114 Peter Stille and Graham Shields
to investigate not only oceanic ferromanganese nodules but also managed to carry
out direct Nd isotope analyses of seawater for the first time. They were able to
show that manganese nodules do indeed incorporate the Nd isotopic composition
of the surrounding seawater and so could be used to establish temporal changes in
seawater ~q'd.
Studies on seawater today as well as studies on marine authigenic minerals
from ancient oceans have shown that these oceans also were never as well mixed
with respect to Nd as to Sr. As kinetic fractionation cannot be the reason, their
respective residence times in seawater are likely to be very different. Let us look
into the calculation of the residence time of Nd.
5.2.1 Calculation of the Residence Time of Nd in Seawater and of the Time
NeceSsary for the Thorough Mixing of Two Oceans or Water Bodies
The observable differences in Nd isotopic composition between ocean bodies
allows us to calculate mass balances for these oceans as welt as to infer the time
necessary for the water bodies of two oceans to become thoroughly mixed. Let us
begin with the derivation of the residence time of Nd in seawater. The residence
time (1:) is defined as follows:
residence time (z) = size of the reservoir(s) / flux
whe re,
flux = input of a certain mass / time
This equation allows us to calculate for example the residence time of water in the
World's oceans. The oceans contain some 1.4 x 10 :~ kg of seawater. The flux from
the rivers consists of about 4.2 x 10 t'~ kg/year. The residence time can therefore be
estimated at about 3 x 10 + years. If the oceans are well mixed, i.e. if there is
vigorous circulation controlled by high temperature gradients from the poles to the
equatorial regions, the water from rivers and their dissolved loads can be spread
throughout the World's oceans. The time for complete mixing of the oceans is
estimated at about 1.5 - 3 x 103 years (ocean mixing time). From this we can see
that if an element's residence time is shorter than 3000 years, significant
variations will be established in elemental concentrations and isotopic ratios
depending on the proximity to source and the isotopic inhomogeneity between the
various sources. Let us concentrate now on the residence time of Nd in seawater.
As we have seen, the Nd budget is largely controlled by the input of Nd from
the continents. The average Nd concentration in the 'effectively dissolved' load is
about 16.4 ppt, whereas that in the ocean is only about 3.5 ppt (Table 4). The
residence time for Nd may be calculated as shown in equation I:
to investigate not only oceanic ferromanganese nodules but also managed to carry
out direct Nd isotope analyses of seawater for the first time. They were able to
show that manganese nodules do indeed incorporate the Nd isotopic composition
of the surrounding seawater and so could be used to establish temporal changes in
seawater ~q'd.
Studies on seawater today as well as studies on marine authigenic minerals
from ancient oceans have shown that these oceans also were never as well mixed
with respect to Nd as to Sr. As kinetic fractionation cannot be the reason, their
respective residence times in seawater are likely to be very different. Let us look
into the calculation of the residence time of Nd.
5.2.1 Calculation of the Residence Time of Nd in Seawater and of the Time
NeceSsary for the Thorough Mixing of Two Oceans or Water Bodies
The observable differences in Nd isotopic composition between ocean bodies
allows us to calculate mass balances for these oceans as welt as to infer the time
necessary for the water bodies of two oceans to become thoroughly mixed. Let us
begin with the derivation of the residence time of Nd in seawater. The residence
time (1:) is defined as follows:
residence time (z) = size of the reservoir(s) / flux
whe re,
flux = input of a certain mass / time
This equation allows us to calculate for example the residence time of water in the
World's oceans. The oceans contain some 1.4 x 10 :~ kg of seawater. The flux from
the rivers consists of about 4.2 x 10 t'~ kg/year. The residence time can therefore be
estimated at about 3 x 10 + years. If the oceans are well mixed, i.e. if there is
vigorous circulation controlled by high temperature gradients from the poles to the
equatorial regions, the water from rivers and their dissolved loads can be spread
throughout the World's oceans. The time for complete mixing of the oceans is
estimated at about 1.5 - 3 x 103 years (ocean mixing time). From this we can see
that if an element's residence time is shorter than 3000 years, significant
variations will be established in elemental concentrations and isotopic ratios
depending on the proximity to source and the isotopic inhomogeneity between the
various sources. Let us concentrate now on the residence time of Nd in seawater.
As we have seen, the Nd budget is largely controlled by the input of Nd from
the continents. The average Nd concentration in the 'effectively dissolved' load is
about 16.4 ppt, whereas that in the ocean is only about 3.5 ppt (Table 4). The
residence time for Nd may be calculated as shown in equation I: