favored species. It exists primarily as the free ion
with a minor contribution from the FeCl
þ complex.
Oxidation kinetics for Fe(II) are rapid above pH 6
and, as a result, Fe(II) is rapidly oxidized to Fe(III) in
oxygenated sea water.
Dissolved iron has a nutrient-type vertical profile,
and is known to be a required element for phytoplankton growth. Surface water concentrations of
dissolved iron in the Pacific, Atlantic, and Southern
Oceans are typically o0.2 nmol kg
À1 (average 0.07).
A maximum is observed at around 500–700 m (a bit
deeper than the maximum for nutrients, nitrate, or
phosphate) with relatively uniform concentrations
in deep waters. The average concentration below
500 m is 0.6 nmol kg
À1 . Unlike other nutrient-type
metals, there is no significant interocean fractionation for iron; the deep waters do not continue to
accumulate iron as they travel from the Atlantic to
the Pacific. This can be explained by a balance between regeneration of iron from biogenic matter and
subsequent removal by scavenging; the residence
time of iron is estimated to be quite short, on the
order of 100–500 years (Table 1). An alternate hypothesis, that organic ligands control the solubility of
iron and set the deep water concentration at 0.6
nmol kg
À1 has also been proposed.
Dissolved iron enters the ocean via atmospheric,
fluvial, hydrothermal, and sediment pathways. Rapid
removal of iron from fluvial and hydrothermal
sources limits the extent of their influence. Sediments
can provide a more significant source; the flux of iron
out of reducing sediments is large. Upwelling over
the continental shelf brings elevated iron levels to the
surface in many coastal regions. Atmospheric dust is
the dominant source of iron to the surface of the
open ocean. Owing to the low and nearly uniform
levels of dissolved iron in surface waters, which do
not follow variations in dust patterns, as dissolved
aluminum does, the importance of this source has
been questioned. The input of iron from atmospheric
sources was observed in the central North Pacific by
Bruland and colleagues. During a time of unusually
strong stratification, when the upper half of the sunlit
waters were cut off from the nutrient supply, the
removal of iron by phytoplankton was limited to the
lower half of the photic zone, thus allowing the
build-up of iron from atmospheric dust in the upper
waters (Figure 8).
Bismuth
Bismuth (Bi) exists in the þ 3 oxidation state in sea
water, probably as the reactive cationic oxyhydroxide species, BiO
þ and Bi(OH) 2
þ , with a minor
contribution from Bi(OH) 3 . Dissolved bismuth ranges from 25 to 450 fmol kg
À1
, with the lowest concentrations in the deep North Pacific and the highest
at mid-depth (600 m). Vertical profiles of bismuth are
0
1000
2000
3000
4000
5000
Ta (pmol kg
_ 1 )
Depth (m)
0
1000
2000
3000
4000
5000
0
2
4
5
Nb (pmol kg
_ 1 )
Depth (m)
1
3
0
0.1
0.2
0.3
0.4
0.5
Figure 7 Depth profiles of (A) niobium and (B) tantalum in the western North Pacific (451N 1651E; Sohrin et al., 1998).
60 REFRACTORY METALS
with a minor contribution from the FeCl
þ complex.
Oxidation kinetics for Fe(II) are rapid above pH 6
and, as a result, Fe(II) is rapidly oxidized to Fe(III) in
oxygenated sea water.
Dissolved iron has a nutrient-type vertical profile,
and is known to be a required element for phytoplankton growth. Surface water concentrations of
dissolved iron in the Pacific, Atlantic, and Southern
Oceans are typically o0.2 nmol kg
À1 (average 0.07).
A maximum is observed at around 500–700 m (a bit
deeper than the maximum for nutrients, nitrate, or
phosphate) with relatively uniform concentrations
in deep waters. The average concentration below
500 m is 0.6 nmol kg
À1 . Unlike other nutrient-type
metals, there is no significant interocean fractionation for iron; the deep waters do not continue to
accumulate iron as they travel from the Atlantic to
the Pacific. This can be explained by a balance between regeneration of iron from biogenic matter and
subsequent removal by scavenging; the residence
time of iron is estimated to be quite short, on the
order of 100–500 years (Table 1). An alternate hypothesis, that organic ligands control the solubility of
iron and set the deep water concentration at 0.6
nmol kg
À1 has also been proposed.
Dissolved iron enters the ocean via atmospheric,
fluvial, hydrothermal, and sediment pathways. Rapid
removal of iron from fluvial and hydrothermal
sources limits the extent of their influence. Sediments
can provide a more significant source; the flux of iron
out of reducing sediments is large. Upwelling over
the continental shelf brings elevated iron levels to the
surface in many coastal regions. Atmospheric dust is
the dominant source of iron to the surface of the
open ocean. Owing to the low and nearly uniform
levels of dissolved iron in surface waters, which do
not follow variations in dust patterns, as dissolved
aluminum does, the importance of this source has
been questioned. The input of iron from atmospheric
sources was observed in the central North Pacific by
Bruland and colleagues. During a time of unusually
strong stratification, when the upper half of the sunlit
waters were cut off from the nutrient supply, the
removal of iron by phytoplankton was limited to the
lower half of the photic zone, thus allowing the
build-up of iron from atmospheric dust in the upper
waters (Figure 8).
Bismuth
Bismuth (Bi) exists in the þ 3 oxidation state in sea
water, probably as the reactive cationic oxyhydroxide species, BiO
þ and Bi(OH) 2
þ , with a minor
contribution from Bi(OH) 3 . Dissolved bismuth ranges from 25 to 450 fmol kg
À1
, with the lowest concentrations in the deep North Pacific and the highest
at mid-depth (600 m). Vertical profiles of bismuth are
0
1000
2000
3000
4000
5000
Ta (pmol kg
_ 1 )
Depth (m)
0
1000
2000
3000
4000
5000
0
2
4
5
Nb (pmol kg
_ 1 )
Depth (m)
1
3
0
0.1
0.2
0.3
0.4
0.5
Figure 7 Depth profiles of (A) niobium and (B) tantalum in the western North Pacific (451N 1651E; Sohrin et al., 1998).
60 REFRACTORY METALS