- 4o-
(pycnocline).
The deep sea comprises approximately 95 per
cent of the ocean waters.
The principal current systems of the surface water
in the northern hemisphere circulate in the ocean basins in
a clockwise direction.
In the southern hemisphere the cir-
culation is counterclockwise.
The waters move slowly,
the
average speed of the surface currents in the open ocean
being one-half tome mile per hour,
move much more slowly.
but the deeper currents
Below the relatively thin surface
layer, often called the stirred layer, the waters are
stratified into a series of layers of increasing density
and slow movement, with little mixing between layers.
The
direction of flow of the deepest layers may be in counter
direction to the flow of the surface current.
With these
conditions the exchange of deep water and surface water
can be expected to be a slow process,
At the present time
the age of deep ocean water in the Atlantic is being deter-
mined by the cl4 age-dating method.
tion now at hand,
From the best informa-
the time required for the replacement of
Atlantic bottom water with surface water is in the range
of 200 to 500 years, but in the Pacific the time may be as
great at 1,000 years (Schaefer 1958).
An exception to this