Using shipboard incubations to assess the cycling of iron (Fe) and Fe-binding ligands in hydrothermal vent plumes along the mid Atlantic Ridge

Shipboard incubation experiments were carried out using water collected from the Rainbow, TAG and Lucky Strike vent fields along the mid Atlantic Ridge (MAR) during the GEOTRACES cruise GA13 (JC156) in December 2017 and January 2018. A total of four incubations were initiated from a combination of nearfield and farfield vent plume systems, and sampled over a period of 1-3 weeks for soluble (<0.02 μm), dissolved (<0.2 μm), and total dissolvable (unfiltered acidified) trace metals, as well as for soluble and dissolved iron (Fe) speciation. Dissolved metal samples were measured by inductively coupled plasma mass spectrometry (ICP-MS) following either preconcentration with a SeaFAST-pico (ESI) or dilution in quartz distilled nitric acid. Dissolved and soluble Fe-binding organic ligands were determined by competitive ligand exchange absorptive cathodic stripping voltammetry (CLE-AdCSV) using a combination of forward titrations with the added ligand salicylaldoxime (SA) and reverse titrations using the added ligand 1-nitroso-2-naphthol (NN). Observations showed a net loss of ambient Fe-binding ligands concomitant with declining Fe concentrations in the two nearfield incubations and evidence of soluble Fe-binding ligand production followed by the solubilization of labile particulate and colloidal Fe fractions in the farfield incubation. Altogether, incubation results highlight the role of Fe-binding ligands in the dynamic cycling of Fe in hydrothermal vent plume systems.