Gallium
Gallium (Ga) is a trivalent metal that exists as the
strongly hydrolyzed Ga(OH) 4
À ion and to a much
lesser extent as Ga(OH) 3 (o2%) in sea water. Dissolved gallium ranges from 2 to 60 pmol l
À1
, with the
lowest values in the surface waters of the high-latitude North Pacific, and the highest values in the
surface waters of the North Atlantic. Vertical distributions of dissolved gallium are complex. In the Pacific there is a surface minimum, a shallow subsurface
increase then a mid-depth decrease, and concentrations increasing with depth below 1000 m to a
maximum in the bottom waters (Figure 4A). Variable
surface sources and advection of the mid-depth water
masses complicate the distribution in the Atlantic
Ocean. Profiles typically show concentrations increasing with depth without a sub-surface feature; at
some stations there is also a surface maximum. The
residence time of dissolved Ga in the deep ocean,
estimated using VAD scavenging removal models,
varies from 100 to 750 years.
Surface distributions (Figure 2B) show that eolian
input is the dominant source of dissolved gallium to
the surface ocean. The subsurface maximum seen in
the Pacific is not an advective feature, but rather the
result of internal cycles. The similarity in the ionic
radii of gallium and iron, a known nutrient element,
is thought to explain the subsurface maximum for
gallium, which may be taken up along with iron in
the surface waters and regenerated at the nutricline,
with subsequent removal via scavenging in the
deeper waters. There is a strong bottom source for
gallium. The North Atlantic dissolved gallium concentrations are 2–6 times higher than those in the
North Pacific at similar depths, showing a net scavenging with age in the deep waters. The residence
times estimated for dissolved gallium are 3–5 times
higher than those for aluminum at the same locations. A combination of the longer residence time
and the likelihood that gallium is more soluble than
aluminum from crustal materials, owing to its larger
ionic radius, is thought to produce the observed Ga/
Al enrichment in sea water of roughly two orders of
magnitude (Table 2, Figure 1).
Indium
Indium (In) is a trivalent metal that exists as In(OH) 3
in sea water, with a minor contribution from
In(OH) 4
À (6%). Dissolved indium ranges from 0.05
to 4.7 pmol kg
À1
, with the lowest values in the North
Pacific and the highest values in the Mediterranean.
Vertical profiles of dissolved indium in the Pacific
show low concentrations (0.06–0.10 pmol kg
À1
) that
are relatively invariant with depth, with a slight
suggestion of a surface maximum. In the Atlantic the
concentrations increase gradually with depth from
0.6 to 1.7 pmol kg
À1 , and in the Mediterranean the
concentration is about 4 pmol kg
À1 and relatively
0
1000
2000
3000
4000
5000
6000
0
20
30
40
50
Al (nmol kg
_ 1 )
Depth (m)
NW Atlantic
NE Atlantic
N Pacific
10
Figure 3 Depth profiles of dissolved aluminum in the central North Pacific (281N 1551W; Orians and Bruland, 1986), in the eastern
North Atlantic (311W, 261N; Hydes, 1983), and in the western North Atlantic (near Bermuda, EN120 Station 3; Measures et al., 1986).
56 REFRACTORY METALS
Gallium (Ga) is a trivalent metal that exists as the
strongly hydrolyzed Ga(OH) 4
À ion and to a much
lesser extent as Ga(OH) 3 (o2%) in sea water. Dissolved gallium ranges from 2 to 60 pmol l
À1
, with the
lowest values in the surface waters of the high-latitude North Pacific, and the highest values in the
surface waters of the North Atlantic. Vertical distributions of dissolved gallium are complex. In the Pacific there is a surface minimum, a shallow subsurface
increase then a mid-depth decrease, and concentrations increasing with depth below 1000 m to a
maximum in the bottom waters (Figure 4A). Variable
surface sources and advection of the mid-depth water
masses complicate the distribution in the Atlantic
Ocean. Profiles typically show concentrations increasing with depth without a sub-surface feature; at
some stations there is also a surface maximum. The
residence time of dissolved Ga in the deep ocean,
estimated using VAD scavenging removal models,
varies from 100 to 750 years.
Surface distributions (Figure 2B) show that eolian
input is the dominant source of dissolved gallium to
the surface ocean. The subsurface maximum seen in
the Pacific is not an advective feature, but rather the
result of internal cycles. The similarity in the ionic
radii of gallium and iron, a known nutrient element,
is thought to explain the subsurface maximum for
gallium, which may be taken up along with iron in
the surface waters and regenerated at the nutricline,
with subsequent removal via scavenging in the
deeper waters. There is a strong bottom source for
gallium. The North Atlantic dissolved gallium concentrations are 2–6 times higher than those in the
North Pacific at similar depths, showing a net scavenging with age in the deep waters. The residence
times estimated for dissolved gallium are 3–5 times
higher than those for aluminum at the same locations. A combination of the longer residence time
and the likelihood that gallium is more soluble than
aluminum from crustal materials, owing to its larger
ionic radius, is thought to produce the observed Ga/
Al enrichment in sea water of roughly two orders of
magnitude (Table 2, Figure 1).
Indium
Indium (In) is a trivalent metal that exists as In(OH) 3
in sea water, with a minor contribution from
In(OH) 4
À (6%). Dissolved indium ranges from 0.05
to 4.7 pmol kg
À1
, with the lowest values in the North
Pacific and the highest values in the Mediterranean.
Vertical profiles of dissolved indium in the Pacific
show low concentrations (0.06–0.10 pmol kg
À1
) that
are relatively invariant with depth, with a slight
suggestion of a surface maximum. In the Atlantic the
concentrations increase gradually with depth from
0.6 to 1.7 pmol kg
À1 , and in the Mediterranean the
concentration is about 4 pmol kg
À1 and relatively
0
1000
2000
3000
4000
5000
6000
0
20
30
40
50
Al (nmol kg
_ 1 )
Depth (m)
NW Atlantic
NE Atlantic
N Pacific
10
Figure 3 Depth profiles of dissolved aluminum in the central North Pacific (281N 1551W; Orians and Bruland, 1986), in the eastern
North Atlantic (311W, 261N; Hydes, 1983), and in the western North Atlantic (near Bermuda, EN120 Station 3; Measures et al., 1986).
56 REFRACTORY METALS