188
ern boundary of an ocean, for example at the base of
the continental margin (Fig. 5.1 e). They approximately follow the contours of the ocean basin and are
therefore referred to as contour currents. Such currents mayaIso occur on the eastern margin, but they
usually flow poleward.
Upwelling and downwelling also occur in coastal
waters. Onshore surface currents tend to stack up
water along the coast, which will consequently begin
to sink and replace deeper water (coastal
downwelling). Offshore currents generate coastal
upwelling of intermediate or deep water. Due to the
effect of the Eckman spiral, these processes are initiated or intensified by winds blowing parallel to the
co ast line, for example in front of a coastal mountain
range (Fig. 5.2d). These mIes allow predicting of
coastal upwelling or downwelling for locations in the
northern or southern hemisphere when the wind direction is known.
If an ocean extends into polar regions and is partly
covered by sea ice during a glacial period, the highlatitude area of downwelling high-density water, i.e.
the polar front, moves toward lower latitudes. At the
same time, the comparatively stable water stratification at low latitudes will persist. Large quantities of
sea ice may lead to an increase in ocean salinity by 1
to 2%0 and thus stabilize the deep water mass. In addition, temporarily melting ice can locally dilute surface water and further promote density stratification.
As a result of both processes, thermohaline circulation may slow somewhat and oxygen transport into
deep water may become reduced. Further consequences of glaciation are discussed in Sect. 5.6.
5.2.4 Current Speeds, Transport of
Water Masses, Heat, and Nutrients
Surface current speeds can locally reach values as
high as 150 to 250 crn/s (Gulf-Stream, Kuroshio off
Japan), but are often considerably lower. The
present-day Antarctic circumpolar current has a
speed of 4 to 15 crn/s. Deep bottom currents can 10cally develop about the same velocity (contour currents up to 15 to 20 crn/s), but are generally much
slower (1 to 3 crn/s).
Because oceanic currents affect water masses with
very large cross sections (in terms of both width and
depth) they transport enormous volumes of water
(e.g., the Gulf Stream and Kuroshio from 30 to
150 10 6 m 3 /s, the Antarctic circumpolar current
11010 6 m 3 /s, and the northward flowing Antarctic
bottom water in the South Atlantic 1810 6 m 3 /s).
Hence, oceanic circulation also transfers huge quantities of heat from equatorial zones into high latitudes
and, vi ce versa, providcs for cooling in low latitudes
regions. In this way, the oceans also exert a very
large influence on the climate and the conditions of
life on the neighboring continents.
Chapter 5 Oceanic Sediments
Surface currents carry fine-grained, suspended
particles and living organisms, including their larvae,
over long distances and thus distribute life and sediments. Currents, particularly those upwelling from
intermediate and deep water, transfer oxygen and
nutrients released by the mineralization of organic
matter or dissolution of biogenic skeletal particles
(see below) into coastal or equatorial regions. There,
in the euphotic zone, penetrated by sun light, the nutrients enable high phytoplankton productivity and
strong activity by its consumers (cf. Sect. 10.3.2).
5.2.5 Summary (Ocean Water Circulation)
~ Wind action and the exchange of energy (heat)
between the atmosphere and the ocean create
wind-driven surface currents and thermohaline
circulation of water masses at various depths.
~ Surface currents follow the main wind directions and are deflected by Corioli' s forces.
~ Changes in water temperature and salinity
cause density gradients which in turn lead to ±
stable stratified ocean waters (at low-latitude
regions) or to thermohaline circulation when
cold and dense water sinks (at high latitudes).
~ Downwelling cold water masses flow back as
intermediate or bottom currents toward the
equator. Near the equator and in mid-latitude
coastal zones, cold water masses come up the
surface (upwelling).
~ This simplified scenario is strongly modified
by the irregular morphology of the ocean basins and may have been different in the past.
Ocean currents transfer heat, nutrients, and to
some extent also fine-grained sediment. They
are an important factor in controlling the global
climate and the fertility ofthe oceans.
5.3 Hemil>elagic and Pelagic
Deep--Sea Sediments
5.3.1 Sourees, Transport, and Classification of
Deep-Sea Sediments
Sediment Sources
Deep-sea sediments are derived from several sources:
(1) Allochthonous sediments from the continents (see
also Sects. 9.3 and 11.5):
~ Suspended load ofrivers.
~ Eolian dust.
~ Volcaniclastic material (e.g. volcanic ash).
