evaporite basins. The residence time for potassium is
about 10 million years because it is more rapidly
adsorbed onto clay minerals and thereby removed.
Calcium is removed both chemically in evaporites
and biologically and has a residence time of 1 mill.
yr. Rare earths have periods of only a few 100 years.
3.9
Circulation of Water in the Oceans
Ocean currents are driven by:
1. The rotation of the Earth (Coriolis effect)
2. Tidal forces
3. Differences in water density due to variations in
salt content and temperature
4. Wind forces due to atmospheric circulation.
Ocean currents are extremely important for
redistributing heat from low latitudes to higher
latitudes. This circulation is strongly dependent on
the topography of the ocean floor and the distribution
of ocean and continents. Bottom currents in the ocean
basins are very different from those at the surface, and
often flow in opposite directions. While warm surface
water flows from the equator to the poles, cold surface
water sinks at the poles and flows along the ocean floor
to the equator. Both currents are deflected by the
Coriolis effect, towards the right in the northern hemisphere and towards the left in the southern hemisphere. While surface currents (like the Gulf Stream)
will be deflected eastwards, deeper currents will be
deflected towards the west of the oceans (e.g. the
Atlantic Ocean). These deep-sea currents which follow the depth contours may be strong enough to transport silt and fine sand, and the resultant deposits are
called contourites.
The vertical circulation of seawater is highly sensitive to variations in temperature and salt concentration.
In periods with glaciation at the poles, the temperature
gradient in the surface water flowing from the equator
to the poles is far greater than in non-glacial periods
(such as the Mesozoic). Oxygen-rich cold water now
flowing down from the polar regions into the ocean
basins is important for maintaining oxidising
conditions in the deep ocean basins.
Animals and bacteria use oxygen from seawater
continuously (for respiration), and oxidation of dead
organic material also requires oxygen. If we did not
have this downward flow of cold surface water, the
water in the ocean basins would be reducing. Some
ocean basins are isolated from this circulation, and we
may then have a more permanent layering of water
based on temperature and salt content. Warm surface
water with low density can flow over heavier, colder
basal water without the water masses mixing to any
major extent. The boundary between warm and cold
water masses is called a thermocline. If the density
difference is largely due to salt content, we call the
boundary a halocline. A pycnocline is the boundary
between two water masses with different densities,
without a specified cause. Lakes in temperate and
cold regions have good circulation. This is due to
water attaining maximum density at 4
C, below
which temperature density inversion causes turnover.
The addition of freshwater to a basin (e.g. Baltic Sea,
Black Sea) also leads to stratification of the water due to
salt concentration because brackish water flows on top
of marine water with higher salinity. Evaporite basins
produce water which is heavy due to its high salt content
and therefore forms a layer which flows along the bottom. If a salinity stratification becomes established, it
will weaken or destroy the circulation and lead to reducing conditions in the bottom layer. If the density contrast
due to salt concentration is greater than that due to
temperature, this will impede or prevent the downward
flow of cold, oxygen-rich surface water (Fig. 3.17).
There are indications that in previous geological
periods (e.g. the Cretaceous) the bottom water in the
ocean basins was warm, salty water (about 15
C)
compared with the present situation with cold basal
water (2–3
) and a normal salinity. A higher average
temperature in the oceans leads to reduced CO 2 solubility and a deeper carbonate compensation depth
(CCD). The volume of water welling up from deeper
water layers to the surface corresponds to the amount
of downflow. If we have basal water with high salinity,
the reduced density contrast results in less downward
flow and consequently less upwelling, so less nutrients
are added to the surface water.
We have seen that a number of different processes
control the geochemical equilibrium of the ocean.
There must also be an equilibrium between the addition and removal of chemical components for the
ocean water composition to remain relatively constant.
The conditions in which this equilibrium was
maintained, however, have varied through geological
time. During the first part of the Earth’s history, up to
about 2.5 billion years ago, the atmosphere was reducing. Most geochemical processes acted very differently then from the way they do now. Weathering
114
K. Bjørlykke
about 10 million years because it is more rapidly
adsorbed onto clay minerals and thereby removed.
