and therefore essential for many algae and ecosystems. As a group VIA element like chromium, in
oxic sea water dissolved molybdenum should be
found in the þ 6 oxidation state as the hydrolysis
product molybdate, MoO 4
2À , but in anoxic waters it
is reduced to Mo(IV), which in the presence of
hydrogen sulfide forms insoluble MoS 2 . In fact,
molybdenum is enriched in anoxic sediments by this
mechanism and, like vanadium, can be used as a
sediment tracer of past anoxia. In spite of its crucial
biological role, molybdenum in the oxic ocean shows
remarkably conservative behavior (Figure 1B), with
no surface depletion and the highest concentration
(B105 nmol l
À1 ) of any of the trace elements
examined here. This does not mean that phytoplankton are not taking it up, but rather this uptake
is trivial compared to its inputs. Hydrothermal vents
also do not remove molybdenum, and the only
waters where molybdenum removal is observed are
in anoxic basins such as the Black Sea.
Tungsten
Completing the group VIA transition metal series,
tungsten is chemically similar to molybdenum and is
found in oxic sea water as W(VI) in the form of
tungstate, WO 4
2À . Owing to difficulties in determining tungsten in sea water, there are few depth
profiles for this dissolved element. In the North Pacific Ocean (Figure 1C), tungsten displays slightly
higher concentrations in surface waters (average of
58 pmol l
À1
) compared to deep waters (average of 49
pmol l
À1 ). Dissolved tungsten appears to have a
slightly scavenged type of profile, with the surface
enrichment likely due to the deposition of terrestrial
dust (aerosols). In crustal rocks that are the source of
most elements to the ocean by weathering, the
abundance of tungsten is about one-third that of
molybdenum, but its sea water concentration is over
1000 times less (compare Figures 1B and C). Since
tungsten shows no strong removal in the open ocean,
this observation suggests that tungsten must be removed in the coastal ocean. Indeed, profiles of
tungsten in estuaries shows strong removal like that
of iron; molybdenum shows no such strong removal
in estuaries. Thus, while tungsten shows nearly
conservative behavior in the open ocean, when the
whole land–ocean system is considered, tungsten
actually has a very active removal, which lowers its
sea water concentration.
Rhenium
Interest in rhenium is for rather esoteric reasons,
primarily because one of its radioactive isotopes, and
that of its periodic table neighbor osmium, are useful
for dating very ancient (450 billion years) sediments
(i.e., the
187 Re/
187
Os ratio). In oxic sea water,
Re(VII) is the stable oxidation state and after hydrolysis exists as the relatively unreactive perrhenate
ion, ReO 4
À
. In the North Pacific Ocean (Figure 1D),
as well as in the Atlantic, the profile of dissolved
rhenium is quite conservative, with an average of
44.370.3 pmol l
À1
. Using the same type of arguments used for tungsten, the very low concentration
of crustal rhenium but relatively high sea water
concentration (sea water rhenium and tungsten are
nearly identical, but crustal tungsten is B3000 times
more abundant than rhenium) suggests that rhenium
is very unreactive in the entire ocean system.
Nevertheless, in anoxic systems such as the Black
Sea, rhenium does show substantial decreases in
concentration (nonconservative behavior) that have
been attributed to removal at the surface of anoxic
sediments. However, in the modern ocean these anoxic systems are too rare to substantially alter the
distributions of rhenium in the water column.
Osmium
From the prior discussion of rhenium, it would seem
logical to consider the oceanic behavior of the group
VIII element osmium in terms of its use as a dating
tool. In addition, the ratio of two of its isotopes
(
187
Os/
186 Os) can trace inputs from extraterrestrial
sources (e.g., meteors) and terrestrial sources (i.e.,
crustal weathering) to the oceans, which are recorded in marine sediments. There is some debate
about the exact form of osmium in sea water, but
thermodynamic calculations suggest that Os(VIII) as
H 3 OsO 6
2À would be stable in oxic sea water. Determinations of dissolved osmium in sea water are
very difficult, especially since osmium concentrations
are over 1000 times lower than those of rhenium.
Indeed, the profile of osmium in the North Pacific
(Figure 2C) indicates not only that osmium is found
at very low concentrations (38 fmol l
À1
) in surface
waters, but also that its distribution is quite dynamic
with depth (minimum at 460 m and rising to 51 fmol
l
À1 in deep waters). This profile is from the eastern
North Pacific, which has the distinct suboxic layer,
and bears a striking resemblance to that of Cr(VI) in
the same region (Figure 2A). Thus, osmium may
have nutrient-like behavior that is also affected by
oxidation–reduction reactions (i.e., reduced to a
more particle-reactive (but unidentified) form in the
suboxic zone). In this respect, profiles in more oxygenated waters of the Atlantic and Indian Oceans
show no such depletion in the upper water column.
