The organic complexation of dissolved iron along the U.S. GEOTRACES (GA03) North Atlantic Section

Abstract

The organic complexation of dissolved iron was determined from full water column depth profile samples collected on the U.S. GEOTRACES North Atlantic Section cruises in 2010 and 2011 (GEOTRACES GA03). The concentrations of iron-binding ligands and their conditional stability constants were determined using competitive ligand exchange-adsorptive cathodic stripping voltammetry (CLE-ACSV) with salicylaldoxime as the added competitive ligand. Across the basin, iron-binding ligands were found in excess of dissolved iron concentrations in all samples except those with the highest dissolved iron in the Trans-Atlantic Geotraverse (TAG) hydrothermal vent plume, where dissolved iron concentrations exceeded ligand concentrations. Ligand results were categorized based on conditional stability constants into three ligand classes (L1: logKFeLi,Fe′cond>12; L2: logKFeL2,Fe′cond=11–12; L3: logKFeL3,Fe′cond=10–11). The stronger L1-type ligand class tracked closely with dissolved iron, with the strongest ligands (i.e., highest logKFeL1,Fe′cond) found in the vicinity of the Trans-Atlantic Geotraverse (TAG) hydrothermal vent plume. All three ligand classes, including the stronger L1-type ligands, were observed through the water column. These measurements indicate that iron-binding ligands are indeed a ubiquitous feature of iron speciation in the North Atlantic.