Neodymium isotopic variations in North Pacific modern silicate sediment and the insignificance of detrital REE contributions to seawater

Abstract

The neodymium isotopic composition of the silicate fraction of Holocene pelagic sediments from the North Pacific define two provinces: a central North Pacific province characterized by unradiogenic and remarkably homogeneous [epsilon]Nd (-10.2 +/- 0.5) and a narrow circum-Pacific marginal province characterized by more radiogenic and variable [epsilon]Nd (-4.2 +/- 3.8). The silicate fraction in the central North Pacific is exclusively eolian; based on prevailing wind patterns, meteorological data, and neodymium isotopic data, the only significant sediment source is Chinese loess. Leaching experiments on Chinese loess confirm that leachable Nd is isotopically indistinguishable from bulk and residual silicate Nd. Silicates in the circum-North Pacific marginal province comprise eolian loess, volcanic ash, and hemipelagic sediments derived from volcanic arcs. A compilation of Pacific seawater and Mn nodule [epsilon]Nd data shows no clear spatial variation except for a general decrease from surface to deep waters from -3 to -4 and slightly lower [epsilon]Nd in bottom waters along the western North Pacific due to the incursion of Antarctic Bottom Water. The relative homogeneity of bottom water [epsilon]Nd, which contrasts sharply with the distinctive variation in sediment [epsilon]Nd, plus the large difference between the average [epsilon]Nd of bottom waters and the central North Pacific eolian silicates ( -4 vs. -10), suggests that any contribution of REE to seawater from eolian materials is insignificant. Furthermore, leaching of REE from eolian particles as they sink though the water column must be insignificant because Nd in shallow waters is more radiogenic than Nd in deeper waters. That there is no contrast in the Nd isotopic composition of bottom waters that overlie the central and marginal sediment provinces suggests that the ash and hemipelagic sediments derived from Pacific rim volcanic arcs also contribute minimal REE to seawater. The elimination of eolian, ash, and hemipelagic sediments leaves only near-shore riverine particulates as a possibly significant particulate source of REE to seawater.