manganese nodules and phosphorites. Europium can
be reduced to Eu
2þ , a larger cation that can be segregated from other REEs, during magmatic processes. Anomalous concentrations of Eu are not
uncommon in various igneous and sedimentary
rocks. However reduction of Eu does not normally
take place within the ocean, although an Eu enrichment has often be encountered in hydrothermal
fluids venting at midoceanic ridges. The anomalous
behavior of Ce due to oxidation–reduction reactions
can be best and quantitatively evaluated relative to
the trivalent neighbors (La and Pr) in the lanthanides
series without significant influence of the other processes affecting their oceanic distributions. This is a
notable advantage of the element over the other
transition metals, such as Mn and Fe, which behave
individually affected by the oxidation states.
In addition, there are two geochemically important
isotopes of the lanthanides,
143
Nd and
138
Ce. The
143
Nd is produced by decay of
147
Sm with a half-life of
10.6 Â 10
11 years. Natural variation of
143
Nd/
144
Nd
in terrestrial materials occurs depending on mantle/
crust segregation of Sm and Nd, and the age of the
rocks. Thus, the
143
Nd/
144
Nd ratio may be used to
constrain the sources of the REEs and mixing within
the ocean. Likewise, the
138
Ce/
142
Ce ratio may also be
used to constrain homogenization of the element by
oceanic mixing since the
138
Ce is produced by
138
La
decay (half-life, 2.97 Â 10
11 years). However, the
138
Ce/
142
Ce ratio has not been well exploited in marine geochemistry yet, because of its smaller natural
variation as compared to that of Nd isotopes and
analytical difficulty.
History and REE Normalization
The analysis of picomolar REE concentrations in
seawater has been difficult due to lack of sensitivity in
the conventional methods. Earlier attempts to measure REEs relied almost entirely on high-sensitivity
instrumental neutron activation analysis. In 1963
reliable REE concentrations were reported in a few
waters from the Eastern Pacific as well as those in a
manganese nodule and a phosphorite and their significance was recognized. Basic features of the REEs
in seawater were found: a progressive increase across
the lanthanide series from the light Pr to the heaviest
Lu when the seawater concentrations were divided by
those of sediments, and that Ce is markedly depleted
in the seawater but enriched in the manganese nodule
relative to neighboring La and Pr, as expected from its
4 þ valency state. Europium was normal relative to
other trivalent REEs in all the sample. It was also
noted that the concentrations of the heavy REEs (Ho,
Yb and Lu) in the Pacific deep water were
considerably higher than those in the surface water.
Although these earlier findings had to be refined and
confirmed by subsequent workers with more precise
modern techniques, the fundamental aspects of the
REE marine geochemistry were developed for that
time. Prior to 1980, reliable data on the distribution
of REEs in seawater were few. Since the early 1980s,
a growing number of reports on the subject have
become available. Now, several laboratories in the
world are capable of determining REEs in seawater
with precision between one and a few percent by use
of isotope dilution thermal ionization mass spectrometry (ID-TIMS) or inductively coupled plasma
mass spectrometry (ICPMS). Therefore, more detailed arguments are possible regarding geochemical
processes controlling the concentration, distribution,
fractionation and anomalous behaviors in the oceans.
When REE fractionation is discussed, it is common
to normalize the data to the values in shale which are
thought to be representative of the REEs in the upper
continental crust. The shale-normalization not only
helps to eliminate the well-known distinctive even–
odd variation in natural abundance (the Oddo–Harkins effect) of REEs but also visualizes, to a first approximation, fractionation relative to the continental
source. It should be noted, however, that different
shale values in the literature have been employed for
normalization, together with the ones of the PostArchean Australian Sedimentary rocks (PAAS)
adopted here (Table 1). Thus, caution must be paid
on the choice of the shale values if one ought to interpret small anomalies at the strictly trivalent lanthanides such as Gd and Tb. Alternatively, for
detailed arguments concerning fractionation between
different water masses in the ocean, it has been recommended that the data are normalized relative to
the REE values of a distinctive reference water mass,
for example, the North Pacific Deep Water (NPDW,
Table 1). The NPDW-normalization eliminates the
common features of seawater that appeared in the
shale-normalized REE pattern and can single out
fractionation relative to the REEs in the dissolved end
product in the route of the global ocean circulation.