(2) Autochthonous biogenic sediments, mainly calcium carbonate and opaline silica.
ern boundary of an ocean, for example at the base of
the continental margin (Fig. 5.1 e). They approximately follow the contours of the ocean basin and are
therefore referred to as contour currents. Such currents mayaIso occur on the eastern margin, but they
usually flow poleward.
Upwelling and downwelling also occur in coastal
waters. Onshore surface currents tend to stack up
water along the coast, which will consequently begin
to sink and replace deeper water (coastal
downwelling). Offshore currents generate coastal
upwelling of intermediate or deep water. Due to the
effect of the Eckman spiral, these processes are initiated or intensified by winds blowing parallel to the
co ast line, for example in front of a coastal mountain
range (Fig. 5.2d). These mIes allow predicting of
coastal upwelling or downwelling for locations in the
northern or southern hemisphere when the wind direction is known.
If an ocean extends into polar regions and is partly
covered by sea ice during a glacial period, the highlatitude area of downwelling high-density water, i.e.
the polar front, moves toward lower latitudes. At the
same time, the comparatively stable water stratification at low latitudes will persist. Large quantities of
sea ice may lead to an increase in ocean salinity by 1
to 2%0 and thus stabilize the deep water mass. In addition, temporarily melting ice can locally dilute surface water and further promote density stratification.
As a result of both processes, thermohaline circulation may slow somewhat and oxygen transport into
deep water may become reduced. Further consequences of glaciation are discussed in Sect. 5.6.
5.2.4 Current Speeds, Transport of
Water Masses, Heat, and Nutrients
Surface current speeds can locally reach values as
high as 150 to 250 crn/s (Gulf-Stream, Kuroshio off
Japan), but are often considerably lower. The
present-day Antarctic circumpolar current has a
speed of 4 to 15 crn/s. Deep bottom currents can 10cally develop about the same velocity (contour currents up to 15 to 20 crn/s), but are generally much
slower (1 to 3 crn/s).
Because oceanic currents affect water masses with
very large cross sections (in terms of both width and
depth) they transport enormous volumes of water
(e.g., the Gulf Stream and Kuroshio from 30 to
150 10 6 m 3 /s, the Antarctic circumpolar current
11010 6 m 3 /s, and the northward flowing Antarctic
bottom water in the South Atlantic 1810 6 m 3 /s).
Hence, oceanic circulation also transfers huge quantities of heat from equatorial zones into high latitudes
and, vi ce versa, providcs for cooling in low latitudes
regions. In this way, the oceans also exert a very
large influence on the climate and the conditions of
life on the neighboring continents.
Chapter 5 Oceanic Sediments
Surface currents carry fine-grained, suspended
particles and living organisms, including their larvae,
over long distances and thus distribute life and sediments. Currents, particularly those upwelling from
intermediate and deep water, transfer oxygen and
nutrients released by the mineralization of organic
matter or dissolution of biogenic skeletal particles
(see below) into coastal or equatorial regions. There,
in the euphotic zone, penetrated by sun light, the nutrients enable high phytoplankton productivity and
strong activity by its consumers (cf. Sect. 10.3.2).
5.2.5 Summary (Ocean Water Circulation)
~ Wind action and the exchange of energy (heat)
between the atmosphere and the ocean create
wind-driven surface currents and thermohaline
circulation of water masses at various depths.
~ Surface currents follow the main wind directions and are deflected by Corioli' s forces.
~ Changes in water temperature and salinity
cause density gradients which in turn lead to ±
stable stratified ocean waters (at low-latitude
regions) or to thermohaline circulation when
cold and dense water sinks (at high latitudes).
~ Downwelling cold water masses flow back as
intermediate or bottom currents toward the
equator. Near the equator and in mid-latitude
coastal zones, cold water masses come up the
surface (upwelling).
~ This simplified scenario is strongly modified
by the irregular morphology of the ocean basins and may have been different in the past.
Ocean currents transfer heat, nutrients, and to
some extent also fine-grained sediment. They
are an important factor in controlling the global
climate and the fertility ofthe oceans.
5.3 Hemil>elagic and Pelagic
Deep--Sea Sediments
5.3.1 Sourees, Transport, and Classification of
Deep-Sea Sediments
Sediment Sources
Deep-sea sediments are derived from several sources:
(1) Allochthonous sediments from the continents (see
also Sects. 9.3 and 11.5):
~ Suspended load ofrivers.
~ Eolian dust.
~ Volcaniclastic material (e.g. volcanic ash).
(2) Autochthonous biogenic sediments, mainly calcium carbonate and opaline silica.