enrichment at depth is due to regeneration of the
elements from particles back into solution by bacterial oxidation of the biological particulate matter.
Deep waters of the North Pacific and Indian Ocean
typically have higher concentrations of these elements than North Atlantic deep waters owing to biogeochemical cycles and ocean circulation.
Mixed Type
Some transition metals and heavy metals, such as Cu,
Fe, Ga, Zr, Ti, La and other rare earths, have distributions that are influenced by both recycling and
scavenging processes. For example, copper displays
the characteristic surface depletion and deep-sea enrichment of the recycled element type; however, its
concentration increases only gradually (almost linearly) with depth, indicating the effects of scavenging.
Modern Advances
Development of new analytical techniques, especially those that can be used at sea aboard ship, have
enabled oceanographers to make large numbers of
measurements of the concentration of a few transition metals across some ocean basins to construct
two- and three-dimensional horizontal profiles, instead of just presenting an element’s vertical profile.
For example, two-dimensional ocean basin-scale
distribution maps have been produced for aluminum
and iron. These two-dimensional distribution maps
can help identify the input and distribution mechanisms of an element and can be useful as tracers of
water mass movements.
Although such detailed information has been obtained for a few transition metals and heavy metals,
initial measurements of the oceanic concentrations
and distributions need to be made for elements such
as Ti, Ga, Ru, Pd, Ir, Pt, Au, Re Te, Zr, and Hf in
many ocean basins before simple vertical and horizontal profiles can be constructed. Using newly developed analytical techniques, researchers have
begun to obtain initial data on these metals. For
example, the first concentration data on iridium in
sea water (North Pacific) have been reported. Iridium
concentrations ranged from 0.5 Â 10
À15 mol l
À1 in
North Pacific surface waters and increased with
depth to a maximum of 0.8 Â 10
À15 mol l
À1 near the
bottom.
Speciation
Introduction
Knowing the oceanic concentrations and distributions is only part of the picture in understanding
the biological and geochemical interactions of transition metals and heavy metals. Dissolved metals can
exist in different oxidation states and chemical forms
(‘species’). These forms include free solvated ions,
organometallic compounds, organic complexes (e.g.,
metals bound to proteins or humic substances), and
inorganic complexes (e.g., metals bound to Cl
À ,
OH
À , CO
2À
3 , SO
2À
4 , etc.). Knowledge of the concentrations of these various species of a transition
metal or a heavy metal, in conjunction with its distribution and concentration, is critical to understanding how the various chemical species interact
biologically and geochemically. For example, the
nutrient availability and toxicity of several transition
metals have been found to be proportional to the
concentrations of their free metal ions and not their
total concentrations. Complexation of a metal by an
organic ligand will decrease the concentration of the
free ion form of the metal, thereby decreasing its
toxicity or bioavailability. Organic complexation
may also decrease or increase adsorption of metals
onto metal oxide particles. These examples illustrate
Increasing [X]
Increasing depth
Increasing depth
Increasing [X]
V, Mo, W, Re, T1
Mn, Co, Ga, In, Te,
Hg, Pb, Bi, Ce
Nutrient
(or recycled)
(R)
Scavenged
(S)
Conservative
(C)
Increasing [X]
Increasing depth
Type
Profile
Fe, Ni, Zn, Ge, Se,
Y, Ag, Cd, Ba, La
Figure 1 Oceanic profile classifications.
74 TRANSITION METALS AND HEAVY METAL SPECIATION
elements from particles back into solution by bacterial oxidation of the biological particulate matter.
Deep waters of the North Pacific and Indian Ocean
typically have higher concentrations of these elements than North Atlantic deep waters owing to biogeochemical cycles and ocean circulation.
Mixed Type
Some transition metals and heavy metals, such as Cu,
Fe, Ga, Zr, Ti, La and other rare earths, have distributions that are influenced by both recycling and
scavenging processes. For example, copper displays
the characteristic surface depletion and deep-sea enrichment of the recycled element type; however, its
concentration increases only gradually (almost linearly) with depth, indicating the effects of scavenging.
Modern Advances
Development of new analytical techniques, especially those that can be used at sea aboard ship, have
enabled oceanographers to make large numbers of
measurements of the concentration of a few transition metals across some ocean basins to construct
two- and three-dimensional horizontal profiles, instead of just presenting an element’s vertical profile.
For example, two-dimensional ocean basin-scale
distribution maps have been produced for aluminum
and iron. These two-dimensional distribution maps
can help identify the input and distribution mechanisms of an element and can be useful as tracers of
water mass movements.
Although such detailed information has been obtained for a few transition metals and heavy metals,
initial measurements of the oceanic concentrations
and distributions need to be made for elements such
as Ti, Ga, Ru, Pd, Ir, Pt, Au, Re Te, Zr, and Hf in
many ocean basins before simple vertical and horizontal profiles can be constructed. Using newly developed analytical techniques, researchers have
begun to obtain initial data on these metals. For
example, the first concentration data on iridium in
sea water (North Pacific) have been reported. Iridium
concentrations ranged from 0.5 Â 10
À15 mol l
À1 in
North Pacific surface waters and increased with
depth to a maximum of 0.8 Â 10
À15 mol l
À1 near the
bottom.
Speciation
Introduction
Knowing the oceanic concentrations and distributions is only part of the picture in understanding
the biological and geochemical interactions of transition metals and heavy metals. Dissolved metals can
exist in different oxidation states and chemical forms
(‘species’). These forms include free solvated ions,
organometallic compounds, organic complexes (e.g.,
metals bound to proteins or humic substances), and
inorganic complexes (e.g., metals bound to Cl
À ,
OH
À , CO
2À
3 , SO
2À
4 , etc.). Knowledge of the concentrations of these various species of a transition
metal or a heavy metal, in conjunction with its distribution and concentration, is critical to understanding how the various chemical species interact
biologically and geochemically. For example, the
nutrient availability and toxicity of several transition
metals have been found to be proportional to the
concentrations of their free metal ions and not their
total concentrations. Complexation of a metal by an
organic ligand will decrease the concentration of the
free ion form of the metal, thereby decreasing its
toxicity or bioavailability. Organic complexation
may also decrease or increase adsorption of metals
onto metal oxide particles. These examples illustrate
Increasing [X]
Increasing depth
Increasing depth
Increasing [X]
V, Mo, W, Re, T1
Mn, Co, Ga, In, Te,
Hg, Pb, Bi, Ce
Nutrient
(or recycled)
(R)
Scavenged
(S)
Conservative
(C)
Increasing [X]
Increasing depth
Type
Profile
Fe, Ni, Zn, Ge, Se,
Y, Ag, Cd, Ba, La
Figure 1 Oceanic profile classifications.
74 TRANSITION METALS AND HEAVY METAL SPECIATION