184
is overturned until it all reaches the temperature of
maximum density. On further cooling, the surface
water becomes lighter and finally freezes from the
surface downward.
At salinities above 25%0, the maximum density is reached
at approximately -2 oe at the freezing point. Thus, the
overtuming of the water column continues up to this temperature. As a result, freezing is delayed.
5.2.2 Density Stratification and the Pycnocline
Everywhere in the ocean, the most dense water tends
to sink to the bottom and the least dense water comes
up to the surface. Particularly in smalI, closed basins
and basins with restricted water circulation, but also
in equatorial and tropical regions of the present-day
oceans, this process often leads to a rather stable
density-stratified water mass. Characteristically, water density does not increase uniformly with depth.
Usually, a relatively shallow, wave-affected water
layer of lower density is underlain by a thick denser
water mass. The transitional layer between these
zones is called the pycnocline; it may frequently also
act as a thermocline and/or halocline (boundary between water rnasses of different salinity). Here, density increases rapidly with depth (Fig. 5.1d). As long
as the surface water remains less den se than the deep
water body, for example in warm climates, and the
bottom water cannot be replaced, the density stratification and the pycnocline persist for long time periods.
This can also be observed in many lakes in warm, low-latitude regions (Sect. 2.5), in the Black Sea, or in the Eastern
Mediterranean. In all these cases, the surface water may be
characterized by turbulence and circulation, but the bottom
waters are Iittle affected byatmospheric forces.
The pycnocline acts as an effective barrier to water
movement, either downward or upward. It is often associated with a chemocline indicating a major change in water
chemistry. The stability of such an internal boundary layer
is, however, essentially based on the rapid change in water
density with depth.
In mid- and high-Iatitude regions, where surface waters undergo cooling during winter time or are permanently cool, density stratification tends to be less
stable or absent (Fig. 5.1d). Density stratification can
Fig. 5.1. a Variation in surface temperature, salinity,
and average density relative to latitude. b Salinitydepth profiles in the Pacific. c Temperature-depth
profiles, thermocline, and zones of "upper water" and
"deep water". d Typical density-depth profiles for
low and high latitudes. All data in a-d from presentday oceans. (After Pickard and Emery 1982). e Rectangular ocean with idealized, simphfied circulation
system as a result of wind forces and thermohaline
Chapter 5 Oceanic Sediments
be rnaintained if fresh water inflow generates lowdensity surface water (e.g. in some Norwegian fjords
or in parts ofthe Baltic Sea).
5.2.3 Oceanic Water Circulation
Thermohaline circulation
Circulation in the oceans is essentially driven by two
mechanisms: Variations in water density and winds.
Because density is controlled by water temperature
and salinity, the first mechanism is called
thermohaline circulation. It enables deep-water circulation. The second component is wind-driven nearsurface circulation.
Thermohaline circulation is essentially controlled
by the temperature gradient between the poles and
the equator. The horizontal and vertical density distribution in the modem oceans (Fig. 5.1a and d) deviates from the ideal, horizontally stratified water
masses consisting of two layers of constant temperature and salinity indicated above. The density of surface water changes in a north-south section through
the present-day oceans as a function of water
temperature and salinity (Fig. 5.1).
Although salinity decreases from low latitudes toward the
poles, density is greatest at high latitudes due to a lower
temperature and, in places, by the ejecting of salt when ice
freezes. Furtherrnore, density-depth profiles (as weIl as
salinity and temperature ) near the equator and tropical seas
considerably differ frorn those at high latitudes (Fig. 5.1 b, c
and d). Both locations show the expected increase in density with depth, but low latitudes are characterized by a big
jump in density creating a two-Iayer ocean, whereas at high
latitudes, the vertical density gradient is small.
Ocean water displaying such a density distribution
cannot be in a stable condition; it circulates. The relatively dense surface water at high latitudes sinks and
is replaced by surface water from lower latitudes.
The sinking dense water approaches the sea floor,
and then flows as bottom current towards the equator
and then somewhere upwards.
Such a therrnohaline circulation is kept in operation as long
as the nonequilibrium conditions can be maintained. This
is accomplished by continuous heat transfer from the
warmer atmosphere to the low-Iatitude surface waters, or
effects. (Based on different sources). Note the distinct asymmetry of the gyres (STG subtropical gyre;
SPG subpolar gyre) , the intensified surface SC and
bottom BC currents along the western cll1argin of the
basin (longer arrows), and the zones of coastal
upwelling C UW along the eastern rnargin. ED equatorial divergence associated with equatorial
upwelling EUW; STC and PC subtropical and polar
convergence giving rise to downwelling DW
is overturned until it all reaches the temperature of
maximum density. On further cooling, the surface
water becomes lighter and finally freezes from the
surface downward.