Calcium is removed both chemically in evaporites
and biologically and has a residence time of 1 mill.
yr. Rare earths have periods of only a few 100 years.
3.9
Circulation of Water in the Oceans
Ocean currents are driven by:
1. The rotation of the Earth (Coriolis effect)
2. Tidal forces
3. Differences in water density due to variations in
salt content and temperature
4. Wind forces due to atmospheric circulation.
Ocean currents are extremely important for
redistributing heat from low latitudes to higher
latitudes. This circulation is strongly dependent on
the topography of the ocean floor and the distribution
of ocean and continents. Bottom currents in the ocean
basins are very different from those at the surface, and
often flow in opposite directions. While warm surface
water flows from the equator to the poles, cold surface
water sinks at the poles and flows along the ocean floor
to the equator. Both currents are deflected by the
Coriolis effect, towards the right in the northern hemisphere and towards the left in the southern hemisphere. While surface currents (like the Gulf Stream)
will be deflected eastwards, deeper currents will be
deflected towards the west of the oceans (e.g. the
Atlantic Ocean). These deep-sea currents which follow the depth contours may be strong enough to transport silt and fine sand, and the resultant deposits are
called contourites.
The vertical circulation of seawater is highly sensitive to variations in temperature and salt concentration.
In periods with glaciation at the poles, the temperature
gradient in the surface water flowing from the equator
to the poles is far greater than in non-glacial periods
(such as the Mesozoic). Oxygen-rich cold water now
flowing down from the polar regions into the ocean
basins is important for maintaining oxidising
conditions in the deep ocean basins.
Animals and bacteria use oxygen from seawater
continuously (for respiration), and oxidation of dead
organic material also requires oxygen. If we did not
have this downward flow of cold surface water, the
water in the ocean basins would be reducing. Some
ocean basins are isolated from this circulation, and we
may then have a more permanent layering of water
based on temperature and salt content. Warm surface
water with low density can flow over heavier, colder
basal water without the water masses mixing to any
major extent. The boundary between warm and cold
water masses is called a thermocline. If the density
difference is largely due to salt content, we call the
boundary a halocline. A pycnocline is the boundary
between two water masses with different densities,
without a specified cause. Lakes in temperate and
cold regions have good circulation. This is due to
water attaining maximum density at 4
C, below
which temperature density inversion causes turnover.
The addition of freshwater to a basin (e.g. Baltic Sea,
Black Sea) also leads to stratification of the water due to
salt concentration because brackish water flows on top
of marine water with higher salinity. Evaporite basins
produce water which is heavy due to its high salt content
and therefore forms a layer which flows along the bottom. If a salinity stratification becomes established, it
will weaken or destroy the circulation and lead to reducing conditions in the bottom layer. If the density contrast
due to salt concentration is greater than that due to
temperature, this will impede or prevent the downward
flow of cold, oxygen-rich surface water (Fig. 3.17).
There are indications that in previous geological
periods (e.g. the Cretaceous) the bottom water in the
ocean basins was warm, salty water (about 15
C)
compared with the present situation with cold basal
water (2–3
) and a normal salinity. A higher average
temperature in the oceans leads to reduced CO 2 solubility and a deeper carbonate compensation depth
(CCD). The volume of water welling up from deeper
water layers to the surface corresponds to the amount
of downflow. If we have basal water with high salinity,
the reduced density contrast results in less downward
flow and consequently less upwelling, so less nutrients
are added to the surface water.
We have seen that a number of different processes
control the geochemical equilibrium of the ocean.
There must also be an equilibrium between the addition and removal of chemical components for the
ocean water composition to remain relatively constant.
The conditions in which this equilibrium was
maintained, however, have varied through geological
time. During the first part of the Earth’s history, up to
about 2.5 billion years ago, the atmosphere was reducing. Most geochemical processes acted very differently then from the way they do now. Weathering
114
K. Bjørlykke