The Oceanic Distributions
In 1982, the first ‘oceanographically consistent’ vertical profiles were reported for nine out of ten lanthanides that could be measured by the ID-TIMS
method in the North Atlantic. Since then, a significant amount of data on the distribution of REEs have
accumulated from various oceanic regions. For example, Figure 2 shows the station locations where the
REEs were measured in seawater, together with the
40 RARE EARTH ELEMENTS AND THEIR ISOTOPES IN THE OCEAN
be reduced to Eu
2þ , a larger cation that can be segregated from other REEs, during magmatic processes. Anomalous concentrations of Eu are not
uncommon in various igneous and sedimentary
rocks. However reduction of Eu does not normally
take place within the ocean, although an Eu enrichment has often be encountered in hydrothermal
fluids venting at midoceanic ridges. The anomalous
behavior of Ce due to oxidation–reduction reactions
can be best and quantitatively evaluated relative to
the trivalent neighbors (La and Pr) in the lanthanides
series without significant influence of the other processes affecting their oceanic distributions. This is a
notable advantage of the element over the other
transition metals, such as Mn and Fe, which behave
individually affected by the oxidation states.
In addition, there are two geochemically important
isotopes of the lanthanides,
143
Nd and
138
Ce. The
143
Nd is produced by decay of
147
Sm with a half-life of
10.6 Â 10
11 years. Natural variation of
143
Nd/
144
Nd
in terrestrial materials occurs depending on mantle/
crust segregation of Sm and Nd, and the age of the
rocks. Thus, the
143
Nd/
144
Nd ratio may be used to
constrain the sources of the REEs and mixing within
the ocean. Likewise, the
138
Ce/
142
Ce ratio may also be
used to constrain homogenization of the element by
oceanic mixing since the
138
Ce is produced by
138
La
decay (half-life, 2.97 Â 10
11 years). However, the
138
Ce/
142
Ce ratio has not been well exploited in marine geochemistry yet, because of its smaller natural
variation as compared to that of Nd isotopes and
analytical difficulty.
History and REE Normalization
The analysis of picomolar REE concentrations in
seawater has been difficult due to lack of sensitivity in
the conventional methods. Earlier attempts to measure REEs relied almost entirely on high-sensitivity
instrumental neutron activation analysis. In 1963
reliable REE concentrations were reported in a few
waters from the Eastern Pacific as well as those in a
manganese nodule and a phosphorite and their significance was recognized. Basic features of the REEs
in seawater were found: a progressive increase across
the lanthanide series from the light Pr to the heaviest
Lu when the seawater concentrations were divided by
those of sediments, and that Ce is markedly depleted
in the seawater but enriched in the manganese nodule
relative to neighboring La and Pr, as expected from its
4 þ valency state. Europium was normal relative to
other trivalent REEs in all the sample. It was also
noted that the concentrations of the heavy REEs (Ho,
Yb and Lu) in the Pacific deep water were
considerably higher than those in the surface water.
Although these earlier findings had to be refined and
confirmed by subsequent workers with more precise
modern techniques, the fundamental aspects of the
REE marine geochemistry were developed for that
time. Prior to 1980, reliable data on the distribution
of REEs in seawater were few. Since the early 1980s,
a growing number of reports on the subject have
become available. Now, several laboratories in the
world are capable of determining REEs in seawater
with precision between one and a few percent by use
of isotope dilution thermal ionization mass spectrometry (ID-TIMS) or inductively coupled plasma
mass spectrometry (ICPMS). Therefore, more detailed arguments are possible regarding geochemical
processes controlling the concentration, distribution,
fractionation and anomalous behaviors in the oceans.
When REE fractionation is discussed, it is common
to normalize the data to the values in shale which are
thought to be representative of the REEs in the upper
continental crust. The shale-normalization not only
helps to eliminate the well-known distinctive even–
odd variation in natural abundance (the Oddo–Harkins effect) of REEs but also visualizes, to a first approximation, fractionation relative to the continental
source. It should be noted, however, that different
shale values in the literature have been employed for
normalization, together with the ones of the PostArchean Australian Sedimentary rocks (PAAS)
adopted here (Table 1). Thus, caution must be paid
on the choice of the shale values if one ought to interpret small anomalies at the strictly trivalent lanthanides such as Gd and Tb. Alternatively, for
detailed arguments concerning fractionation between
different water masses in the ocean, it has been recommended that the data are normalized relative to
the REE values of a distinctive reference water mass,
for example, the North Pacific Deep Water (NPDW,
Table 1). The NPDW-normalization eliminates the
common features of seawater that appeared in the
shale-normalized REE pattern and can single out
fractionation relative to the REEs in the dissolved end
product in the route of the global ocean circulation.
The Oceanic Distributions
In 1982, the first ‘oceanographically consistent’ vertical profiles were reported for nine out of ten lanthanides that could be measured by the ID-TIMS
method in the North Atlantic. Since then, a significant amount of data on the distribution of REEs have
accumulated from various oceanic regions. For example, Figure 2 shows the station locations where the
REEs were measured in seawater, together with the
40 RARE EARTH ELEMENTS AND THEIR ISOTOPES IN THE OCEAN