Depth distribution of dissolved zirconium in the (sub)tropical Atlantic

Dissolved zirconium (Zr) is present in open-ocean seawater in low pmol/kg concentrations and studies for the Pacific Ocean reported an increase in concentration with depth although Zr is characterized as being particle-reactive. We observed the same pattern in this study and report the first data on dissolved Zr in the (sub)tropical Atlantic Ocean. We analyzed dissolved Zr in 194 Atlantic seawater samples (0.2 µm filtered) collected at 16 stations located in the (sub)tropical Atlantic during GEOTRACES cruise M81/1 (GEOTRACES cruise GA11, RV Meteor, 04.02.-08.03.2010). Seawater samples were collected with a trace metal clean CTD and filtration, subsampling and acidification was performed in the clean laboratory container onboard. Measurements were done using our newly developed online-preconcentration method for the SeaFAST-system (ESI, seaFAST 1) with subsequent analysis by quadrupole inductively coupled plasma-mass spectrometry (Perkin-Elmer, ELAN DRC-e). Details on the preconcentration of Zr and the analytical measurement are provided in Poehle et al. 2015 (doi:10.1016/j.dsr.2014.11.014). Our results showed an increase in dissolved Zr with depth with some distinct maxima at intermediate depths along a NE-SW transect (30°N to 12°S) in the Atlantic. Surface depletion was less pronounced along the Atlantic SE-NW transect (12°S to 8°N) and disappeared almost completely at the northernmost station at 8°N. We suggest that dust deposited off West Africa scavenged Zr 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. Remineralization of Si-particulates in deep waters release Zr which seems to be stabilized in the dissolved phase by complexation with e.g. organic ligands. This is supported by a sequential filtration approach (0.2 µm and 0.015 µm filter pore size) on seawater samples collected in the Pacific Ocean (research cruise SO229, 2011) showing that up to 91% of dissolved Zr in deep waters are present in the fraction < 0.015 µm. Hence, truly dissolved or colloidally bound Zr seems to a reasonable explanation for the observed increase in concentration with depth. Submitted data include the date of sample collection, geographic coordinates, depth and the concentration of dissolved Zr in pmol/kg.