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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 During aerobic oxidation of methane (CH₄) in seawater, a process which mitigates atmospheric emissions, the¹²C‐isotopologue reacts with a slightly greater rate constant than the¹³C‐isotopologue, leaving the residual CH₄isotopically fractionated. Prior studies have attempted to exploit this systematic isotopic fractionation from methane oxidation to quantify the extent that a CH₄pool has been oxidized in seawater. However, cultivation‐based studies have suggested that isotopic fractionation fundamentally changes as a microbial population blooms in response to an influx of reactive substrates. Using a systematic mesocosm incubation study with recently collected seawater, here we investigate the fundamental isotopic kinetics of aerobic CH₄oxidation during a microbial bloom. As detailed in a companion paper, seawater samples were collected from seep fields in Hudson Canyon, U.S. Atlantic Margin, and atop Woolsey Mound (also known as Sleeping Dragon) which is part of lease block MC118 in the northern Gulf of Mexico, and used in these investigations. The results from both Hudson Canyon and MC118 show that in these natural environments isotopic fraction for CH₄oxidation follows a first‐order kinetic process. The results also show that the isotopic fractionation factor remains constant during this methanotrophic bloom once rapid CH₄oxidation begins and that the magnitude of the fractionation factor appears correlated with the first‐order reaction rate constant. These findings greatly simplify the use of natural stable isotope changes in CH₄to assess the extent that CH₄is oxidized in seawater following seafloor release. 

Abstract Microbial aerobic oxidation is known to be a significant sink of marine methane (CH₄), contributing to the relatively minor atmospheric release of this greenhouse gas over vast stretches of the ocean. However, the chemical kinetics of aerobic CH₄oxidation are not well established, making it difficult to predict and assess the extent that CH₄is oxidized in seawater following seafloor release. Here we investigate the kinetics of aerobic CH₄oxidation using mesocosm incubations of fresh seawater samples collected from seep fields in Hudson Canyon, U.S. Atlantic Margin and MC118, Gulf of Mexico to gain a fundamental chemical understanding of this CH₄sink. The goals of this investigation were to determine the response or lag time following CH₄release until more rapid oxidation begins, the reaction order, and the stoichiometry of reactants utilized (i.e., CH₄, oxygen, nitrate, phosphate, trace metals) during CH₄oxidation. The results for both Hudson Canyon and MC118 environments show that CH₄oxidation rates sharply increased within less than one month following the CH₄inoculation of seawater. However, the exact temporal characteristics of this more rapid CH₄oxidation varied based on location, possibly dependent on the local circulation and biogeochemical conditions at the point of seawater collection. The data further suggest that methane oxidation behaves as a first‐order kinetic process and that the reaction rate constant remains constant once rapid CH₄oxidation begins.