The residence time of dissolved titanium in the highlatitude North Pacific is estimated to be 100–200
years, by VAD scavenging models. This is a region of
high particle fluxes, where other elements are known
to have a shorter than usual residence times. A
comparison of titanium with aluminum and gallium
at the same location shows that the residence time
for titanium is about three times as long as for
aluminum and 50% longer than for gallium. A global average is therefore expected to be higher – perhaps 500–700 years. The very small observed
Ti/Al enrichment in sea water (Table 2) suggests that
the residence time for titanium cannot be much
longer than that for aluminum and that titanium is
probably less soluble from continental materials. The
interocean differences in surface concentrations
suggest that atmospheric sources are likely to be
important, as seen for aluminum and other metals in
this group, but rivers have also been shown to be a
significant source of titanium to the oceans, even
after estuarine removal.
Zirconium and Hafnium
Zirconium (Zr) and hafnium (Hf) exist in the þ 4
oxidation state as Zr(OH) 5
À and Hf(OH) 5
À (with less
than 2% in the neutral forms, Zr(OH) 4 or Hf(OH) 4 ).
Owing to the lanthanide contraction, these two
elements are very similar in their size and chemical
properties. In most geological samples, their ratio remains nearly constant. In sea water, however, the ratio
varies considerably. Both elements show a surface
minimum, with concentrations gradually increasing to
a maximum at the bottom (Figure 6A, B). The source
to the bottom is similar to that seen for gallium and
titanium. Surface distributions (Figure 2C) indicate
that fluvial and/or reducing shelf sediments may be a
significant source for zirconium and hafnium.
The concentration range for zirconium with depth
is much larger than that for hafnium. This leads to a
Zr/Hf atom ratio that increases from a near-crustal
value (75–100) in the surface waters of the Pacific
(even in the elevated costal waters shown in
Figure 2C) to B350 in the deep waters (Figure 6C).
In the high-latitude North Atlantic, the ratio is
higher in the surface (180–200), but not as high in
the deep waters (B240, and quite variable from
place to place). It appears that the Zr/Hf enrichment
increases with the age of the water. Residence times
estimated from river input suggest that zirconium has
a longer residence time than hafnium (5600 versus
1300 y); estimates using VAD scavenging removal
models, while shorter, lead to the same conclusion
(see Table 1). The difference in residence time,
0
10
15
20
25
Sc (pmol kg
_ 1 )
0
1000
2000
3000
4000
5000
0
100
200
300
Ti (pmol kg
_ 1 )
Depth (m)
5
0
1000
2000
3000
4000
5000
Depth (m)
Figure 5 Depth profiles of (A) dissolved scandium in the central North Pacific (solid symbols; 281N 1221W; Spencer et al., 1970) and
in the western North Atlantic (open symbols; 361N 681W; Brewer et al., 1972), and (B) dissolved titanium in the North Pacific (solid
symbols; 501N 1451W; Orians et al., 1990) and the western North Atlantic (321N 641W; Orians et al., 1990).
58 REFRACTORY METALS
years, by VAD scavenging models. This is a region of
high particle fluxes, where other elements are known
to have a shorter than usual residence times. A
comparison of titanium with aluminum and gallium
at the same location shows that the residence time
for titanium is about three times as long as for
aluminum and 50% longer than for gallium. A global average is therefore expected to be higher – perhaps 500–700 years. The very small observed
Ti/Al enrichment in sea water (Table 2) suggests that
the residence time for titanium cannot be much
longer than that for aluminum and that titanium is
probably less soluble from continental materials. The
interocean differences in surface concentrations
suggest that atmospheric sources are likely to be
important, as seen for aluminum and other metals in
this group, but rivers have also been shown to be a
significant source of titanium to the oceans, even
after estuarine removal.
Zirconium and Hafnium
Zirconium (Zr) and hafnium (Hf) exist in the þ 4
oxidation state as Zr(OH) 5
À and Hf(OH) 5
À (with less
than 2% in the neutral forms, Zr(OH) 4 or Hf(OH) 4 ).
Owing to the lanthanide contraction, these two
elements are very similar in their size and chemical
properties. In most geological samples, their ratio remains nearly constant. In sea water, however, the ratio
varies considerably. Both elements show a surface
minimum, with concentrations gradually increasing to
a maximum at the bottom (Figure 6A, B). The source
to the bottom is similar to that seen for gallium and
titanium. Surface distributions (Figure 2C) indicate
that fluvial and/or reducing shelf sediments may be a
significant source for zirconium and hafnium.
The concentration range for zirconium with depth
is much larger than that for hafnium. This leads to a
Zr/Hf atom ratio that increases from a near-crustal
value (75–100) in the surface waters of the Pacific
(even in the elevated costal waters shown in
Figure 2C) to B350 in the deep waters (Figure 6C).
In the high-latitude North Atlantic, the ratio is
higher in the surface (180–200), but not as high in
the deep waters (B240, and quite variable from
place to place). It appears that the Zr/Hf enrichment
increases with the age of the water. Residence times
estimated from river input suggest that zirconium has
a longer residence time than hafnium (5600 versus
1300 y); estimates using VAD scavenging removal
models, while shorter, lead to the same conclusion
(see Table 1). The difference in residence time,
0
10
15
20
25
Sc (pmol kg
_ 1 )
0
1000
2000
3000
4000
5000
0
100
200
300
Ti (pmol kg
_ 1 )
Depth (m)
5
0
1000
2000
3000
4000
5000
Depth (m)
Figure 5 Depth profiles of (A) dissolved scandium in the central North Pacific (solid symbols; 281N 1221W; Spencer et al., 1970) and
in the western North Atlantic (open symbols; 361N 681W; Brewer et al., 1972), and (B) dissolved titanium in the North Pacific (solid
symbols; 501N 1451W; Orians et al., 1990) and the western North Atlantic (321N 641W; Orians et al., 1990).
58 REFRACTORY METALS