Use of Neodymium in Paleoceanography
A part answer to your question.

According to the article Palaeoceanography: The briny deep (Martin, 2008), explains the reason for the use of the neodymium ratio as being 


  The isotopic signature of a water mass is imparted from weathering of continental material in the region where the water mass sinks below the surface. Radiogenic Nd isotope values reflect a contribution from relatively young volcanic material, whereas non-radiogenic values are a consequence of inputs from old continental crust. The water mass carries its distinctive signature as it circulates through the oceans.


Its stability is described in the paper Temporal stability of the neodymium isotope signature of the Holocene to glacial North Atlantic (de Flierdt et al. 2006), as being due to the observations that Nd isotopes


  are not thought to be altered in any significant way by biological processes, and thus can serve as a quasi-conservative water mass tracer.