Neodymium isotopic composition of the Mediterranean outflow and the eastern North Atlantic

Abstract

The neodymium concentration and isotopic composition of the Mediterranean outflow in the Strait of Gibraltar has been directly determined. The outflow has a dissolved Nd concentration of ~23 pmoles/kg with ϵ_Nd(0) = −9.4. These results are in clear disagreement with previous values inferred for the Mediterranean. This concentration and ϵ_Nd(0) do not sharply contrast with North Atlantic Deep Water (NADW) values and can produce isotopic anomalies no larger than 1 ϵ unit in the North Atlantic. A new profile in the eastern North Atlantic does not exhibit an ϵ_Nd(0) anomaly at the depth of the Mediterranean Water core, in accordance with the above inferences, while concentrations and ϵNd(0) at this site indicate that Nd is conservative in the deep eastern North Atlantic. A previously reported ϵNd(0) anomaly in the eastern North Atlantic, originally associated with the Mediterranean outflow, has a different source. The direct contribution of the Mediterranean outflow to the NADW Nd budget is small, ~10% of the high latitude sources. However, the outflow plus entrained water supply ~30% of the total Nd associated with northern latitude deep water sources. Concentrations of dissolved Nd in the outflow are enriched compared to the Atlantic inflow, which has 16 pmoles/ kg with ϵ_Nd(0) = −11.8. The principal source of the excess dissolved Nd is inferred to be partial dissolution of detrital particles. Simple mass balance calculations suggest that ~0.6% of the Nd associated with detrital river-transported sediments and/or ~3.5% of the Nd associated with eolian particles delivered to the Mediterranean is mobilized during passage into the marine environment. If general, these mobilization efficiencies imply that the dissolution of detrital particles is a significant source of dissolved Nd (and other rare earth elements) in seawater.

Additional details