Organic iron-binding ligands mediate dissolved-particulate exchange in hydrothermal vent plumes along the Mid-Atlantic Ridge

Abstract

Hydrothermal vents are important contributors to the dissolved iron (Fe) inventory in the ocean. Investigating the processes underlying Fe behavior in hydrothermal plumes is challenging, but important for constraining deep ocean Fe cycling. Field studies suggest that the retention of hydrothermal Fe in the deep ocean is primarily supported by two mechanisms: the formation of colloidal nanoparticles and the stabilization of Fe by organic ligands. Here we present a novel dataset from shipboard incubation experiments designed to investigate the interplay between these two processes and how they contribute to the stabilization of Fe away from ridge axes. Filtered and unfiltered water collected from the hydrothermal plumes of three vent fields along the Mid-Atlantic Ridge as part of GEOTRACES cruise GA13 was incubated in the dark and regularly sampled over time (up to 3 weeks) for concentrations of size-fractionated Fe and Fe-binding ligands, for dissolved Fe isotopic composition, and for microbial community composition. We observed rapid exchange of Fe between physicochemical phases that appeared to be mediated in part by organic Fe-binding ligands at each stage of plume evolution. Weaker Fe-binding ligands sources from the vents were largely lost to the particulate phase with colloidal Fe phases via aggregation early in plume development, similar to the loss of Fe and organic matter commonly observed in estuarine systems. Soluble organic ligand production was observed in later stages of all unfiltered incubations followed by mobilization of particulate and colloidal Fe into the soluble phase in the longer incubations, revealing a potentially important mechanism for generating the persistent Fe observed in long-range plumes.