Complete characterization of the physicochemical speciation of hydrothermal dissolved iron, as revealed by iron isotopes: Southern East Pacific Rise (GEOTRACES GP16)

The physicochemical speciation of iron (Fe) is vital to understanding bioavailability and scavenging fate of dissolved Fe (dFe). However, analytical methods to characterize the chemical speciation of Fe in the colloidal (cFe) and soluble (sFe) size fractions are lacking, especially for the basic differentiation of organically-complexed Fe from small mineral nanoparticulate phases. Here, we show that measurements of size-fractioned (soluble vs. colloidal) dissolved Fe isotopes allows characterization of the dFe speciation within the neutrally-buoyant hydrothermal plume off the Southern East Pacific Rise (SEPR) at 15°S. Using an objective model for assigning Fe isotope end-members for each Fe phase, we were able to resolve the relative concentrations of Fe-oxyhydroxide nanoparticles, hydrothermal Fe bound to organic ligands (Fe_ht-L), and background ocean Fe-L complexes in each of the soluble, colloidal, and summed dissolved phases. We found that Fe-oxyhydroxide nanoparticles are formed in the plume as both larger cFe and, surprisingly, smaller sFe, but that these nanoparticles quickly settle out of the plume, presumably via aggregation. In contrast, hydrothermal Fe bound to ligands enters the plume only as smaller sFe, but down-plume this pool is transformed into the larger colloidal phase, in alignment with background ocean Fe-L complexes that are more equally partitioned between soluble and colloidal size fractions. We will use this rare, fully resolved Fe physicochemical speciation dataset to calculate relative lifetimes of different Fe phases in hydrothermal plumes, which will better inform global iron biogeochemical models, especially as they determine scavenging fate of dFe down-plume and potential upwelling to surface ocean phytoplankton. We conclude that this novel approach of size-fractionated Fe isotope modeling has the potential to open the black box of Fe speciation in other regions of the world’s oceans in future projects.