1'40 Peter Stille and Graham Shields
5.4 Ce Isotopic Composition of Seawater: Another Potential
Tracer of Ocean Circulation?
In recent years advances on the analytic side of isotope geochemistry have meant
that much isotope analysis has become standard procedure. New isotopic systems
of elements whose low abundances have made their application impossible up to
nov,' are being incorporated into geologic and environmental research. Let us
concentrate now on an isotopic system that is still at the very beginning of its
development and application due largely to technical problems: La-Ce.
Lanthanum and cerium are socalled 'rare earth elements' of slightly lower atomic
mass and greater ionic radius than their sisters Sm and Nd. About one third of
138La decays result in the production of 138Ce As a potential tracer of circulation
patterns in the ocean for example, the Ce isotopic ratio 138Ce/1"12Ce has several
advantages over the isotopic systems looked at so far:
1) Ce ought to have a short residence time, far less than a thousand years, as
the residence time of Ce is likely to be even shorter than that of Nd. The reason
for this is that it can exist in two valency states in nature. 3" and 4". All other rare
earth elements (or lanthanides~ exist normally as 3" ions except for europium (Eu),
.which can also exist as 2" ions under extremely reducing conditions and at high
temperatures. In its oxidized form, cerium is prefentially removed from seawater
complexed with Fe and Mn oxyhydroxide phases. Therefore. seav,ater shows a
strong depletion in Ce (negative Ce anomaly) compared with the concentration of
the other REE normalized against average shale. Because of this rapid removal,
cerium could be used for fine isotopic distinction of water masses.
2) Another consequence of this rapid removal is that water masses have very
different Ce concentrations depending on the age of the water mass and their
oxidation state. This cerium anomaly, which is measured as the relative depletion
in cerium relative to the neighbouring REE, ka and Nd, can tell us about the redox
conditions of deposition and diagenesis of sediments.
3) As a result, variable La/Ce ratios are typical of sedimentary authigenic or
diagenetic minerals. This provides an as yet unexplored potential for dating of
ancient marine minerals, which preferentially incorporate REE. e.g. phosphates,
Fe-Mn oxide phases.
The La-Ce geochronometer was first introduced as a dating method in 1982 by
Tanaka and Masuda. Ever since it has largely been the domain of Japanese
scientists. The technical problems involved stem from the low relative abundance
of t38La in nature (0.09%) and its long half-life which results in a low natural
range of 138Ce/142Ce ratios.
Tanaka et al. (1986) were the first to publish ~Ce values from ferromanganese
nodules found on the seafloor today. They established that there is likely to be a
difference between Ce isotopic ratios in the Pacific and Atlantic oceans and this
work was continued by Amakawa et al. (1996) who more fully explored the
potential of this system in marine research. These latter authors analyzed samples
5.4 Ce Isotopic Composition of Seawater: Another Potential
Tracer of Ocean Circulation?
In recent years advances on the analytic side of isotope geochemistry have meant
that much isotope analysis has become standard procedure. New isotopic systems
of elements whose low abundances have made their application impossible up to
nov,' are being incorporated into geologic and environmental research. Let us
concentrate now on an isotopic system that is still at the very beginning of its
development and application due largely to technical problems: La-Ce.
Lanthanum and cerium are socalled 'rare earth elements' of slightly lower atomic
mass and greater ionic radius than their sisters Sm and Nd. About one third of
138La decays result in the production of 138Ce As a potential tracer of circulation
patterns in the ocean for example, the Ce isotopic ratio 138Ce/1"12Ce has several
advantages over the isotopic systems looked at so far:
1) Ce ought to have a short residence time, far less than a thousand years, as
the residence time of Ce is likely to be even shorter than that of Nd. The reason
for this is that it can exist in two valency states in nature. 3" and 4". All other rare
earth elements (or lanthanides~ exist normally as 3" ions except for europium (Eu),
.which can also exist as 2" ions under extremely reducing conditions and at high
temperatures. In its oxidized form, cerium is prefentially removed from seawater
complexed with Fe and Mn oxyhydroxide phases. Therefore. seav,ater shows a
strong depletion in Ce (negative Ce anomaly) compared with the concentration of
the other REE normalized against average shale. Because of this rapid removal,
cerium could be used for fine isotopic distinction of water masses.
2) Another consequence of this rapid removal is that water masses have very
different Ce concentrations depending on the age of the water mass and their
oxidation state. This cerium anomaly, which is measured as the relative depletion
in cerium relative to the neighbouring REE, ka and Nd, can tell us about the redox
conditions of deposition and diagenesis of sediments.
3) As a result, variable La/Ce ratios are typical of sedimentary authigenic or
diagenetic minerals. This provides an as yet unexplored potential for dating of
ancient marine minerals, which preferentially incorporate REE. e.g. phosphates,
Fe-Mn oxide phases.
The La-Ce geochronometer was first introduced as a dating method in 1982 by
Tanaka and Masuda. Ever since it has largely been the domain of Japanese
scientists. The technical problems involved stem from the low relative abundance
of t38La in nature (0.09%) and its long half-life which results in a low natural
range of 138Ce/142Ce ratios.
Tanaka et al. (1986) were the first to publish ~Ce values from ferromanganese
nodules found on the seafloor today. They established that there is likely to be a
difference between Ce isotopic ratios in the Pacific and Atlantic oceans and this
work was continued by Amakawa et al. (1996) who more fully explored the
potential of this system in marine research. These latter authors analyzed samples