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Abstract Deep-sea hydrothermal vent geochemistry shapes the foundation of the microbial food web by fueling chemolithoautotrophic microbial activity. Microbial eukaryotes (or protists) play a critical role in hydrothermal vent food webs as consumers and hosts of symbiotic bacteria, and as a nutritional source to higher trophic levels. We measured microbial eukaryotic cell abundance and predation pressure in low-temperature diffuse hydrothermal fluids at the Von Damm and Piccard vent fields along the Mid-Cayman Rise in the Western Caribbean Sea. We present findings from experiments performed under in situ pressure that show cell abundances and grazing rates higher than those done at 1 atmosphere (shipboard ambient pressure); this trend was attributed to the impact of depressurization on cell integrity. A relationship between the protistan grazing rate, prey cell abundance, and temperature of end-member hydrothermal vent fluid was observed at both vent fields, regardless of experimental approach. Our results show substantial protistan biomass at hydrothermally fueled microbial food webs, and when coupled with improved grazing estimates, suggest an important contribution of grazers to the local carbon export and supply of nutrient resources to the deep ocean. 

Dissolved organic carbon (DOC) constitutes the largest pool of reduced carbon in the global ocean, with important contributions from both recently formed and aged, biologically refractory DOC (RDOC). The mechanisms regulating RDOC transformation and removal remain uncertain though hydrothermal vents have been identified as sources and sinks. This study examines RDOC sinks in the deep Pacific Ocean, highlighting the role of submarine hydrothermal systems. Geochemical survey data from GO‐SHIP and GEOTRACES projects, alongside specific investigations of Pacific hydrothermal systems, suggest that particulate iron introduced by hydrothermal systems plays a key role in scavenging DOC and delivering it to the seafloor, leaving a deficit in the RDOC of the deep ocean. Dilution of the oceanic water column by hydrothermal fluids exhibiting low DOC concentrations likely plays a secondary role. 

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. 

The process of seeking, sampling, and characterizing deep hydrothermal systems is benefited by the use of autonomous underwater vehicles (AUVs) equipped with in situ sensors. Traditional AUV operations require multiple deployments with manual data analysis by ship-board scientists. Development of advanced autonomous methods that analyze in situ data in real-time and allow the vehicle itself to make decisions would improve the efficiency of operations and enable new frontiers in exploration at hydrothermal systems on Ocean Worlds. Adaptive robotic decision making is facilitated by computational models of hydrothermal systems and selected in situ sensors used to refine and validate these predictions. Improving autonomous missions requires better models, and thus an understanding of how different sensors respond to hydrothermally altered seawater. During cruise AT50-15 (Juan De Fuca Ridge, 2023), we performed surveys of the hydrothermal plumes at the Endeavour Segment with AUV Sentry to investigate the utility of in situ sensors measuring tracers such as oxidation-reduction potential, optical backscatter, methane abundance, conductivity, and temperature, for building working models of plume dynamics. We investigated length scales of under 1 km to 5 km with a focus on reoccupying locations over varying time scales. Persistent deep current data were available through the Ocean Networks Canada mooring array. Using these datasets, we investigate two questions: (1) how reliably and at what length scales can real-time current information be used to predict the location and source of a hydrothermal plume? (2) How does the relative age (hence, biogeochemical maturation) of the hydrothermal plume fluid affect the response of different in situ sensors? These results will be used to inform the development of autonomous plume detection algorithms that use real-time, in situ data with the purpose of improving AUV exploration of hydrothermal plumes on Earth and other Ocean Worlds. 

Deep-sea hydrothermal vents inject dissolved and particulate metals, dissolved gasses, and biological matter into the water column, creating plumes several hundred meters above the seafloor that can be traced thousands of kilometers. To understand the impact of these plumes, rosettes equipped with sample bottles and in situ instruments, e.g., for turbidity, oxidation-reduction potential, and temperature, have been key tools for collecting water column fluid for informative ex situ analysis. However, deploying rosettes strategically in distal (>1km) plume-derived fluids is difficult when plume material is entrained rapidly with background water and transported by complicated bathymetric, internal, and/or tidal currents. This problem is exacerbated when the controlling dynamics are also poorly constrained (e.g., no persistent monitoring, few historical data) and data collected while in the field to estimate or compensate for these dynamics are only available to be analyzed hours or days following an asset deployment. Autonomous underwater vehicles (AUVs) equipped with equivalent in situ instruments to rosettes excel at exploration missions and creating highly-resolved maps at different spatial scales. Utilization of AUVs for hydrothermal plume charting and strategic sampling with rosettes is at a techno-scientific frontier that requires new data transmission and visualization interfaces for supporting real-time evidence-based operational decisions made at sea. We formulated a method for monitoring in situ water properties while an AUV is underway that (1) builds situational awareness of deep fluid mass distributions, (2) allows scientists-in-the-loop to rapidly identify fluid distribution patterns that inform adaptations to AUV missions or deployments of other assets, like rosettes, for targeted sample collection, and (3) supports robust formulation of working hypotheses of plume dynamics for in-field investigation. We will present a description of the method with preliminary results from cruise AT50-15 (Juan de Fuca Ridge, 2023) using AUV Sentry and discuss how supervised autonomy will improve ocean robotics for future science missions. 

A multiscale numerical framework has been developed to investigate the dispersion of deep-sea hydrothermal plumes that originate from the Endeavour Segment of the Juan de Fuca Ridge located in the Northeast Pacific. The analysis of simulation outputs presented in this study provides insights into the influences of tidal forcing and the buoyancy flux associated with hydrothermal venting on ocean circulation and plume dispersion in the presence of pronounced seafloor topography. The results indicate that tidal forcing drives anti-cyclonic circulation near the ridge-axis, while hydrothermal venting induces cyclonic circulation around vent fields within the axial rift valley. Tidal forcing has a notable impact on plume dispersion, particularly near the large topographic features to the north of the Endeavour Segment. Furthermore, plume dispersion exhibits notable inter-annual variability, with a northbound trajectory in 2016 and a southbound trajectory in 2021. The study also reveals that both buoyancy fluxes and tidal forcing enhance the mixing of hydrothermal plumes with ambient seawater.