Diagnostic Morphology and Solid-State Chemical Speciation of Hydrothermally Derived Particulate Fe in a Long-Range Dispersing Plume

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.