5.2 Water Circulation
5.1 Introduction
The ocean basins are the ultimate sink for all the material transported by rivers or blown by winds into
the sea. In addition, the oceans produce large quantities of autochthonous biogenic material. In their areal
extent, the sediments of the present-day deep ocean
basins surpass by far all the other sedimentary environments. Even measured by volume and for a certain time slice, deep-sea sediments have presumably
predominated over other sediment types, for example
shelf deposits, for the last millions of years.
However, in the ancient record, a large proportion of
former deep-sea sediments is missing (cf. Sect. 1l.7).
They were either subducted at convergent plate margins, or incorporated into accretionary prisms and
orogenic belts and partially transformed into metamorphic rocks. After uplift, most of these rocks were
eroded. Nevertheless, even in ancient rock sequences
exposed on the continents, nonmetamorphic deep-sea
sediments playa significant part. Their identification
and interpretation are an important objective in basin
studies and paleogeographic reconstructions.
As a resu1t of intensive research during the last few decades, including deep-sea drilling, our know1edge about
deep-sea sediments in the present and ancient oceans has
increased enormous1y. Much has .been written on the distribution of marine sediments in space and time in relation to
the changing configuration of the ocean basins, varying
climate, and pa1eoceanographic conditions (e.g., Kuenen
1950; Emery 1960; Shepard and Dill 1966; Shepard 1973;
Schopf 1980; Berger 1981; Emiliani 1981; Kennett 1982;
Pickering et al. 1986; Rart1ey and Prosser 1995; Seibo1d
and Berger 1996; Stowet al. 1996). This brief summary
emphasizes some general princip1es for the generation and
distribution of deep-sea sediments and includes an overview of the most important processes controlling oceanic
circu1ation (Sect. 5.2).' Without an e1ementary know1edge
of these processes, many sedimentary features and their
regional variations cannot be properly understood. Special
case studies from both present-day and ancient ocean basins are mentioned in the additional references.
5.2 Water Circulation in the Oceans
5.2.1 Salinity and Density of Ocean Water
Circulation of water masses in the oceans is the result of an interplay between the atmosphere and the
oceans. Both atmospheric and oceanic circulation are
driven by the energy provided by the Sun's radiation.
The nature of these two circulation systems is, however, quite different. Atmospheric circulation, which
affects a medium of very low density, is extremely
fast and complex, and is not restricted to certain basins with strict boundaries like the oceans. Circulation
in the oceans is much slower, which is mainly caused
by the higher density of water in comparison with air.
183
Thus, much more energy is needed to move thick
water masses than to drive the atmosphere.
Besides absolute density, variations in density
also play an important part in controlling circulation
systems (e.g., Pickard and Emery 1982). The density
variations of sea water are controlled by salinity (S),
water temperature (T), and to a minor extent by pressure. The average salinity of present-day ocean water
is about 35 gof salts per kg of sea water.
For exact determinations all carbonate has to be converted
to oxide, while bromine and iodine are rep1aced by ch10rine. Sa1inity is usually written as S = 35%0 or 35 ppt (parts
per thousand).
Whereas the total concentration of disso1ved salts, i.e.,
the salinity, varies from p1ace to p1ace and at different
depths, the ratios of the main components of sea water are
found to be almost constant in all parts of the oceans. This
fact is explained by the permanent mixing ofwater masses
not only within certain oceanic basins, but also by water
exchange from one basin to another. On the other hand,
this homogenization of sea water is counteracted by processes continually di1uting or concentrating the salt content
of sea water in specific areas, for example by entering fresh
water from the continents, by forming or melting sea ice, or
by high evaporation in warm, shallow seas.
In the large oceans, salinity only varies between
about 33 and 370/00 (Fig. 5.la and b). Therefore its
influence on the density of sea water is generally
lower than that of temperature variations (Fig. 5.2a).
The surface temperatures of present-day ocean waters vary between about -2 and +30 °C (Fig. 5.la and
c). In low latitude regions the difference in temperature of surface water and deep water (> 1000 m
depth) is greater than 20°C. The combined effects of
salinity and temperature on the density of sea water
are generally depicted in Fig. 5.2a. At high temperatures, the density change with t1 T becomes more pronounced than at lower temperatures. The change in
density with t1S is about the same at all temperatures.
Provided salinity and pressure are kept constant, a increase
in temperature, d T, leads to a decrease in the density of sea
water. Example: for d T = 30°C and S = 35%0, the density
changes from about 1.028 to 1.022 glcm 3, Fig. 5.2a).
Although the effect of pressure changes on the density
of sea water may be re1atively high, for examp1e between
surface waters and deep ocean waters, its influence on water circulation is usually neglected. The reason for this is
that water masses at the same depths are subject to the
same pressure. Thusno horizontal density gradient due to
pressure is generated which may initiate or enhance lateral
flow. This topic is not discussed further here.