The nutrient-like distribution does not mean that it is
used as a nutrient or nutrient substitute, but rather
METALLOIDS AND OXYANIONS 67
oxic sea water dissolved molybdenum should be
found in the þ 6 oxidation state as the hydrolysis
product molybdate, MoO 4
2À , but in anoxic waters it
is reduced to Mo(IV), which in the presence of
hydrogen sulfide forms insoluble MoS 2 . In fact,
molybdenum is enriched in anoxic sediments by this
mechanism and, like vanadium, can be used as a
sediment tracer of past anoxia. In spite of its crucial
biological role, molybdenum in the oxic ocean shows
remarkably conservative behavior (Figure 1B), with
no surface depletion and the highest concentration
(B105 nmol l
À1 ) of any of the trace elements
examined here. This does not mean that phytoplankton are not taking it up, but rather this uptake
is trivial compared to its inputs. Hydrothermal vents
also do not remove molybdenum, and the only
waters where molybdenum removal is observed are
in anoxic basins such as the Black Sea.
Tungsten
Completing the group VIA transition metal series,
tungsten is chemically similar to molybdenum and is
found in oxic sea water as W(VI) in the form of
tungstate, WO 4
2À . Owing to difficulties in determining tungsten in sea water, there are few depth
profiles for this dissolved element. In the North Pacific Ocean (Figure 1C), tungsten displays slightly
higher concentrations in surface waters (average of
58 pmol l
À1
) compared to deep waters (average of 49
pmol l
À1 ). Dissolved tungsten appears to have a
slightly scavenged type of profile, with the surface
enrichment likely due to the deposition of terrestrial
dust (aerosols). In crustal rocks that are the source of
most elements to the ocean by weathering, the
abundance of tungsten is about one-third that of
molybdenum, but its sea water concentration is over
1000 times less (compare Figures 1B and C). Since
tungsten shows no strong removal in the open ocean,
this observation suggests that tungsten must be removed in the coastal ocean. Indeed, profiles of
tungsten in estuaries shows strong removal like that
of iron; molybdenum shows no such strong removal
in estuaries. Thus, while tungsten shows nearly
conservative behavior in the open ocean, when the
whole land–ocean system is considered, tungsten
actually has a very active removal, which lowers its
sea water concentration.
Rhenium
Interest in rhenium is for rather esoteric reasons,
primarily because one of its radioactive isotopes, and
that of its periodic table neighbor osmium, are useful
for dating very ancient (450 billion years) sediments
(i.e., the
187 Re/
187
Os ratio). In oxic sea water,
Re(VII) is the stable oxidation state and after hydrolysis exists as the relatively unreactive perrhenate
ion, ReO 4
À
. In the North Pacific Ocean (Figure 1D),
as well as in the Atlantic, the profile of dissolved
rhenium is quite conservative, with an average of
44.370.3 pmol l
À1
. Using the same type of arguments used for tungsten, the very low concentration
of crustal rhenium but relatively high sea water
concentration (sea water rhenium and tungsten are
nearly identical, but crustal tungsten is B3000 times
more abundant than rhenium) suggests that rhenium
is very unreactive in the entire ocean system.
Nevertheless, in anoxic systems such as the Black
Sea, rhenium does show substantial decreases in
concentration (nonconservative behavior) that have
been attributed to removal at the surface of anoxic
sediments. However, in the modern ocean these anoxic systems are too rare to substantially alter the
distributions of rhenium in the water column.
Osmium
From the prior discussion of rhenium, it would seem
logical to consider the oceanic behavior of the group
VIII element osmium in terms of its use as a dating
tool. In addition, the ratio of two of its isotopes
(
187
Os/
186 Os) can trace inputs from extraterrestrial
sources (e.g., meteors) and terrestrial sources (i.e.,
crustal weathering) to the oceans, which are recorded in marine sediments. There is some debate
about the exact form of osmium in sea water, but
thermodynamic calculations suggest that Os(VIII) as
H 3 OsO 6
2À would be stable in oxic sea water. Determinations of dissolved osmium in sea water are
very difficult, especially since osmium concentrations
are over 1000 times lower than those of rhenium.
Indeed, the profile of osmium in the North Pacific
(Figure 2C) indicates not only that osmium is found
at very low concentrations (38 fmol l
À1
) in surface
waters, but also that its distribution is quite dynamic
with depth (minimum at 460 m and rising to 51 fmol
l
À1 in deep waters). This profile is from the eastern
North Pacific, which has the distinct suboxic layer,
and bears a striking resemblance to that of Cr(VI) in
the same region (Figure 2A). Thus, osmium may
have nutrient-like behavior that is also affected by
oxidation–reduction reactions (i.e., reduced to a
more particle-reactive (but unidentified) form in the
suboxic zone). In this respect, profiles in more oxygenated waters of the Atlantic and Indian Oceans
show no such depletion in the upper water column.
The nutrient-like distribution does not mean that it is
used as a nutrient or nutrient substitute, but rather
METALLOIDS AND OXYANIONS 67