Geology Reference
In-Depth Information
matrix is invariant, both spatially and temporally. Thus, the technique
cannot be reliably employed in marine boundary environments where the
seawater composition differs to the bulk characteristics.
There are two types of conductometric procedures commonly used.
Firstly, a Wheatstone Bridge circuit can be set up, whereby the ratio of
the resistance of unknown seawater to standard seawater balances the
ratio of a fixed resistor to a variable resistor. The system uses alternating
current to minimise electrode fouling. Alternatively, the conductivity
can be measured by magnetic induction, in which case the sensor
consists of a plastic tube containing sample seawater that links two
transformers. An oscillator establishes a current in one transformer that
induces current flow within the tube, the magnitude of which depends
upon the salinity of the sample. This in turn induces a current in the
second transformer, which can then be measured. This design has been
exploited for in situ conductivity measurements.
4.1.4 Oceanic Circulation
The distribution of chemical components within the ocean is determined
by both transportation and transformation processes. A brief outline of
oceanic circulation is necessary to ascertain the relative influences. Two
main flow systems must be considered. Surface circulation is established
by tides and the prevailing wind patterns and deep circulation is deter-
mined by gravitational forces. Both are modified by Coriolis force, the
acceleration due to the earth's rotation. It acts to deflect moving fluids
(i.e., both air and water) to the right in the northern hemisphere and to
the left in the southern hemisphere. The magnitude of the effect is a
function of latitude, being nil at the equator and increasing poleward.
Surface oceanic circulation is depicted in Figure 4. For the most part,
the circulation patterns describe gyres constrained by the continental
boundaries. The prevailing winds acting under the influence of Coriolis
force result in clockwise and counter-clockwise flow in the northern and
southern hemispheres, respectively. The flow fields are non-uniform,
exhibiting faster currents along the western margins. These are manifest,
for example, as the Gulf Stream, Kuroshio Current and Brazil Current.
Circulation within the Indian Ocean is exceptional in that there are
distinct seasonal variations in accord with the monsoons. The absence of
other continents within the immediate boundary region of Antarctica
gives rise to a circumpolar current within the Southern Ocean.
The surface circulation is restricted to the upper layer influenced by the
wind, typically about 100 m. However, underlying water can be trans-
ported up into this zone when horizontal advection is insufficient to
