than 0.1 at the oxic/aoxic boundary of B100 m
depth to B1.0 (no anomaly) in reducing waters
below 200 m (Figure 8). Even positive Ce anomaly
can be found in the deep Cariaco Trench. Cerium
reduction also takes place in coastal and hemipelagic
sediment pore waters and can affect the distribution
of Ce in the overlying waters.
Inputs of REEs to the Ocean
The most obvious source of REEs in the ocean is
riverine input. The concentrations of dissolved REEs
in river waters are significantly higher than those of
seawaters. The behaviors of the dissolved REEs in
rivers and estuaries have been intensively studied as a
link of REE geochemistry between the crust and the
ocean. Shale-normalized REE patterns in river and
estuarine waters often show nonflat patterns indicating that fractionation takes place with respect to
continental crust. One of the prominent features
derived from those works is that large-scale removal
of dissolved REEs (in particular, light REEs) occurs
in the estuarine mixing zone. Planktonic uptake,
coprecipitation with iron hydroxides, and salt-induced coagulation of colloids have been suggested as
the removal mechanism. This reduces the effective
fluvial flux of dissolved REEs to the ocean considerably and affects the budget calculation in the
ocean. For example, the river flux of dissolved Nd is
estimated to be 4.6 Â 10
6 mol year
À1 (Table 3), being
lowered by a factor of 3–4 in the estuaries, and the
mean oceanic residence time with respect to the river
input is about 10
4 years. This Nd mean residence
time appears too long in the light of heterogeneous
distribution of its isotopic composition observed in
the different oceanic basins.
The
143 Nd/
144
Nd isotopic composition is generally
expressed as,
e Nd ¼
143 Nd=
144 Nd
measured
0:512638
À 1
2
4
3
5 Â 10
4
where the value of 0.512638 is referred to the
143 Nd/
144
Nd ratio in the chondritic uniform reservoir (CHUR). Rivers entering into the Atlantic
Ocean are influenced by old continental crust and
have a relatively nonradiogenic e Nd value of À 12,
whereas values for rivers entering into the Pacific
which is surrounded by young oceanic islands and
island arcs are more radiogenic, À 3 to À 4. Rivers
draining into the Indian Ocean have intermediate
values around À 9. The Nd isotopic compositions in
the riverine input are almost faithfully reflected in
seawater. The measured e Nd values for seawater also
average À 1272.5 for the Atlantic, À 872 for the
Indian Ocean, and À 372 for the Pacific Ocean.
Thus, the mean residence time of Nd must be short
0
0.2
0.4
0.6
0.8
La
Pr
Pm Eu
Tb Ho
Tm Lu
Yb
Er
Dy
Gd
Sm
Nd
Ce
Y
(A)
La
Pr
Pm Eu
Tb Ho
Tm Lu
Yb
Er
Dy
Gd
Sm
Nd
Ce
(B)
(Seawater/PAAS)
× 10
6
5 m
100 m
200 m
400 m
600 m
1000 m
1200 m
1750 m
2250 m
0
0.2
0.4
0.6
0.8
1.0
Seawater/NPDW
Figure 4 The PAAS-normalized (A) and the NPDW-normalized (B) patterns of dissolved REEs in the western North Pacific (Alibo
and Nozaki, 1999).
RARE EARTH ELEMENTS AND THEIR ISOTOPES IN THE OCEAN 45
depth to B1.0 (no anomaly) in reducing waters
below 200 m (Figure 8). Even positive Ce anomaly
can be found in the deep Cariaco Trench. Cerium
reduction also takes place in coastal and hemipelagic
sediment pore waters and can affect the distribution
of Ce in the overlying waters.
Inputs of REEs to the Ocean
The most obvious source of REEs in the ocean is
riverine input. The concentrations of dissolved REEs
in river waters are significantly higher than those of
seawaters. The behaviors of the dissolved REEs in
rivers and estuaries have been intensively studied as a
link of REE geochemistry between the crust and the
ocean. Shale-normalized REE patterns in river and
estuarine waters often show nonflat patterns indicating that fractionation takes place with respect to
continental crust. One of the prominent features
derived from those works is that large-scale removal
of dissolved REEs (in particular, light REEs) occurs
in the estuarine mixing zone. Planktonic uptake,
coprecipitation with iron hydroxides, and salt-induced coagulation of colloids have been suggested as
the removal mechanism. This reduces the effective
fluvial flux of dissolved REEs to the ocean considerably and affects the budget calculation in the
ocean. For example, the river flux of dissolved Nd is
estimated to be 4.6 Â 10
6 mol year
À1 (Table 3), being
lowered by a factor of 3–4 in the estuaries, and the
mean oceanic residence time with respect to the river
input is about 10
4 years. This Nd mean residence
time appears too long in the light of heterogeneous
distribution of its isotopic composition observed in
the different oceanic basins.
The
143 Nd/
144
Nd isotopic composition is generally
expressed as,
e Nd ¼
143 Nd=
144 Nd
measured
0:512638
À 1
2
4
3
5 Â 10
4
where the value of 0.512638 is referred to the
143 Nd/
144
Nd ratio in the chondritic uniform reservoir (CHUR). Rivers entering into the Atlantic
Ocean are influenced by old continental crust and
have a relatively nonradiogenic e Nd value of À 12,
whereas values for rivers entering into the Pacific
which is surrounded by young oceanic islands and
island arcs are more radiogenic, À 3 to À 4. Rivers
draining into the Indian Ocean have intermediate
values around À 9. The Nd isotopic compositions in
the riverine input are almost faithfully reflected in
seawater. The measured e Nd values for seawater also
average À 1272.5 for the Atlantic, À 872 for the
Indian Ocean, and À 372 for the Pacific Ocean.
Thus, the mean residence time of Nd must be short
0
0.2
0.4
0.6
0.8
La
Pr
Pm Eu
Tb Ho
Tm Lu
Yb
Er
Dy
Gd
Sm
Nd
Ce
Y
(A)
La
Pr
Pm Eu
Tb Ho
Tm Lu
Yb
Er
Dy
Gd
Sm
Nd
Ce
(B)
(Seawater/PAAS)
× 10
6
5 m
100 m
200 m
400 m
600 m
1000 m
1200 m
1750 m
2250 m
0
0.2
0.4
0.6
0.8
1.0
Seawater/NPDW
Figure 4 The PAAS-normalized (A) and the NPDW-normalized (B) patterns of dissolved REEs in the western North Pacific (Alibo
and Nozaki, 1999).
RARE EARTH ELEMENTS AND THEIR ISOTOPES IN THE OCEAN 45