Geology Reference
In-Depth Information
of a marine origin. If EF
X
4 1, then it is enriched with respect to
seawater. Conversely, depletion is signified when values EF
X
o
1. An-
other example of the application of inter-element ratios can be found in
examining the geochemical cycle of sulfur. Concentrations of SO
4
2
and
Na
1
in ice cores and marine aerosols exhibit a SO
4
2
/Na
1
greater than
that observed in seawater. This excess can be readily calculated and is
known as non-sea salt sulfate (NSSS). Contributions to NSSS include
SO
2
derived from both volcanic and anthropogenic sources, together
with dimethylsulfide (DMS) of marine biogenic origin.
Not all the major constituents consistently exhibit conservative be-
haviour in the ocean. The most notable departures occur in deepwaters,
where Ca
21
and HCO
3
exhibit anomalously high concentrations due
to the dissolution of calcite. The concept of relative constant composi-
tion does not apply in a number of atypical environments associated
with boundary regions. Inter-element ratios for major constituents can
be quite different in estuaries and near hydrothermal vents. Obviously,
these are not solutions of sea salt with the implication that accuracy
of salinity measurements by chemical and conductometric means is
limited.
The residence times for some elements are presented in Table 2. The
major constituents normally have long residence times. The residence
time is a crude measure of a constituent's reactivity in the reservoir. The
aqueous behaviour and rank ordering can be appreciated simply in
terms the ionic potential given by the ratio of electronic charge to ionic
radius (Z/r). Elements with Z/r
o
3 are strongly cationic. The positive
charge density is relatively diffuse, but sufficient to attract an envelope
of water molecules to form a hydrated cation. As the ionic potential
increases, the force of attraction towards the water similarly rises to the
extent that one oxygen - hydrogen bond in the molecule breaks. This
causes the solution pH to fall and metal hydroxides to form. Neutral
hydroxides tend to be relatively insoluble and so precipitate. However,
in the more extreme case for which Z/r 4 12, the attraction towards the
oxygen is so great that both bonds in the associated water molecules are
broken. The reaction product is an oxyanion, usually quite soluble
because of the associated anionic charge. Thus in seawater, those
elements (Al, Fe) having a tendency to form insoluble hydroxides have
short residence times. This is also true for elements that exist preferen-
tially as neutral oxides (Mn, Ti). Hydrated cations (Na
1
,Ca
21
) and
strongly anionic species (Cl
,Br
,UO
2
(CO
3
)
2
4
) have long residence
times. This treatment is, of course, somewhat of an over-simplification
ignoring the rather significant role that biological organisms play in
nutrient and trace element chemistry.
