Deep-sea hydrothermal vents are a source of Fe to the ocean with potential impact on surface ocean primary productivity. Long-range horizontal transport of hydrothermally derived Fe in suspended particles (pFe) has been demonstrated recently. However, the biogeochemical mechanisms allowing for this sustained transport of Fe, in a size class that should otherwise sink to the sediments, are unknown. In this study, we measured particle morphology and pFe speciation in the far-field Southern East Pacific Rise (SEPR) neutrally buoyant hydrothermal plume to understand the properties of pFe transported over 1000s of km. Particles were collected by in situ filtration over an 8000 km transect that included the 4300 km SEPR neutrally buoyant plume. Particle morphology was investigated using scanning electron microscopy (SEM) with elemental analysis. Solid-state pFe speciation was measured by microfocused X-ray absorption near edge structure (μXANES) spectroscopy, microfocused extended X-ray absorption fine structure (μEXAFS) spectroscopy, and bulk EXAFS spectroscopy. We identified two diagnostic hydrothermal signatures for plume pFe emanating from the SEPR. First, the morphological signature is best described as large rounded aggregates (∼3 μm in diameter) composed of mostly Fe nanoparticles (≤100 nm in diameter). Second, the chemical speciation signature is best described as an Fe(III) oxyhydroxide nanomineral having short-range structural order. This pFe speciation signature has properties consistent with precipitation in the presence of organic or inorganic ligands or within a microbial biofilm. To our knowledge, this study is the first of its kind in terms of the overall length and coherency of the far-field hydrothermal plume sampled, as well as the use of EXAFS to describe Fe speciation in the particulate size class. Our findings can be used to design controlled experiments to investigate processes and their rates, such as precipitation, aggregation/disaggregation, and settling. The outcomes of carefully designed experiments should lead to more realistic representations of transport and bioavailability of hydrothermally derived Fe in ocean biogeochemical models.
Bibliographical noteFunding Information:
The research for this project was funded primarily through National Science Foundation (NSF) grants OCE-1232986 to BMT and PJL, and OCE-1234827 to RMS and CRG. Ship time for the project was funded separately through NSF OCE-1235248. This research used resources of the Advanced Light Source, a U.S. Department of Energy (DOE) Office of Science User Facility under contract no. DE-AC02–05CH11231. This research used resources of the Advanced Photon Source, a DOE Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under contract no. DE-AC02-06CH11357. Parts of this work were carried out in the Characterization Facility, University of Minnesota, which receives partial support from NSF through the MRSEC program. University of Minnesota Doctoral Dissertation Fellowship and Thesis Travel Grant. The University of Minnesota-College of Earth Sciences for the Frances Gibson Fellowship. The University of Minnesota-College of Biological Sciences for the Moos Graduate Fellowship for Aquatic Biology. The University of Minnesota-Council of Graduate Students Community Development Grant. The Geological Society of America Graduate Student Research Grant. And last, the Midwest branch of the America Federation of Mineralogical Societies scholarship.
- Extended X-ray absorption fine structure (EXAFS) spectroscopy
- X-ray absorption near edge structure (XANES) spectroscopy
- neutrally buoyant hydrothermal plume
- particulate iron, U.S. GEOTRACES-EPZT (GP16)
- scanning electron microscopy (SEM)