Dissolved zirconium and hafnium distributions across a shelf break in the northeastern Atlantic Ocean

Abstract

Dissolved Zr and Hf distributions have been determined for five stations located across the shelf break in the northeastern Atlantic Ocean on the Celtic Approaches. For stations in deep water, the range in Zr concentrations has been found to be 70–180 pmol/kg, and the range in hafnium concentrations 0.4–1.1 pmol/kg. The distributions with depth are indicative of a nutrient-like scavenging-regeneration behavior in seawater, although the dissimilarity with silicate and nitrate for samples below the nutricline suggest that other mechanisms are also important in defining the oceanic distribution of these elements. In contrast, the concentrations of Zr and Hf at the station located closest to land are highest in surface waters and decrease with depth, although salinity does not indicate dilution of seawater with freshwater with high Zr and Hf concentrations. The ratio of dissolved Zr to Hf in seawater is higher and more variable than is found for the majority of terrestrial rocks, or predicted from thermodynamic considerations. Fractionation due to small differences in their interactions with particulate material is implicated.