# Use of Neodymium in Paleoceanography

$\varepsilon_{Nd}$ (i. e. the relative deviation of $\frac{^{143}Nd}{^{144}Nd}$ from a standard) is often used as a proxy in paleoceanography to identify the basin of origin of oceanic currents.

But what is the model behind this idea? Is this proxy purely empiric?
Why would the Neodymium isotopic ratio vary among basins and why would this variation be stable in (geological) time?