136 Peter Stille and Graham Shields
in the case of Nd. Chow and Patterson were already able to show in 1962 that
such differences in Pb isotopic compositions do exist between ocean basins.
By virtue of the short residence time of Pb in seawater, the Pb isotope system
ought to react even more sensitively to local variations in Pb input through space
and time than does the Nd isotope system for Nd. Pb isotopes also represent a
further opportunity to trace ocean currents and the extent of exchange between
various oceans through time.
The direct measurement of Pb isotope ratio in natural fluids is extremely
difficult to perform as Pb concentrations are generally very low indeed. Until
today, scarcely one direct analysis of seawater Pb isotope ratios has been carried
out successfully and published. Nevertheless, Chow and Patterson (1962) were
able to show that marine autbigenic Mn ores incorporate seawater Pb isotopic
ratios. Although this pioneering study showed the way, it was not until the recent
studies of Abouchami and Goldstein (1995) that the behaviour of Pb isotopes in
the marine environment began to be investigated using manganese nodules.
E ,.r
r
0,.
r ~
O~
&
0.3
Nd ~'10-1Zg/g )
Nd \
\\ Sm
&
I.
,
, . I
O.&
5
0.6
Sm (lO-lZg/g)
Fig. 5.31. The Sm-Nd concentrations change with water depth at Gibraltar. (Piepgras and
Wasserburg 1983)