118
Effects of Waves and Currents
dunes is less well known. These features may be several meters high, and tens, even
hundreds, of meters apart. Some resemble the barchan dunes of the great deserts.
What kind of currents are necesary to produce such dunes? Do they only form where
certain types of sediment are available? Are the dunes relicts from periods of unusual
activity on the sea floor? These questions are still open at the present time.
In coastal areas with a sufficiently large tidal range, alternation between high and
low tide produces tidal currents. Considerable erosion can take place where tidal
currents are confined because of the high velocities attained and the large volumes of
water which are commonly involved. In a number of harbors in estuaries, the tidal
currents help flush out the entrance and prevent if from silting up. The inlets of
lagoons on the East Coast are kept open by fast tidal currents. As these currents slow
on either side of the inlet, tidal deltas form (see Fig. 3.9).
4.3.3 Upwelling. So far, we have contemplated the effects of horizontal currents on
the sea floor. Vertical water motion also affects the sea floor, in more than one way.
The overall mixinR rate of the ocean is ultimately tied to vertical motion, and this rate
is closely tied to the productivity of the ocean. In tum, the sea's productivity influences what types of biogenous sediments (carbonate, silica, phosphates) will end
up on what part of the sea floor.
A more obvious example for the effects of vertical motion is upwellinR. Off coasts
with eastern boundary currents, surface water has a tendency to move seaward, due
to deflection by the Coriolis Force which results from the rotation of the Earth.
Deflection is to the right in the nothern hemisphere, and to the left on the southern
one (Fig. 4.19a). The seaward motion is reinforced when winds blow off-shore.
Outward-bound surface waters are replaced by cold, nutrient-rich waters from within
the thermocline (100 to 200 m depth) (see Fig. 4.19b). Hence the low water temperatures off Northwest and Southwest Africa, California, Peru, and Chile, and hence also
the high production of algal plankton, which feeds zooplankton, fish, and even birds
up the food chain. The intensity of upwelling changes seasonally and also from year
to year. For example, during "El Niiio"-years, upwelling is greatly reduced both off
California and off Peru (Sect. 7.7.3). Precipitation tends to be significantly increased
during such periods, along the North Central Coast of Peru large floods can occur and
have been documented in river flood deposits for the last several thousand years. The
upwelling variations are being followed by satellite sensing of temperature and chlorophyll abundance at the sea surface.
The sediments below areas with strong upwelling are typically rich in organic
matter. For example, sediments in the Walvis Bay area, Southwest Africa, have up to
20 % of organic carbon (Corg). Opal also is usually abundant: off Walvis Bay one
finds up to 70 % opal from the frustules of diatoms. Fish debris and other vertebrate
remains also are increased in abundance, presumably delivering part of the phosphate
necessary for phosphorite formation (see Fig. 10.7).
The sediment contains many detailed clues to the intensity of upwelling. The
planktonic species (foraminifera, diatoms) tend to indicate cold water. The high supply of organic matter depresses the oxygen content, due to decay in the deep water
and on the sea floor. In extreme cases anaerobic sediments with annual layers
Effects of Waves and Currents
dunes is less well known. These features may be several meters high, and tens, even
hundreds, of meters apart. Some resemble the barchan dunes of the great deserts.
What kind of currents are necesary to produce such dunes? Do they only form where
certain types of sediment are available? Are the dunes relicts from periods of unusual
activity on the sea floor? These questions are still open at the present time.
In coastal areas with a sufficiently large tidal range, alternation between high and
low tide produces tidal currents. Considerable erosion can take place where tidal
currents are confined because of the high velocities attained and the large volumes of
water which are commonly involved. In a number of harbors in estuaries, the tidal
currents help flush out the entrance and prevent if from silting up. The inlets of
lagoons on the East Coast are kept open by fast tidal currents. As these currents slow
on either side of the inlet, tidal deltas form (see Fig. 3.9).
4.3.3 Upwelling. So far, we have contemplated the effects of horizontal currents on
the sea floor. Vertical water motion also affects the sea floor, in more than one way.
The overall mixinR rate of the ocean is ultimately tied to vertical motion, and this rate
is closely tied to the productivity of the ocean. In tum, the sea's productivity influences what types of biogenous sediments (carbonate, silica, phosphates) will end
up on what part of the sea floor.
A more obvious example for the effects of vertical motion is upwellinR. Off coasts
with eastern boundary currents, surface water has a tendency to move seaward, due
to deflection by the Coriolis Force which results from the rotation of the Earth.
Deflection is to the right in the nothern hemisphere, and to the left on the southern
one (Fig. 4.19a). The seaward motion is reinforced when winds blow off-shore.
Outward-bound surface waters are replaced by cold, nutrient-rich waters from within
the thermocline (100 to 200 m depth) (see Fig. 4.19b). Hence the low water temperatures off Northwest and Southwest Africa, California, Peru, and Chile, and hence also
the high production of algal plankton, which feeds zooplankton, fish, and even birds
up the food chain. The intensity of upwelling changes seasonally and also from year
to year. For example, during "El Niiio"-years, upwelling is greatly reduced both off
California and off Peru (Sect. 7.7.3). Precipitation tends to be significantly increased
during such periods, along the North Central Coast of Peru large floods can occur and
have been documented in river flood deposits for the last several thousand years. The
upwelling variations are being followed by satellite sensing of temperature and chlorophyll abundance at the sea surface.
The sediments below areas with strong upwelling are typically rich in organic
matter. For example, sediments in the Walvis Bay area, Southwest Africa, have up to
20 % of organic carbon (Corg). Opal also is usually abundant: off Walvis Bay one
finds up to 70 % opal from the frustules of diatoms. Fish debris and other vertebrate
remains also are increased in abundance, presumably delivering part of the phosphate
necessary for phosphorite formation (see Fig. 10.7).
The sediment contains many detailed clues to the intensity of upwelling. The
planktonic species (foraminifera, diatoms) tend to indicate cold water. The high supply of organic matter depresses the oxygen content, due to decay in the deep water
and on the sea floor. In extreme cases anaerobic sediments with annual layers