Depth distribution of Zr and Nb in seawater: The potential role of colloids or organic complexation to explain non-scavenging-type behavior

Dissolved zirconium (Zr) and niobium (Nb) are present in open-ocean seawater in low pmol/kg concentrations and studies for the Pacific Ocean reported an increase in concentration with depth although both metals are characterized as being particle-reactive. We analyzed dissolved Zr and Nb in 194 Atlantic seawater samples (0.2 mu m filtered) collected at 16 stations located in the (sub)tropical Atlantic during GEOTRACES cruise M81/1 (GA11) and 6 Pacific samples (0.2 pm and 0.015 mu m filtered) collected from a hydrothermal plume and the background seawater during S0229. Measurements were done using our newly developed online-preconcentration method for the SeaFAST-system with subsequent analysis by inductively coupled plasma-mass spectrometry. Our results showed an increase in dissolved Zr with depth with some distinct maxima at intermediate depths along a NE-SW transect (30 degrees N to 12 degrees S) in the Atlantic. Surface depletion was less pronounced along the Atlantic SE-NW transect (12 degrees S to 8 degrees N) and disappeared almost completely at the northernmost station at 8 N. Dissolved Nb was almost conservatively distributed except off West Africa where a surface depletion was observed. We suggest that dust deposited off West Africa scavenged Zr and Nb from surface waters and riverine discharge from the Amazon may represent a source to the West Atlantic. The correlation of Zr with Si hints to a coupling of Zr to the uptake and release on and from Si particles such as biogenic opal. Analysis of the size-fractionated Pacific samples ruled out a significant colloidal (0.015 mu m-0.2 mu m) component as host for Zr and Nb. We suggest that dissolved Zr and Nb might be stabilized by organic complexation in deep waters because both elements form very stable complexes with organic molecules such as siderophores. (C) 2016 Elsevier B.V. All rights reserved.