METALLOIDS AND OXYANIONS
G. A. Cutter, Old Dominion University, Norfolk, VA,
USA
Copyright & 2001 Elsevier Ltd.
Introduction
The concentrations and distributions of dissolved
trace elements (typically called trace ‘metals,’ though
not all trace elements are metals) in the world’s
oceans are due to a complex interaction between
their purely chemical behavior (e.g., acid/base properties, oxidation state, solubility), the way in which
they are delivered to the ocean (atmosphere, rivers,
submarine hydrothermal vents), biological reactions,
and water circulation (e.g., currents). To organize
this somewhat chaotic and confusing situation, the
kinds of trace element behavior are classified into
four types: conservative, nutrient-like or recycled,
scavenged, and hybrid or mixed. A conservative
trace element behaves like the major dissolved
elements that make up the bulk of the ocean’s salinity
(e.g., Na
þ
). These elements are only effected by the
physical processes of mixing, or the addition (dilution) or removal (evaporation) of water. Since there
are no chemical or biological reactions that affect
these elements, they have rather uniform concentrations with ocean depth. In contrast, the nutrientlike trace element is taken up by phytoplankton in
surface waters during photosynthesis (like the nutrient nitrate), and this organic matter-bound element begins to gravitationally settle into deep waters.
However, organic matter is a precious commodity in
the open and deep sea, so many levels of the food
web (bacteria to zooplankton) consume this organic
detritus, releasing some fraction of the bound trace
element back into the water column. This recycling
makes the nutrient-like trace element concentration
lower at the surface and higher at depth, with the
exact shape of the profile depending on the rate at
which it is recycled. Many dissolved trace elements
have high charge to atomic radius ratios, and electrostatically adsorb to particle surfaces; this process
is loosely termed ‘scavenging.’ Thus, scavenged
elements have distributions that depend on the
number and type of particles (e.g., clay, phytoplankton) and the mode of introduction (e.g., atmosphere, hydrothermal vents). An excellent
example of this interaction is given by lead, which is
very particle-reactive and is introduced from atmosphere, with the resulting distribution showing a
surface maximum and rapid decrease with depth.
Finally, many trace elements display features of both
scavenged and nutrient-like elements, with the distribution of the micronutrient iron being a good
example (particle reactive, but also recycled).
Most of these classifications were developed for
the metals that are cations (positively charged) in
solution, but there are elements in periodic groups
IVA, VA and B, VIA and B, VIIA, and VIII that actually form oxygen-containing anions. In general,
these elements display their maximum potential
oxidation state in sea water, and in aqueous solution
undergo hydrolysis (e.g., Mo
þ6 þ 4H 2 O-MoO 4
2À þ
8H
þ
). The metalloid elements (antimony, Sb; arsenic,
As; germanium, Ge; selenium, Se; tellurium, Te) all
exist as oxyanions, as do the transition metals
chromium (Cr), molybdenum (Mo), osmium (Os),
rhenium (Re), tungsten (W), and vanadium (V). In
addition to existing as anions, most of these trace
elements can be found in multiple oxidation states
(e.g., As(III) and As(V)), ensuring that the oxyanions
probably have the most diverse behaviors of any
trace elements in the ocean. Interestingly, the form in
which an oxyanion exists in sea water, the ‘chemical
speciation,’ strongly affects its biological and chemical reactivity.
In this review, the oceanic distributions of each
element will be discussed in terms of its purely
chemical properties, known biological behavior, and
general geochemical considerations (inputs and outputs). The focus will be primarily on the dissolved
ions rather than those associated with particles, since
these are free to move with the water molecules and
are available to the first trophic level in the ocean,
phytoplankton. Because deeper waters in the Pacific
Ocean are much older and have undergone more
mixing than those in the Atlantic, most data will
come from the Pacific to focus on the biological and
chemical processes affecting the element, and not the
mixing of different water masses. In addition, all
concentrations will be expressed in fractions of a
mole per liter rather than as mass per liter. This
allows direct comparisons between elements (i.e.,
atom to atom) and is consistent with the principles of
chemical and biological reactions (e.g., to make CO 2 ,
it takes one atom of C and 2 atoms of O). As trace
elements, the concentrations units will be nanomoles
per liter (nmol l
À1
; 10
À9 mol l
À1
), picomoles per liter
(pmol l
À1
; 10
À12 mol l
À1
), and femptomoles per liter
64
G. A. Cutter, Old Dominion University, Norfolk, VA,
USA
Copyright & 2001 Elsevier Ltd.
