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Cabled deep-sea observatories can deliver continuous power and communications linkages to seafloor instruments and sampling devices. Building on this capability, a nine-month time series of hydrothermal vent fluids was recently collected from the well-studied Main Endeavour Field on the Juan de Fuca Ridge (northeast Pacific Ocean) using a novel remotely triggered vent fluid sampling system connected to Ocean Network Canada's NEPTUNE observatory (Seyfried et al., 2022). These samples exhibit very low Mg concentrations (Mg = 0.19–3.07 mmol/kg), indicative of little-to-no contamination by ambient seawater, providing excellent insight into sub-seafloor hydrothermal processes. Here, we present analyses of these samples for transition metals identified as biological nutrients (V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo, Cd, and W) and identify possible controlling processes. Overall, nutrient transition metal concentrations in collected samples reflect dynamic responses to subtle deep-seated and near-surface changes in the hydrothermal system. Roughly two months into the deployment, inmixing of a Mg- and sulfate-rich fluid is decoupled from observed changes in vent fluid temperature but coincides with noticeable decreases in fluid Co and Mo concentrations, likely indicating subtle or more deep-seated cooling of the system and subsurface deposition of these temperature-sensitive metals. Several months later, a ∼20 °C drop in vent fluid temperature from 304 °C to 280–285 °C over ∼20 hours is accompanied by ∼90% decreases in Cu, Zn, and Cd concentrations and an additional decrease in Mo attributable to precipitation of metal sulfides, presumably in the shallow subsurface. Relative stability in concentrations of other metals (V, Cr, Mn, Ni, W) suggests more deeply seated higher-temperature controls, though covariations in Cr and Ni concentrations decoupled from vent fluid temperature suggest subtle, temporally variable lithologic controls. Molybdenum concentrations (29–220 nmol/kg) are higher than expected based on previous analyses of seafloor hydrothermal vent fluids and do not reflect contamination by modern Mo-rich seawater. This finding has implications for understandings of hydrothermal Mo delivery to the ocean, relevant to hypotheses about the evolution of Mo-dependent biological pathways among early life forms in anoxic and Mo-poor ocean environments thought to be prevalent throughout the Archean Eon. • A deep-sea remote sampler collected a time series of seafloor vent fluids with little-to-no seawater contamination. • Fluid metal concentrations reflect variability in vent fluid exit temperatures and subseafloor mineral deposition. • Molybdenum concentrations indicate a net source of Mo to the ocean, in contrast to previous studies. • Hydrothermal Mo is important to the biogeochemical evolution of Mo-dependent methanogenesis and nitrogen fixation pathways. [ABSTRACT FROM AUTHOR]

Seafloor hydrothermal fluids play a critical role in regulating a wide range of geochemical and biological processes. Chemical and physical changes to venting fluids undoubtedly occur in response to geological events, at depth or near the seafloor, with corresponding effects on vent fluid chemistry, mineralization, and biological activity. However, the uncertain timing of such events makes it impractical to anticipate their occurrence. Thus, the temporal evolution of seafloor vent fluids and coexisting mineral deposits is most often inferred from observations made using conventional deep submergence assets that may be non-continuous for a specific vent field. The recent development of submarine volcanic observatories operated by Ocean Networks Canada (ONC) and the Ocean Observatory Initiative (OOI) in the U.S. that deliver power to instruments on the seafloor and communications via high-speed fiber optic cable permits real-time communication and monitoring of seafloor vents. Here, we describe the development and application of a remotely operated hydrothermal fluid sampler that enables repeat on-demand sampling of high-temperature vent fluids triggered through the internet. The sampler was deployed from September 2019 to June 2020 at the S&M vent area, in the southern portion of Main Endeavour Field (MEF), Juan de Fuca Ridge, on ONC's NEPTUNE ocean observatory. Nine vent fluid samples were acquired over a period of nine months. Analyses of these samples confirm the moderately high-temperature origin of the source fluid, as indicated, for example, by dissolved chloride depletion relative to seawater (vapor-rich) and moderately high silica concentrations. These and other dissolved species, such as methane, a known hallmark of MEF vent fluids, are in excellent agreement with the reported composition of S&M vent fluids over the past 15 years or more, suggesting overall chemical stability and rock-dominated alteration processes. On the other hand, short-term variability of dissolved Mg, sulfate and barium indicate entrainment of conductively heated and partially reacted seawater. Both before and after the incursion of the "secondary" seawater derived fluid, however, samples reveal the lowest Mg and sulfate concentrations yet reported for seafloor hydrothermal vent fluid samples. Accordingly, these data provide new insight on the solubility of these and other elements coexisting with minerals close to the Axial Magma Chamber (AMC), from which the primary source fluid is ultimately derived. A separate noteworthy event characterized by ∼20 °C decrease in temperature and associated 24–77% decrease in dissolved Fe concentration was observed toward the end of the deployment, with implications for hydrothermal transition metal fluxes and linked biogeochemical processes. On-demand remote acquisition of a continuous series of high-temperature vent fluid samples from a single vent permits the temporal evolution of heat and mass transfer processes to be studied with a heretofore unavailable perspective. Furthermore, deployment of this sampler within the NEPTUNE ocean observatory opens the door to future studies based on comparative analysis of contemporaneous datasets. • A new remotely operated fluid sampler collected a nine-month time series of seafloor vent fluids from Main Endeavour Field. • Samples contain minimal seawater, allowing close examination of sub-seafloor hydrothermal processes. • Comparison with a ∼30-year record of vent fluid samples suggests stable high-temperature reaction conditions. • Short-term variations in magnesium, sulfate and barium indicate transient incursions of partially reacted seawater. • A rapid ∼20 °C cooling, decoupled from magnesium and sulfate, is consistent with increased cooling during vent fluid up flow. [ABSTRACT FROM AUTHOR]