Hafnium and neodymium isotope variations in NE Atlantic seawater

The Hf-176/Hf-177 ratio of seawater is measured directly with Nd-143/Nd-144 in a composite vertical profile in the NE Atlantic. The value of epsilon(Hf) of intermediate and deep water exhibits little variation and averages +1.0 +/- 0.8; surface water shows a wider range, -5.7 to +3.3. While intermediate and deep water samples plot on, or close to, the seawater Hf-Nd isotope array, defined by ferromanganese deposits, near-surface seawater samples plot between the seawater and terrestrial rock epsilon(Hf) - epsilon(Nd) arrays. It is likely that the shallow water variability in epsilon(Hf) arises from a short residence time of Hf in surface water and isotopically heterogeneous inputs. The difference between surface and intermediate-deep water epsilon(Hf) may be maintained by the release of surface scavenged Hf at depth from a subset of sinking particles; the composition of epsilon(Nd) at middepths reflects overlying surface water and water advected along isopycnals from areas where they outcrop in winter. Uniform epsilon(Hf) values below 1000 m suggest the residence time of Hf may be longer than that of Nd. The radiogenic isotopic composition of seawater Hf relative to Nd and terrestrial rocks suggests that minerals with high Lu/Hf ratios may be preferentially weathered, providing a source of radiogenic Hf to the oceans.