Furthermore, it is of interest that both the temperature of maximum density as well as the freezing point
of sea water drop with increasing salinity. Low saline
water sinks before freezing, because its maximum
density is already reached at temperatures above the
freezing point. Consequently, the total water column
5.1 Introduction
The ocean basins are the ultimate sink for all the material transported by rivers or blown by winds into
the sea. In addition, the oceans produce large quantities of autochthonous biogenic material. In their areal
extent, the sediments of the present-day deep ocean
basins surpass by far all the other sedimentary environments. Even measured by volume and for a certain time slice, deep-sea sediments have presumably
predominated over other sediment types, for example
shelf deposits, for the last millions of years.
However, in the ancient record, a large proportion of
former deep-sea sediments is missing (cf. Sect. 1l.7).
They were either subducted at convergent plate margins, or incorporated into accretionary prisms and
orogenic belts and partially transformed into metamorphic rocks. After uplift, most of these rocks were
eroded. Nevertheless, even in ancient rock sequences
exposed on the continents, nonmetamorphic deep-sea
sediments playa significant part. Their identification
and interpretation are an important objective in basin
studies and paleogeographic reconstructions.
As a resu1t of intensive research during the last few decades, including deep-sea drilling, our know1edge about
deep-sea sediments in the present and ancient oceans has
increased enormous1y. Much has .been written on the distribution of marine sediments in space and time in relation to
the changing configuration of the ocean basins, varying
climate, and pa1eoceanographic conditions (e.g., Kuenen
1950; Emery 1960; Shepard and Dill 1966; Shepard 1973;
Schopf 1980; Berger 1981; Emiliani 1981; Kennett 1982;
Pickering et al. 1986; Rart1ey and Prosser 1995; Seibo1d
and Berger 1996; Stowet al. 1996). This brief summary
emphasizes some general princip1es for the generation and
distribution of deep-sea sediments and includes an overview of the most important processes controlling oceanic
circu1ation (Sect. 5.2).' Without an e1ementary know1edge
of these processes, many sedimentary features and their
regional variations cannot be properly understood. Special
case studies from both present-day and ancient ocean basins are mentioned in the additional references.
5.2 Water Circulation in the Oceans
5.2.1 Salinity and Density of Ocean Water
Circulation of water masses in the oceans is the result of an interplay between the atmosphere and the
oceans. Both atmospheric and oceanic circulation are
driven by the energy provided by the Sun's radiation.
The nature of these two circulation systems is, however, quite different. Atmospheric circulation, which
affects a medium of very low density, is extremely
fast and complex, and is not restricted to certain basins with strict boundaries like the oceans. Circulation
in the oceans is much slower, which is mainly caused
by the higher density of water in comparison with air.
183
Thus, much more energy is needed to move thick
water masses than to drive the atmosphere.
Besides absolute density, variations in density
also play an important part in controlling circulation
systems (e.g., Pickard and Emery 1982). The density
variations of sea water are controlled by salinity (S),
water temperature (T), and to a minor extent by pressure. The average salinity of present-day ocean water
is about 35 gof salts per kg of sea water.
For exact determinations all carbonate has to be converted
to oxide, while bromine and iodine are rep1aced by ch10rine. Sa1inity is usually written as S = 35%0 or 35 ppt (parts
per thousand).
Whereas the total concentration of disso1ved salts, i.e.,
the salinity, varies from p1ace to p1ace and at different
depths, the ratios of the main components of sea water are
found to be almost constant in all parts of the oceans. This
fact is explained by the permanent mixing ofwater masses
not only within certain oceanic basins, but also by water
exchange from one basin to another. On the other hand,
this homogenization of sea water is counteracted by processes continually di1uting or concentrating the salt content
of sea water in specific areas, for example by entering fresh
water from the continents, by forming or melting sea ice, or
by high evaporation in warm, shallow seas.
In the large oceans, salinity only varies between
about 33 and 370/00 (Fig. 5.la and b). Therefore its
influence on the density of sea water is generally
lower than that of temperature variations (Fig. 5.2a).
The surface temperatures of present-day ocean waters vary between about -2 and +30 °C (Fig. 5.la and
c). In low latitude regions the difference in temperature of surface water and deep water (> 1000 m
depth) is greater than 20°C. The combined effects of
salinity and temperature on the density of sea water
are generally depicted in Fig. 5.2a. At high temperatures, the density change with t1 T becomes more pronounced than at lower temperatures. The change in
density with t1S is about the same at all temperatures.
Provided salinity and pressure are kept constant, a increase
in temperature, d T, leads to a decrease in the density of sea
water. Example: for d T = 30°C and S = 35%0, the density
changes from about 1.028 to 1.022 glcm 3, Fig. 5.2a).
Although the effect of pressure changes on the density
of sea water may be re1atively high, for examp1e between
surface waters and deep ocean waters, its influence on water circulation is usually neglected. The reason for this is
that water masses at the same depths are subject to the
same pressure. Thusno horizontal density gradient due to
pressure is generated which may initiate or enhance lateral
flow. This topic is not discussed further here.
Furthermore, it is of interest that both the temperature of maximum density as well as the freezing point
of sea water drop with increasing salinity. Low saline
water sinks before freezing, because its maximum
density is already reached at temperatures above the
freezing point. Consequently, the total water column