Tracing the impacts and imprints of sulfide precipitation on iron export in hydrothermal plumes

Hydrothermal venting at ocean spreading centers releases iron and other metals to the global oceans and contributes to the formation of critical metal deposits. Upon venting, some of the iron forms dense sulfide minerals that fall to nearby sediments. As vent fluids mix with oxygen-rich seawater, most of the remaining iron forms iron oxide minerals that settle out of a spreading hydrothermal plume, while a small amount of the iron remains in solution and can be transported thousands of kilometers from the vent source. Understanding the fate of iron from hydrothermal venting is important to topics ranging from critical mineral deposits to the productivity of ocean life. The investigators have developed a new method using stable isotopes of iron to determine how much iron is deposited as sulfide minerals and trace the transport of iron away from vent sites. They will apply this technique to samples collected from three hydrothermal vent fields. The project will support graduate and undergraduate student researchers. Non-technical lectures will engage public audiences in ocean discovery. Despite its importance to determining hydrothermal iron export efficiency, the average fraction of iron removed to near vent mineral sinks is hard to quantify. Sulfide sinks are particularly elusive as their deposition is localized, their traditional iron stable isotopic signatures are overprinted by later oxidation in the plume, and sulfide precipitation does not scale simply with the iron to sulfide ratio of vent fluids. The team will use a novel triple iron isotope method that enables specific tracing and quantification of the iron sulfide precipitation history, from analysis of accessible, spatially averaged neutrally buoyant plume sediment. They will i) quantify the fraction of iron removed to near vent sulfides at three vent fields spanning a large range in iron to sulfide ratios and determine the scaling between these values; and ii) assess whether nanoparticulate pyrite proposed to be exported in the dispersing neutrally buoyant plume is detectable and quantifiable in sediments. Clarifying links between iron/sulfide ratio and iron export efficiency will reveal how spreading ridge magmatic activity impacts net iron fluxes to the ocean, because phase separation driven by volcanic eruptions modulates vent fluid iron/sulfide ratios. Ridge magmatic activity may impact iron export on timescales from the several-yearly eruption cadence on fast spreading ridges, to sea-level-impacted glacial-interglacial timescales, which can be assessed with down core sedimentary records. Finally, the potential to detect the contribution of nanoparticulate pyrite in neutrally buoyant plume sediments will allow the investigators to assess the relative importance of this hypothesized carrier of iron to the open oceans, compared to organically stabilized iron. This is critical for better modeling global hydrothermal iron export fluxes and the availability of exported iron to marine life. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.