Geology Reference
In-Depth Information
Analytical difficulties are readily comprehensible when it is appreciated
that the concentration for some of these elements can be extremely low,
i.e.,afewpgL
1
for platinum group metals.
19
Some trace elements, such
as Cs
1
, behave conservatively and therefore absolute concentrations
depend upon salinity. More often, the elements are non-conservative
and their distributions in both surface waters and the water column vary
greatly, reflecting the differing source strengths and removal processes in
operation. Generalisations regarding residence times cannot be made in
many cases because biologically active elements are removed from seawa-
ter relatively rapidly. Nevertheless, conservative constituents and platinum
group metals have rather long residence times on the order of 10
5
years.
Considering firstly the distribution in surface waters, several elements
exhibit high concentrations in coastal waters in comparison to levels in
the centres of oceanic gyres. Typically, this distribution arises because
the elements originate predominantly from riverine inputs or through
diffusion from coastal sediments. However, as they are effectively
removed from the surface waters in the coastal regions, little material
is advected horizontally to the open sea. Examples of elements that
behave in this way are Cd, Cu and Ni. In contrast, the concentration of
Pb, including
210
Pb, is greater in the gyres. This results from a strong
widespread aeolian input coupled with less effective removal from
surface waters in the gyres.
Clearly, the removal mechanisms have an appreciable effect on dis-
solved elemental abundances. The two major processes in operation are
uptake by biota and scavenging by suspended particulate material. In the
first instance, the constituent mimics the behaviour of nutrients. This is
evident in the metal/nutrient correlation for Cd/P and Zn/Si (Figure 12).
No consistent pattern for depth profiles of trace elements exists.
Conservative elements trend with salinity variations provided they have
no significant submarine sources. Non-conservative elements may ex-
hibit peak concentrations at different depths in oxygenated waters as
(i) surface enrichment,
(ii) maximum at the O
2
minimum,
(iii) mid-depth maximum not associated with the O
2
minimum, and
(iv) bottom enrichment.
The criteria for an element, such as Pb, to exhibit a maximum concen-
tration in surface waters are that the only significant input must be at the
surface (aeolian supply) and it must be effectively removed from the
water column. Constituents such as As, Ba, Cd, Ni and Zn exhibit
nutrient type behaviour. Those elements (Cd) associated with the soft
parts of the organism are strongly correlated with phosphate and are