At salinities above 25%0, the maximum density is reached
at approximately -2 oe at the freezing point. Thus, the
overtuming of the water column continues up to this temperature. As a result, freezing is delayed.
5.2.2 Density Stratification and the Pycnocline
Everywhere in the ocean, the most dense water tends
to sink to the bottom and the least dense water comes
up to the surface. Particularly in smalI, closed basins
and basins with restricted water circulation, but also
in equatorial and tropical regions of the present-day
oceans, this process often leads to a rather stable
density-stratified water mass. Characteristically, water density does not increase uniformly with depth.
Usually, a relatively shallow, wave-affected water
layer of lower density is underlain by a thick denser
water mass. The transitional layer between these
zones is called the pycnocline; it may frequently also
act as a thermocline and/or halocline (boundary between water rnasses of different salinity). Here, density increases rapidly with depth (Fig. 5.1d). As long
as the surface water remains less den se than the deep
water body, for example in warm climates, and the
bottom water cannot be replaced, the density stratification and the pycnocline persist for long time periods.
This can also be observed in many lakes in warm, low-latitude regions (Sect. 2.5), in the Black Sea, or in the Eastern
Mediterranean. In all these cases, the surface water may be
characterized by turbulence and circulation, but the bottom
waters are Iittle affected byatmospheric forces.
The pycnocline acts as an effective barrier to water
movement, either downward or upward. It is often associated with a chemocline indicating a major change in water
chemistry. The stability of such an internal boundary layer
is, however, essentially based on the rapid change in water
density with depth.
In mid- and high-Iatitude regions, where surface waters undergo cooling during winter time or are permanently cool, density stratification tends to be less
stable or absent (Fig. 5.1d). Density stratification can
Fig. 5.1. a Variation in surface temperature, salinity,
and average density relative to latitude. b Salinitydepth profiles in the Pacific. c Temperature-depth
profiles, thermocline, and zones of "upper water" and
"deep water". d Typical density-depth profiles for
low and high latitudes. All data in a-d from presentday oceans. (After Pickard and Emery 1982). e Rectangular ocean with idealized, simphfied circulation
system as a result of wind forces and thermohaline
Chapter 5 Oceanic Sediments
be rnaintained if fresh water inflow generates lowdensity surface water (e.g. in some Norwegian fjords
or in parts ofthe Baltic Sea).
5.2.3 Oceanic Water Circulation
Thermohaline circulation
Circulation in the oceans is essentially driven by two
mechanisms: Variations in water density and winds.
Because density is controlled by water temperature
and salinity, the first mechanism is called
thermohaline circulation. It enables deep-water circulation. The second component is wind-driven nearsurface circulation.
Thermohaline circulation is essentially controlled
by the temperature gradient between the poles and
the equator. The horizontal and vertical density distribution in the modem oceans (Fig. 5.1a and d) deviates from the ideal, horizontally stratified water
masses consisting of two layers of constant temperature and salinity indicated above. The density of surface water changes in a north-south section through
the present-day oceans as a function of water
temperature and salinity (Fig. 5.1).
Although salinity decreases from low latitudes toward the
poles, density is greatest at high latitudes due to a lower
temperature and, in places, by the ejecting of salt when ice
freezes. Furtherrnore, density-depth profiles (as weIl as
salinity and temperature ) near the equator and tropical seas
considerably differ frorn those at high latitudes (Fig. 5.1 b, c
and d). Both locations show the expected increase in density with depth, but low latitudes are characterized by a big
jump in density creating a two-Iayer ocean, whereas at high
latitudes, the vertical density gradient is small.
Ocean water displaying such a density distribution
cannot be in a stable condition; it circulates. The relatively dense surface water at high latitudes sinks and
is replaced by surface water from lower latitudes.
The sinking dense water approaches the sea floor,
and then flows as bottom current towards the equator
and then somewhere upwards.
Such a therrnohaline circulation is kept in operation as long
as the nonequilibrium conditions can be maintained. This
is accomplished by continuous heat transfer from the
warmer atmosphere to the low-Iatitude surface waters, or
effects. (Based on different sources). Note the distinct asymmetry of the gyres (STG subtropical gyre;
SPG subpolar gyre) , the intensified surface SC and
bottom BC currents along the western cll1argin of the
basin (longer arrows), and the zones of coastal
upwelling C UW along the eastern rnargin. ED equatorial divergence associated with equatorial
upwelling EUW; STC and PC subtropical and polar
convergence giving rise to downwelling DW