Near-field iron and carbon chemistry of non-buoyant hydrothermal plume particles, Southern East Pacific Rise 15°S

Abstract Iron (Fe)-poor surface waters limit phytoplankton growth and their ability to remove carbon (C) from the atmosphere and surface ocean. Over the past few decades, research has focused on constraining the global Fe cycle and its impacts on the global C cycle. Hydrothermal ventshave become a highly debated potential source of Fe to the surface ocean. Two main mechanisms for transport of Fe over long distances have been proposed: Fe-bearing nanoparticles and organic C complexation with Fe in the dissolved (dFe) and particulate (pFe) pools. However, the ubiquity and importance of these processes is unknown at present, and very few vents have been investigated for Fe-Corg interactions or the transport of such materials away from the vent. Here we describe the near-field contributions (first ~100 km from ridge) of pFe and Corg to the Southern East Pacific Rise (SEPR) plume, one of the largest known hydrothermal plumefeatures in the global ocean. Plumeparticles (>0.2 μm) were collected as part of the U.S. GEOTRACESEastern Pacific Zonal Transect cruise (GP16) by in-situ filtration. Sediment cores were also collected to investigate the properties of settling particles. In this study, X-ray absorption near edge structure(XANES) spectroscopy was used in two complementary X-ray synchrotron approaches, scanning transmission X-ray microscopy(STXM) and X-ray microprobe, to investigate the Fe and C speciation of particles within the near-field non-buoyant SEPR plume. When used in concert, STXM and X-ray microprobe provide fine-scale and representative information on particle morphology, elemental co-location, and chemical speciation. Bulk chemistry depth profiles for particulate Corg (POC), particulate manganese (pMn), and pFe indicated that the source of these materials to the non-buoyant plumeis hydrothermal in origin. The plumeparticles at stations within the first ~100 km down-stream of the ridge were composites of mineral (oxidized Fe) and biological materials (organic C, Corg). Iron chemistry in the plumeand in the core-top sediment fluff layer were both dominated by Fe(III) phases, such as Fe(III) oxyhydroxides and Fe(III) phyllosilicates. Particulate sulfur (pS) was a rare component of our plumeand sediment samples. When pS was detected, it was in the form of an Fe sulfide mineralphase, composing ≤0.4% of the Fe on a per atom basis. The sediment fluff layer contained a mixture of inorganic ( coccolithfragments) and Corg bearing (lipid-rich biofilm-like) materials. The particle morphology and co-locationof C and Fe in the sediment was different from that in plumeparticles. This indicates that if the Fe-Corg composite particles settle rapidly to the sediments, then they experience strong alteration during settling and/or within the sediments. Overall, our observations indicate that the particles within the first ~100 km of the laterally advected plumeare S-depleted, Fe(III)-Corg composites indicative of a chemically oxidizing plumewith strong biological modification. These findings confirm that the Fe-Corg relationships observed for non-buoyant plumeparticles within ~100 m of vent sites are representative of particles within the first ~100 km of the advecting non-buoyant plume, and demonstrate that the export of hydrothermal pFe is facilitated through physical-chemical association with Corg.