The isotopic composition of Nd in different ocean masses

Abstract

Sm-Nd data for authigenic ferromanganese sediments from the oceans indicate that the Atlantic, Pacific, and Indian Oceans each have a distinct range in Nd isotopic composition which are characteristic of each ocean basin and reflect the dissolved load of Nd in the water mass. Within each basin, Nd isotopic compositions show some variability but are relatively well defined. Isotopic compositions of Nd measured in these samples are all far less than the 143Nd/ 144Nd ratios of source rocks with oceanic affinities. Direct measurements of the Nd isotopic composition of seawater presented here support the view that REE in ferromanganese sediments are derived by the direct precipitation of these elements out of seawater. Nd isotopic variations in ferromanganese sediments cannot be explained by contributions from continental detritus. It is therefore believed that the Nd isotopic variations found for ferromanganese sediments represent true variations in the isotopic composition of Nd dissolved in seawater in various ocean masses. These variations reflect primarily the age and 147Sm/ 144Nd of the continental masses being sampled, which is believed to be the major source of REE in seawater. These variations indicate that the residence time of Nd in seawater must be very short relative to the mixing rates between ocean masses. Nd isotopic studies, both in seawater and sediments should, therefore, be useful as a monitor of ocean currents and interocean mixing over the past several million years.