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 vents have 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 plume features in the global ocean. Plume particles (>0.2 μm) were collected as part of the U.S. GEOTRACES Eastern 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 plume is hydrothermal in origin. The plume particles 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 plume and 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 plume and sediment samples. When pS was detected, it was in the form of an Fe sulfide mineral phase, composing ≤0.4% of the Fe on a per atom basis. The sediment fluff layer contained a mixture of inorganic (coccolith fragments) and Corg bearing (lipid-rich biofilm-like) materials. The particle morphology and co-location of C and Fe in the sediment was different from that in plume particles. 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 plume are S-depleted, Fe(III)-Corg composites indicative of a chemically oxidizing plume with strong biological modification. These findings confirm that the Fe-Corg relationships observed for non-buoyant plume particles 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.
Bibliographical noteFunding Information:
We thank the captain and crew of R/V Thomas G. Thompson and Co-Chief Scientist James Moffett for supporting our research during the EPZT GP16 cruise. This research was supported by grants from the National Science Foundation to BMT ( OCE-1232986 ), PJL ( OCE-1518110 ), CRG ( OCE-1235248 ), and RMS ( OCE-1234827 ). We thank Sirine Fakra and Matthew Marcus, beamline scientists at the Advanced Light Source (ALS) 10.3.2, and David Kilcoyne, beamline scientist at ALS 22.214.171.124 for their mentorship, patience, and guidance in STXM and X-ray microprobe; Adam Gillespie, Tom Regier, and James Dynes, beamline scientists at the Canadian Light Source (CLS) SGM, and Yongfeng Hu, Aimee MacLennan, and Qunfeng Xiao, beamline scientists at the CLS SXRMB for their mentorship, patience, and guidance in bulk XANES measurements; Brandi Cron Kamermans and Rebecca Sims for support in beamtime data collection; Andy Scobbie, Thor Sellie, Brandi Cron Kamermans, Rebecca Sims, Michael Ottman, Sofia Oufqir, and Reba VanBusekom for cruise preparation support; and James Brandes for sharing carbon XANES standard spectra for carbonate minerals. The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. Research described in this paper was performed at the Canadian Light Source, which is supported by the Canada Foundation for Innovation, Natural Sciences and Engineering Research Council of Canada, the University of Saskatchewan, the Government of Saskatchewan, Western Economic Diversification Canada, the National Research Council Canada, and the Canadian Institutes of Health Research.
- Eastern Pacific Zonal Transect
- GEOTRACES GP16
- Scanning transmission X-ray microscopy (STXM)
- X-ray absorption near edge structure (XANES) spectroscopy
- X-ray microprobe