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One of the most exciting results from the GEOTRACES program’s zonal and meridional sections has been the recognition that hydrothermally sourced Fe may persist long enough to be upwelled along shoaling isopycnals and act as an essential micronutrient, stimulating primary productivity at high latitudes. In Aug-Sep 2023 our team used a combination of predictive plume dispersion modelling, real-time current meter data from the Ocean Networks Canada observatory, and in situ sensing and sampling from the AUV Sentry to guide biogeochemical sampling of dispersing hydrothermal plumes above the Juan de Fuca Ridge. A key motivation for this study was to investigate what sets the export flux of dissolved Fe and Mn away from ridge-axis venting. We specifically targeted hydrothermal vents in the NE Pacific for this study, at the far end of the thermohaline circulation, to maximize predicted Fe oxidation times within the dispersing plume and, hence, optimize our ability to reveal distinct processes that may contribute to regulating Fe flux as a function of time and distance down-plume. We also targeted an overlooked gap in the length-scale over which hydrothermal processes may regulate export fluxes, between the ≤1km range typical of submersible-based investigations and the ~100km spacing for GEOTRACES Section stations. Over 3 weeks on station we were able to use the Sentry AUV equipped with an in situ oxidation-reduction potential (ORP) sensor, an optical backscatter sensor (OBS) and two methane sensors (METS, SAGE) to track predicted plume dispersion trajectories and guide a telescopically-expanding program of water column sampling for dissolved, soluble, colloidal and particulate species of Fe, Mn and other metals, at <0.1, 0.25, 0.50, 1, 2, 5 and 10km down-plume from the High Rise and Main Endeavour vent-sites. We will present results from Sentry sensor data revealing length scales over which hydrothermal plume signatures attenuated, together with complementary TEI data, all set within the context of our dispersing plume model. Our approach will ultimately allow us to assign both effective distances down-plume from source, for each sample collected, and model dispersion ages. This will provide insights into both the processes active within a dispersing hydrothermal plume and the rates at which those processes occur. 

Abstract Distinctively‐light isotopic signatures associated with Fe released from anthropogenic activity have been used to trace basin‐scale impacts. However, this approach is complicated by the way Fe cycle processes modulate oceanic dissolved Fe (dFe) signatures (δ⁵⁶Fe_diss) post deposition. Here we include dust, wildfire, and anthropogenic aerosol Fe deposition in a global ocean biogeochemical model with active Fe isotope cycling, to quantify how anthropogenic Fe impacts surface ocean dFe and δ⁵⁶Fe_diss. Using the North Pacific as a natural laboratory, the response of dFe, δ⁵⁶Fe_diss, and primary productivity are spatially and seasonally variable and do not simply follow the footprint of atmospheric deposition. Instead, the effect of anthropogenic Fe is regulated by the biogeochemical regime, specifically the degree of Fe limitation and rates of primary production. Overall, we find that while δ⁵⁶Fe_dissdoes trace anthropogenic input, the response is muted by fractionation during phytoplankton uptake, but amplified by other isotopically‐light Fe sources.