114
Effects of Waves and Currents
Surface currents and associated temperature distributions control not only plankton patterns, but also the growth of benthic assemblages, especially coral reefs (see
Sect. 7.4). The western boundary currents bring warm water to high latitudes, the
eastern ones bring cool water to low latitudes. Thus, the belt of tropical coral reefs is
much wider in the west than in the east, within each ocean basin.
The drift of icebergs and hence the transport paths of their load of rocks is controlled by currents, as ar the paths of various other drifting materials such as trees - some
of which are thought to be responsible for the dispersal of land animals on islands.
The drifting larvae of benthic organisms (meroplankton) reach their uncertain destinations with the aid of currents, and disperse throughout the ocean basins by islandhopping if necessary.
Finally, surface currents can affect the sea floor directly, through erosion on
shelves and even on upper slopes. The deep-reaching Gulf Stream sweeps fine material off the Blake Plateau east of Florida (see Fig. 1.3), keeping the manganese
pavements there clean.
4.3.2 Current Markers. How can we detect the action of currents by studying the
sea floor? The winnowing of fines from remaining coarser material, that is the "sorting" of sediment, was discussed earlier (Sect. 4.1.2). Usually, it is necessary to make
grain size determinations to show that sorting occurred. However, sometimes effects
of winnowing can be seen in bottom photography, which shows pavements of
cobbles, nodules, shells, or other winnowed layers of coarse residual material. Also,
scour marks behind obstacles are common indicators of currents.
The use of side-scan sonar, on the shelf and more recently in the deep sea, has
greatly increased our knowledge about the activity of near-bottom currents, from
their effects on sediment patterns and morphology of the sea floor (Fig. 4.15). Streaks
of coarse material in fine sediments, indicating current action, were discovered in the
entrance to the Baltic Sea by such acoustic sensing (Fig. 4.16).
The direction of a current can be readily deduced from photography and acoustic
scanning. Current strength is more difficult to deduce. Direct observations on the
effects of current strength are possible in tidal flats. The German and Dutch Wadden
flats of the North Sea have long been a natural laboratory for these kinds of observations (Fig. 4. I 7).
We have seen some features which parallel currents. Some are at right angles
also: the current ripples. Much like wind dunes, ripples have a gentle up-current side
and a steep down-current slope (Fig. 4. I 8). This type of ripple differs from the
oscillation type mentioned earlier (Sect. 4.2. I). The internal laminae of the current
ripples consist of the down-current slopes made by material falling over the edge at
the crest. Thus, fossil ripple marks (cross-bedding) yield clues to ancient current
systems in contact with the sea floor (Fig. 5.3c).
The relationship between current velocities and ripple formation has been studied
in the laboratory in some detail. Ripple characteristics depend both on the type of
sediment present, and on velocity distributions in a complex manner. The formation
of ripples of dimensions of centimeters to decimeters starts at current velocities of
about 25 to 100 cm/s. The origin and architecture of giant ripples and underwater
Effects of Waves and Currents
Surface currents and associated temperature distributions control not only plankton patterns, but also the growth of benthic assemblages, especially coral reefs (see
Sect. 7.4). The western boundary currents bring warm water to high latitudes, the
eastern ones bring cool water to low latitudes. Thus, the belt of tropical coral reefs is
much wider in the west than in the east, within each ocean basin.
The drift of icebergs and hence the transport paths of their load of rocks is controlled by currents, as ar the paths of various other drifting materials such as trees - some
of which are thought to be responsible for the dispersal of land animals on islands.
The drifting larvae of benthic organisms (meroplankton) reach their uncertain destinations with the aid of currents, and disperse throughout the ocean basins by islandhopping if necessary.
Finally, surface currents can affect the sea floor directly, through erosion on
shelves and even on upper slopes. The deep-reaching Gulf Stream sweeps fine material off the Blake Plateau east of Florida (see Fig. 1.3), keeping the manganese
pavements there clean.
4.3.2 Current Markers. How can we detect the action of currents by studying the
sea floor? The winnowing of fines from remaining coarser material, that is the "sorting" of sediment, was discussed earlier (Sect. 4.1.2). Usually, it is necessary to make
grain size determinations to show that sorting occurred. However, sometimes effects
of winnowing can be seen in bottom photography, which shows pavements of
cobbles, nodules, shells, or other winnowed layers of coarse residual material. Also,
scour marks behind obstacles are common indicators of currents.
The use of side-scan sonar, on the shelf and more recently in the deep sea, has
greatly increased our knowledge about the activity of near-bottom currents, from
their effects on sediment patterns and morphology of the sea floor (Fig. 4.15). Streaks
of coarse material in fine sediments, indicating current action, were discovered in the
entrance to the Baltic Sea by such acoustic sensing (Fig. 4.16).
The direction of a current can be readily deduced from photography and acoustic
scanning. Current strength is more difficult to deduce. Direct observations on the
effects of current strength are possible in tidal flats. The German and Dutch Wadden
flats of the North Sea have long been a natural laboratory for these kinds of observations (Fig. 4. I 7).
We have seen some features which parallel currents. Some are at right angles
also: the current ripples. Much like wind dunes, ripples have a gentle up-current side
and a steep down-current slope (Fig. 4. I 8). This type of ripple differs from the
oscillation type mentioned earlier (Sect. 4.2. I). The internal laminae of the current
ripples consist of the down-current slopes made by material falling over the edge at
the crest. Thus, fossil ripple marks (cross-bedding) yield clues to ancient current
systems in contact with the sea floor (Fig. 5.3c).
The relationship between current velocities and ripple formation has been studied
in the laboratory in some detail. Ripple characteristics depend both on the type of
sediment present, and on velocity distributions in a complex manner. The formation
of ripples of dimensions of centimeters to decimeters starts at current velocities of
about 25 to 100 cm/s. The origin and architecture of giant ripples and underwater