Collaborative Research: Hydrothermal Fluxes and their Impact on Ocean Biogeochemistry: An Integrated pre-and post-eruption Study at the EPR, 9-10 degrees N

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Intellectual Merit: The objective of this proposal is to investigate the impact that hydrothermal fluxes from high-temperature seafloor venting may have on the biogeochemistry of the overlying ocean. One particular focus is the fate of iron, which is strongly enriched (ca. one million-fold) in vent-fluids when compared to open-ocean waters. Prior studies estimated that dissolved Fe released from venting is quantitatively precipitated close to seafloor vent-sites and demonstrated that the process of polymetallic sulfide and Fe-oxyhydroxide particle formation had the potential to significantly modify gross hydrothermal fluid fluxes. In the limit, it has been argued, uptake onto hydrothermal particles could even represent a significant sink for some dissolved chemical budgets. To investigate this, we have proposed an analytical study of settling hydrothermal plume particles collected in sediment-traps deployed directly adjacent to two known vent-sites on the East Pacific Rise at 9 50'N. The samples we will analyze date from both before and after a recent volcanic eruption at this site (Winter 2005-06). While it is known that such eruption events" can trigger rapidly evolving physical, chemical and biological hydrothermal responses, what remains unknown is how significant these transient effects might be to the time-averaged hydrothermal flux from any given system. By comparing samples collected at weekly intervals during the months immediately following the latest EPR 9-10 N eruptions with those collected during the year leading up to the eruption we will be uniquely well positioned to answer that question. We will investigate the mineralogy and chemical composition of polymetallic sulfide and Fe-oxyhydroxide particles collected within the sediment traps, as well as associated biogenic material. We will use elemental and isotopic techniques in our analyses including use of Fe isotopes to potentially fingerprint" hydrothermally-sourced Fe and U-series radionuclides as unambiguous tracers of uptake from seawater. As an integral part of our interdisciplinary study, we will complement our geochemical work with parallel microbial investigations of the same samples. Our microbial studies will, in particular, concentrate upon the role of Fe-oxidizing bacteria in controlling the biogeochemical fate of deep-ocean, hydrothermally-sourced iron. Broader Impacts: This research is a multi-disciplinary inter-institutional collaboration, which supports the research of three faculty, two graduate students, and 2-3 undergraduates. Our research offers additional opportunities in support of two other early-career postdoctoral researchers (one female) who are developing state-of-the-art analytical techniques pertinent to the analyses of deep-ocean mineral phases. In terms of impact on the broader science community, this project will directly address two of seven key questions from the NSF Ridge 2000 program concerning the limits of the hydrothermal biosphere and the impact of hydrothermal fluxes on ocean chemistry, physics and biology. One current hypothesis to be tested through our Fe-isotopic studies is that hydrothermally-sourced Fe not only dominates dissolved Fe budgets in the deep ocean but, through upwelling, also contributes significantly as an essential micro-nutrient for surface-ocean productivity. Thus, the potential exists that this project will help to establish a direct connection between solid-earth processes at Mid-Ocean Ridges and processes that are significant to global ocean carbon cycles and climate.