Introduction
The concentrations and distributions of dissolved
trace elements (typically called trace ‘metals,’ though
not all trace elements are metals) in the world’s
oceans are due to a complex interaction between
their purely chemical behavior (e.g., acid/base properties, oxidation state, solubility), the way in which
they are delivered to the ocean (atmosphere, rivers,
submarine hydrothermal vents), biological reactions,
and water circulation (e.g., currents). To organize
this somewhat chaotic and confusing situation, the
kinds of trace element behavior are classified into
four types: conservative, nutrient-like or recycled,
scavenged, and hybrid or mixed. A conservative
trace element behaves like the major dissolved
elements that make up the bulk of the ocean’s salinity
(e.g., Na
þ
). These elements are only effected by the
physical processes of mixing, or the addition (dilution) or removal (evaporation) of water. Since there
are no chemical or biological reactions that affect
these elements, they have rather uniform concentrations with ocean depth. In contrast, the nutrientlike trace element is taken up by phytoplankton in
surface waters during photosynthesis (like the nutrient nitrate), and this organic matter-bound element begins to gravitationally settle into deep waters.
However, organic matter is a precious commodity in
the open and deep sea, so many levels of the food
web (bacteria to zooplankton) consume this organic
detritus, releasing some fraction of the bound trace
element back into the water column. This recycling
makes the nutrient-like trace element concentration
lower at the surface and higher at depth, with the
exact shape of the profile depending on the rate at
which it is recycled. Many dissolved trace elements
have high charge to atomic radius ratios, and electrostatically adsorb to particle surfaces; this process
is loosely termed ‘scavenging.’ Thus, scavenged
elements have distributions that depend on the
number and type of particles (e.g., clay, phytoplankton) and the mode of introduction (e.g., atmosphere, hydrothermal vents). An excellent
example of this interaction is given by lead, which is
very particle-reactive and is introduced from atmosphere, with the resulting distribution showing a
surface maximum and rapid decrease with depth.
Finally, many trace elements display features of both
scavenged and nutrient-like elements, with the distribution of the micronutrient iron being a good
example (particle reactive, but also recycled).
Most of these classifications were developed for
the metals that are cations (positively charged) in
solution, but there are elements in periodic groups
IVA, VA and B, VIA and B, VIIA, and VIII that actually form oxygen-containing anions. In general,
these elements display their maximum potential
oxidation state in sea water, and in aqueous solution
undergo hydrolysis (e.g., Mo
þ6 þ 4H 2 O-MoO 4
2À þ
8H
þ
). The metalloid elements (antimony, Sb; arsenic,
As; germanium, Ge; selenium, Se; tellurium, Te) all
exist as oxyanions, as do the transition metals
chromium (Cr), molybdenum (Mo), osmium (Os),
rhenium (Re), tungsten (W), and vanadium (V). In
addition to existing as anions, most of these trace
elements can be found in multiple oxidation states
(e.g., As(III) and As(V)), ensuring that the oxyanions
probably have the most diverse behaviors of any
trace elements in the ocean. Interestingly, the form in
which an oxyanion exists in sea water, the ‘chemical
speciation,’ strongly affects its biological and chemical reactivity.
In this review, the oceanic distributions of each
element will be discussed in terms of its purely
chemical properties, known biological behavior, and
general geochemical considerations (inputs and outputs). The focus will be primarily on the dissolved
ions rather than those associated with particles, since
these are free to move with the water molecules and
are available to the first trophic level in the ocean,
phytoplankton. Because deeper waters in the Pacific
Ocean are much older and have undergone more
mixing than those in the Atlantic, most data will
come from the Pacific to focus on the biological and
chemical processes affecting the element, and not the
mixing of different water masses. In addition, all
concentrations will be expressed in fractions of a
mole per liter rather than as mass per liter. This
allows direct comparisons between elements (i.e.,
atom to atom) and is consistent with the principles of
chemical and biological reactions (e.g., to make CO 2 ,
it takes one atom of C and 2 atoms of O). As trace
elements, the concentrations units will be nanomoles
per liter (nmol l
À1
; 10
À9 mol l
À1
), picomoles per liter
(pmol l
À1
; 10
À12 mol l
À1
), and femptomoles per liter
64