Geology Reference
In-Depth Information
sedimentation acts as the major removal mechanism. The mechanism is a
geological/physical process, essentially the deposition of suspended parti-
cles in response to changes in the hydrodynamic regime. Such removal is
especially important in coastal environments. Marine organisms play an
important role in the open sea through the sinking of shells of microor-
ganisms and faecal pellets. The chemical precipitation of salts from seawa-
ter occurs only under special conditions. CaCO
3
can precipitate in warm
tropical lagoons and Mn
21
canbeoxidisedtotherelativelyinsoluble
MnO
2
, a process of importance near deep-sea hydrothermal vents.
Some definitions help in the interpretation of chemical phenomena in
the ocean. Conservative behaviour signifies that the concentration of a
constituent or absolute magnitude of a property varies only due to
mixing processes. Components or parameters that behave in this manner
can be used as conservative indices of mixing. Examples are salinity and
potential temperature, the definitions for which are presented in subse-
quent sections. In contrast, non-conservative behaviour indicates that
the concentration of a constituent may vary as a result of biological or
chemical processes. Examples of parameters that behave non-conserv-
atively are dissolved oxygen and pH.
Residence time, t, is defined as
A
dA
=
dT
t
¼
ð
4
:
1
Þ
ð
Þ
where A is the total amount of constituent A in the reservoir and dA/dT
can be either in rate of supply or the rate of removal of A. This
represents the average lifetime of the component in the system and is,
in effect, a reciprocal rate constant (his concept is discussed further in
Chapter 7).
The photic zone refers to the upper surface of the ocean in which
photosynthesis can occur. This is typically taken to be the depth at
which sunlight radiation has declined to 1% of the magnitude at the
surface. This might typically be 4100 m for visible light or photosyn-
thetically active radiation (PAR), but generally
o
20 m for UV wave-
lengths, of recent interest due to the enhanced input invoked by
stratospheric ozone depletion.
4.1.2 Properties of Water and Seawater
Water is a unique substance, with unusual attributes because of its struc-
ture. The molecule consists of a central oxygen atom with two attached
hydrogen atoms forming a bond angle of about 1051. As oxygen is more
