that do not accentuate the tide. Nevertheless, there are local areas in both the Arctic and Antarctic with favorable geomorphology that have macrotidal ranges. Thus, the latitudinal distribution of large tides is contingent on the plate-tectonic and sea-level history of the earth and changes over geologic time as the configuration of the ocean basins and the geometry of the flooded shelves change. The latitudinal variation of the strength of the Coriolis effect has a second-order influence on tidal dynamics, with the degree of tidal-range asymmetry across a basin potentially being larger at higher latitudes. The offshore extent of large coastal tidal ranges decreases at higher latitudes because the increased Coriolis effect leads to the tidal wave being more strongly banked-up against the shoreline. Diurnal, topographically trapped vorticity waves that can generate large tidal currents in shelf-edge water depths are also limited to middle to high latitudes. The presence of ice in polar areas also has an influence on tidal dynamics. Sea ice causes a small decrease in tidal range, whereas thick, floating ice shelves can cause dramatic increases in tidal range and tidal-current speeds, at least locally, as a result of the decrease in the cross-sectional area of the water beneath the ice shelves. Because coastal sedimentation is controlled by the relative importance of tidal currents and waves, the abundance of tide-dominated deposits might not reflect perfectly the latitudinal distribution of large tides. Thus, the small size of waves in the equatorial zone appears to cause preferential development of tide-dominated coastal zones near the equator, whereas wave dominance might be higher at midlatitudes because of the higher level of storminess, regardless of the latitudinal distribution of large tides.

Abstract The submerged sill in the Strait of Messina, which is located today at a minimum depth of 81 m below sea level (bsl), represents the only land connection between Sicily and mainland Italy (and thus Europe) during the last lowstand when the sea level locally stood at about 126 m bsl. Today, the sea crossing to Sicily, although it is less than 4 km at the narrowest point, faces hazardous sea conditions, made famous by the myth of Scylla and Charybdis. Through a multidisciplinary research project, we document the timing and mode of emergence of this land connection during the last 40 kyr. The integrated analysis takes into consideration morphobathymetric and lithological data, and relative sea-level change (both isostatic and tectonic), resulting in the hypothesis that a continental land bridge lasted for at least 500 years between 21.5 and 20 cal ka BP. The emergence may have occurred over an even longer time span if one allows for seafloor erosion by marine currents that have lowered the seabed since the Last Glacial Maximum (LGM). Modelling of palaeotidal velocities shows that sea crossings when sea level was lower than present would have faced even stronger and more hazardous sea currents than today, supporting the hypothesis that earliest human entry into Sicily most probably took place on foot during the period when the sill emerged as dry land. This hypothesis is compared with an analysis of Pleistocene vertebrate faunas in Sicily and mainland Italy, including a new radiocarbon date on bone collagen of an Equus hydruntinus specimen from Grotta di San Teodoro (23–21 cal ka BP), the dispersal abilities of the various animal species involved, particularly their swimming abilities, and the Palaeolithic archaeological record, all of which support the hypothesis of a relatively late land-based colonization of Sicily by Homo sapiens .

In the past decade, several large programs that monitor currents and transport patterns for periods from a few months to a few years were conducted by a consortium of university, federal, state, and municipal agencies in the central Southern California Bight, a heavily urbanized section of the coastal ocean off the west coast of the United States encompassing Santa Monica Bay, San Pedro Bay, and the Palos Verdes shelf. These programs were designed in part to determine how alongshelf and cross-shelf currents move sediments, pollutants, and suspended material through the region. Analysis of the data sets showed that the current patterns in this portion of the Bight have distinct changes in frequency and amplitude with location, in part because the topography of the shelf and upper slope varies rapidly over small spatial scales. However, because the mean, subtidal, and tidal-current patterns in any particular location were reasonably stable with time, one could determine a regional pattern for these current fields in the central Southern California Bight even though measurements at the various locations were obtained at different times. In particular, because the mean near-surface flows over the San Pedro and Palos Verdes shelves are divergent, near-surface waters from the upper slope tend to carry suspended material onto the shelf in the northwestern portion of San Pedro Bay. Water and suspended material are also carried off the shelf by the mean and subtidal flow fields in places where the orientation of the shelf break changes abruptly. The barotropic tidal currents in the central Southern California Bight flow primarily alongshore, but they have pronounced amplitude variations over relatively small changes in alongshelf location that are not totally predicted by numerical tidal models. Nonlinear internal tides and internal bores at tidal frequencies are oriented more across the shelf. They do not have a uniform transport direction, since they move fine sediment from the shelf to the slope in Santa Monica Bay, but carry suspended material from the mid-shelf to the beach in San Pedro Bay. It is clear that there are a large variety of processes that transport sediments and contaminants along and across the shelf in the central Southern California Bight. However, because these processes have a variety of frequencies and relatively small spatial scales, the dominant transport processes tend to be localized and have dissimilar characteristics even in adjacent regions of this small part of the coastal ocean.