5 Isotopic Composition of Seawater 135
E
J= 2
Q
t
I
1
-13
-1~
-n
-~o
-9
ENd (0)
Fig. 5.30. Nd isotopic variation as a function of depth at Gibraltar. The exchange between
the water bodies of the Mediterranean and the Adarttic can be shown with the help of Nd
~sotopes. (Piepgras and Wasserburg 1983)
The difference allows us to surmise that this Nd must be derived from a source
other than that for the waters above and below this layer. The maximum value lies
2t units from that of the other water samples and implies that Mediterranean water
with a eNd value of about -9.8 has flowed into the Atlantic, which has ENd value
of about - 12. A eNd value of -9.8 makes it clear that some other source with much
higher (more mantle influenced) eNd than the Atlantic must supply the
Mediterranean with REE (Frost et al. 1986).
5.3.2 Ocean Currents and Pb Isotopes
Pb concentrations in marine bottom waters are also vanishingly small: around 1
ng/L (Flegal et ah 1986, 1989; Ftegal and Patterson 1983). This is what makes Pb
such a sensitive tracer of anthropogenic contamination in the environment (see
Sect. 4.1; such widespread contamir~ation is a cause for concern in every
radiogenic isotope laboratory!). The residence time for Pb in seawater is on the
order of 80 to 100 years and so is even shorter than that of Nd (Craig et al. 1973).
We would therefore expect that the Pb isotopic ratios vary from ocean to ocean as
E
J= 2
Q
t
I
1
-13
-1~
-n
-~o
-9
ENd (0)
Fig. 5.30. Nd isotopic variation as a function of depth at Gibraltar. The exchange between
the water bodies of the Mediterranean and the Adarttic can be shown with the help of Nd
~sotopes. (Piepgras and Wasserburg 1983)
The difference allows us to surmise that this Nd must be derived from a source
other than that for the waters above and below this layer. The maximum value lies
2t units from that of the other water samples and implies that Mediterranean water
with a eNd value of about -9.8 has flowed into the Atlantic, which has ENd value
of about - 12. A eNd value of -9.8 makes it clear that some other source with much
higher (more mantle influenced) eNd than the Atlantic must supply the
Mediterranean with REE (Frost et al. 1986).
5.3.2 Ocean Currents and Pb Isotopes
Pb concentrations in marine bottom waters are also vanishingly small: around 1
ng/L (Flegal et ah 1986, 1989; Ftegal and Patterson 1983). This is what makes Pb
such a sensitive tracer of anthropogenic contamination in the environment (see
Sect. 4.1; such widespread contamir~ation is a cause for concern in every
radiogenic isotope laboratory!). The residence time for Pb in seawater is on the
order of 80 to 100 years and so is even shorter than that of Nd (Craig et al. 1973).
We would therefore expect that the Pb isotopic ratios vary from ocean to ocean as